JPH09122966A - Solder paste, joining method using this solder paste and semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to decrease the void defects in a joint part by forming solder powder of which >=20wt.% has at least 90μm grain size and specifying the content of a flux component to >=6 to <=12wt.% of solder paste. SOLUTION: This solder paste is formed by mixing the solder powder and the flux component and >=20wt.% of the solder particles have at least 90μm grain size. The content of the flux component is specified to >=6 to <=12wt.%. The number of particles of the solder powder and the density of the solder paste decrease and the content of the flux component is low as well. Then, even if the printing quantity of the solder paste in joining increases, the amt. of the flux is sufficiently decreased and voids are decreased. The gaps among the particles of the solder powder increase and the discharge of the gases generated from the flux component is easy. The generation of air bubbles which are the cause for the voids is suppressed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solder paste used for bonding a member having a relatively large bonding area such as a semiconductor chip or a semiconductor device substrate to a base material.

[0002]

2. Description of the Related Art FIG. 3 shows a semiconductor device of high heat generation and large current consumption, which uses an insulating substrate of ceramics, as described in, for example, "Electronic Ceramics" (November 1988) issued by Gakudonsha. It is a cross-sectional view showing the structure of. In the figure, 1 is an insulating substrate made of ceramics (hereinafter referred to as ceramics substrate), 2a and 2b are metal layers formed on the front and back surfaces of the ceramics substrate 2, and 3 is a metal layer 2a in a predetermined region on the surface of the ceramics substrate 1. A semiconductor chip with a large heat generation, which is bonded to the semiconductor chip and mounted on the ceramic substrate 1.
Is a metal substrate which is bonded to the metal layer 2b on the back surface of the ceramic substrate 1 and has good thermal conductivity for radiating the heat generated in the semiconductor chip 3 to the outside. 5 electrically connects the semiconductor chip 3 and the metal layer 2a. Wires 6a and 6b are metal layers 2a and 2b formed on the front and back surfaces of the ceramic substrate 1,
It is a solder layer for joining formed between the semiconductor chip 3 and the metal substrate 4 joined thereto.

In the semiconductor device constructed as described above, the solder layers 6a and 6b for joining are made of a solder paste composed of a solder powder mixed with a flux component. The solder layer 6a is supplied with the above-mentioned solder paste by screen printing to a predetermined region on the metal layer 2a formed on the front surface of the ceramic substrate 1 having the metal layers 2a and 2b formed on the front and back surfaces thereof, and the solder paste layer 7a.
After forming (not shown), the semiconductor chip 3 is mounted on the place where the solder paste layer 7a is formed, and heat treatment is performed by a heating device to melt the solder powder in the solder paste layer 7a and to make a flux component. Is discharged to the outside, and subsequently, a cooling process is performed to solidify the molten solder to form the solder. Further, the solder layer 6b is formed on the metal substrate 4 in a predetermined area by screen printing to supply the solder paste to form a solder paste layer 7b (not shown), and then the ceramic substrate 1 is formed with the solder paste layer 7b. The solder layer 6a is formed in the same process as the above-mentioned solder layer 6a by mounting the solder layer 6a in a different place and performing heat treatment with a heating device. The residue of the solid component of the flux (hereinafter referred to as the flux residue) attached to the outside of the semiconductor device when the solder layers 6a and 6b are formed is removed by a cleaning liquid. The two solder layers 6a and 6b may be formed individually or simultaneously. The solder paste used for the two solder layers 6a and 6b may be the same or different.

By the way, the above-mentioned solder layers 6a, 6
The conventional solder paste used for joining by b is, for example, "PRODUCTS" issued by Nippon Superior Co., Ltd.
As described in "GUIDE Soldering Department", it is a mixture of flux components with a content of 9 to 15 wt% in order to secure printability and wettability to solder powder having a particle size distribution of 75 μm or less. It was

[0005]

When joining is performed using the conventional solder paste as described above, since the surface of the solder paste supplied by screen printing has irregularities, if a member is mounted on it A gap is formed between the surface of the solder paste and the member. When heating starts and the temperature rises to around the softening point of the solid component of the flux, the fluidity of the solder paste increases, and the solder paste flows to fill the above gap due to the capillary phenomenon, increasing the contact area between the solder paste and the mounting member. I will do it. However, at this time, a part of the atmosphere existing in the above-mentioned gap is confined, and innumerable bubbles are formed between the solder paste and the mounting member. When time passes and the temperature rises, some of the bubbles move in the solder paste and are discharged to the outside to disappear, but some of them are pushed away by the surrounding solder paste due to the expansion of the atmosphere trapped inside the bubbles. The size of the solder paste is increased to form bubbles that penetrate the front and back surfaces of the solder paste. When the temperature further rises and the solvent of the flux component in the solder paste begins to vaporize actively, some of the vaporized gas is trapped in the solder paste and bubbles are formed. It occurs remarkably when done.

In the bubbles formed as described above, the solid components of the flux and the gas generated from the flux components are selectively confined, and the solder powder is melted at a part of the places where such bubbles are formed. Even so, the contact between the molten solder and the members to be joined was impeded to cause non-wetting, and a joint defect called a void was formed after solidification of the solder. In the case of a member having a large bonding area, such voids due to the residual flux components are likely to occur because the printing amount of the solder paste is large and the flux amount of the bonding portion is large.
The void defect may occupy 10% or more of the area of the joint portion. Especially, in the large current semiconductor device as described above, when a large number of voids are formed in the solder layers 6a and 6b, the semiconductor chip 3 and the metal substrate are not formed. There is a problem that the heat flow path between the semiconductor device and the device 4 is narrowed, the heat dissipation capability is significantly reduced, and the reliability of the semiconductor device is deteriorated.

In order to reduce the void defects as described above, a scrubbing step has been conventionally performed before the solder solidifies in the cooling step after the melting of the solder in the above-mentioned joining step of the high-current semiconductor device. This scrubbing process
The semiconductor chip 3 or the ceramics substrate 1 is manually
In this step, the gas and solid components inside the voids are discharged to the outside to reduce the void defects in the solder layers 6a and 6b. However, such a scrubbing process has a large effect difference depending on the degree of skill of the operator and there is a problem in reliability, and there are problems such as a decrease in productivity due to an increase in the number of processes and an increase in manufacturing cost due to personnel costs. It was Also,
Especially when it is desired to control the atmosphere during heating and bonding, there is also a problem that the scrubbing process cannot be performed to maintain airtightness.

The present invention has been made in order to solve the above problems, and provides a highly reliable solder paste which can stably reduce void defects in the joint portion, and also has high heat generation and large heat generation. It is an object of the present invention to improve the heat dissipation of a semiconductor device which consumes current and to improve the reliability.

[0009]

A solder paste according to claim 1 of the present invention is a mixture of a solder powder and a flux component, and 20 wt% or more of the solder particles of the solder powder have a particle size of at least 90 μm. Has a diameter
Moreover, the content of the above-mentioned flux component is 6 wt% or more, 1
It is 2 wt% or less.

A joining method according to claim 2 of the present invention is
The solder paste in the solder paste is melted by supplying the solder paste according to claim 1 to a first member to be joined by screen printing, mounting a second member to be joined thereon, and then performing heat treatment. At the same time, the flux component is discharged to the outside, and then the cooling process is performed to solidify the molten solder to form a joint by the solder layer.

The joining method according to claim 3 of the present invention is
In the second aspect of the present invention, after the cooling process is performed to solidify the molten solder, the residue of the solid component of the flux adhered to the outside of the member to be joined and the solder layer is removed by using a cleaning liquid.

According to a fourth aspect of the present invention, there is provided a semiconductor device, a metal substrate, an insulating substrate having metal layers formed on the front and back surfaces of the metal substrate, and a semiconductor chip mounted on the insulating substrate. And a joint between the metal substrate and the insulating substrate and a joint between the insulating substrate and the semiconductor chip are formed by using the solder paste according to claim 1.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 FIG. Hereinafter, Embodiment 1 of the present invention will be described. Spherical solder powder having a composition of Sn27 / Sb3 / Pb70 and various particle sizes was sieved through a mesh having a mesh size of 63 μm, and the solder powder passing through the mesh was taken out to prepare a solder powder having a particle size distribution of 63 μm or less. . Similarly, it is sieved with a mesh having a mesh size of 150 μm, and the powder that has passed through the mesh is further sieved with a mesh having a mesh size of 90 μm. I prepared.

Next, the chlorine content of 0.20 wt.% Is added to the solder powder having the particle diameter of 63 μm or less and the solder powder having the particle diameter of 90 to 150 μm added at different mixing ratios.
% Flux component was mixed at 8 wt% to prepare several kinds of solder paste. At this time, the particle size is 90 to 150 μ
The mixing ratio of the solder powder of m is 20 as an example of the present invention.
wt%, 30 wt%, 40 wt% and 50 wt%, and 0 wt% and 10 wt% as comparative examples.

On a metal substrate 4 having a nickel plate on the surface of a copper plate having a width of 45 mm, a length of 91 mm and a thickness of 3 mm,
The prepared solder paste was supplied by screen printing to form a solder paste layer 7b having a width of 38 mm, a length of 60 mm and a thickness of 0.4 mm. Next, a thickness of 0.
A wiring pattern was formed by a 3 mm copper / nickel metal layer 2a, and a copper / nickel metal layer 2b having a width of 38 mm, a length of 60 mm and a thickness of 0.3 mm was formed on the back surface.
A ceramic substrate 1 having a width of 39 mm, a length of 61 mm, and a thickness of 1 mm is prepared, and the ceramic substrate 1 is superposed on the metal substrate 4 on which the solder paste layer 7b is formed.
After that, the furnace is filled with nitrogen so that the oxygen concentration is 100 ppm.
250 ℃ or more in the reflow furnace (maximum temperature 275 ℃)
Hold for 3 minutes to melt the solder powder in the solder paste and cool without performing a scrubbing process to obtain the solder layer 6b.
Was solidified and joined.

The joint portion of the sample prepared as described above is observed with an ultrasonic microscope, and the area ratio of voids present on the joint surface (hereinafter referred to as the void ratio) is measured. The results are shown in FIG. As shown in FIG. 1, the content of the flux component is 8 wt.
% Solder paste, the mixing ratio of solder powder having a particle size of 90 μm or more is 0 wt% and 10 w, which are comparative examples.
At t%, the void ratio exceeds 5% at the maximum, but it is 20 wt% or more (20 wt%, 30 wt%) which is an example of the present invention.
%, 40 wt% and 50 wt%), the void ratio was 3% or less at the maximum, and void defects could be reduced.

By the way, the conventional solder paste is a solder powder having a particle size distribution of 75 μm or less and a flux component mixed in a content of 9 to 15 wt%, as described above. In order to reduce the generation of voids due to the residual flux component, the particle size of the solder powder is left unchanged and the content of the flux component is reduced. For example, solder paste containing 8 wt% of the flux component is used for bonding. However, the flux amount at the joint was not sufficiently reduced, and a large effect was not obtained. Further, with the solder paste containing 7 wt% of the flux component, the surface oxide film of the solder powder was not sufficiently removed, and voids increased due to poor wetting.

In the above-described first embodiment according to the present invention, 90
The mixing ratio of the solder powder having a particle diameter of μm or more was 20 wt% or more, the solder powder having a large particle diameter was used, and the content of the flux component was 8 wt%, which was lower than the conventional one. From the viewpoint of the printability of the solder paste, it is desirable that the maximum particle size of the solder powder be about ½ or less of the thickness of the print mask. Also in the first embodiment, the particle size distribution of the solder powder is 90 to 1
It was set to 50 μm.

By using the large-diameter solder powder in this way, the number of particles of the solder powder is reduced and the density of the solder paste is also reduced. In addition, the content of flux component is 8
Since it is as low as wt%, the flux amount per unit volume of the solder paste can be greatly reduced. For this reason, in joining members having a large joining area, the flux amount at the joining portion can be sufficiently reduced even when the printing amount of the solder paste is large, and thus voids due to the residual flux components can be reduced. Even if the amount of flux at the joint is reduced, the surface area of the entire solder powder at the joint is reduced because the particle size of the solder powder is large, and the surface oxide film of the solder powder can be sufficiently removed. Furthermore, since the solder powder has a large particle size, the gap between the solder powders becomes large, and the gas generated from the atmosphere entrained in the joint and the flux component is easily discharged to the outside, which may cause voids. Occurrence can be suppressed. In addition, the solid components of the flux are easily discharged to the outside. By such a synergistic effect, it becomes possible to significantly reduce the occurrence of voids,
The results shown in FIG. 1 were obtained. In addition, it was found from a comparative example that when the mixing ratio of the solder powder having a particle diameter of 90 μm or more is less than 20 wt%, the above-described action and effect cannot be sufficiently obtained.

Embodiment 2 Next, a second embodiment of the present invention will be described. Composition is Sn27 / Sb3 / Pb
70 wt% of the solder powder having a particle size distribution of 63 μm or less is mixed with 20 wt% of a solder powder having a particle size distribution of 90 to 150 μm, and a flux component having a chlorine content of 0.20 wt% is mixed and mixed. I made several kinds of solder paste. At this time, the content of the flux component is 6 wt%, 8 wt% and 12 wt% as an example of the present invention,
As comparative examples, 13 wt% and 15 wt% were used. Similar to the first embodiment, the ceramic substrate 1 in which the solder paste layer 7b made of the above-mentioned solder paste formed on the metal substrate 4 is formed and the metal layers 2a and 2b are formed on the front and back surfaces are formed.
After superposing, the solder powder in the solder paste was melted by heat treatment, and then cooled to solidify the solder layer 6b for joining.

The joint portion of the sample prepared as described above is observed with an ultrasonic microscope to measure the void ratio of the joint surface, and the result is shown in FIG. As shown in FIG. 2, in the solder paste in which 20 wt% of solder powder having a particle diameter of 90 μm or more is mixed, when the content of the flux component is 13 wt% and 15 wt% which are comparative examples, the maximum void ratio is 4 to 5 wt. %, But it is 6 wt% to 12 wt% (6
wt%, 8 wt% and 12 wt%), the void ratio was about 3% at maximum, and void defects could be reduced.
When the content of the flux component was less than 6 wt%, the printability was remarkably reduced and the solder paste did not function.

Also in the second embodiment, as in the first embodiment, the content of the flux component in the solder paste is as low as 6 to 12 wt% and the solder powder having a large particle size is used. The amount of flux per unit volume of paste can be greatly reduced, and the amount of flux at the joint can be sufficiently reduced. Also, since the solder powder has a large particle size, the atmosphere caught in the joint, the gas generated from the flux component, and the solid component of the flux are easily discharged to the outside, which can significantly reduce the occurrence of voids. Then, the results shown in FIG. 2 were obtained. Further, it was found from the comparative example that when the content of the flux component exceeds 12 wt%, the reduction of the flux amount at the joint is not sufficient and the generation of voids due to the residual flux component cannot be suppressed.

Embodiment 3 FIG. Next, a third embodiment of the invention will be described. The composition is Sn63 / Pb37 and spherical solder powder with various grain sizes has a mesh of 106
It was sifted with a mesh having a mesh size of .mu.m, passed through the mesh, further sieved with a mesh having a mesh size of 90 .mu.m, and passed through the mesh with a mesh having a mesh size of 45 .mu.m. Next, the solder powder having a particle size of 45 to 90 μm and the solder powder having a particle size of 90 to 106 μm are mixed with a particle size of 90 to 1
The solder paste A, which is an example of the present invention, was prepared by mixing so as to have a content of 06 μm so as to be 40 wt%, and mixing 8 wt% of a flux component having a chlorine content of 0.20 wt%. Further, 14 wt% of a flux component having a chlorine content of 0.20 wt% was mixed with a solder powder having a particle diameter of 45 μm or less to prepare a solder paste B in the related art as a comparative example.

In the sample prepared in the first embodiment (ceramics substrate 1 bonded to metal substrate 4), the mixing ratio of solder powder having a particle size of 90 to 150 μm is 40 wt.
% Of the solder paste, that is, solder powder having a particle size of 63 μm or less and solder powder having a particle size of 90 to 150 μm.
Sample A used in this invention example is a solder paste prepared by mixing 0 wt% and 8 wt% of a flux component having a chlorine content of 0.20 wt%. Similarly, the mixing ratio of the solder powder having a particle size of 90 to 150 μm is 0 w.
Sample B used in the comparative example is a solder paste having t%, that is, a solder paste prepared by mixing 8 wt% of a flux component having a chlorine content of 0.20 wt% with a solder powder having a particle diameter of 63 μm or less. To do.

Next, solder pastes A and B were respectively screen-printed to predetermined regions on the metal layer 2a on the surface of the ceramics substrate 1 of the samples A and B, and the width was 6.1 mm.
Solder paste layer 7 having a length of 6.1 mm and a thickness of 0.2 mm
a was formed. At this time, the solder paste A is the sample A,
The solder paste B is used for the sample B. Next, on each of the samples A and B on which the solder paste layer 7a was formed, six transistor chips 3 as a semiconductor chip having a width of 6.1 mm, a length of 6.1 mm and a thickness of 0.25 mm were mounted,
After that, the furnace is filled with nitrogen so that the oxygen concentration is 100 ppm.
200 ℃ or more in the reflow furnace (maximum temperature 235 ℃)
Was held for 2 minutes to melt the solder powder in the solder pastes A and B, and to cool without performing the scrubbing process to solidify the solder layer 6a for joining. This results in a large current
A semiconductor device with high heat generation was formed. After that, the flux residue attached to the outside of the semiconductor device was removed by a cleaning liquid.

With respect to the two types of semiconductor devices produced as described above, the joint portion between the transistor chip 3 and the ceramics substrate 1 was observed with an ultrasonic microscope, and the void ratio of the joint surface was measured. Further, in order to examine the heat dissipation performance, the metal layer 2 on the surface of the transistor chip 3 and the ceramic substrate 1 is examined.
a is connected by a wire 5 (see FIG. 3), then a voltage of 50 V is applied between the collector and the base of the transistor chip 3, and a current of 1 A is applied between the collector and the emitter for 10 seconds to heat the transistor chip 3. The transistor characteristics were measured to examine the destruction rate of the transistor chip 3 due to heat generation. Table 1 shows the results.

[0027]

[Table 1]

As shown in Table 1, the semiconductor device, which is an example of the present invention prepared by using the sample A and the solder paste A, has a void ratio of 1% or less, and the transistor chip 3 is not destroyed by heat generation. The reliability is remarkably improved as compared with the comparative example prepared using the sample B and the solder paste B. In Sample A, the void ratio in the bonding between the metal substrate 4 and the ceramics substrate 1 is also 1% or less (see FIG. 1), and if the generation of voids is reduced in this way,
The heat flow path between the transistor chip 3 and the metal substrate 4 is not narrowed, the heat dissipation capability is improved, and a highly reliable semiconductor device can be obtained. Further, since the solder paste A has a sufficiently reduced flux component, it is possible to reduce the flux residue attached to the outside of the semiconductor device after the joining is completed, thereby extending the life of the cleaning liquid for cleaning the flux residue. did it.

[0029]

As described above, according to the present invention, 20 wt% or more of the solder powder of the solder paste is at least 9%.
It has a particle size of 0 μm and contains 6 wt% of flux component.
% Or more and 12 wt% or less, it is possible to stably reduce void defects in the joint portion using the solder paste without impairing the printability and wettability of the solder paste. Further, such an effect can be easily obtained without lowering productivity and increasing manufacturing cost.
As a result, the reliability of the joining of the members having a relatively large joining area can be significantly improved.

Further, according to the present invention, the solder paste is supplied by screen printing, and after the members to be joined are mounted thereon, the solder powder in the solder paste is melted and the flux component is discharged to the outside by heat treatment. After that, since the molten solder is solidified by the cooling treatment and the joining is performed, the above-mentioned effect is surely obtained.

Further, according to the present invention, since the flux residue is removed by using the cleaning liquid after the bonding with the solder paste, the flux residue can be reduced and the life of the cleaning liquid can be extended.

Further, according to the present invention, since the joint between the metal substrate and the insulating substrate and the joint between the insulating substrate and the semiconductor chip are formed by using the solder paste,
A void defect in the joint can be reduced, the heat dissipation capability of the semiconductor device is improved, and a highly reliable semiconductor device can be obtained. In particular, a great effect can be obtained in a semiconductor device with large current and high heat generation.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an effect in a first embodiment of the present invention.

FIG. 2 is a diagram for explaining effects in the second embodiment of the present invention.

FIG. 3 is a cross-sectional view showing the structure of a semiconductor device.

[Explanation of symbols]

1 insulating substrate, 2a, 2b metal layer, 3 semiconductor chip, 4 metal substrate, 6a, 6b solder layer.

Claims (4)

[Claims] 1. In a solder paste in which a solder powder and a flux component are mixed, 20 wt% or more of the solder particles of the solder powder have a particle size of at least 90 μm, and the content of the flux component is 6 wt% or more, Solder paste characterized by being 12 wt% or less. 2. The solder paste according to claim 1 is supplied to a first member to be joined by screen printing, and a second member to be joined is mounted on the first member to be joined, followed by heat treatment. The method of joining according to claim 1, wherein the solder powder is melted, the flux component is discharged to the outside, and then a cooling treatment is performed to solidify the molten solder to form a joint by a solder layer. 3. The solidified residue of the flux adhered to the outside of the members to be joined and the solder layer is removed using a cleaning liquid after the molten solder is solidified by cooling. The joining method described. 4. A metal substrate, an insulating substrate mounted on the metal substrate, having metal layers formed on front and back surfaces, and a semiconductor chip mounted on the insulating substrate. One or both of the joint between the insulating substrate and the joint between the insulating substrate and the semiconductor chip.
A semiconductor device formed using the described solder paste.