JPH05129771A - Spare solder for printed wiring board - Google Patents

Abstract

(57) [Abstract] [Purpose] Preliminary solder covering the conductor pattern of a printed wiring board, more specifically, regarding its solder alloy composition, to prevent oxidation of the conductor pattern (copper) and to secure solder wettability. Although it is a pre-solder, its function is further improved and its thickness is made thinner. In a preliminary solder used for coating a conductor pattern formed on a printed wiring board, the solder is a tin-lead alloy to which one or more of indium, copper and germanium are added. The added amount of indium is 0.5% by weight or less, the added amount of copper is within the solid solution limit of the tin-lead alloy, and the added amount of germanium is 0.1.
It is less than or equal to weight%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pre-solder for coating a conductor pattern of a printed wiring board, and more particularly to a solder alloy composition thereof. Due to the necessity of processing a large amount of information at high speed, information processing apparatuses have been downsized and increased in capacity, and in order to achieve this, electronic components have been downsized and high-density mounting has been performed. And a heat-resistant synthetic resin as a wiring board for mounting such electronic parts,
Multilayer substrates made of ceramics, glass ceramics, etc. are used.

In the case of a printed wiring board made of glass epoxy or the like, a fine conductor pattern is formed on a copper-clad substrate by a photolithography technique (photolithography) to form a unit substrate, and a plurality of these are prepared and a prepreg is sandwiched therebetween. A multilayer printed wiring board is obtained by thermocompression bonding and integration. In the case of a glass ceramics (or ceramics) substrate, a multilayer printed wiring board is obtained by stacking green sheets on which conductor patterns are screen-printed and heating and firing.

In most cases, a conductor pattern forming an electronic circuit on a wiring board is made of copper (C) in terms of electrical conductivity.
u) is used, but pre-solder (solder coat) is applied on the conductor pattern on the substrate surface because it is easily oxidized.
Is formed thin to prevent the conductive pattern from being oxidized and to ensure the solder wettability with the terminals of the element mounted here.

[0004]

2. Description of the Related Art Preliminary solder for preventing oxidation of a copper conductor pattern of a wiring board and ensuring solderability is to immerse the wiring board in a solder bath and blow hot air when pulling it up to remove excess solder. Is formed on the conductor pattern. If the electronic component to be mounted is a semiconductor device such as an IC or LSI, the size of the pad (of the conductor pattern) for welding the lead terminal has been 0.65 mm wide until now. , 0.3 due to miniaturization
It has been reduced to the mm width and the pad spacing has been shortened. Therefore, in order to ensure the insulation from the adjacent pads, it is necessary to make the thickness of the preliminary solder coated on the pads thinner than the conventional 0.8 to 80 μm.

[0005] Conventionally, tin-
Lead (Sn-Pb) eutectic solder is commonly used.
The formed pre-solder is often oxidized under the influence of heat treatment performed before surface mounting of a semiconductor device or the like. Hereinafter, a conventional example in which a printed wiring board made of a heat resistant resin is coated with preliminary solder will be described.

[0006] In order to coat the surface copper conductor pattern of a printed wiring board manufactured according to a usual process with preliminary solder, the printed wiring board is immersed in a Sn-Pb solder bath and hot air is blown when pulling it up. Excessive solder is scattered. At this time, the printed wiring board is heated by the solder bath, and the solder is oxidized in the cooling process. In addition, soldering may be performed by flow soldering after mounting individual components such as resistors and capacitors in the through holes of the printed wiring board. In this case as well, the board is heated and the preliminary solder is oxidized. ..

The phenomenon of oxidation of the pre-solder proceeds as follows. Since tin (Sn) forming the solder alloy is more easily oxidized than lead (Pb), tin is oxidized. 2Sn + O ₂ → 2SnO On the other hand, the lead component is converted to a single lead due to the decrease of the tin component, and is deposited on the surface and oxidized.

2Pb + O ₂ → 2PbO As described above, the pre-solder is affected by heating, and an oxide is generated on the surface, so that the wettability is deteriorated. Furthermore, the copper (Cu) of the conductor pattern diffuses into the solder and reacts with tin to form Cu ₃
Intermetallic compounds such as Sn and Cu ₆ Sn ₅ are generated, and these grow up to the surface of the pre-solder to reduce the wettability. These reductions in wettability also reduce solderability.

Conventionally, as a countermeasure against the problem, the preliminary solder layer has been thickened, but as described above, it is difficult to thicken the semiconductor device with higher integration. Therefore, before soldering, a convenient method may be adopted in which the joint portion (pad) is surface-polished to be thin. However, such surface polishing is troublesome and difficult to carry out.

[0010]

Although it is a preliminary solder for preventing the oxidation of the conductor pattern (copper) of the printed wiring board and ensuring solder wettability, it is required to further improve its function. Further, it is required to reduce the thickness of the preliminary solder in accordance with the high integration of the mounted semiconductor device.

It is an object of the present invention to provide a presolder that meets these needs.

[0012]

SUMMARY OF THE INVENTION The above object is to provide a pre-solder used for coating a conductor pattern formed on a printed wiring board, wherein the solder is a tin-lead alloy containing indium, copper or germanium. This is achieved by pre-soldering for a printed wiring board, which is characterized by adding one kind or two or more kinds.

These additive elements are added to the tin-lead alloy.
The amount of indium added is 0.1 to 0.5% by weight, the amount of copper added is 0.1% by weight or more and is within the solid solubility limit of the tin-lead alloy, and the amount of germanium added is 0.1%. 005-0.
It is preferably 1% by weight. The tin-lead alloy is composed of 50 to 70% by weight of tin and the balance of lead and inevitable impurities, and most preferably has a eutectic solder composition of 63% by weight tin.

[0014]

In the present invention, the following additive element effect is utilized in order to prepare a pre-solder which can secure solder wettability even if it is thin. Metals (indium and germanium) are added to cover the tin-lead (Sn-Pb) solder and prevent oxidation without impairing the solder wettability.

The amount of indium added exceeds 0.5% by weight,
Alternatively, when the addition amount of germanium exceeds 0.1% by weight, the surface tension of indium is extremely lowered and a solder bridge is formed. Further, Ge has a problem that the melting point of the solder rises and the surface of the preliminary solder becomes rough. Copper diffuses into the solder, forming intermetallic compounds,
This is because the growth is a cause of impairing wettability,
A metal (copper) that suppresses the diffusion of copper is added.

The amount of copper added is within the solid solution limit of solder,
The solid solubility limit of tin-lead eutectic solder at 240 ℃ is 0.4.
It is 2% by weight. Pre-contain copper in pre-solder
This can prevent copper diffusion from the conductor pattern.
Cu₃Sn and Cu₆Sn _FiveIntermetallic compounds such as
It is possible to prevent the solder from growing to the surface.

When these indium, copper, and germanium are combined and added in appropriate amounts, the effect of addition appears as it is,
Good soldering can be performed.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the accompanying drawings by way of embodiments and comparative examples of the present invention. Example 1 (when indium was added) As a test piece, a copper plate (shape dimension: 50 × 5 × 1 mm) plated with copper having a thickness of 20 μm was prepared. Sn-P
b 0.1 to 1.0% by weight of In is added to the eutectic solder,
Alternatively, it was melted into a preliminary solder bath without addition.
The test piece is dipped in these molten solder baths for 5 seconds to obtain a thickness of 0.
A 3 μm pre-solder layer was coated. In order to investigate the effect of pre-solder surface oxidation on wettability, 1
It heated at 70 degreeC for 1 hour. Then, one end of the test piece is Sn
The solder wetting time was examined by the meniscograph wettability test method of immersing in a -Pb eutectic solder bath and pulling it up. A printed board including pads formed at a pitch of 0.3 mm was solder-coated and the solderability was examined. The test results are shown in Table 1.

[0019]

[Table 1]

As can be seen from Table 1, the In addition has the best results (shortest) when the wetting time is 0.5% by weight.
Is obtained. When the amount of In added is larger than that, the wetting time is relatively short, but the surface tension of the solder is lowered, and a bridge is easily generated. This is data obtained by immersing a printed board in a solder bath, which includes pads formed with a pitch of 0.3 mm. The bridge means a bridging of solder that connects the adjacent pads of the conductor pattern and connects them, and in this case, the interval is 0.5 μm pitch (line and space). In addition, a defect is a case where the solder is repelled (partly wet).

Example 2 (when copper was added) A copper-plated copper plate test piece was prepared in the same manner as in Example 1. Cu was added to the Sn-Pb eutectic solder with or without addition of 0.1 to 5% by weight, and melted to prepare a preliminary solder bath. A test piece was coated with a preliminary solder layer in the same manner as in Example 1 and heated in the atmosphere. Then, one end of the test piece was dipped in a Sn-Pb eutectic solder bath, and the pull-up meniscograph wettability test method was used to examine the solder wettability and solderability. The test results are shown in Table 2.

[0022]

[Table 2]

As can be seen from Table 2, the Cu addition amount is best near the solid solution limit (0.3%) (wetting time is short), and if it is more than that, the wetting time does not change, but the solder Since the viscosity increases, bridges are likely to occur. Example 3 (in the case of adding germanium) A test piece of a copper plate plated with copper was prepared in the same manner as in Example 1. Ge from 0.005 to Sn-Pb eutectic solder
With or without addition of 0.3% by weight, it was melted to prepare a preliminary solder bath. A test piece was coated with a preliminary solder layer in the same manner as in Example 1 and heated in the atmosphere. Then, one end of the test piece was dipped in a Sn-Pb eutectic solder bath, and the pull-up meniscograph wettability test method was used to examine the solder wettability and solderability. The test results are shown in Table 3.

[0024]

[Table 3]

As can be seen from Table 3, the Ge addition gives the best results (shortest) at a wetting time of 0.02% by weight. If the addition amount is larger than that, although the wetting time is relatively short, the surface tension of the solder increases and the thickness tends to vary. Example 4 (in the case of adding indium and copper) As in Example 1, a copper-plated copper plate test piece was prepared. In is 0.1 to 1.0 for Sn-Pb eutectic solder
% By weight and 0.3 to 5% by weight of copper, or neither of them was added and the mixture was melted to prepare a preliminary solder bath. A test piece was coated with a preliminary solder layer in the same manner as in Example 1 and heated in the atmosphere. Then, one end of the test piece was dipped in a Sn-Pb eutectic solder bath and pulled up to examine the solder wettability and solderability by a meniscograph wettability test method. The test results are shown in Table 4.

[0026]

[Table 4]

As can be seen from Table 4, the amount of In added is 0.5.
The best results are obtained when the weight% and the amount of Cu added are 0.3% by weight. The optimum value of the addition amount corresponds to that of Examples 1 and 2, and the solder wetting time is 0.76 seconds, which is shorter than that of Examples 1 and 2. Example 5 (in the case of adding germanium and copper) A test piece of a copper plate plated with copper was prepared in the same manner as in Example 1. Ge from 0.005 to Sn-Pb eutectic solder
0.05% by weight and 0.3-5% by weight of copper were added, or neither of them was added to obtain a preliminary solder bath.
A test piece was coated with a preliminary solder layer in the same manner as in Example 1,
Heated in air. And, one end of the test piece is Sn-Pb
The solder wetting time and solderability were examined by the meniscograph wettability test method of immersing in a eutectic solder bath and pulling it up. The test results are shown in Table 5.

[0028]

[Table 5]

As can be seen from Table 5, the Ge addition amount is 0.0
The best results are obtained when the amount of Cu added is 2% by weight and the amount of Cu added is 0.3% by weight. The optimum value of the added amount corresponds to that of Examples 2 and 3, and the solder wetting time is 0.75 seconds, which is shorter than those of Examples 2 and 3. Example 6 (of the addition of indium, germanium and copper)
Case) A copper-plated copper plate test piece was prepared in the same manner as in Example 1. In is 0.1 to 0.5 for Sn-Pb eutectic solder
% By weight, 0.01 to 0.02% by weight of Ge and 0.3% by weight of copper were added, or none of the three was added to obtain a pre-solder bath. A test piece was coated with a preliminary solder layer in the same manner as in Example 1 and heated in the atmosphere. Then, one end of the test piece was dipped in a Sn-Pb eutectic solder bath and pulled up to examine the solder wettability and solderability by a meniscograph wettability test method. The test results are shown in Table 6.

[0030]

[Table 6]

As can be seen from Table 6, the amount of In added is 0.5.
% By weight, the amount of Ge added is 0.02% by weight and the amount of Cu added is 0.02%.
Best results are obtained at 3% by weight. The optimum value of the addition amount corresponds to that of Examples 1 to 5, and the solder wetting time is 0.69 seconds, which is shorter than that of Examples 1 to 5. In solderability, the generation of bridges (due to the decrease in surface tension) due to the addition of In is suppressed and improved by the addition of Ge (the effect of increasing the surface tension). In this way, the effect of composite addition appears.

[0032]

As described above, with the preliminary solder according to the present invention, good solder wettability can be obtained even if the thickness of the conductor pattern of the printed wiring board is reduced to 0.3 μm. Therefore, a printed wiring board suitable for mounting a highly integrated semiconductor device can be provided.

Claims (3)

[Claims] 1. Preliminary solder used for coating a conductor pattern formed on a printed wiring board,
A pre-solder for a printed wiring board, characterized in that the solder is made by adding one or more of indium, copper and germanium to a tin-lead alloy. 2. The amount of indium added is 0.1 to 0.5% by weight, and the amount of copper added is 0.1% with respect to the tin-lead alloy.
The pre-solder according to claim 1, characterized in that the content is 1% by weight or more and is within the solid solution limit of the tin-lead alloy, and the addition amount of germanium is 0.005-0.1% by weight. 3. The tin-lead alloy is tin 50 to 70% by weight.
The pre-solder according to claim 1, characterized by comprising: and a balance of lead and unavoidable impurities.