JP4362764B2 - Conductive paste and method of manufacturing wiring board using the same - Google Patents

Description

The present invention relates to a conductive paste particularly suitable for forming an inner via hole connection and a method for manufacturing a wiring board having an inner via hole connection using the conductive paste.

  In order to conduct conductive connection between the circuits on both sides of the double-sided wiring board, or between the circuits of each layer of the multilayer wiring board, conventionally, fine through holes formed in the insulating substrate that is the basis of the wiring board Through-hole connection, in which the inner peripheral surface of the metal plate is plated and made conductive, has recently been used as a simpler connection, and through holes (via holes) formed in an insulating substrate are filled with a conductive paste by a printing method. After that, a method of curing the binder resin contained in the conductive paste to form an inner via hole connection for conductive connection is becoming widespread.

  The conductive paste used for such inner via hole connection is required to have a low viscosity in order to improve the fillability of fine via holes. In addition, since it is necessary to prevent the void from being generated in the inner via hole connection formed by filling the via hole with a conductive paste and curing it, it is necessary to use a solventless type that does not contain a solvent. It is preferred.

And in order to satisfy these conditions, in patent document 1, as a conductive filler and binder resin, it has one or more epoxy groups in 1 molecule, and the total amount of a hydroxyl group, an amino group, and a carboxyl group Inner via-hole connection using a solvent-free conductive paste containing 5 mol% or less of the epoxy group and an epoxy equivalent of 100 to 350 and a liquid epoxy compound having a relatively low molecular weight and low viscosity Has been proposed to form.
Japanese Patent No. 3038210 (Claim 1, columns 0010 to 0011)

  However, the inner via-hole connection formed by using the conventional conductive paste is used to form a circuit on a polyimide resin sheet or film, or an insulating substrate, which is often used as an insulating substrate particularly in a flexible printed wiring board. When the flexible printed wiring board is bent, for example, the inner via hole connection is first peeled off from the inner peripheral surface of the through hole, and then the flexible printed wiring board. There is a problem that the resistance value rises due to peeling from the circuit closely contacting the substrate.

  In addition, the conventional conductive paste gradually cures the epoxy resin and increases the molecular weight with time, and the viscosity also increases accordingly.For example, when performing continuous printing using a screen printing machine, In addition to the problem that the printing characteristics decrease with increasing viscosity, there is also the problem that long-term storage is difficult even if stored at 0 ° C. or lower.

The object of the present invention is to form an inner via-hole connection having excellent adhesion to a metal surface of a circuit formed on an insulating substrate made of polyimide resin or the surface thereof, etc. A novel conductive paste that can maintain good printing characteristics in many times of printing than before and can be stored for a longer period of time than before, and a method of manufacturing a wiring board having an inner via hole connection using the same And to provide.

The invention according to claim 1 is a conductive filler, an epoxy resin having an epoxy equivalent of 5000 or more and 10,000 or less, an epoxy resin having a molecular weight of 20000 or more and 70000 or less, and a solvent having a boiling point of 240 ° C. or more and 250 ° C. or less. Is a conductive paste characterized by containing.
According to a second aspect of the present invention, the solvent is removed by filling the conductive paste of the first aspect into a via hole penetrating the insulating substrate and heat-treating in a reduced pressure atmosphere of 1.3 × 10 ² Pa or less. The wiring board manufacturing method is characterized in that an inner via hole connection is formed through a process.

The invention according to claim 3 is the method for manufacturing a wiring board according to claim 2, wherein the heat treatment in a reduced-pressure atmosphere is performed at 40 to 80 ° C.
The invention according to claim 4 is the method for manufacturing a wiring board according to claim 2 , including a step of curing the epoxy resin by heating the insulating substrate while pressing after removing the solvent by heat treatment .
The invention according to claim 5 is the method for manufacturing a wiring board according to claim 2, wherein the insulating substrate is made of polyimide resin.

A high-molecular weight epoxy resin having an epoxy equivalent of 5000 or more and 10,000 or less and having a molecular weight of 20000 or more and 70000 or less is used as a binder resin in the conductive paste of the invention of claim 1. In addition, the ability to form a hydrogen bond between the surface of an insulating substrate (such as the inner peripheral surface of a via hole) and a metal surface forming a circuit is higher than that of a low molecular weight epoxy compound. For this reason, according to the conductive paste of the invention described in claim 1, since the surface energy is particularly small compared with the insulating substrate made of fiber reinforced epoxy resin used in Patent Document 1, it is difficult to obtain high adhesion. It is possible to form an inner via hole connection or the like that adheres more firmly than before to a resin insulating substrate or a metal surface of a circuit formed on the surface of the resin insulating substrate. Moreover, since the adhesive force with respect to the metal surface of a circuit improves as mentioned above, according to the said electrically conductive paste, especially the resistance value of an inner via-hole connection can also be reduced more than before.

Further, the conductive paste is constituted by dissolving the epoxy resin in a high boiling point solvent having a boiling point of 240 ° C. or higher and 250 ° C. or lower and dispersing a conductive filler,
・ The epoxy resin has a high molecular weight and is less reactive than conventional low molecular weight epoxy compounds.
-Since the epoxy resin is dissolved in the solvent, it is difficult for the viscosity to increase suddenly even if the curing reaction proceeds to some extent.
・ Since the solvent has a high boiling point and is difficult to evaporate, it is difficult for the viscosity to increase due to such evaporation.
As a result, the degree of increase in viscosity during continuous printing or long-term storage can be made significantly smaller than before. Therefore, the conductive paste according to the first aspect of the invention can be stored for a longer period of time than before, and if such a conductive paste is used, good printing characteristics can be maintained in many times of printing. It becomes possible.

  According to the invention of claim 2, since the high-boiling solvent is removed by filling the conductive paste of the invention of claim 1 into a via hole and then heating in a reduced pressure atmosphere, It is possible to reliably prevent a void from being generated due to the residual and an increase in the resistance value of the inner via hole connection.

  The heat treatment in a reduced pressure atmosphere more reliably prevents voids from being generated in the inner via hole connection by quickly evaporating the high boiling point solvent while suppressing the epoxy resin curing reaction from proceeding. If it considers, it is preferable to carry out at 40-80 degreeC as described in Claim 3. After removing the solvent, the inner via-hole connection can be formed by curing the epoxy resin by heating while pressing the insulating substrate as described in claim 4. Furthermore, by using an insulating substrate made of polyimide resin as described in claim 5, it is possible to form an inner via-hole connection that is more firmly adhered to the insulating substrate made of polyimide resin than ever before.

<Conductive paste>

As described above, the conductive paste of the present invention has a conductive filler, an epoxy equivalent of 5000 or more and 10,000 or less, an epoxy resin having a molecular weight of 20000 or more and 70000 or less, and a boiling point of 240 ° C. or more and 250 ° C. or less. It is characterized by including the solvent which is.
(Conductive filler)

  Among these, as a conductive filler, it has favorable electroconductivity and conventionally well-known various conductive fillers can be used for an electrically conductive paste. Examples of such conductive fillers include, but are not limited to, metals having excellent conductivity such as Au, Ag, Cu, Pt, Pd, Rb, Rh, and Re, two or more alloys of the metals, and the metals. On the surface of powders having various shapes such as spherical, granular, needle-like and other shapes made of an alloy of metal and other metals, or powders having various shapes such as spherical, granular and needle-like shapes made of other metals, Examples thereof include a powder having a composite structure in which one or more layers of the above-described metal having excellent conductivity or an alloy thereof are coated.

  The particle size of the conductive filler is not particularly limited. For example, for inner via hole connection, considering the formation of an inner via hole connection having good conductivity, the particle size is set to 0.5 mm of the inner diameter of the via hole. It is preferable to make it -20%. That is, when the particle size of the conductive filler exceeds 20% of the inner diameter of the via hole, the filling rate of the conductive filler in the inner via hole connection formed by filling the via hole with the conductive paste by printing or the like decreases. The resistance value may increase.

  Conversely, when the particle size of the conductive filler is less than 0.5% of the inner diameter of the via hole, although the filling rate increases, the connection resistance between the conductive fillers increases, so the resistance value of the inner via hole connection also increases. In addition, since the filling rate of the epoxy resin is relatively lowered, there is a possibility that the adhesion of the inner via hole connection, particularly to the insulating substrate made of polyimide resin, may be lowered.

  In consideration of further decreasing the resistance value of the inner via hole connection by increasing the filling rate, the particle size of the conductive filler is preferably 1% or more of the inner diameter of the via hole, even within the above range, More preferably, it is 1.5% or more. In addition, the resistance value of the inner via hole connection is further reduced by suppressing an increase in the connection resistance between the conductive fillers, and the adhesive strength of the inner via hole connection to the insulating substrate is further improved. Even within the above range, the particle diameter is preferably 10% or less of the inner diameter of the via hole, more preferably 5% or less.

  The blending ratio of the conductive filler is not limited, but especially when combined with an insulating substrate made of polyimide resin, the adhesion value to the insulating substrate is improved while preventing the resistance value such as inner via hole connection from increasing. In consideration of the above, the blending ratio of the conductive filler is preferably 800 to 2300 parts by weight with respect to 100 parts by weight of the epoxy resin. That is, if the blending ratio of the conductive filler is less than 800 parts by weight with respect to 100 parts by weight of the epoxy resin, the effect of imparting conductivity by the conductive filler is reduced, so that the resistance value such as inner via hole connection may increase. On the other hand, when the amount exceeds 2300 parts by weight, the proportion of the epoxy resin is relatively reduced, so that there is a possibility that the adhesion to an insulating substrate made of polyimide resin, such as inner via hole connection, may be reduced.

In consideration of further reducing the resistance value of the inner via hole connection or the like, the blending ratio of the conductive filler is preferably 900 parts by weight or more with respect to 100 parts by weight of the epoxy resin, even within the above range. More preferably, it is 950 parts by weight or more. In consideration of further improving the adhesion to the insulating substrate such as inner via hole connection, the blending ratio of the conductive filler is 2100 parts by weight or less with respect to 100 parts by weight of the epoxy resin, even within the above range. It is preferable that it is 2000 parts by weight or less.
(Epoxy resin)

Examples of the epoxy resin include various grades such as bisphenol A type, bisphenol F type, bisphenol AD type, phenol type, cresol type, novolac type, glycidyl ether type, glycidyl ester type, glycidyl amine type, and heterocyclic type. Among epoxy resins, it is necessary to select and use one having curability and having an epoxy equivalent of 5000 or more and 10,000 or less and a molecular weight of 20000 or more and 70000 or less .

When the epoxy equivalent of the epoxy resin is less than 5000 , the ability to form a hydrogen bond between the surface of the insulating substrate (such as the inner peripheral surface of the via hole) and the metal surface such as copper foil is insufficient. There is a problem in that an inner via hole connection having excellent adhesion cannot be formed on a resin insulating substrate or a metal surface of a circuit formed on the surface thereof .

Further, when the molecular weight is less than 20000, the reactivity becomes high and the curing reaction is likely to occur naturally at a low temperature, so that the degree of increase in viscosity during continuous printing or long-term storage increases. And the problem that the period which can be preserve | saved becomes short or the frequency | count of printing which can maintain a favorable printing characteristic reduces arises.

However, if the epoxy equivalent of the epoxy resin is too large, the reactivity becomes low and there is a risk of causing adverse effects such as a long curing time. Therefore, the epoxy equivalent needs to be not more than 10,000 even within the above range. In particular, it is more preferably 9000 or less.

Further by suppressing the reactivity, considering that to more reliably prevent the curing reaction spontaneously at a low temperature, the molecular weight of the epoxy resin, and even more preferably at JP four 0000 or even within the above range. However, if the molecular weight of the epoxy resin is too large, the solubility in a solvent may be reduced, and a conductive paste having a uniform viscosity may not be formed, or even if formed, the viscosity may be too high. If the amount of the solvent is increased in order to solve these problems, it is not easy to remove the entire amount promptly, so that a void may be generated in the inner via hole connection and the resistance value may be increased. Therefore, in consideration of preventing these problems from occurring, the molecular weight of the epoxy resin needs to be 70000 or less even in the above range, and more preferably 60000 or less.

Specific examples of the epoxy resin that satisfies these conditions include, but are not limited to, the following, for example, among the Epicoat (registered trademark) series manufactured by Japan Epoxy Resin Co., Ltd.
Epicoat 1256 (bisphenol A type, epoxy equivalent 7000-8500, molecular weight 53000 )
· Epikote 4250 (bisphenol A / F mixed type, epoxy equivalent 7,000 to 10,000, molecular weight 60000)
Epicoat 4275 (bisphenol A / F mixed type, epoxy equivalent of 7000 to 10,000, molecular weight of 60000)
(solvent)

As a solvent for constituting a conductive paste together with the conductive filler and the epoxy resin, the epoxy resin can be uniformly dissolved, and the boiling point at normal pressure, that is, 1 atm (= 1.01325 × 10 ⁵ Pa) is Thus, it is necessary to select and use one having a temperature of 240 ° C. or higher and 250 ° C. or lower .

Since the solvent having a boiling point of less than 240 ° C. is likely to evaporate during continuous printing or long-term storage, there is a problem that the viscosity of the conductive paste is likely to increase .

In addition, since the solvent having a boiling point exceeding 250 ° C. is not easily evaporated, even when heated in a reduced pressure atmosphere as described later, the entire amount cannot be removed quickly, and voids are generated by the remaining solvent, There is a problem that the resistance value of the inner via hole connection increases.

As such a high-boiling solvent, and the like di ethylene glycol monobutyl ether acetate For example (boiling point 246.8 ° C.).
(Additive)

  Examples of the additive include a curing agent for curing the epoxy resin. Examples of the curing agent include amine-based, polyaminoamide-based, acid and acid anhydride-based, and other conventionally known curing agents. Various curing agents capable of curing the high molecular weight epoxy resin can be selected and used. What is necessary is just to set the addition amount of a hardening | curing agent suitably according to the equivalent of the epoxy of an epoxy resin.

Examples of other additives include a coupling agent for further improving the adhesion between the epoxy resin and the conductive filler or the metal surface of the circuit.
(Conductive paste)

What is necessary is just to set the viscosity of the electrically conductive paste containing said each component to the suitable range according to the printing method etc. to employ | adopt. For example, in the screen printing method, the initial viscosity (measurement temperature: 25 ° C.) of the conductive paste may be set to 100 Pa · s or less, preferably about 50 to 90 Pa · s. For this purpose, the amount of the solvent may be adjusted.
< Manufacturing method of wiring board >

In the method for manufacturing a wiring board according to the present invention using the conductive paste, first, the conductive paste is filled into a via hole penetrating the insulating substrate by a printing method such as a screen printing method as in the prior art.

Next, the insulating substrate is heat-treated in a reduced-pressure atmosphere to remove the solvent from the conductive paste filled in the via hole. The decompression condition at this time needs to be set to 1.3 × 10 ² Pa or less in the present invention. When the depressurization condition exceeds 1.3 × 10 ² Pa, the solvent cannot be removed efficiently. In order to remove the solvent as efficiently as possible, it is possible to increase the temperature of the heat treatment or lengthen the time. In such a case, however, the epoxy resin partially undergoes a curing reaction. It may be difficult to remove the solvent. The remaining solvent causes a problem that voids are generated in the inner via hole connection formed through the curing process and the resistance value is increased.

  Although it does not specifically limit about other conditions, It is preferable that the temperature of heat processing is 40-80 degreeC. If the temperature of the heat treatment is less than 40 ° C., the solvent may not be efficiently removed, and if the treatment time is increased in order to remove the solvent as much as possible, the epoxy resin may partially undergo a curing reaction. On the other hand, when the temperature of the heat treatment exceeds 80 ° C., the epoxy resin may partially undergo a curing reaction. Therefore, in any of these cases, it is difficult to remove the solvent in the same manner as described above, and the residual solvent may cause voids in the inner via hole connection formed through the curing process, thereby increasing the resistance value. There is. In consideration of removing the solvent as efficiently as possible, the temperature of the heat treatment is preferably 50 ° C. or higher, and more preferably 60 ° C. or higher, even within the above range.

  Moreover, considering that the partial curing reaction of the epoxy resin proceeds and removing the solvent as much as possible while improving the efficiency of the treatment, the heat treatment time is within 1 hour, particularly 20 to 40 minutes. It is preferable that it is about.

  Next, when the insulating substrate is a so-called copper-clad substrate in which a metal foil such as a copper foil is laminated on one surface, the metal foil or the metal foil side of a similar copper-clad laminate is laminated on the opposite surface. When the insulating substrate is a raw insulating substrate in which metal foil is not laminated on both sides, the metal foil or the metal foil side of the copper-clad laminate is laminated on both sides. Moreover, you may form a circuit in each metal foil by etching etc. in the arbitrary steps before and after performing said lamination | stacking operation | work as needed.

Then, this lamination operation and circuit formation operation are repeated for the required number of layers in the case of a multilayer wiring board to produce a laminate, and this laminate is then heated while pressing to cure the epoxy resin. IVH connection are formed, wiring board Ru manufactured Te. The circuit formation on the outermost metal foil of the multilayer wiring board or single-layer wiring board may be performed after the press working.

In the wiring board thus manufactured, the inner via hole connection has no voids as described above, and the inner peripheral surface of the via hole of the insulating substrate and the metal surface of the circuit formed from the metal foil due to the characteristics of the epoxy resin described above. It has excellent adhesion to the surface. For this reason, in the case of a flexible printed wiring board, when the flexible printed wiring board is bent, the resistance value rises more reliably than before, for example, the inner via hole connection is peeled off from both sides. It becomes possible to do. In addition, since the adhesion to the metal surface of the circuit is improved as described above, the resistance value of the inner via hole connection can be further reduced.

  The configuration of the present invention is not limited to the example described above. For example, the conductive paste is not limited to the formation of the inner via hole connection, and can be used, for example, for forming a circuit having a width of 300 μm or more, for a jumper line, or for a through-through hole. In addition, appropriate changes can be made without departing from the scope of the present invention.

The present invention will be described below based on examples and comparative examples.
Example 1
(Preparation of conductive paste (i))

Silver powder [Ag-cL made by Fukuda Metal Foil Powder Co., Ltd.] as an electrically conductive filler, epoxy resin [Epicoat 1256 made by Japan Epoxy Resin Co., Ltd., bisphenol A type, epoxy equivalent 7000-8500, molecular weight 53000], Curing agent [NOVACURE (registered trademark) HX3941 manufactured by Asahi Kasei Epoxy Co., Ltd.], silane coupling agent [KBM403 manufactured by Shin-Etsu Chemical Co., Ltd.], and diethylene glycol monobutyl ether acetate (boiling point: 246.8 ° C., solvent) (Abbreviated as “DGMBEA”) was mixed at the following ratio, and kneaded with a three-roll mixer to prepare a conductive paste (i). The volume ratio of silver powder to epoxy resin was 50:50. Further, the viscosity (measurement temperature: 25 ° C.) of the conductive paste (i) immediately after preparation was 60 Pa · s.
(Ingredients) (Parts by weight)
・ Silver powder 970
・ Epoxy resin 100
・ Curing agent 1
・ Silane coupling agent 5
・ DGMBEA 200
( Manufacture of wiring boards )

  Prepare a copper-clad laminate in which a copper foil (thickness 12 μm) is laminated on one side of a polyimide resin sheet (thickness 25 μm), and laminate the copper foil on the polyimide resin sheet of the copper-clad laminate; A number of via holes (inner diameter 100 μm) reaching the copper foil were penetrated and formed from the opposite surface.

Next, after filling the via holes with the conductive paste (i) prepared above by screen printing, the copper-clad laminate was heated to 70 ° C. in a reduced pressure atmosphere of 1.3 × 10 ² Pa or less. Then, heat treatment was performed for 30 minutes. And the laminated body is produced by laminating the copper foil side of the same copper-clad laminate on the opposite surface of the polyimide resin sheet to the side laminated with the copper foil of the copper-clad laminate after the heat treatment. This was heated at 200 ° C. for 30 minutes while being pressed at a pressure of 3.9 MPa to cure the epoxy resin, thereby forming an inner via hole connection and manufacturing a wiring board . When the resistance value of the formed inner via hole connection was measured by the four probe method, it was 10.0 mΩ per piece.
(Evaluation of peel strength)

The conductive paste (i) was applied to one side of a polyimide resin sheet (thickness 25 μm) and then heat-treated in a reduced-pressure atmosphere under the same conditions as described above. Then, the same polyimide resin sheet is laminated on the conductive paste layer after the heat treatment to produce a laminate, and this laminate is heated at 200 ° C. for 30 minutes while being pressed under the same conditions as described above. Was cured to prepare a sample for evaluation of peel strength. And about this sample, when the 180 degree peeling strength was measured based on the method described in Japanese Industrial Standard JIS C5016, it was 193.0 N / m.
(Repeated printing characteristics test)

The viscosity of the conductive paste (i) on the printing machine after conducting the work of continuously filling the conductive paste (i) in a large number of via holes formed on the same copper-clad laminate as described above by screen printing, 1000 times. It was 63 Pa · s when measured (measurement temperature 25 ° C.).
(Long-term storage stability test)

  When the conductive paste (i) is placed in a translucent container and the container is hermetically sealed, it is stored at a temperature of −5 ° C. or lower while periodically checking the fluidity from the outside of the container. (i) did not lose its liquidity. Therefore, when the conductive paste (i) was taken out of the container and the viscosity (measurement temperature 25 ° C.) was measured, it was 65 Pa · s.

Example 2
( Manufacture of wiring boards )

A wiring board was manufactured by forming an inner via hole connection in the same manner as in Example 1 except that the conductive paste (i) was used and the heat treatment in a reduced pressure atmosphere was omitted. When the resistance value of the formed inner via hole connection was measured by the four probe method, it was 13.0 mΩ per piece. Then, from this result and the result of Example 1, when the heat treatment in a reduced pressure atmosphere is performed to remove the solvent in the conductive paste (i) to prevent voids from being formed in the inner via hole connection, It was found that the resistance value of the connection can be lowered.
(Evaluation of peel strength)

  A sample for peeling strength evaluation was prepared in the same manner as in Example 1 except that the conductive paste (i) was used and the heat treatment in a reduced pressure atmosphere was omitted. And about this sample, when the 180 degree peel strength was measured based on the method described in Japanese Industrial Standard JIS C5016, it was 180.0 N / m. Then, from this result and the result of Example 1, when the heat treatment in a reduced pressure atmosphere is performed to remove the solvent in the conductive paste (i) to prevent the generation of voids, the cured inner via hole connection It was found that the adhesion strength of can be improved.

Comparative Example 1
(Preparation of conductive paste (ii))

Silver powder as conductive filler [Ag-cL manufactured by Fukuda Metal Foil Powder Co., Ltd.], liquid epoxy resin [Epicoat 828 manufactured by Japan Epoxy Resin Co., Ltd., bisphenol A type, epoxy equivalents 184 to 194, molecular weight 380] A curing agent [Novacure (registered trademark) HX3941 manufactured by Asahi Kasei Epoxy Co., Ltd.], a silane coupling agent [KBM403 manufactured by Shin-Etsu Chemical Co., Ltd.], and dimethylacetamide (boiling point 165 ° C.) as a solvent The mixture was mixed at a ratio and kneaded with a three-roll mixer to prepare a conductive paste (ii). The volume ratio of silver powder to epoxy resin was 50:50. The viscosity (measurement temperature: 25 ° C.) immediately after preparation of the conductive paste (ii) was 162 Pa · s.
(Ingredients) (Parts by weight)
・ Silver powder 1180
・ Epoxy resin 100
・ Curing agent 25
・ Silane coupling agent 5
・ Dimethylacetamide 40

This conductive paste (ii) uses the liquid epoxy resin presented in Example 5 of Patent Document 1 to reproduce the one described in Patent Document 1, but it can be kneaded without a solvent. A small amount of low boiling point solvent was added. In Patent Document 1, copper powder was used as the conductive filler. However, in Comparative Example 1, silver powder was used to prevent a difference due to the difference in the conductive filler during resistance value measurement. .
( Manufacture of wiring boards )

An inner via hole connection was formed in the same manner as in Example 2 except that the conductive paste (ii) was used to manufacture a wiring board . When the resistance value of the formed inner via hole connection was measured by the four probe method, it was 29.0 mΩ per piece. And from this result and the results of Examples 1 and 2, replacing the conventional epoxy resin used in Comparative Example 1 with the high molecular weight epoxy resin used in Examples 1 and 2, the inner via hole connection, It has been found that the resistance can be lowered by improving the adhesion of the circuit to the metal surface.
(Evaluation of peel strength)

A sample for peel strength evaluation was prepared in the same manner as in Example 2 except that the conductive paste (ii) was used. And about this sample, when the 180 degree peel strength was measured based on the method described in Japanese Industrial Standard JIS C5016, it was 6.0 N / m. And from this result and the results of Examples 1 and 2, replacing the conventional epoxy resin used in Comparative Example 1 with the high molecular weight epoxy resin used in Examples 1 and 2, the inner via hole connection, It was found that the adhesion to the metal surface of the circuit can be improved.
(Repeated printing characteristics test)

The viscosity of the conductive paste (ii) on the printing machine after conducting the work of continuously filling the conductive paste (ii) into a large number of via holes formed in the same copper-clad laminate as described above by screen printing, 1000 times. It was 430 Pa * s when (measurement temperature 25 degreeC) was measured. And from this result and the result of Example 1, the conventional epoxy resin used in Comparative Example 1 is replaced with the high molecular weight epoxy resin used in Examples 1 and 2 to improve the number of times of printing. It was found that a conductive paste capable of maintaining excellent printing characteristics can be obtained.
(Long-term storage stability test)

  The conductive paste (ii) was placed in a translucent container, and when the container was sealed, it was stored at a temperature of −5 ° C. or lower while periodically checking the fluidity from the outside of the container. The conductive paste (ii) Lost its liquidity in two months. And by replacing the conventional epoxy resin used in Comparative Example 1 with the high molecular weight epoxy resin used in Examples 1 and 2 based on this result and the result of Example 1, it can be stored for a longer period of time. It was found that a conductive paste was obtained.

Comparative Example 2
( Manufacture of wiring boards )

An inner via hole connection was formed in the same manner as in Example 1 except that the conductive paste (ii) was used to manufacture a wiring board . When the resistance value of the formed inner via hole connection was measured by the four probe method, it was 23.0 mΩ per piece. From this result and the results of Examples 1 and 2 and Comparative Example 1, when the conductive paste (ii) using the conventional epoxy resin was used, the solvent was removed by heat treatment in a reduced pressure atmosphere. However, it was found that the resistance value of the inner via hole connection cannot be lowered as much as the conductive paste (i) is used.
(Evaluation of peel strength)

  A sample for peeling strength evaluation was produced in the same manner as in Example 1 except that the conductive paste (ii) was used. And about this sample, when the 180 degree peel strength was measured based on the method described in Japanese Industrial Standard JIS C5016, it was 7.5 N / m. From this result and the results of Examples 1 and 2 and Comparative Example 1, when the conductive paste (ii) using the conventional epoxy resin was used, the solvent was removed by heat treatment in a reduced pressure atmosphere. However, it was found that the adhesion of the inner via hole connection to the metal surface of the circuit could not be improved as much as the conductive paste (i) was used.

＊１：ジャパンエポキシレジン(株)製のエピコート１２５６、ビスフェノールＡ型、エポキシ当量７０００〜８５００、分子量５３０００
＊２：ジャパンエポキシレジン(株)製のエピコート８２８、ビスフェノールＡ型、エポキシ当量１８４〜１９４、分子量３８０
The above results are summarized in Table 1.

* 1: Epicoat 1256 manufactured by Japan Epoxy Resin Co., Ltd., bisphenol A type, epoxy equivalent 7000-8500, molecular weight 53000
* 2: Japan Epoxy Resin Co., Ltd. Epicoat 828, bisphenol A type, epoxy equivalent 184-194, molecular weight 380

Claims (5)

A conductive filler, having an epoxy equivalent of 5000 or more and 10,000 or less, an epoxy resin having a molecular weight of 20000 or more and 70000 or less, and a solvent having a boiling point of 240 ° C. or more and 250 ° C. or less. Conductive paste. Inner via-hole connection through a step of filling the conductive paste of claim 1 into a via hole penetrating the insulating substrate and a step of removing the solvent by heat treatment in a reduced pressure atmosphere of 1.3 × 10 ² Pa or less. A method of manufacturing a wiring board, comprising: forming a wiring board . The method for producing a wiring board according to claim 2, wherein the heat treatment in a reduced-pressure atmosphere is performed at 40 to 80 ° C. 3. The method for manufacturing a wiring board according to claim 2 , further comprising a step of curing the epoxy resin by pressing the insulating substrate after removing the solvent by heat treatment . The method for manufacturing a wiring board according to claim 2, wherein the insulating substrate is made of polyimide resin.