JP6506046B2 - Solder bump reflow method - Google Patents

Description

  The present invention relates to a method of reflowing solder bumps formed on a workpiece.

  BACKGROUND ART Conventionally, in the manufacture of electronic devices and semiconductors, a bonded structure in which a plurality of bonded portions, for example, electronic components and a work such as a substrate or a silicon wafer are bonded is used. In such a joint structure, a solder layer is formed between a plurality of joints, and the joints are joined via the solder layer. And when manufacturing such a bonded structure, the method of forming a solder bump on a workpiece | work and bonding a several to-be-joined part via this solder bump is used.

  In the formation of such a solder bump, if the wettability of a solder material such as solder paste is insufficient, voids (air bubbles) may be generated in the formed solder bump. Such a void causes a reduction in the bonding property between the parts to be bonded and a reduction in the heat dissipation property, leading to a reduction in the reliability of the electronic device or semiconductor.

  Therefore, as a method of suppressing the generation of such a void, for example, a method of heating in a highly reducing atmosphere such as hydrogen gas to improve the wettability of the solder material, or heating a portion to be joined via the solder material Then, the method of defoaming the void in the solder alloy has been used by melting the solder alloy and then setting the inside of the container containing the solder under a vacuum atmosphere.

  In addition, a method is also widely used in which residual states of voids in the manufactured connection structure and solder bumps are inspected using an X-ray inspection apparatus, and those exceeding predetermined reference values are excluded as defective products.

  As a method of defoaming a void in the solder alloy, for example, a method of narrowing the size of the void by repeating depressurization and pressurization a plurality of times in a molten state of solder (Patent Document 1), once from atmospheric pressure After reducing the pressure to a low first pressure, the solder is melted, and while raising the temperature, raise the pressure higher than the first pressure to a second pressure that does not exceed the atmospheric pressure, and while raising the temperature, increase the third pressure and lower than the second pressure A method of reducing pressure to pressure and then returning to a pressure that does not exceed atmospheric pressure and then lowering the temperature to below the melting point of the solder (Patent Document 2). The method (patent document 3) etc. which repeat the pressure change process returned to atmospheric pressure in multiple times are disclosed.

JP-A-6-69387 Patent No. 4404000 JP 2007-915 A

In the case of the methods disclosed in Patent Document 1 and Patent Document 3, voids are ruptured at the time of degassing in the process of depressurizing and pressurizing a plurality of times, and the wettability of the solder (solder) is deteriorated to cause sufficient voids. May not be able to degas.
Next, in the case of the method disclosed in Patent Document 2, since the temperature is raised above the melting temperature of the solder alloy under reduced pressure, when the solder paste is used, the flux contained therein is likely to be volatilized and the solder alloy There is a possibility that the meltability of
Moreover, the method of defoaming the void in the conventional solder alloy and the technique disclosed by patent document 1 to patent document 3 are related to the void suppression at the time of the solder joint of a to-be-joined part, and the solder formed already There is no disclosure or suggestion on how to cope with the occurrence of a void in the bump.

  The present invention relates to a solder bump reflow method capable of efficiently reducing the solder bumps formed on a work, particularly even when the solder bumps contain voids of a certain standard or more. Its purpose is to provide.

(1) The method for reflowing a solder bump according to the present invention includes the steps of applying a flux to the surface of a solder bump formed on a work, and heating the work under a first pressure near atmospheric pressure. And reducing the pressure around the work to a second pressure lower than the first pressure after the heating temperature for heating the work reaches or exceeds the liquidus temperature of the solder alloy constituting the solder bump. A step of pressurizing the pressure around the work to a third pressure higher than the second pressure while the heating temperature is higher than the liquidus temperature of the solder alloy; and the heating temperature is higher than that of the solder alloy Pressurizing the pressure around the work to the first pressure in a state above the liquidus temperature, and heating the heating temperature to the liquidus phase of the solder alloy after the pressure around the work reaches the first pressure Degrees below to include a step of solidifying the solder bumps melted, and its characterized in that said second plurality of times and a step of pressurizing to said third pressure and a step of pressure reduction to a pressure.

(2) In the configuration described in (1), the second pressure is 50 Pa to 100 Pa.

(3) In the configuration described in (1) or (2) above, in the step of pressurizing up to the third pressure, the heating temperature at the time when the pressure around the work reaches the third pressure is It is characterized in that the temperature is 10 ° C. or higher than the liquidus temperature of the solder alloy.

(4) In the configuration described in any one of (1) to (3) above, the third pressure is characterized by being 10,000 Pa to 50,000 Pa.

(5) In the configuration described in any one of (1) to (3) above, the step of reducing the pressure to the second pressure and the step of pressurizing the pressure to the third pressure are performed twice to four times. It is characterized by what it does.

(6) In the configuration described in the above (4), the step of reducing the pressure to the second pressure and the step of pressurizing the pressure to the third pressure are performed twice to six times.

(7) In the configuration described in any one of (1) to (6) above, the pressure is reduced from the first pressure to the second pressure and then is increased to the third pressure. It is characterized in that one cycle time taken is 10 seconds to 40 seconds.

  According to the above configuration, the method of reflowing solder bumps according to the present invention efficiently removes the solder bumps formed on the work, particularly when the solder bumps contain voids of a certain standard or more. Can be reduced.

It is a schematic diagram showing change on a substrate from solder bump formation to flux residue removal about a reflow method of a solder bump concerning one embodiment of the present invention. It is a figure which shows the temperature profile at the time of formation of a solder bump in the embodiment, an example, a comparative example, and a reference example. It is a figure which shows the change of the temperature profile and pressure in a vacuum chamber at the time of reflow of a solder bump concerning the same embodiment.

  Hereinafter, one embodiment of the method for reflowing solder bumps of the present invention will be described in detail. The present invention is of course not limited to the embodiment.

The reflow method of the present embodiment will be described with reference to FIG.
In the reflow method of the present embodiment, first, the substrate 10 on which the solder bumps 20 are formed is prepared. As a method of forming such a substrate 10, there may be mentioned, for example, a method of forming a solder bump 20 by applying a solder paste containing a solder alloy powder and a flux onto the substrate 10 and reflowing it. Note that the method for forming the solder bumps 20 is not limited to this, and any method may be used as long as the solder bumps can be formed on the substrate 10, such as a method of mounting by reflowing solder balls on the substrate. Even if it can be used.
Although the substrate 10 is used as a work in the present embodiment, any substrate such as a printed circuit board, a silicon wafer, etc. can be used as a work as long as it is used for mounting and mounting of electronic components. .

In addition, as a flux used for the said solder paste, the thing containing base resin, a solvent, and an activator is mentioned, for example.
Further, examples of the base resin include rosin resins and acrylic resins, and examples of the activator include organic acids, amines, and halides of these. Moreover, the flux which added antioxidant and a thixo agent to these is also used preferably.

  The composition of the solder alloy is also not particularly limited, but is preferably lead-free solder from the environmental point of view. In the present embodiment, a lead-free solder alloy having a composition of Sn-3Ag-0.5Cu is used.

  Furthermore, in the present embodiment, the substrate 10 coated with the solder paste is placed in a reflow apparatus, and reflow is performed under a nitrogen atmosphere (oxygen concentration 100 ppm) at atmospheric pressure and a temperature profile shown in FIG. The conditions at the time of reflow are not limited to this, and can be appropriately changed depending on the substrate 10, the type of the solder paste, and the like.

  Since the flux of the solder paste is attached to the residue on the substrate 10 after the reflow, the substrate 10 on which the solder bumps 20 are formed is manufactured by cleaning this using a cleaning solution or the like (FIG. 1 (a )reference).

Next, a flux 30 is applied to the surface of the formed solder bump 20 (see FIG. 1B). The component of the flux 30 may be the same as or different from the flux used at the time of formation of the solder bump 20. For example, in TG-DTA measurement, the weight loss at a temperature of 250 ° C. is 40% or less Is preferably used. The conditions for this TG-DTA measurement are as follows.
Sample amount: about 10 mg, temperature rising rate: 10 ° C./min, atmosphere: nitrogen The method of applying the flux 30 to the solder bump 20 is not particularly limited as long as the flux 30 can be applied to the surface of the solder bump 20 A coating method using a metal or a coating method using a metal mask or the like is preferably used.

Next, the substrate 10 coated with the flux 30 on the surface of the solder bump 20 is accommodated, for example, in a vacuum chamber provided in a vacuum reflow soldering apparatus.
The inside of the vacuum chamber containing the substrate 10 is adjusted to a first pressure (P1). In this adjustment, for example, once the inside of the vacuum chamber is evacuated by drawing a vacuum from atmospheric pressure at room temperature, then an inert gas such as nitrogen or a reducing gas such as hydrogen is supplied to It is preferable to set the pressure (P1) of The first pressure (P1) is preferably near atmospheric pressure.

Then, the substrate 10 is heated by heating the inside of the vacuum chamber while keeping the inside of the vacuum chamber under the first pressure (P1). Changes in the temperature profile and the pressure in the vacuum chamber (= around the substrate 10) in the present embodiment are shown in FIG. As shown in FIG. 3, the preheating temperature (150 ° C. in FIG. 3) may be maintained for a predetermined time.
In the present embodiment, heating under the first pressure (P1) is performed in the vacuum chamber, but for example, heating under the first pressure (P1) is performed in the heating zone of the reflow apparatus, Thereafter, the substrate 10 may be accommodated in the vacuum chamber.

After the heating temperature to the substrate 10 becomes equal to or higher than the liquidus temperature of the solder alloy constituting the solder bump 20, the pressure in the vacuum chamber is a second pressure (P2) lower than the first pressure (P1). Depressurize to).
In the present embodiment, after the temperature profile shown in FIG. 3 has become equal to or higher than the liquidus temperature of the solder alloy, that is, the temperature (D) in the vacuum chamber is 220 ° C. (D1) which is the liquidus temperature of the solder alloy. When this occurs, the pressure in the vacuum chamber is reduced to a second pressure (P2) lower than the first pressure (P1). As a method of reducing pressure, for example, there is a method of evacuating gas from the vacuum chamber using a pump (not shown) provided in the vacuum chamber. In particular, it is preferable to reduce the pressure in the vacuum chamber to a second pressure (P2) after the temperature in the vacuum chamber reaches the liquidus temperature or more.

Thus, the timing of the pressure reduction to the second pressure (P2) was such that the temperature in the vacuum chamber became close to the liquidus temperature (220.degree. C. (D1) which is the liquidus temperature in this embodiment) By doing this, it is possible to prevent the decrease in the melting property of the solder alloy due to the decrease in the amount of flux used for the solder paste. That is, if the pressure in the vacuum chamber is reduced for a long time before the solder alloy melts, the amount of flux tends to decrease, which may affect the meltability of the solder alloy.
In the present embodiment, the vicinity of the liquidus temperature means a temperature at which 50% or more of the solder alloy melts.

  In the present embodiment, the second pressure (P2) is preferably 50 Pa to 100 Pa. This range can be appropriately selected depending on the application of the substrate 10.

  The higher the degree of vacuum at the second pressure (P2), the larger the expansion size of the voids contained in the solder alloy, and the larger the difference in pressure at the time of subsequent pressurization, the solder alloy The void discharge effect from water is enhanced. However, in the case of the present embodiment, even if the degree of vacuum of the second pressure (P2) is 50 Pa or more, the void reduction effect can be efficiently exhibited.

Next, the pressure in the vacuum chamber is increased to a third pressure (P3) higher than the second pressure (P2). As a method of pressurization, for example, it is preferable to adjust a third pressure (P3) by supplying an inert gas such as nitrogen or the like and a reducing gas such as hydrogen into the vacuum chamber.
Here, the temperature (D2) in the vacuum chamber when the third pressure (P3) is reached is preferably equal to or higher than the liquidus temperature of the solder alloy. When the solder alloy starts to solidify and is pressurized to the third pressure (P3), there is a possibility that the void is confined in the solder bump in the expanded state.

  In addition, when supplying gas, such as nitrogen, in the said vacuum chamber, the temperature in the said vacuum chamber may fall once by the difference with the temperature of these gas. In this case, when the difference between the temperature (D2) in the vacuum chamber and the liquidus temperature is small, a drop in the internal temperature generates a solid phase in a part of the solder alloy, and the void is discharged in the solder alloy. There is a possibility that the effect may be hindered and the void may be confined in the expanded state in the solder bump after reflow. Such a phenomenon is likely to occur particularly when a solder alloy having a narrow range of solid phase temperature and liquid phase temperature is used. This phenomenon can be suppressed by setting the temperature (D2) in the vacuum chamber to the liquidus temperature of the solder alloy or more, particularly 10 ° C or more than the liquidus temperature.

The third pressure (P3) is preferably 10,000 Pa to 50,000 Pa. By setting the third pressure (P3) in this range, it is possible to shorten the cycle time of depressurization and pressurization, and a plurality of depressurization and pressurization can be performed in a short time. Can be reduced. Further, in this case, it is possible to suppress the reduction of the amount of flux contained in the solder paste and the re-oxidation of the surface of the solder alloy due to this.
Generally, if the inside of the vacuum chamber is depressurized to vacuum pressure, the expansion size of the void contained in the solder alloy becomes large and it is easy to be discharged outside, and then the inside of the vacuum chamber is pressurized to atmospheric pressure If this is done, the void shrinks to make the solder more fluid and improve the defoaming effect. On the other hand, if the difference between the pressure at the time of depressurization and the pressure at the time of pressurization is large, there is a possibility that the solder alloy melted together with the void may be scattered at the time of defoaming. However, when the third pressure (P3) is in the above range, such scattering can be suppressed. As described later, in the embodiment, since the decompression and pressurization are repeated a plurality of times, voids in the solder alloy can be efficiently reduced, and the third pressure (P3) is within the above range. In this case, the scattering phenomenon due to repetition can be suppressed.

  Decompression from the first pressure (P1) to the second pressure (P2) and pressurization from the second pressure (P2) to the third pressure (P3) in one cycle. The cycle time is preferably 10 seconds to 40 seconds. By setting one cycle time to this time, it is possible to suppress the reduction of the amount of flux and the re-oxidation of the surface of the solder alloy due to this. On the other hand, when the cycle time per one cycle is longer than 40 seconds, the amount of flux contained in the solder paste is likely to be reduced, which may make it difficult to eliminate voids from the solder alloy.

  In addition, the depressurizing process from the first pressure (P1) to the second pressure (P2) and the pressurizing process from the second pressure (P2) to the third pressure (P3) are performed a plurality of times. Thus, it is possible to accelerate the degassing of voids in the molten solder alloy by repeating decompression and pressurization a plurality of times, and to reduce the voids remaining in the solder bumps after reflow.

The number of times of the pressure reduction step and the pressure step is preferably 2 to 4 times. Although it is preferable that the number of times of the pressure reduction step and the pressure step is large, if the heating time in the vacuum chamber, particularly the peak heating time, is increased to increase the number, the amount of flux contained in the solder paste decreases. As a result, the surface of the solder alloy may be reoxidized.
In addition, since pressure reduction and pressurization can be performed in a short time when the third pressure (P3) is in the range of 10,000 Pa to 50,000 Pa as described above, the pressure reduction process and the pressure application process can be performed. The number of times can be from 2 to 6 times.

  After the final decompression, the pressure in the vacuum chamber is increased to a third pressure (P3), which is further increased to a first pressure (P1). Although the first pressure (P1) shown in FIG. 3 is represented by the same value as the first pressure (P1), the pressure may be the same as or near the atmospheric pressure. . Further, as shown in FIG. 3, the pressurization to the third pressure (P3) and the pressurization to the first pressure (P1) may be performed continuously, and may be once added to the third pressure (P3). After the pressure is applied, the pressure may be increased to a first pressure (P1). As mentioned above, in order not to extend the heating time in the vacuum chamber, the former continuous pressurization is preferable.

  Then, after the pressure in the vacuum chamber reaches the first pressure (P1), the temperature in the vacuum chamber is cooled to below the liquidus temperature of the solder alloy, and the temperature returns to around room temperature Thereafter, the substrate 10 with the melted solder bumps 20 solidified is taken out from the vacuum chamber.

  Since the flux residue 40 remains on the substrate 10 taken out of the vacuum chamber (see FIG. 1 (c)), the flux residue 40 is removed by a cleaning solution or the like as needed.

  In the present embodiment, particularly, the time (T) shown in FIG. 3, that is, the pressure reduction from the first pressure (P1) to the second pressure (P2) and the second pressure (P2) to the third pressure Wetting the solder alloy by pressurizing the (P3) and shortening the time required for repressurization to the first pressure (P1) by pressurization, and repeating decompression and pressurization several times during this time The voids can be efficiently reduced without affecting the properties and the like. This time (T) is preferably, for example, about 120 seconds to 210 seconds.

  In the present embodiment, the reflow is performed without using the bonded portion. However, for example, a solder in which the bonded portion is mounted on the solder bump 20 coated with the flux 30 on the surface may be reflowed.

  Hereinafter, the present invention will be described in detail by way of examples, comparative examples and reference examples. The present invention is not limited to these examples.

<Preparation of solder paste>
A base resin, an activator and a solvent are appropriately mixed to prepare a flux, which is mixed with 12% by weight and 88% by weight of Sn-3Ag-0.5Cu solder alloy powder (liquid phase temperature 220 ° C.), and then soldered. I got a paste.

<Formation of solder bumps>
Examples 4, 6 and 7, Reference Examples 1 'to 3' and 5 ' , Comparative Examples 1 to 3 and Reference Examples 1 to 3 for each FR 4 substrate (Cu-OSP treatment, substrate resist film 20 μm , Electrode pitch 130 μm, electrode aperture diameter 75 μm). Solder paste prepared using a metal mask (mask thickness 30 μm) having a pattern corresponding to the FR4 substrate was printed by hand.
The respective substrates were reflowed using a reflow apparatus (product name: SMT Scope SK-5000 manufactured by Sanyo Seiko Co., Ltd.) under nitrogen atmosphere (oxygen concentration 100 ppm) at atmospheric pressure and temperature profile conditions shown in FIG. 2 .
Next, the semi-aqueous cleaning solution (product name: Pine Alpha ST-100 SX, manufactured by Arakawa Chemical Co., Ltd.) is placed in a beaker, which is heated to about 60 ° C. with a hot plate stirrer and stirred while being in the state of stirring. Each of the performed substrates was placed, and the residual flux remaining on the substrate was cleaned. Thereafter, deionized water is put into a beaker, which is stirred while being heated to 60 ° C. with a hot plate stirrer, and each washed substrate is put into rinse and washed, it is taken out from a beaker and dried with hot air. did.
When the area ratio of voids to the solder bumps formed was evaluated using an X-ray observation apparatus (product name: XD7600 Diamond, manufactured by Nordson Corporation) for each substrate on which the solder bumps were formed, the average void area was evaluated. The rate was 20%.

About each substrate in which the said solder bump was formed, after applying the flux used for preparation of solder paste on the surface of a solder bump using a brush, reflow apparatus (product name: SMT Scope SK-5000 made by Sanyo Seiko Co., Ltd. Each test substrate is prepared by reflowing under the nitrogen atmosphere (oxygen concentration 100 ppm), the temperature profile shown in FIG. 2, and the conditions shown in Tables 1 and 2 under atmospheric pressure except at the time of depressurization and pressurization. did.
Moreover, the area ratio of the void occupied to the formed solder bump was evaluated using X-ray observation apparatus (product name: XD7600 Diamond, product made by Nordson Corporation) about each said test board | substrate. The results are shown in Tables 1 and 2.
In Tables 1 and 2, the pressure reduction timing to the second pressure indicates the time at which the temperature in the reflow apparatus is reached at the time of temperature rise.

As shown in Tables 1 and 2, although the void area ratio of the solder bumps before reflow was 20%, Examples 4, 6 and 7 and Reference Examples 1 'to 3' and 5 ' In any case, the void area ratio is less than 10%, and it can be seen that the voids in the solder bumps which have already occurred can be reduced.
Also, as in Example 6 and Example 7, when the third pressure is set to 10,000 Pa to 50,000 Pa, good void reduction effect is exhibited even if the number of times of pressure reduction and pressurization is performed in a short time six times. be able to.
Furthermore, particularly when the third pressure is 10,000 Pa and the number of decompression / pressurization is four times as in Example 4, the solder bumps formed on the substrate are lower in pressure than the first pressure and higher in temperature than the first pressure. It can be seen that by repeating the depressurization-pressurization while reducing the time of exposure to the lower side (the melting temperature of the solder alloy), it is possible to exhibit a better void reduction effect while preventing the reduction of the amount of flux.
On the other hand, in Comparative Example 3, since the solder alloy constituting the solder pump solidifies in the reduced pressure atmosphere, the void expanded in the reduced pressure atmosphere is confined within the solder bump without shrinkage, and rather the solder It can be seen that the void area ratio in the bumps is increased.

  As described above, according to the reflow method of the present invention, it is possible to reduce the voids in the solder bumps that have already occurred. Therefore, for example, even a so-called defective product that is determined to have a void greater than or equal to a reference value in an X-ray inspection or the like in a solder bump formation process can be reflowed by using the reflow method of the present invention. The voids in the solder bumps can be reduced and sent to the next step as a non-defective product, and the manufactured workpiece can be used without waste.

10: Substrate 20: Solder bump 30: Flux 40: Flux residue

Claims (5)

Applying a flux to the surface of a solder bump formed on a workpiece;
Heating the workpiece under a first pressure, which is near atmospheric pressure;
Reducing the pressure around the work to a second pressure lower than the first pressure after the heating temperature for heating the work becomes equal to or higher than the liquidus temperature of the solder alloy forming the solder bump; ,
Pressurizing the pressure around the work to a third pressure higher than the second pressure while the heating temperature is higher than the liquidus temperature of the solder alloy;
Pressurizing the pressure around the work to the first pressure while the heating temperature is equal to or higher than the liquidus temperature of the solder alloy;
After the pressure around the workpiece reaches the first pressure, the heating temperature is set to a liquidus temperature of the solder alloy or less to solidify the melted solder bumps;
The timing of the pressure reduction to the second pressure is performed when the heating temperature reaches the liquidus temperature of the solder alloy constituting the solder bump,
The third pressure is 10,000 Pa to 50,000 Pa,
A method for reflowing solder bumps, comprising the steps of: reducing the pressure to the second pressure; and pressurizing the pressure to the third pressure multiple times.   The solder bump reflow method according to claim 1, wherein the second pressure is 50 Pa to 100 Pa.   In the step of pressurizing to the third pressure, the heating temperature at the time when the pressure around the work reaches the third pressure is 10 ° C. or higher than the liquidus temperature of the solder alloy. The reflow method of the solder bump of Claim 1 or Claim 2.   The solder bump according to any one of claims 1 to 3, wherein the step of reducing the pressure to the second pressure and the step of pressing the pressure to the third pressure are performed twice to six times. Reflow method. One cycle time taken to reduce the pressure from the first pressure to the second pressure and pressurize the pressure to the third pressure is 10 seconds to 40 seconds. The solder bump reflow method according to any one of claims 1 to 4 .

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