JP6629507B2 - Printed wiring board and liquid level sensor - Google Patents

Description

  The present invention relates to a printed wiring board used for a capacitance type liquid level sensor, and a liquid level sensor using the printed wiring board.

2. Description of the Related Art A liquid level sensor that detects a liquid level based on a change in capacitance is known.
For example, the liquid level sensor described in Patent Literature 1 detects capacitance using a pair of electrodes formed on a base material. This electrode is immersed in a fuel such as gasoline.

  When the liquid level changes, in a region where the electrodes are formed (electrode region), the ratio of the region where the fuel is immersed (fuel occupancy) changes, and accordingly, the capacitance between the electrodes changes. Therefore, in this technique, the liquid level is obtained by detecting the capacitance.

JP 2012-225788 A

  In this technique, it is assumed that the actual liquid level and the liquid level calculated based on the capacitance are within a predetermined error range. However, according to various tests, the error is larger than the predetermined range. It turns out that it can be large. For example, even when the occupation ratio of the fuel in the electrode region is equal, the capacitance between the electrodes may be different depending on the state of the base material. In this case, the capacitance is calculated based on the actual liquid level and the capacitance. The liquid level greatly deviates.

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide a printed wiring board having at least a pair of electrodes, when the occupation ratio of the liquid to the electrode region is a predetermined value, the electrostatic capacitance between the electrodes is reduced. An object of the present invention is to provide a printed wiring board having a constant capacity and a liquid level sensor provided with the printed wiring board.

  The printed wiring board according to the present invention includes a base material, a wiring pattern formed on the base material, and a coverlay that covers the wiring pattern, and the wiring pattern detects at least capacitance between electrodes. Including a pair of electrodes, each of the base material and the coverlay has at least one layer of a liquid crystal polymer and a fluororesin.

  According to the present invention, it is possible to provide a printed wiring board having a constant capacitance between electrodes when a liquid occupation ratio is a predetermined value in an electrode region, and a liquid level sensor including the printed wiring board. it can.

FIG. 2 is a schematic view of the printed wiring board according to the first embodiment. FIG. 2 is a sectional view taken along line AA of FIG. 1. It is a schematic diagram of a liquid level sensor. FIG. 4A is a schematic view showing a immersion state before refueling, and FIG. 4B is a schematic view showing an immersion state immediately after refueling, with respect to the printed wiring board of the liquid level sensor. (c) is a schematic diagram showing an immersion state after a lapse of a predetermined time from refueling. FIG. 5A is a plan view of a printed wiring board according to the second embodiment, and FIG. 5B is an enlarged view near the electrode pair of the printed wiring board. FIG. 6 is a plan view of a modified example of the printed wiring board according to the second embodiment. FIG. 7A is a cross-sectional view of a printed wiring board according to the third embodiment, and FIG. 7B is a side view of the printed wiring board. It is sectional drawing of the printed wiring board which concerns on 4th Embodiment. It is sectional drawing of the printed wiring board which concerns on 5th Embodiment. It is sectional drawing of the printed wiring board which concerns on 6th Embodiment. It is sectional drawing of the printed wiring board which concerns on 7th Embodiment. It is sectional drawing of the printed wiring board which concerns on 8th Embodiment. It is sectional drawing of the printed wiring board which concerns on 9th Embodiment. It is sectional drawing of the printed wiring board which concerns on 10th Embodiment.

[Description of Embodiment of the Present Invention]
First, embodiments of the present invention will be listed and described.
A printed wiring board according to the present invention includes (1) a base material, a wiring pattern formed on the base material, and a cover lay covering the wiring pattern, wherein the wiring pattern reduces the capacitance between electrodes. The substrate and the coverlay each include at least one pair of electrodes to be detected, and each of the substrate and the coverlay has at least one layer of a liquid crystal polymer and a fluororesin. Here, the liquid crystal polymer layer indicates a layer containing a liquid crystal polymer as a main component (a layer containing a liquid crystal polymer in a mass ratio of 50% by mass or more), and the fluororesin layer includes a fluororesin as a main component. 1 shows a layer (a layer containing 50% by mass or more of a fluororesin in mass ratio).

The capacitance between the electrodes changes due to the presence of the surrounding material.
When the printed wiring board is immersed in a liquid, the capacitance changes in a region where the electrodes are formed (hereinafter, referred to as an “electrode region”) according to a ratio of a portion occupied by the liquid.

  By the way, in the state where the base material and the cover lay absorb liquid, and in the state where the base material and the cover lay do not absorb liquid, even if the proportion of the liquid occupied in the electrode region is the same, Due to the influence of the absorbed liquid, the capacitance between the electrodes differs in both states. Then, as the liquid absorptivity of the base material and the coverlay increases, the difference in capacitance between the two states increases. That is, the capacitance changes depending on the liquid absorption rate.

  In the above configuration, each of the substrate and the cover lay of the printed wiring board has at least one layer of a liquid crystal polymer and a fluororesin. Liquid crystal polymers and fluororesins have lower water and fuel absorptivity than resins used for printed wiring boards, such as polyimide resins and epoxy resins. Here, the fuel means a combustible liquid such as petroleum such as gasoline, light oil, or heavy oil, and vegetable oil or alcohol.

  For this reason, the printed wiring board according to the present invention has a smaller characteristic change caused by the absorption of water or fuel than the printed wiring board having the conventional structure. That is, when the ratio of the portion occupied by the liquid in the electrode region is the same in the state where the base material and the coverlay absorb the liquid and the state where the base material and the coverlay do not absorb the liquid, The difference in capacitance between the electrodes in both states is smaller than that of a printed wiring board having a conventional structure. That is, even when the printed wiring board is immersed in the liquid or the immersion time is different, the capacitance when the occupation ratio of the liquid to the electrode region is a predetermined value is constant.

(2) In the printed wiring board, it is preferable that both the base material and the cover lay are formed of a liquid crystal polymer.
In this configuration, the base material and the coverlay are formed of a liquid crystal polymer. Liquid crystal polymers have lower water and fuel absorption rates than resins used for printed wiring boards, such as polyimide resins and epoxy resins.

  For this reason, the printed wiring board has a smaller characteristic change caused by the absorption of water or fuel than the conventional printed wiring board. That is, when the ratio of the portion occupied by the liquid in the electrode region is the same in the state where the base material and the coverlay absorb the liquid and the state where the base material and the coverlay do not absorb the liquid, The difference in capacitance between the electrodes in both states is smaller than that of a printed wiring board having a conventional structure. That is, even when the printed wiring board is immersed in the liquid or the immersion time is different, the capacitance when the occupation ratio of the liquid to the electrode region is a predetermined value is constant.

  (3) In the printed wiring board, the base material and the coverlay are both formed of a liquid crystal polymer, and edges of the pair of electrodes of the wiring pattern are separated by a predetermined interval. According to this configuration, since the edges of the pair of electrodes are separated by a predetermined distance, the capacitance of the liquid can be detected by the pair of electrodes.

(4) In the printed wiring board, it is preferable that the coverlay is laminated on the base material via an adhesive layer formed of a liquid crystal polymer.
According to this configuration, since the adhesive is interposed between the cover lay and the base material, bubbles or voids are formed at the periphery of the wiring pattern sandwiched between the cover lay and the base material during manufacturing. As a result, bubbles and voids existing between the coverlay and the base material are reduced. As a result, deterioration due to peeling or the like caused by bubbles or voids is small, and a change in characteristics over time or a change in characteristics due to a change in temperature is reduced.

(5) In the printed wiring board, preferably, the melting point of the adhesive layer is lower than the melting point of the base material and lower than the melting point of the cover lay.
According to this configuration, at the time of manufacturing, when the adhesive layer is softened by thermocompression bonding to adhere the base material and the cover lay, deformation of the base material and the cover lay can be suppressed. Therefore, the dimensional accuracy of the printed wiring board is high. Further, since the adhesive layer is softened at the time of thermocompression bonding, bubbles and voids existing between the coverlay and the base material are reduced as compared with the structure without the adhesive layer.

  (6) In the printed wiring board, the base is covered with a first fluororesin layer formed of a fluororesin having a higher melting point than the base, and the coverlay has a higher melting point than the coverlay. It is preferable to be covered with a second fluororesin layer formed of fluororesin.

  According to this configuration, the fluororesin has a lower absorptivity of alcohol, particularly methyl alcohol, than the liquid crystal polymer. For this reason, in the printed wiring board, the characteristic change caused by the absorption of methyl alcohol is reduced.

  (7) In the printed wiring board, at least one of the first fluororesin layer and the second fluororesin layer preferably includes at least one of a filler, a nonwoven fabric, and a fiber woven sheet.

  According to this configuration, the rigidity of the printed wiring board is increased, and expansion and contraction between the electrodes due to deformation of the printed wiring board is suppressed. Therefore, the detection accuracy of the capacitance is improved. In the manufacturing stage, a fluororesin sheet including a filler, a nonwoven fabric, and a fiber woven sheet is used as a material for forming the first fluororesin layer or the second fluororesin layer. Since such a sheet has high rigidity and is hard to wrinkle, the above-described configuration improves the productivity of the printed wiring board.

  (8) In the printed wiring board, it is preferable that the wiring pattern is formed on the base material via a first adhesive layer, and the coverlay is laminated on the base material via a second adhesive layer. According to this configuration, in the manufacturing process, bubbles and voids are unlikely to enter between the base material and the coverlay or between the electrodes. For this reason, this printed wiring board has few bubbles and voids. As a result, deterioration due to peeling or the like due to bubbles or voids is small, and a change in characteristics over time or a change in characteristics due to a change in temperature is reduced.

(9) In the printed wiring board, it is preferable that an end of the base material and an end of the cover lay are sealed so that the first and second adhesive layers are not exposed.
In the state where the first and second adhesive layers absorb liquid and the state where the first and second adhesive layers do not absorb liquid, even when the ratio of the portion occupied by the liquid in the electrode region is equal, Due to the effect of the liquid, the capacitance between the electrodes becomes different in both states. For this reason, when the printed wiring board has the first and second adhesive layers, it is preferable to configure the printed wiring board so that the first and second adhesive layers do not absorb liquid. Therefore, in the above configuration, the end of the base material and the end of the cover lay are sealed so that the first and second adhesive layers are not exposed. Thereby, it is possible to suppress the first and second adhesive layers from absorbing the liquid.

  (10) In the printed wiring board, it is preferable that an end of the base material and an end of the cover lay are sealed with a liquid crystal polymer member. According to this configuration, the end of the base material and the end of the cover lay can be easily sealed.

  (11) In the printed wiring board, a configuration is employed in which an end of the base material is folded back and pressed against the coverlay, or an end portion of the coverlay is turned back and pressed against the base material. You can also. Also in this configuration, the end of the base material and the end of the cover lay can be easily sealed.

(12) In the printed wiring board, it is preferable that each of the base material and the cover lay is formed of a fluororesin.
In this configuration, the base material and the coverlay are formed of a fluororesin. Fluororesins have lower water and fuel absorption rates than resins used for printed wiring boards, such as polyimide resins and epoxy resins.

  For this reason, the printed wiring board has a smaller characteristic change caused by the absorption of water or fuel than the conventional printed wiring board. As a result, even when the printed wiring board is immersed in the liquid or the immersion time is different, the capacitance when the occupation ratio of the liquid to the electrode region is a predetermined value becomes constant.

(13) In the printed wiring board, the cover lay is laminated to the base material via an adhesive layer formed of a liquid crystal polymer having a lower melting point than a fluororesin.
According to this configuration, at the time of manufacturing, when the adhesive layer is softened by thermocompression bonding to adhere the base material and the cover lay, deformation of the base material and the cover lay can be suppressed. Therefore, the dimensional accuracy of the printed wiring board is high. In addition, since the liquid crystal polymer is used as the adhesive layer, the water absorption of the adhesive layer is lower than that in which an epoxy resin is used. This reduces the change in capacitance when the occupation ratio of the liquid to the electrode region is a predetermined value.

(14) In the printed wiring board, each of the base material and the cover lay includes at least one of a filler, a nonwoven fabric, and a fiber woven sheet.
According to this configuration, the rigidity of the printed wiring board is increased, and expansion and contraction between the electrodes due to deformation of the printed wiring board is suppressed. Therefore, the detection accuracy of the capacitance is improved. In the manufacturing stage, a fluororesin sheet including a filler, a nonwoven fabric, and a fiber woven sheet is used as a material for forming the first fluororesin layer or the second fluororesin layer. Since such a sheet has high rigidity and is hard to wrinkle, the above-described configuration improves the productivity of the printed wiring board.

  (15) In the printed wiring board, the wiring pattern has a pair of electrodes formed on the same plane, and each of the electrodes is arranged so as to have a constant width from one end to the other end. And the flow direction of the liquid crystal polymer of the base material and the coverlay preferably coincides with each other.

  A substrate or cover lay formed of a liquid crystal polymer has anisotropy in linear expansion coefficient. That is, the linear expansion coefficient of the liquid crystal polymer in the flow direction is smaller than the linear expansion coefficient in the direction perpendicular to the flow direction.

On the other hand, when a printed wiring board is used for detecting a change in liquid level as a change in capacitance, the printed wiring board is arranged such that the liquid level moves along the direction in which the electrodes extend.
In order to reduce the dispersion of the measured values every time in the measurement of the liquid level, it is preferable that the dimensional change of the dimension along the extension direction of the electrode on the substrate and the coverlay is small. Therefore, in this configuration, the direction in which the electrodes extend and the direction in which the liquid crystal polymer flows on the base material and the coverlay are the same. This makes it difficult for the base material and the cover lay to extend in the extension direction of the electrode. As a result, when the printed wiring board is used for a liquid level sensor, it is possible to reduce the variation in the measured value of the liquid level caused by the temperature change.

  (16) In the printed wiring board, the wiring pattern may be configured such that the wiring pattern has a plurality of pairs of electrodes. In this case, a change in capacitance can be detected in each of the electrode pairs.

When a printed wiring board is used for detecting a change in liquid level as a change in capacitance, the printed wiring board is arranged so that the liquid level moves along the arrangement direction of the electrode pairs.
When the printed wiring board is used for such a purpose, it is possible to detect the presence or absence of a liquid such as fuel in each of the electrode pairs. For this reason, the liquid level can be detected with constant accuracy regardless of the amount of fuel absorbed by the base material and the coverlay.

  (17) In the printed wiring board, the wiring pattern may include a pair of electrodes arranged to face each other. With such a configuration, the liquid level of fuel or the like can be detected based on the capacitance between the electrodes.

  (18) A liquid level sensor according to the present invention has the printed wiring board. The capacitance of the printed wiring board is constant when the occupation ratio of the liquid to the electrode region is a predetermined value. For this reason, a liquid level detection error in the liquid level sensor can be reduced.

[Details of the embodiment of the present invention]
A specific example of a printed wiring board according to an embodiment of the present invention will be described below with reference to the drawings. It should be noted that the present invention is not limited to these exemplifications, but is indicated by the scope of the claims, and includes the meaning equivalent to the scope of the claims and all modifications within the scope of the claims. Is intended.

[Printed wiring board]
A printed wiring board 1 according to a first embodiment will be described with reference to FIGS. As shown in FIGS. 1 and 2, the printed wiring board 1 includes a base material 2, a wiring pattern 4, and a cover lay 3.

The base material 2 and the coverlay 3 are both formed of a rectangular resin sheet.
The wiring pattern 4 is formed on the substrate 2, and the wiring pattern 4 (excluding the terminal portions 4 b) is covered with the coverlay 3.

  The base material 2 and the coverlay 3 are both formed of a resin having a low absorption rate of the fuel 30. In addition to this configuration, it is preferable that both the base material 2 and the cover lay 3 have high heat resistance. Further, it is preferable that both the base material 2 and the cover lay 3 have a small linear expansion coefficient.

  For example, the base material 2 and the coverlay 3 are both formed of a resin containing a liquid crystal polymer as a main component. Here, the main component indicates that the ratio of the liquid crystal polymer to the resin is 50% by mass or more. Preferably, the substrate 2 and the coverlay 3 are formed of a resin having a liquid crystal polymer of 90% by mass or more. Suitable liquid crystal polymers for forming the base material 2 and the coverlay 3 include polycondensates of ethylene terephthalate and parahydroxybenzoic acid, polycondensates of phthalic acid, biphenol and parahydroxybenzoic acid, 6-hydroxy And polycondensates of 2-naphthoic acid and parahydroxybenzoic acid.

  The thickness of the substrate 2 is preferably 20 μm or more and 100 μm or less. More preferably, the thickness of the substrate 2 is 25 μm or more and 50 μm or less. The thickness of the coverlay 3 is set in the same manner as the thickness of the base material 2. That is, the thickness of the cover lay 3 is preferably 20 μm or more and 100 μm or less, and more preferably 25 μm or more and 50 μm or less.

  Such a setting is because when the thickness of the base material 2 or the cover lay 3 becomes too large, the capacitance between the electrodes 4a becomes too small, and the detection accuracy of the fuel 30 becomes too low. Further, the detection accuracy of the fuel 30 is reduced due to an increase in the amount of fuel absorbed by the base material 2 or the coverlay 3. On the other hand, if the thickness of the base material 2 or the cover lay 3 is too small, the sealing function of the base material 2 or the cover lay 3 is reduced. For example, if the base material 2 or the cover lay 3 is damaged, fuel or air may enter from this portion and the wiring pattern 4 may be deteriorated.

  Further, in these liquid crystal polymers, when a mixture of isooctane and toluene in a volume ratio of 1: 1 is used as the fuel 30, the absorption rate of the fuel 30 is preferably 2% by mass or less. Is preferably 1.0% by mass or less. Thereby, the change in the capacitance between the electrodes 4a due to the absorption of the fuel 30 can be suppressed.

  Here, the absorptivity indicates a value (expressed as a percentage) obtained by dividing the mass when the mass of the liquid crystal polymer does not change after being immersed in the fuel 30 by the mass of the liquid crystal polymer when dried. The reason for defining the properties of the liquid crystal polymer using the fuel 30 in which isooctane and toluene are mixed at a volume ratio of 1 to 1 is that there are various types of general fuels 30 on the market and the same name is used. This is because the components may be different even in the case of the fuel 30, and the characteristics cannot be defined using such a general fuel 30.

In these liquid crystal polymers, the linear expansion coefficient in the flow direction DF is 10 × 10 ⁻⁵ or less and the linear expansion coefficient in the perpendicular direction DR is 20 × 10 ⁻⁵ or less in the range of −20 ° C. to 240 ° C. preferable. More preferably, in the range of −20 ° C. to 240 ° C., the linear expansion coefficient in the flow direction DF is 2 × 10 ⁻⁵ or less, and the linear expansion coefficient in the perpendicular direction DR is 10 × 10 ⁻⁵ or less. Here, the flow direction DF indicates the flow direction of the liquid crystal polymer formed at the time of molding (that is, the orientation direction of the liquid crystal polymer), and the perpendicular direction DR indicates the direction perpendicular to the flow direction. .

The wiring pattern 4 has a pair of electrodes 4a. These electrodes 4 a are arranged on the substrate 2. That is, the electrodes 4a are arranged on the same plane.
Each electrode 4a is provided with a terminal portion 4b for connecting to an electric circuit of the measuring section 11 (described later). Each of the terminal portions 4 b is arranged on one end side of the base material 2.

  The two electrodes 4a are formed in parallel with each other so that the interval Sa has a constant width from one end to the other end. The constant width means that the interval Sa between the electrodes 4a from one end to the other end (see FIG. 1; the interval Sa between the edge of one electrode 4a and the edge of the other electrode 4a) is within a predetermined distance range. Indicates that there is. For example, the interval Sa is set to 200 μm ± 100 μm.

  The maximum value of the interval Sa (the maximum value of the set dimension of the interval Sa) is preferably 500 μm, and the minimum value (the minimum value of the set dimension of the interval Sa) is preferably 50 μm. If the interval Sa exceeds 500 μm, the capacitance between the electrodes 4a becomes too small, and the accuracy of detecting the liquid level of the fuel 30 is too low. On the other hand, if the interval Sa is smaller than 50 μm, it becomes difficult to flow the liquid crystal polymer between the electrodes 4a, so that the reliability as a sensor is reduced due to an increase in detection variation. Preferably, each of the pair of electrodes 4a is a rectangle having the same shape and the same area.

Further, it is preferable that each of the electrodes 4a is formed such that its extending direction DE (longitudinal direction) and the flow direction DF of the liquid crystal polymer coincide with each other.
This is for the following reason. The liquid crystal polymer resin sheet (base material 2 and cover lay 3) has anisotropy in linear expansion coefficient. That is, in the resin sheet, the coefficient of linear expansion in the flow direction DF is smaller than the coefficient of linear expansion in the perpendicular direction DR. On the other hand, when the printed wiring board 1 as a component of the liquid level sensor 10 is placed in the fuel tank 20 (see FIG. 3), the printed wiring board 1 is placed so that its electrodes 4a are along the vertical direction DL. . With such an arrangement, the liquid level of the fuel 30 moves along the extension direction DE of the electrode 4a of the printed wiring board 1. However, when the dimension of the electrode 4a in the extension direction DE changes, the accuracy of detecting the liquid level decreases. For this reason, it is preferable that this type of printed wiring board 1 has a small dimensional change in the dimension along the electrode 4a (extension direction DE). For this reason, for each of the base material 2 and the cover lay 3, it is preferable that the flow direction DF and the extension direction DE of the electrode 4a match.

  The wiring pattern 4 is formed of copper, gold, silver, nickel, or a laminate thereof. Such a wiring pattern 4 is formed by a subtractive method or a semi-additive method. It is preferable that the thickness of the wiring pattern 4 is 12 μm or more and 18 μm or less.

The wiring pattern 4 is sealed by the base material 2 and the cover lay 3.
For example, the base material 2 on which the wiring pattern 4 is formed and the cover lay 3 are pressure-bonded in a vacuum chamber. Thereby, the wiring pattern 4 is sealed so that no bubbles or voids remain between the base material 2 and the coverlay 3.

  In addition, in order to ensure the sealing of the wiring pattern 4, the distance L (see FIG. 1) between the edge of the electrode 4 a of the wiring pattern 4 and the edge of the base material 2 and the coverlay 3 is set to 0. It is preferable to set it to 5 mm or more. When the distance is smaller than 0.5 mm, the width of the bonding portion between the base material 2 and the cover lay 3 becomes too narrow, and the possibility that a gap is generated between the base material 2 and the cover lay 3 increases. .

[Liquid level sensor]
FIG. 3 shows a schematic diagram of the liquid level sensor 10. FIG. 3 shows an example in which the liquid level sensor 10 is arranged in the fuel tank 20.

  The liquid level sensor 10 includes the printed wiring board 1 according to the above embodiment, and a measuring unit 11 that measures the capacitance between the electrodes 4a. The measuring unit 11 and the printed wiring board 1 are connected by a conducting wire 12.

  The measuring unit 11 periodically measures the capacitance between the electrodes 4a. The capacitance is measured by, for example, a resonance method or a method of inputting a frequency signal. In the resonance method, a coil is arranged in series or in parallel with a pair of electrodes 4a. This coil is arranged inside the measuring section 11 in FIG.

  The printed wiring board 1 is disposed in the fuel tank 20 such that the extension direction DE of the electrode 4a matches the vertical direction. Therefore, when the liquid level of the fuel 30 changes, the liquid level moves along the extension direction DE of the electrode 4a.

  Further, when the liquid level of the fuel 30 changes, a region corresponding to the pair of electrodes 4a (a region corresponding to the portion where the wiring pattern 4 is formed; a region surrounded by a broken line in FIG. 3). In the “electrode area AE”, the ratio of the occupied portion SA of the air (the occupation ratio of the air) and the ratio of the occupied portion SG of the fuel 30 (ie, the occupation ratio of the fuel 30) change. This ratio correlates with the liquid level. That is, when the liquid level rises, the ratio of the occupied portion SG of the fuel 30 increases and the ratio of the occupied portion SA of the air decreases.

  The capacitance between the electrodes 4a is the sum of the capacitance of the occupied portion SA of the air and the capacitance of the occupied portion SG of the fuel 30 in the electrode region AE. Therefore, the liquid level of the fuel 30 and the capacitance between the electrodes 4a are substantially linearly related.

FIG. 4 shows the immersion state of the printed wiring board 1 before the liquid level changes, after the liquid level changes, and after a lapse of a predetermined time after the liquid level changes.
FIG. 4A shows a state in which the printed wiring board 1 is immersed in a state where the liquid level before refueling is low. FIG. 4B shows a state in which the printed wiring board 1 is immersed in a state where the liquid level is high due to oil supply. FIG. 4C shows a state where the printed wiring board 1 is immersed when a predetermined time has elapsed without using the fuel 30 after refueling.

  When the liquid level in the fuel tank 20 rises due to refueling, the occupied portion SG of the fuel 30 in the electrode region AE increases. Therefore, the capacitance between the electrodes 4a changes. Thereafter, if the liquid level does not change, the capacitance between the electrodes 4a should not change.

  However, in the conventional liquid level sensor having a printed wiring board, even if the fuel 30 is not used (that is, even if the liquid level does not change), the time elapses after the liquid level in the fuel tank 20 is increased by refueling. Then, the capacitance between the electrodes 4a may change. For this reason, the measuring unit 11 detects the capacitance between the electrodes 4a as a different value at both the time immediately after refueling and the time after a predetermined time has elapsed, even if the liquid levels are equal. As a result, the liquid level sensor 10 outputs different liquid levels even though the actual liquid levels are equal at both times.

The cause of such an event is that the base material 2 and the coverlay 3 absorb the fuel 30.
Hereinafter, this point will be described.

  Before the fuel 30 is supplied, when the amount of the fuel 30 gradually decreases, the occupied portion SA of the air in the electrode area AE dries, so that the fuel 30 in this portion (the portion indicated by the symbol SA in FIG. Low absorption.

  As shown in FIG. 4B, when the fuel 30 is refueled and the liquid level of the fuel 30 rises rapidly, the portion occupied by the air SA immediately before refueling changes to the portion SG occupied by the fuel 30. In such a case, the fuel 30 is not absorbed in the base material 2 and the cover lay 3 in this portion (that is, the air occupying portion SA just before refueling; hereinafter, referred to as “immediate air occupying portion SAB”). In FIG. 4B, a portion below the immediately preceding air occupying portion SAB in the electrode region AE is shown as a portion SGB immersed in the fuel 30 before and after refueling.

After the refueling, when the fuel 30 is not used and a predetermined time has elapsed, the base material 2 and the coverlay 3 in the immediately preceding air occupying portion SAB are in a state of absorbing the fuel 30.
That is, there is no change in the liquid level between the time when the fuel 30 is refueled and the time after the fuel 30 is refueled, but the fuel absorption state of the immediately preceding air occupying portion SAB is different. When the fuel 30 is supplied, the fuel 30 is present at a position separated from the surface of the electrode 4a by the thickness of the base material 2 or the coverlay 3.

  On the other hand, at a point in time after a predetermined time has elapsed since the fuel 30 was supplied, the base material 2 or the cover lay 3 absorbs the fuel 30, and thus is separated from the surface of the electrode 4 a by the thickness of the base material 2 or the cover lay 3. The fuel 30 (substance composing the fuel 30) exists from a position closer to the electrode 4a side.

  In this way, at the time when the fuel 30 is supplied and the time after a predetermined time has elapsed after the fuel 30 is supplied, the fuel 30 is separated from the electrode 4a in the direction of the substrate thickness (or the thickness of the coverlay). The difference is that (the substance constituting the fuel 30) starts to exist. For this reason, even if the liquid level is equal at both times, the capacitance will be different.

  A situation in which the capacitances have different values in spite of the same liquid level does not occur only in such a situation. For example, such a situation also occurs due to a difference in temperature conditions around the printed wiring board 1. Since the base material 2 and the coverlay 3 have different absorption rates of the fuel 30 depending on the temperature, even when the liquid level is at a predetermined position, the capacitance detected in winter and the capacitance detected in summer are different. Are different.

The following can be seen from these events.
The capacitance between the electrodes 4a when the fuel 30 is not absorbed by the base material 2 and the cover lay 3, and the capacitance between the electrodes 4a when the fuel 30 is absorbed by the base material 2 and the cover lay 3. Different from capacity. For this reason, it is preferable that the base material 2 and the coverlay 3 have a low absorption rate of the fuel 30.

  Therefore, in the present embodiment, the base material 2 and the cover lay 3 that do not easily absorb the fuel 30 are employed. That is, the influence of the increase or decrease in the amount of fuel contained in the base material 2 and the coverlay 3 on the capacitance between the electrodes 4a is reduced. Thereby, the capacitance can be made constant when the occupation ratio of the liquid to the electrode region AE is a predetermined value. That is, according to this configuration, the variation in capacitance when the occupation ratio of the liquid to the electrode region AE is a predetermined value can be reduced as compared with the conventional printed wiring board.

  Note that “constant” indicates that the error rate of the capacitance is 1% or less, preferably 0.5% or less. Here, the error rate is defined as “(actual measurement value−reference value) / reference value × 100”. The reference value is measured immediately after arranging the printed wiring board 1 using the printed wiring board 1 in which the base material 2 and the cover lay 3 are sufficiently dried, so that the printed wiring board 1 is immersed in the electrode area AE by an area ratio of 100%. 3 shows the measured capacitance. In a state where the printed wiring board 1 is arranged so as to be immersed in the electrode region AE in an area ratio of 100%, the static electricity measured after the substrate 2 and the cover lay 3 are immersed until the fuel absorption reaches a saturated state. Indicates the capacity.

According to the printed wiring board 1, the following effects can be obtained.
(1) The substrate 2 and the coverlay 3 of the printed wiring board 1 are both formed of a liquid crystal polymer. The liquid crystal polymer has a lower water and fuel 30 absorptivity than a resin used for a conventional printed wiring board such as a polyimide resin or an epoxy resin.

  For this reason, the printed wiring board 1 has a smaller characteristic change caused by the absorption of water and the fuel 30 than the printed wiring board having the conventional structure. That is, the comparison between the state in which the base material 2 and the cover lay 3 absorb water and the fuel 30 and the state in which the base material 2 and the cover lay 3 do not absorb water and the fuel 30 indicate that the fuel When the ratio of the portions occupied by 30 is the same value (predetermined value), the difference in capacitance between the electrodes 4a in both states is smaller than in the printed wiring board having the conventional structure. For this reason, the capacitance of the printed wiring board 1 becomes constant when the occupation ratio of the water or the fuel 30 with respect to the electrode region AE is a predetermined value regardless of the immersion time, the immersion state, or the temperature condition.

  (2) In the printed wiring board 1, the wiring pattern 4 has a pair of electrodes 4a formed on the same plane, and the interval between the electrodes 4a is a constant width from one end to the other end of the electrodes 4a. Preferably, there is. According to this configuration, as compared with a configuration in which each of the electrodes 4a does not have a constant width from one end to the other end, the capacitance of the electrode 4A with the change in the occupancy of the water or the fuel 30 with respect to the electrode region AE increases. The rate of change becomes constant, and the relationship between the two approaches a linear expression.

(3) In the printed wiring board 1, it is preferable that the extension direction DE of the electrode 4a and the flow direction DF of the liquid crystal polymer of the base material 2 and the coverlay 3 match.
According to this configuration, the base material 2 and the cover lay 3 are unlikely to extend in the extension direction DE of the electrode 4a. As a result, when the printed wiring board 1 is used for the liquid level sensor 10, it is possible to reduce the variation in the measured value of the liquid level caused by the temperature change.

  (4) The liquid level sensor 10 has the printed wiring board 1 of the present embodiment. The capacitance of the printed wiring board 1 is constant when the occupation ratio of the water or the fuel 30 to the electrode region AE is a predetermined value. As a result, the detection error of the liquid level in the liquid level sensor 10 is reduced, and the detection accuracy of the liquid level is improved.

[Other embodiments]
The above embodiment is an example of the present technology.
Hereinafter, other embodiments will be described.

-The printed wiring board 101 according to the second embodiment will be described with reference to FIG.
The printed wiring board 101 is different from the printed wiring board 1 according to the first embodiment in the structure of the wiring pattern 4. FIG. 5A is a plan view of the printed wiring board 101. FIG. FIG. 5B is an enlarged view near the electrode pair 42 of the printed wiring board 101. In FIG. 5A, some wirings 41 are indicated by dots, and their description is omitted.

The wiring pattern 4 includes a plurality of wiring pairs 40. The wiring pair 40 indicates two wirings 41 that are simultaneously energized when measuring the capacitance.
As shown in FIG. 5A, the wiring 41 has an electrode 41a, a wiring part 41b, and a terminal part 41c. The electrode 41a is arranged at the tip of the wiring part 41b. The terminal portion 41c is arranged at an end of the wiring portion 41b. In the wiring pair 40, the edge of the electrode 41a of the one wiring 41 (the part closest to the other wiring 41) and the edge of the electrode 41a of the other wiring 41 (the part closest to the one wiring 41) are at a predetermined distance. Are arranged at intervals. The predetermined interval is a distance at which the liquid can be detected by the pair of electrodes 41a. The form of the interval is not particularly limited. The interval between the pair of electrodes 41a may be enlarged in the middle from one end to the other end, may be reduced in the middle, or may be a structure in which the enlarged portion and the reduced portion are alternately repeated. It may be constant from one end to the other end. For example, in the wiring pair 40, the electrode 41a of one wiring 41 and the electrode 41a of the other wiring 41 are arranged in the direction DR perpendicular to the liquid crystal polymer so that the interval Sb (see FIG. 5B) has a constant width. It is arranged along. The electrodes 41a form an electrode pair 42. The electrode pairs 42 are arranged in a line along the flow direction DF of the liquid crystal polymer on the base material 2 and the coverlay 3. It is preferable that the maximum value of the interval Sb is 500 μm and the minimum value is 50 μm. If the interval Sb exceeds 500 μm, the capacitance between the electrodes 41 a becomes too small, and the accuracy of detecting the liquid level of the fuel 30 is too low. On the other hand, if the distance Sb is smaller than 50 μm, it becomes difficult for the liquid crystal polymer to flow between the electrodes 41a, so that the reliability as a sensor decreases, for example, the detection variation increases. Here, the interval Sb is an average of the plurality of electrode pairs 42. Further, it is desirable that the variation in the distance Sb between the plurality of electrode pairs 42 provided on the printed wiring board 101 is small. For example, for these electrode pairs 42, the minimum value of the interval Sb may be 0.5 times the average value of the interval Sb, and the maximum value of the interval Sb may be 1.5 times the average value of the interval Sb. desirable.

  In FIG. 5, each of the wiring portions 41b includes a portion along the direction DR of the liquid crystal polymer and a portion along the flow direction DF of the liquid crystal polymer. It is not limited to the form. For example, each of the wiring portions 41b is configured to extend obliquely with respect to the flow direction DF of the liquid crystal polymer, or may be configured to have a curved line due to a limitation on the size of the base material.

According to such a configuration, the liquid level can be detected with constant accuracy regardless of the amount of fuel 30 absorbed by the base material 2 and the coverlay 3. This will be described below.
Since each electrode pair 42 is electrically independent, the capacitance between the electrodes 41a can be measured independently. For example, a predetermined voltage is applied between the wiring pairs 40 in order from the outermost wiring pair 40 toward inward or in the reverse order. Thus, the capacitance between the electrodes 41a of the wiring pair 40 is measured. Then, it is determined whether or not the fuel 30 exists in the portion between the electrodes 41a based on whether or not the capacitance is equal to or larger than the threshold. In this determination, a determination value indicating whether fuel is present or not is output.

  This determination is made for all of the wiring pairs 40, or until the fuel 30 is determined from the “present” state to the “absent” state, or until the fuel 30 is determined from the “absent” state to the “present” state. Done. By such a determination operation, for the row of the wire pairs 40, the wire pair 40 that switches from the “absent” state to the “present” state of the fuel 30 or the wire pair 40 that switches from the “present” state to the “absent” state of the fuel 30. Is detected. The position of the wire pair 40 detected in this manner indicates the liquid level of the fuel 30.

  By the way, in the printed wiring board 1 according to the first embodiment, the liquid level is estimated based on the magnitude of the capacitance between the electrodes 4a. However, in the case of this configuration, the concentration of the substance (for example, the fuel 30) existing between or near the electrodes 4a (change in the concentration gradient of the substance in the thickness direction of the base material 2 or the coverlay 3) is changed. The capacitance between the electrodes 4a changes. Even if the base material 2 and the cover lay 3 are formed of a member having a low absorption rate of the fuel 30, the measurement accuracy for the liquid level is small because the fuel 30 is absorbed to some extent as long as the resin is used as this member. It is considered that there is a limit in further increasing the value of the criterion.

On the other hand, according to the structure of the printed wiring board 101 shown in FIG. 5, a change in capacitance can be detected in each of the electrode pairs 42.
For this reason, if an appropriate threshold value is set, the portion between the electrodes 41a is changed irrespective of the change in the capacitance between the electrodes 41a due to the change in the concentration of the material (material constituting the fuel 30) existing between the electrodes 41a. Can accurately determine the presence or absence of the fuel 30. Thus, the liquid level can be detected with a constant accuracy regardless of the amount of the fuel 30 absorbed by the base material 2 and the coverlay 3.

  In addition, since the base material 2 and the cover lay 3 are formed of a liquid crystal polymer, the capacitance between the electrodes becomes constant, so that the accuracy of determining whether or not fuel exists in each wiring pair 40 is determined. Increase. Thereby, the detection accuracy of the liquid level is improved.

-A modified example of the printed wiring board 101 according to the second embodiment will be described with reference to FIG.
In the printed wiring board 101 according to the second embodiment, the wiring pattern 4 is formed only on one surface of the base material 2. In the printed wiring board 102 according to this modification, the wiring pattern 4 is formed on both surfaces of the base material 2. Is formed. The wiring portion 41b of the wiring pattern 4 has a first wiring portion 41bx arranged on the same surface side as the electrode 41a, and a first wiring portion 41bx connected to the first wiring portion 41bx via a through hole or a via hole, and a surface on which the electrode 41a is arranged. And the second wiring portion 41by disposed on the opposite surface. The terminal portion 41c is provided at an end of the second wiring portion 41by.

Further, the structure of the wiring pattern 4 differs from the wiring pattern 4 of the printed wiring board 101 according to the second embodiment in the following points.
The electrode 41a group on one side of the plurality of electrode pairs 42 arranged in a line is divided into a predetermined number of sets, and each electrode 41a in the same set is connected to one second wiring portion 41by. With such a configuration, the number of the second wiring portions 41by is reduced, and the area of the printed wiring board 102 is reduced.

-The printed wiring board 201 according to the third embodiment will be described with reference to FIG.
7A is a cross-sectional view of the printed wiring board 201, and FIG. 7B is a side view of the printed wiring board 201.

  This printed wiring board 201 differs from the printed wiring board 1 according to the first embodiment in the electrode arrangement structure. In the printed wiring board 1 shown in FIG. 1, two electrodes 4a are arranged on the same plane. In the printed wiring board 201, two electrodes 51a are arranged so as to face each other, and the respective opposing surfaces are provided. Are set in parallel. Such a configuration is realized by bending the printed wiring board 201 at an intermediate portion between the electrodes 51a. The bending positions are not limited to the positions shown in FIG. 7, and the bending positions may be determined by individual designs so that the paired electrodes 51a face each other. In order to obtain such a structure that requires bending, the printed wiring board 201 preferably has flexibility. Also, in the structure shown in FIGS. 3 to 6, since the printed wiring boards 1 and 101 have flexibility, the printed wiring is formed along a non-planar structure, for example, a wall surface of the cylindrical fuel tank 20. Since the plates 1 and 101 can be tightly fixed in close contact, highly accurate liquid level detection is possible. For this reason, it is preferable that the printed wiring boards 1, 101, and 102 have flexibility.

  Each of the electrodes 51a is formed in a rectangular shape. Each of the electrodes 51a is configured such that its extension direction DE (longitudinal direction) coincides with the flow direction DF of the base material 2 and the coverlay 3.

  The two electrodes 51a are arranged so that the interval Sc has a constant width from one end to the other end. The constant width indicates that the distance Sc between the two electrodes 51a from one end to the other end is within a predetermined distance range. In order to realize this, a spacer 52 is disposed between the two electrodes 51a around both ends of the electrode 51a. The fuel 30 flows freely in the space between the electrodes 51a.

The interval Sc between the two electrodes 51a is set in consideration of the following.
If the distance Sc between the two electrodes 51a is too small, the fuel 30 spreads between the two electrodes 51a due to capillary action. In consideration of this, the interval Sc is set so that the fluctuation of the liquid level due to the capillary phenomenon does not become too large. On the other hand, when the interval Sc between the electrodes 51a is too large, the measured capacitance becomes small, and the measurement accuracy of the liquid level decreases. Note that the electrode area may be increased to compensate for a decrease in the liquid level measurement accuracy.

According to the printed wiring board 201, similarly to the printed wiring board 1 according to the first embodiment, the liquid level of the fuel 30 or the like can be detected based on the capacitance between the electrodes 51a.
Further, since the printed wiring board 201 has a pair of electrodes 51a facing each other on the facing surface, the maximum capacitance between the electrodes 51a can be changed by changing the electrode area. For this reason, in this configuration, it is possible to relatively easily change the measurement accuracy of the capacitance.

Note that the printed wiring board 201 according to the third embodiment can also be configured as follows.
Each of the electrodes 51a is formed on a separate substrate, and these substrates are arranged in parallel with each other. This configuration also has the same effect as the printed wiring board 201 shown in FIG.

-A printed wiring board 301 according to the fourth embodiment will be described with reference to FIG.
This printed wiring board 301 differs from the above printed wiring board 1 in the following points. That is, the printed wiring board 1 according to the first embodiment has a structure in which the base material 2 and the cover lay 3 are pressure-bonded, and the printed wiring board 301 has the first structure between the base material 2 and the wiring pattern 4. It has an adhesive layer 5a, and has a second adhesive layer 5b between the coverlay 3 and the wiring pattern 4. Since the first and second adhesive layers 5a and 5b adhere to each other, it is difficult for moisture to enter the inside from the adhesive surfaces of the first and second adhesive layers 5a and 5b. The first and second adhesive layers 5a and 5b are formed of a resin having a low absorption rate of the fuel 30.

  According to such a configuration, bubbles and voids are unlikely to enter between the base material 2 and the cover lay 3 or between the electrodes 4a in the manufacturing process. This is because the first and second adhesive layers 5a and 5b flow during the pressure bonding between the base material 2 and the cover lay 3 and spread between the base material 2 and the cover lay 3 and between the electrodes 4a. This is for pouring. For this reason, the printed wiring board 301 has fewer bubbles and voids compared to the case where the base material 2 and the cover lay 3 are laminated without interposing an adhesive layer. As a result, the printed wiring board 301 is less likely to deteriorate due to peeling or the like caused by bubbles or voids, and has little change in characteristics over time or temperature due to bubbles or voids.

  Further, it is preferable that the thickness of the first adhesive layer 5a and the thickness of the second adhesive layer 5b be larger than the thickness of the wiring pattern 4. According to this configuration, it is possible to further suppress the generation of bubbles and voids between the edge of the wiring pattern 4 and the two electrodes 4a in the manufacturing process. For example, when the thickness of the wiring pattern 4 is 18 μm, the thickness of the first adhesive layer 5a and the thickness of the second adhesive layer 5b are both set to 25 μm.

  The configuration in which the base material 2 and the cover lay 3 are bonded to each other via the first adhesive layer 5a and the second adhesive layer 5b is based on the printed wiring board 101 according to the second embodiment and the printed wiring board 101 according to the third embodiment. The same applies to 201.

  In addition, the configuration regarding the thickness of the first adhesive layer 5a and the second adhesive layer 5b can be applied to the printed wiring board 401 according to the fifth embodiment and the printed wiring board 501 according to the sixth embodiment described below. Is preferred.

-The printed wiring board 401 according to the fifth embodiment will be described with reference to FIG.
The printed wiring board 401 differs from the printed wiring board 1 according to the first embodiment in the following points. That is, the printed wiring board 1 according to the first embodiment has a structure in which the base material 2 and the coverlay 3 are pressure-bonded, but the printed wiring board 401 according to the present embodiment has a structure in which the base material 2 and the wiring pattern 4 There is a first adhesive layer 5a between them, and a second adhesive layer 5b between the coverlay 3 and the wiring pattern 4. The first and second adhesive layers 5a and 5b are sealed so as not to be exposed. The configurations of the base material 2 and the coverlay 3 are the same as those of the printed wiring board 1 according to the first embodiment.

  The first and second adhesive layers 5a and 5b are formed of an epoxy resin or the like. Since the first and second adhesive layers 5a and 5b have a high absorptivity of the fuel 30, the ends of the base material 2 and the cover lay 3 are coated with the liquid crystal polymer member 6 so that the first and second adhesive layers 5a and 5b are not exposed. Is sealed. Like the base material 2 and the coverlay 3, the liquid crystal polymer member 6 is formed of a liquid crystal polymer having an absorptivity of the fuel 30 of 2% by mass or less, preferably 1.0% by mass or less.

  In the state where the first and second adhesive layers 5a and 5b are absorbing the fuel 30, and the state where the first and second adhesive layers 5a and 5b are not absorbing the fuel 30, the fuel 30 is in the electrode region AE. Even when the proportions of the occupied portions are equal, the capacitance between the electrodes 4a differs in both states due to the influence of the absorbed fuel 30.

  Therefore, in the above configuration, the end of the base material 2 and the end of the cover lay 3 are sealed with the liquid crystal polymer member 6 so that the first and second adhesive layers 5a and 5b are not exposed. With this configuration, it is possible to prevent the first and second adhesive layers 5a and 5b from absorbing the fuel 30.

  The configuration in which the ends of the base material 2 and the cover lay 3 are sealed with the liquid crystal polymer member 6 is based on the configuration in which an adhesive layer is interposed between the base material 2 and the cover lay 3 according to the second embodiment. The present invention is also applied to the wiring boards 101 and 102 and the printed wiring board 201 according to the third embodiment.

-A printed wiring board 501 according to the sixth embodiment will be described with reference to FIG.
This printed wiring board 501 is obtained by modifying the printed wiring board 401 according to the fifth embodiment as follows. In the printed wiring board 401 according to the fifth embodiment, the ends of the base material 2 and the coverlay 3 are covered with the liquid crystal polymer member 6 so that the first and second adhesive layers 5a and 5b are not exposed. Instead of the configuration, in the printed wiring board 301 according to the sixth embodiment, the end 2 a of the base material 2 is folded back and pressed against the cover lay 3. Note that a configuration in which the end of the cover lay 3 is folded back and pressure-bonded to the base material 2 may be employed. Even with this configuration, the end of the base material 2 and the end of the cover lay 3 can be easily sealed.

  The configuration in which the end portion of the printed wiring board 501 is sealed by folding back the end portion of the base material 2 or the cover lay 3 is described in the second embodiment in which an adhesive layer is interposed between the base material 2 and the cover lay 3. The present invention is also applied to the printed wiring board 101 according to the embodiment and the printed wiring board 201 according to the third embodiment.

-A printed wiring board 601 according to the seventh embodiment will be described with reference to FIG.
This printed wiring board 601 differs from the printed wiring board 1 according to the first embodiment in the following points. That is, the printed wiring board 1 according to the first embodiment has a structure in which the base material 2 and the cover lay 3 are directly adhered to each other (see FIG. 2).

  On the other hand, in the printed wiring board 601, the coverlay 3 is laminated on the base material 2 via the adhesive layer 5. The adhesive layer 5 is made of a liquid crystal polymer. Preferably, the liquid crystal polymer constituting the adhesive layer 5 is lower than the melting point of the base material 2 and lower than the melting point of the coverlay 3.

  With such a configuration, the water absorption of the printed wiring board 601 decreases. Further, when bonding the base material 2 and the cover lay 3 by thermocompression bonding during manufacturing, the heating temperature can be lower than the melting points of the base material 2 and the cover lay 3. Then, even at such a temperature setting, the adhesive layer 5 can be sufficiently softened, and the adhesive (material of the adhesive layer 5) can be filled in the gap between the wiring portions 41b and the gap between the electrodes 41a. Become. This suppresses the formation of bubbles and voids between the base material 2 and the coverlay 3 or between the electrodes 4a.

  For this reason, the printed wiring board 601 has an improved characteristic that the capacitance is constant when the occupation ratio of the liquid to the electrode region is a predetermined value, and the printed wiring board 601 is less likely to be degraded by peeling or the like due to bubbles or voids. In addition, a change in characteristics over time or a change in characteristics due to a temperature change is reduced.

-The printed wiring board 701 according to the eighth embodiment will be described with reference to FIG.
The printed wiring board 701 differs from the printed wiring board 1 according to the first embodiment in the following points. That is, the printed wiring board 701 according to the eighth embodiment includes a first fluororesin layer 7 covering the base material 2 and a coverlay in addition to the configuration of the printed wiring board 1 according to the first embodiment (see FIG. 2). And a second fluororesin layer 8 covering the second fluororesin layer 3. The first fluororesin layer 7 and the second fluororesin layer 8 are layers containing a fluororesin as a main component. Here, the main component indicates that the ratio of the fluororesin to the resin is 50% by mass or more. Preferably, the first fluororesin layer 7 and the second fluororesin layer 8 are formed of a resin having a fluororesin content of 90% by mass or more.

  Fluororesins have lower absorptivity of alcohol, especially methyl alcohol, than liquid crystal polymers. For this reason, in the printed wiring board 701 having the above configuration, the characteristic change caused by the absorption of methyl alcohol is small, and the printed wiring board 701 has a capacitance when the liquid occupation ratio with respect to the electrode region is a predetermined value. The property of being constant is improved.

  The thickness of the first fluororesin layer 7 is preferably 25 μm or more and 200 μm or less. More preferably, the thickness of the first fluororesin layer 7 is 50 μm or more and 100 μm or less. The thickness of the second fluororesin layer 8 is set similarly to the thickness of the first fluororesin layer 7.

  This is because if the thickness of the first fluororesin layer 7 or the second fluororesin layer 8 is too large, the capacitance between the electrodes 4a is too small, and the detection accuracy of the fuel 30 is too low. On the other hand, if the thickness of the first fluororesin layer 7 or the second fluororesin layer 8 is too small, the function of the first fluororesin layer 7 or the second fluororesin layer 8 to prevent methyl alcohol is reduced. . For example, when the first fluororesin layer 7 or the second fluororesin layer 8 is damaged, methyl alcohol may enter from this portion and the base material 2 or the coverlay 3 may absorb the methyl alcohol.

  Examples of the fluororesin include PTFE (polytetrafluoroethylene), PFA (polytetrafluoroethylene / perfluoroalkylvinyl ether copolymer), FEP (tetrafluoroethylene / hexafluoropropylene copolymer), and ethylene tetrafluoride / 6 Fluoroethylene copolymer, polyvinylidene fluoride, polychlorotrifluoroethylene, chlorotrifluoroethylene / ethylene copolymer, polyvinyl fluoride, THV (three kinds of monomers of tetrafluoroethylene, hexafluoropropylene, vinylidene fluoride Thermoplastic fluororesin), fluoroelastomer, and the like. Further, a mixture or a copolymer containing these compounds is used as a material constituting the fluororesin.

  More preferably, at least one of the first fluororesin layer 7 and the second fluororesin layer 8 includes a fiber woven sheet. Examples of the fiber woven sheet include a glass cloth sheet and a carbon sheet. Instead of or in addition to the fiber woven sheet, the first fluororesin layer 7 and the second fluororesin layer 8 may include a filler and a nonwoven fabric. Examples of the filler include a glass filler and a carbon filler. Examples of the nonwoven fabric include a glass nonwoven fabric and a carbon fiber nonwoven fabric. The fluororesin sheet as a member for forming the first fluororesin layer 7 and the second fluororesin layer 8 including the fiber woven sheet or nonwoven fabric is formed by impregnating the fiber woven sheet or nonwoven fabric with the fluororesin, It is formed by hot pressing with a fluororesin sheet or the like. Accordingly, the rigidity of the printed wiring board 701 is increased, and expansion and contraction between the electrodes due to deformation of the printed wiring board 701 is suppressed. Further, the productivity of the printed wiring board 701 is improved for the reasons described above.

-The printed wiring board 801 according to the ninth embodiment will be described with reference to FIG.
This printed wiring board 801 differs from the printed wiring board 1 according to the first embodiment in the following points. In the printed wiring board 1 according to the first embodiment, the base material 2 and the coverlay 3 are formed of a resin containing a liquid crystal polymer as a main component, whereas in the printed wiring board 801 according to the ninth embodiment, The base material 2 and the coverlay 3 are formed of a resin containing a fluororesin as a main component. Here, the main component indicates that the ratio of the fluororesin to the resin is 50% by mass or more. Preferably, the base material 2 and the cover lay 3 are formed of a resin containing 90% by mass or more of a fluororesin.

  It is preferable that the thickness of the base material 2 is 25 μm or more and 200 μm or less. More preferably, the thickness of the substrate 2 is 50 μm or more and 100 μm or less. The thickness of the coverlay 3 is set in the same manner as the thickness of the base material 2. That is, the thickness of the cover lay 3 is preferably 25 μm or more and 200 μm or less, and more preferably 50 μm or more and 100 μm or less.

  Such a setting is because when the thickness of the base material 2 or the cover lay 3 becomes too large, the capacitance between the electrodes 4a becomes too small, and the detection accuracy of the fuel 30 becomes too low. Further, the detection accuracy of the fuel 30 is reduced due to an increase in the amount of fuel absorbed by the base material 2 or the coverlay 3. On the other hand, if the thickness of the base material 2 or the cover lay 3 is too small, the sealing function of the base material 2 or the cover lay 3 is reduced. For example, if the base material 2 or the cover lay 3 is damaged, fuel or air may enter from this portion and the wiring pattern 4 may be deteriorated.

As the fluorine resin, the fluorine resin described in the description of the printed wiring board 701 according to the eighth embodiment can be used.
Furthermore, at least one of the substrate 2 and the coverlay 3, preferably both of them (see FIG. 13), include a fiber woven sheet 9. Examples of the fiber woven sheet 9 include a glass cloth sheet and a carbon sheet. Instead of or in addition to the fiber woven sheet 9, the base material 2 and the cover lay 3 may include the same filler as described above and the same nonwoven fabric as described above. Accordingly, the rigidity of the printed wiring board 801 is increased, and expansion and contraction between the electrodes due to deformation of the printed wiring board 801 is suppressed. Further, the productivity of the printed wiring board 801 is improved for the reasons described above.

  According to the above configuration, both the substrate 2 and the cover lay 3 are formed of a fluororesin. Fluororesins have lower absorptivity of water and fuel 30 than resins used in conventional printed wiring boards, such as polyimide resins and epoxy resins.

  Therefore, the same effect as in the first embodiment can be obtained. That is, the capacitance of the printed wiring board 801 is constant when the occupation ratio of the water or the fuel 30 with respect to the electrode region AE is a predetermined value regardless of the immersion time, the immersion state, or the temperature condition.

-The printed wiring board 901 according to the tenth embodiment will be described with reference to FIG.
This printed wiring board 901 differs from the printed wiring board 801 according to the ninth embodiment in the following points. The printed wiring board 801 according to the ninth embodiment has a structure in which the base material 2 and the coverlay 3 are directly adhered to each other (see FIG. 13). On the other hand, in the printed wiring board 901, the coverlay 3 is laminated on the base material 2 via the adhesive layer 5. The adhesive layer 5 is made of a liquid crystal polymer. It is preferable that the thickness of the adhesive layer 5 is larger than the thickness of the wiring pattern 4. Preferably, the liquid crystal polymer constituting the adhesive layer 5 is lower than the melting point of the base material 2 and lower than the melting point of the coverlay 3. With such a configuration, the water absorption of the printed wiring board 601 decreases. Further, when the base material 2 and the cover lay 3 are bonded to each other by thermocompression bonding during manufacturing, the heating temperature can be lower than the melting points of the base material 2 and the cover lay 3. Thereby, the same effect as in the eighth embodiment can be obtained.

[Other embodiments]
-In each of the above embodiments and modifications, the structure of the electrode 41a of the wiring pattern 4 is not limited. In the first and second embodiments, the two electrodes 41a forming the electrode pair 42 are set so that the interval Sb is a constant width. The edges need not be parallel to each other but may be enlarged or reduced in part.

  That is, the two electrodes 41a constituting the electrode pair 42 may be arranged so as to be close to each other to such an extent that the capacitance between them can be detected by applying a predetermined voltage. Further, the shape of the two electrodes 41a is not limited to a rectangle. For example, the electrode 41a is formed in a comb shape, and the two electrodes 41a are arranged so that the teeth of one electrode 41a and the teeth of the other electrode 41a are alternately positioned. According to this configuration, the portion of the electrode pair 42 where the two electrodes 41a approach each other becomes longer, and thus the capacitance between the electrodes 41a increases.

-About the liquid level sensor 10, the following structures can also be employ | adopted.
The liquid level sensor 10 includes the printed wiring board 1 according to the first embodiment. Instead of the printed wiring board 1, any one of the printed wiring boards 101 to 901 and 102 according to the second to tenth embodiments is used. The liquid level sensor 10 may be configured so as to be provided. The liquid level sensor 10 has the same effect as the liquid level sensor 10.

The printed wiring boards 1, 101 to 901 and 102 are useful when used for a liquid level sensor 10 for measuring capacitance.
The liquid level sensor 10 is useful when used as a measuring device for measuring the liquid level of the fuel 30.

1, 101, 102, 201, 301, 401, 501, 601, 701, 801, 901 Printed wiring board 2 Base material 2a End 3 Coverlay 4 Wiring pattern 4a Electrode 4b Terminal 5 Adhesive layer 5a First adhesive layer 5b Second adhesive layer 6 Liquid crystal polymer member 7 First fluororesin layer 8 Second fluororesin layer 9 Fiber woven sheet 10 Liquid level sensor 11 Measurement unit 12 Conductive wire 20 fuel tank 30 fuel 40 wiring pair 41 wiring 41a electrode 41b wiring part 41bx first wiring part 41by second wiring part 41c terminal part 42 electrode pair 51a electrode 52 spacer AE electrode Area DF: Flow direction DR: Right angle direction DE: Extension direction

Claims (10)

A base material formed of a liquid crystal polymer and having a thickness of 25 μm or more and 50 μm or less ;
A wiring pattern formed on the base material and having at least a pair of electrodes having a thickness of 12 μm or more and 18 μm or less and detecting capacitance between the electrodes ,
Wherein not covering the wiring pattern, and a cover lay is thick is formed by the liquid crystal polymer is 25μm or 50μm or less,
A first fluororesin layer formed of a fluororesin having a higher melting point than the base material and covering the base material;
A second fluororesin layer formed of a fluororesin having a higher melting point than the coverlay and covering the coverlay,
Wherein at least one of the first fluororesin layer and the second fluorocarbon resin layer is viewed including nonwovens, and at least one of the fiber fabric sheet,
The fluororesin of the first fluororesin layer and the fluororesin of the second fluororesin layer may be made of thermoplastic fluorine made of three kinds of monomers such as polyvinyl fluoride and THV (tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride). Resin), and at least one of fluoroelastomer,
The base material and the cover lay include a polycondensate of ethylene terephthalate and parahydroxybenzoic acid,
A printed wiring board , wherein, for a fuel in which isooctane and toluene are mixed at a volume ratio of 1: 1, the fuel absorptivity of the base material and the fuel absorptivity of the coverlay are each 2% by mass or less . The printed wiring board according to claim 1, wherein edges of the pair of electrodes of the wiring pattern are separated by a predetermined interval. The printed wiring board according to claim 1, wherein the coverlay is laminated on the base material via an adhesive layer formed of a liquid crystal polymer. The printed wiring board according to claim 3, wherein a melting point of the adhesive layer is lower than a melting point of the base material and lower than a melting point of the cover lay. A base material formed of a fluororesin and having a thickness of 50 μm or more and 100 μm or less ,
A wiring pattern formed on the base material and having at least a pair of electrodes having a thickness of 12 μm or more and 18 μm or less and detecting capacitance between the electrodes ,
Not covering the wiring pattern, the thickness is formed by fluorine resin comprises a coverlay and is 50μm or more 100μm or less,
Each of the base and the cover lay, nonwoven fabric, and saw including at least one of the fiber fabric sheet,
A printed wiring board containing at least one of the fluorine resin, polyvinyl fluoride, THV (a thermoplastic fluorine resin composed of three kinds of monomers of tetrafluoroethylene, hexafluoropropylene, and vinylidene fluoride), and a fluoroelastomer . The printed wiring board according to claim 5, wherein the coverlay is laminated on the base material via an adhesive layer formed of a liquid crystal polymer having a lower melting point than a fluororesin. The wiring pattern has a pair of electrodes formed on the same plane, the interval between the two electrodes is a constant width from one end to the other end,
The printed wiring board according to any one of claims 1 to 4, wherein an extension direction of the electrode and a flow direction of the liquid crystal polymer of the base material and the cover lay coincide with each other. The printed wiring board according to any one of claims 1 to 6, wherein the wiring pattern has a plurality of pairs of electrodes. The printed wiring board according to any one of claims 1 to 6, wherein the wiring pattern has a pair of electrodes arranged to face each other.   A liquid level sensor comprising the printed wiring board according to any one of claims 1 to 9.