JPS6286895A - Soldering of electronic part - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a method for soldering electronic components suitable for joining electronic components to a circuit board at low temperatures and improving the reliability of the joint.

- [Background of the Invention] In conventional electronic circuit devices, a method of connecting electronic components to a circuit board by soldering is often used from the viewpoint of mass productivity and reliability.

In this method, solder inserted between an electronic component and a circuit board is heated and melted to form an alloy layer between the solder and the terminal metallization, such as Cu or Ni, for connection. At this time, the solder itself is in a cast structure state in which the elongation is small due to segregation and defects in the alloy composition during the cooling process [Refer to Reference 9, Journal of the Japan Institute of Metals, Vol. 49, No. 1 (1985), pp. 26 to 33] becomes.

However, this cast state has low elongation and non-uniform deformation in response to external forces, so it has poor fatigue properties and is susceptible to various stresses during use, such as thermal stress due to the difference in thermal expansion between electronic components and substrates. On the other hand, there was a problem that the solder was destroyed in a relatively short period of time.

In addition, as described in Japanese Patent Application Laid-Open No. 54-50269, it has been proposed to prevent unevenness and inclination of solder thickness between two substrates on which semiconductor chips such as diode power transistors are mounted, thereby improving reliability. In order to achieve a good bond, a method has been proposed in which a high melting point solder is sandwiched with a low melting point solder and only the low melting point solder is melted and bonded.

However, although it may be possible to obtain good flatness with this method, on the other hand, the area near the interface between the melting low-melting point solder and the unmelting high-melting point solder becomes a metallographically very brittle layer. There was a problem that they were easily destroyed during thermal cycles.

Furthermore, as described in Japanese Unexamined Patent Publication No. 51-118372, when soldering a silicon diode, a support electrode, and a lead wire, low melting point Sn and high melting point PB must be completely melted and joined. Accordingly, a method has been proposed in which a sufficient amount of Sn is supplied to the bonding interface to increase the strength of the interface and to produce a solder with a high Pb composition that can withstand high-temperature use.

However, in this method, similar to the first method mentioned above, a cast solder structure is formed over the entire surface, so there is a problem that fatigue failure easily occurs due to thermal cycles during use.

[Purpose of the invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for soldering electronic components that solves the above-mentioned conventional problems and enables highly reliable solder connections.

[Summary of the invention]

The present invention was invented to achieve the above object, and the background to the invention is as follows.

That is, as a result of studying the above-mentioned conventional problems, the present inventor found that when a low melting point eutectic solder is melted and bonded to a high Pb-Sn high melting point solder, a sixth
As is clear from the Pb-Sn alloy phase diagram shown in the figure, a solder alloy in the shaded range in which pb and Sn are continuous is formed continuously.

On the other hand, the tensile strength-elongation properties of the cast Pb-Sn alloy are determined by the primary solid solution of pb (αp
When b) has an intermediate composition excluding the eutectic structure, dendrites (
D andrite)-like structure and grain boundaries are hard;
It has become clear that the formation of a Sn-rich layer with a low melting point makes it extremely brittle, as shown in FIG.

Since the intermediate structure between the primary solid solution (αpb) and the eutectic structure, which is inevitably formed during the bonding process between low-melting point solder and high-melting point solder, has a very brittle property, Fracture occurs from the brittle layer due to thermal cycles and thermal shock.

Therefore, in order to prevent this brittle intermediate structure, the present invention forms a thin low melting point solder layer and a thick high melting point solder layer, so that sufficient unmelted high melting point solder remains during melting, and during melting or afterward. During heat treatment, the intermediate layer Sn formed between the low melting point solder and the high melting point solder is uniformly high in pb.
The height ratio of the low melting point solder and the high melting point solder is set so that the solder diffuses into the high melting point solder layer, that is, the composition of the primary solid solution (αpb) is on the high melting point side. It is.

[Embodiments of the invention]

Embodiments of the present invention will be explained below with reference to FIGS. 1 to 5, which show embodiments of the present invention.

FIG. 1 is a cross-sectional view showing a junction of a semiconductor device having the same length as a semiconductor chip using high melting point solder made according to the present invention, and FIG. 3(a) to 3(C) are enlarged sectional views of part A in FIG. 1 for explaining the method of soldering a semiconductor substrate according to the present invention, and FIG. 4 is a cross-sectional view showing the joint part of the device. FIG. 5 is a diagram showing another example of the relationship between the volume ratio of high melting point solder and low melting point solder and PB concentration,
FIG. 6 is a Pb-Sn alloy state diagram, FIG. 7 is a typical stress-strain curve diagram of cast solder, and FIG. 8 is a stress-strain curve diagram of processed solder.

As shown in FIGS. 1 and 2, the present invention is formed in advance by processing and forming between a semiconductor chip 1 and a substrate 2 through electrodes 4 and 5, and a low melting point eutectic solder (a third solder) is applied to both upper and lower end surfaces. After inserting the high melting point solder 3 with the symbols 6 and 7) shown in the figure and melting only the low melting point eutectic solder, the electrodes 4 and 5 and the high melting point solder 3 are melted and bonded through a diffusion reaction. The bonding is performed by maintaining the melting temperature of the low melting point eutectic solder or at a high temperature below the melting point to uniformly diffuse Sn in the bonding portion.

Next, the manufacturing process of the semiconductor substrate shown in FIGS. 1 and 2 will be explained with reference to FIGS. 3(a) to 3(C).

First, as shown in FIG. 3(a), a substrate 2 made of various materials
Electrodes 5 are formed thereon using a method suitable for each material. That is, for example, when the substrate 2 is made of alumina ceramic, the electrode 5 is formed by printing and firing a conductive paste such as Ag-PB and W. Also, the substrate 2 is made of Si.
and if it is made of glass, the electrode 5 is made of Cr
A thin film of -Cu, Cr-Cu-Ni, Ti-Cu, etc. is deposited by vacuum evaporation or sputtering. This vapor deposition method is also used for the electrode 4 on the semiconductor chip 1 side.

Printing a solder paste of low melting point solder such as eutectic solder such as Pb-Sn or Au-5n or pure Sn on the electrodes 4 and 5 thus formed;
The low melting point solder layers 6 and 7 are formed by supplying solder formed in a riff port or a plate shape, solder balls, vacuum evaporation, or by using a riff port or a solder dip (Dip). In this embodiment, the thickness of the low melting point solder layers 6 and 7 is approximately 30 μm.

Next, the electrodes 4 and 5 are placed between these low melting point solder layers 6 and 7.
A processed solder 3 having a high melting point and formed into a shape similar to the above is inserted. This processed solder 3 is used by molding, for example, 95%Pb-5wt%Sn that has been processed and heat treated.

The above processing and heat treatment involved melting, casting, rolling into a plate shape, processing to a reduction rate of 90%, and heat treatment at 100°C for 5 hours in an inert atmosphere.
The strain curve approximately corresponds to the characteristic marked ■ in the stress-strain curve diagram of the processed material shown in Figure 8, and the elongation is significantly improved compared to the 95Pb characteristic in the stress-strain curve diagram of the as-cast material. It is understood that what is being done.

In this way, high melting point processing and
After inserting the heat-treated high melting point solder 3 with a thickness of about 1 ma+ to obtain the state shown in FIG. 3(a), the low melting point solder 6 is inserted.
, 7 to a temperature just above the melting point to obtain the result shown in FIG. 3(b). In this case, P b-S is used as the low melting point solder 6.7
When n eutectic solder is used, it is heated to a temperature of 200°C. In addition, at the initial stage of heating, the low melting point solder 6, 7 reacts with the electrode 4.5 by diffusion 9 and forms Sn and the electrode 4,
5 and intermetallic compounds 8, for example, Cu and Sn for Cu electrodes, and Ni and Sn for Ni electrodes.
However, if it is an Ag electrode, Ag3Sn etc. will be generated.

On the other hand, processed solder 3 with a high melting point has a 6th layer in the reaction/diffusion region.
As shown in the figure, an intermediate layer 9 is formed continuously between the primary solid solution of pb (αpb) and the eutectic structure.

If heating is continued further, approximately 30
The μm low melting point solder layers 6 and 7 completely react with the electrodes 4 and 5 and processed solder 3, and the intermediate layer 9 almost disappears, becoming almost a primary solid solution of PB (αpb) and forming the bonding interface. In this case, only the reaction layer 8 with the electrodes 4 and 5 is formed, and the high melting point solder 3
A good joint can be formed between the electrodes 4 and 5 and the electrodes 4 and 5. In this case, pb is used for the low melting point solder layers 6 and 7.
As shown in Figure 4, the volume of the high melting point solder 3 required to form the primary solid solution (αPb) is 95wt%Pb-5wt when each solder is uniformly diffused and mixed.
%Sn, the volume ratio becomes about 8 times or more (solid arrow in the figure). In addition, the good solder composition in the same figure means the seventh
As is clear from the figure, 90wt%Pb is soft and has excellent elongation.
- Refers to a composition with higher pb than 10wt%Sn. Furthermore, the fourth
In the figure, high melting point solder 3 is 98st%pb-2vt%
The case of Sn is also shown, but in this case a volume ratio of 4.3 or more is required, which is larger than the arrow ■.

The total heating holding time at the heating temperature of 200°C is approximately 40
It was done in seconds, but it goes without saying that it was done at a higher temperature and in a shorter time.

Further, when heat treatment is performed at about 150° C. just below the melting point of the eutectic solder, the same diffusion and reaction as in the above case can be obtained by holding the heat for about 30 minutes.

In the semiconductor substrate shown in FIG. 1 manufactured in this way, the power transistor is subjected to a temperature cycle of -55 to +1.
When the lifespan was evaluated by conducting a test at 50° C. and 1 cycle/hr, it was found that the fatigue life was improved by about 2 to 5 times compared to conventional joints.

In addition, in the above-mentioned example, Pb-8n-based solder was described, but it is not limited to this.
For example, In-Pb type, In-8n type.

It goes without saying that similar effects can be obtained with materials such as I n-P b-S n-based materials.

〔Effect of the invention〕

As described above, the present invention makes it possible to eliminate the intermediate solder layer that is hard and has little elongation, thereby making it possible to obtain a uniform solder connection.

It is possible to obtain highly reliable electronic circuit devices with easy operation, and to make a major contribution to the enhancement of the functionality of electronic circuit devices in the field of surface mounting, where higher density and higher reliability are required in the future. It has the effect of

[Brief Description of the Drawings] Fig. 1 is a sectional view showing a semiconductor device made according to the present invention, Fig. 2 is a sectional view showing a semiconductor device made of a plurality of high melting point solders made according to the present invention, Figure 3 (a) to (
c) is an enlarged sectional view of part A in FIG. 1 for explaining the method of soldering a semiconductor device according to the present invention, and FIG. 4 shows an example of the relationship between the volume ratio of high melting point solder and low melting point solder and PB concentration. Figure 5 is a diagram showing another example of the relationship between the volume ratio of high melting point solder and low melting point solder and PB concentration, Figure 6
The figure is a phase diagram of a Pb-Sn alloy, FIG. 7 is a typical stress-strain curve diagram of cast solder, and FIG. 8 is a stress-strain curve diagram of processed solder. DESCRIPTION OF SYMBOLS 1... Semiconductor chip, 2... Substrate, 3... High melting point solder, 4, 5... Electrode, 6, 7... Low melting point solder layer, 8... Intermetallic compound, 9... ...middle class.

Claims (1)

[Claims] 1. In a method of soldering electronic components such as semiconductors to a circuit board by soldering, between the electronic components and circuit board and the high melting point solder inserted between them after processing and molding. A low melting point solder is attached to the solder, and the height ratio of these low melting point solders and high melting point solders is set in advance so that sufficient unmelted high melting point solder remains during melting, and during melting or subsequent heat treatment. A method for soldering electronic components, characterized in that an intermediate layer formed between a low melting point solder and a high melting point solder is expanded into a uniform high melting point solder layer.