JP6506035B2 - Method of forming solder bumps - Google Patents

Description

  The present invention relates to a method of forming 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.

  As a method of suppressing the generation of such voids, for example, a method of improving the wettability of a solder material by heating in a highly reducing atmosphere such as hydrogen gas, or heating a portion to be joined via the solder material A method of degassing the air in the solder alloy has been used by melting the solder alloy and then setting the inside of a container containing the solder under a vacuum atmosphere.

  Moreover, the method of narrowing the size of a void by repeating pressure reduction and pressurization a plurality of times in a molten state of solder (Patent Document 1), once the pressure is reduced to a first pressure lower than atmospheric pressure, the solder is melted The temperature is raised to a second pressure which is higher than the first pressure and does not exceed the atmospheric pressure, and the temperature is raised to a third pressure which is higher than the first pressure and lower than the second pressure, and then the pressure which does not exceed the atmospheric pressure Method to lower the temperature below the melting point of the solder after returning to the (Patent Document 2), a method of melting the solder under approximately atmospheric pressure, depressurizing this, and then repeating the pressure change process several times to return to approximately atmospheric pressure ( Patent Document 3) and the like 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.
Further, 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, the flux contained in the solder paste is easily volatilized when the solder paste is used. There is a possibility that the meltability may be reduced.

  An object of the present invention is to provide a method of forming a solder bump capable of efficiently reducing voids in the solder bump.

(1) The method for forming a solder bump according to the present invention comprises the steps of: printing a solder paste consisting of flux and a solder alloy at a predetermined position on a work; and under the first pressure near the atmospheric pressure. Heating, and reducing the pressure around the workpiece to a second pressure lower than the first pressure after the heating temperature for heating the workpiece reaches or exceeds the liquidus temperature of the solder alloy. The step of pressing the pressure around the work to a third pressure higher than the second pressure when the heating temperature is higher than the liquidus temperature of the solder alloy; and the heating temperature is a liquidus phase of the solder alloy A step of pressurizing the pressure around the work to the first pressure in a state of temperature or higher; and, after the pressure around the work reaches the first pressure, the heating temperature is higher than the liquidus temperature of the solder alloy And forming a solder bump on the workpiece in the, 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 for forming a solder bump according to the present invention can efficiently devoid the void in the solder bump.

It is a schematic sectional drawing which shows a part of board | substrate which has a solder bump formed using the formation method of the solder bump concerning one Embodiment of this invention. It is a figure which concerns on the same embodiment and shows the change of the temperature profile and pressure in a container. It is a figure which concerns on the Example of this invention and which shows the temperature profile in formation of a solder bump.

  Hereinafter, an embodiment of a method of forming a solder bump of the present invention will be described in detail. The present invention is of course not limited to the embodiment.

  First, solder bumps formed by the forming method of the present embodiment will be described with reference to FIG. Although the substrate 11 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 electronic components. As shown in FIG. 1, the solder bumps 10 are formed on the substrate 11.

Next, in the formation method of the present embodiment, first, a solder paste in which a solder alloy and a flux are mixed is applied to a predetermined position on the substrate 11. Examples of such flux include those containing a base resin, a solvent and an activator.
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.

  By using such a flux, it is possible to improve the wettability of the solder alloy and to suppress the occurrence of voids in the formed solder bumps.

  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.

Next, the substrate 11 coated with the solder paste is housed, for example, in a vacuum chamber provided in a vacuum reflow soldering apparatus.
The inside of the vacuum chamber containing the substrate 11 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 11 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 container (= around the substrate 11) in the present embodiment are shown in FIG. As shown in FIG. 2, the preheating temperature (150 ° C. in FIG. 2) 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 11 may be accommodated in the vacuum chamber.

After the heating temperature to the substrate 11 becomes equal to or higher than the liquidus temperature of the solder alloy, the pressure in the vacuum chamber is reduced to a second pressure (P2) lower than the first pressure (P1).
In the present embodiment, after the temperature profile shown in FIG. 2 becomes higher than the liquidus temperature of the solder alloy or higher, 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 according to the application of the substrate (work) on which the solder bumps are formed.

  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 is hindered and the void is confined in the expanded state in the formed solder bump. 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 formed solder bumps.

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. 2 is represented by the same value as the first pressure (P1), the pressure may be the same as or near the atmospheric pressure. . In addition, as shown in FIG. 2, the pressurization to the third pressure (P3) and the pressurization to the first pressure (P1) may be performed continuously, and once applied 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 11 is taken out of the vacuum chamber.
Thereafter, the flux residue (not shown) remaining on the substrate 11 is removed by a cleaning solution or the like, and the substrate 11 on which the solder bumps 10 shown in FIG. 1 are formed is completed.

  In the present embodiment, in particular, the time (T) shown in FIG. 2, 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.

  Hereinafter, the present invention will be described in detail by way of examples and comparative 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, and this is mixed with 12% by weight and 88% by weight of Sn-3Ag-0.5Cu solder alloy powder (liquidus temperature 220 ° C.), and then implemented. Solder pastes according to examples and comparative examples were obtained.

Bumps (Examples 4, 6 and 7, Reference Examples 1 'to Reference Example 3', Reference Example 5 ', Comparative Comparative Example 1 Example 3, Reference Example 3 from Example 1)
An FR4 substrate (Cu-OSP treatment, substrate resist film 20 μm, electrode pitch 130 μm, electrode aperture diameter 75 μm) was used. Each solder paste was printed by hand using a metal mask (mask thickness 30 μm) having a pattern corresponding to the FR4 substrate.

Next, using the reflow apparatus (product name: SMT Scope SK-5000 manufactured by Sanyo Seiko Co., Ltd.), the above-mentioned substrates are shown in FIG. Reflow was performed under the conditions shown in the profiles and Tables 1 and 2 to produce each test substrate. 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, the void area ratio is less than 10% in any of Examples 4, 6 and 7 and Reference Examples 1 ′ to 3 ′ and 5 ′ , and the void reduction effect is I know that there is.
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 times of decompression and pressurization is four as in Example 4, the solder paste applied onto the substrate is under pressure lower than the first pressure and higher 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.

10: Solder bumps 11: substrate

Claims (5)

A method of forming solder bumps on a workpiece,
Printing a solder paste consisting of flux and solder alloy at a predetermined position on the work;
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 reaches or exceeds the liquidus temperature of the solder alloy;
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 work reaches the first pressure, making the heating temperature equal to or lower than the liquidus temperature of the solder alloy to form a solder bump on the work;
The third pressure is 10,000 Pa to 50,000 Pa,
A method of forming a solder bump, comprising performing the step of reducing pressure to the second pressure and the step of pressing to the third pressure multiple times.   The method for forming a solder bump according to claim 1, wherein the second pressure is 50 Pa to 100 Pa. In the step of pressurizing to said third pressure, wherein the heating temperature at which the pressure of the workpiece surrounding reaches the third pressure and wherein a liquidus temperature than 10 ° C. or more higher than the temperature of the solder alloy The formation 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. Formation 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 method for forming a solder bump according to any one of claims 1 to 4 .

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