JPH08296050A - Tin plating bath for preventing whiskering and method for preventing whiskering of tin plating - Google Patents

Abstract

PURPOSE: To prevent the whiskering of a tin plating film in the case of plating a fine pattern of copper, etc., with tin without need for annealing, etc., while keeping the bonding strength between the tin plating and the gold bumping electrode of a semiconductor chip. CONSTITUTION: A tin plating bath contg. the soluble metallic salt prepared by adding a salt of a low-m.p. metal such as bismuth, indium, lead or antimony is used to form a tin plating film on a fine pattern so that the content of the low-m.p. metal in the deposited film is controlled to 0.1-3.0wt.% and the thickness to 0.1-1.0μm. Since the tin film having a low content of a specified trace metal is deposited in a specified thickness, annealing, etc., are not needed, and a whiskering preventive effect and a high bonding strength are attained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a tin plating bath for preventing whiskers, and a method for preventing whisker of tin plating.
When applying tin plating on a fine pattern such as copper,
For example, it is possible to effectively prevent the generation of whiskers in a tin-plated film while maintaining high bonding strength between the tin plating and the gold bump electrodes of the semiconductor chip and without requiring special treatment such as annealing treatment.

[0002]

2. Description of the Related Art In recent years, with the miniaturization of electronic and electrical equipment, COB (Chip On Board) semiconductor chip mounting methods have become widespread. As a mounting method, a semiconductor chip is mounted on a printed wiring board using a wire, one for each electrode.
There is a wire bonding method for bonding one by one, but recently, regardless of the number of electrodes, a TAB (Tape Automated Bonding) method for collectively bonding electrodes of a semiconductor chip and inner leads of a film carrier via bumps is described below. It is widely used as a high-density mounting method because of the following reasons. Economical, highly reliable, electrical characteristics can be inspected on the tape, and the packaging yield is not reduced. Flexibility of the tape can be used for bending and three-dimensional mounting. On the same tape. Can be equipped with multiple chips.

In the TAB method, a fine pattern of copper or copper alloy called an inner lead is bonded to a gold bump electrode on a semiconductor chip by thermocompression bonding using a bonding tool (hereinafter, this step will be referred to as ILB (Inner)). Lead Bondin
(g)), if tin plating is applied to the inner leads during this ILB, a gold-tin eutectic alloy is generated between the tin film and the bump electrodes, increasing the bonding strength between the bump electrodes and the inner leads. Usually, such a tin-plated film is previously formed on the leads (that is, a fine pattern such as a copper foil).

However, when the above tin plating is applied,
It is known that, over the course of several days to several weeks, a large number of peen called genuine tin whiskers and needle-like crystals extending straight from the plating film grow, and the lead wire width is extremely narrow and high-density mounting is possible. In the fine pattern for use, this whisker causes a short circuit, and significantly reduces the reliability of the film carrier.

Therefore, for example, Japanese Patent Laid-Open No. 148658/1993.
As shown in Japanese Patent Publication No. 3 (page 46, left column, lines 46 to 48) and Japanese Patent Application Laid-Open No. 5-33187 (page 2, left column, line 26), 100 to 150 ° C. after tin plating. , 30
The annealing treatment is performed under the condition of about 120 minutes to take measures to prevent the generation of whiskers.

However, when this annealing treatment is performed, a tin-copper intermetallic compound layer is formed by the interdiffusion of copper and tin at the interface between the copper and tin plating of the fine pattern of the film carrier, and the tin plating is correspondingly formed. There is a problem that the film thickness becomes thin.

For this reason, it is possible to preliminarily deposit an additional plating film to solve the above problem, but the actual plating method is an electroless plating method from the viewpoint of eliminating variations in the plating thickness. , This electroless plating is
Since the basic principle is to carry out a chemical substitution reaction between copper and tin by reversing the electrode potential, as the thickness of the tin plating film increases, this substitution reaction proceeds and the copper metal (that is, the lead) It becomes ions and elutes excessively. As a result, the strength of the inner lead itself formed of the copper foil is reduced, so there is a limit to the miniaturization of the pattern for the purpose of making the inner lead thin, thin, and narrowing the line width. I will end up.

[0008]

2. Description of the Related Art Japanese Unexamined Patent Publication (Kokai) No. 5-51760 (hereinafter referred to as "conventional technique 1") describes a technique for plating on a fine pattern such as copper without the annealing treatment. That is, a method of applying a lead-tin alloy plating film on a fine pattern by an electroless method with a thickness of 0.3 to 2.1 μm and a lead content of 13 ± 4% is disclosed. It is said that whiskers in the tin-plated film can be prevented without performing annealing treatment later.

Further, in JP-A-5-247683 (hereinafter, referred to as Prior Art 2), when tin plating is applied to the inner lead of a film carrier, first, a solder having a thickness of 0.05 to 0.4 μm is used as a base. Plating, tin plating on it, the total thickness of the solder plating and tin plating layers is 0.5-1 μm, then 60-16
Heat treatment at 0 ° C for 0.5-2 hours to obtain a plating thickness of 0.3-
A technology of 0.5 μm is disclosed, which states that whiskers can be prevented.

[0010]

In the above-mentioned prior art 1,
Although the inner leads are solder-plated, the lead content of this solder-plated film reaches 13 ± 4%, so that a gold-tin eutectic alloy is generated between the gold bump electrode of the semiconductor chip and the plated film. It is difficult to obtain good bonding strength due to the hindrance.

Further, in the above-mentioned prior art 2, the following (1) to (3)
There is a problem. (1) Substantially annealed, the process is complicated and the productivity is low. (2) Since the solder plating and the tin plating are combined, the tin plating amount increases, and as described above, the electroless plating causes the copper foil pattern to be excessively eluted. (3) Lead content of the solder plating film for the base is about 30 to 5
Since it reaches 0% (see Table 1 of the publication), as in the case of the prior art 1, the formation of the gold-tin eutectic alloy is hindered, and it is difficult to obtain good bonding strength.

According to the present invention, when tin plating is applied to a fine pattern of copper or the like, the tin plating and the gold bump electrode of the semiconductor chip are maintained in the bonding strength without any special treatment such as annealing treatment, and the tin plating film is formed. It is a technical issue to achieve both prevention of whiskers.

[0013]

[Means for Solving the Problems] In the field of plating, it is generally accepted that whiskers are less likely to occur when the tin plating film becomes thinner, and it was thought that whiskers could be effectively prevented by making the film thinner. As a result, tin whiskers are generated with time even in a thin tin-plated film, and thus the annealing treatment is required as described above (and as a result, the above-mentioned problems occur).

As a result of intensive studies based on the above-mentioned known knowledge, the present inventors have specified both the type and the content of trace metals in the tin plating film and the thickness of the deposited film. It has been found that the chemical composition has a strong relationship not only with the whisker prevention effect but also with the degree of bonding strength between the tin film and the gold bump electrode. In particular, when the film thickness is in a specific thin range and the content of a predetermined trace metal is in a specific low range, whiskers can be prevented by the trace metal without performing special treatment such as annealing. The present invention has been completed by discovering that the effect and high bonding strength are both manifested.

That is, the present invention 1 contains a mixture of soluble metal salts obtained by adding a salt of a low melting point metal selected from the group consisting of bismuth, indium, lead and antimony to stannous salt, and in the deposited film. The whisker preventing tin plating bath is characterized in that the bath can be adjusted so that the total composition of the low melting point metal is 0.1 to 3.0% by weight.

The present invention 2 uses the tin plating bath of the above invention 1, and the composition of the low melting point metal in the deposited film is 0.1 to 3.0% by weight in total, and the thickness of the deposited film is 0. .1
The tin-plated whisker prevention method is characterized in that a tin-plated film is formed on a fine pattern of either copper or copper alloy so as to have a thickness of 1.0 μm.

A third aspect of the present invention is the method for preventing whiskers of tin plating according to the second aspect, wherein the tin plating film is applied by the electroless plating method according to the second aspect of the invention.

In the present invention 1, the composition ratio of tin to the low melting point metal in the bath does not matter, but is characterized by the composition ratio in the deposited film. Generally, in the plating field, the metal composition of the bath and the composition of the deposited film are often not the same. In practice, the composition of the low melting point metal in the deposited film of tin should be 0.1 to 3.0% by weight. In addition, there is a high degree of freedom in selecting the composition ratio of tin and the low melting point metal in the bath stage, and a tin deposition film that matches the composition ratio of the above conditions from a mixed soluble salt of tin and the low melting point metal having various composition ratios. Will be generated.

As the mixture of the above-mentioned stannous salt and the salt of a low melting point metal, any soluble salt can be used. For example, a mildly reacting free organic sulfonic acid is used as a basic bath.
Salts of the organic sulfonic acid can be used. That is, the tin plating bath of the present invention generally comprises a metal source consisting of tin and a soluble salt of a low melting point metal, and a free organic sulfonic acid (or an inorganic acid) as a basic component, and a surfactant. Various additives such as a smoothing agent or an antioxidant can be added. For example, in the case of an electroless plating bath, a complexing agent or a reducing agent is further added.

The above-mentioned low melting point metals can be used alone or in combination, and the mixed soluble salt of the present invention 1 is, for example, tin and (a) bismuth and lead, indium and lead, or two kinds such as lead and antimony. Mixed soluble salt with low melting point metal, or (b)
It may be a mixed soluble salt with three or more kinds of low melting point metals.

The method of applying the plating film of the present invention 2 is as follows.
It does not matter whether it is electroless plating or electroplating. However, the electroless plating method of the present invention 3 is more practical than the electroplating method from the viewpoints that the variation in the plating thickness is small, the film thickness is uniform, and it is not necessary to form a current conduction path. .

The fine pattern of copper or copper alloy according to the second aspect of the present invention includes, for example, the inner lead of TAB mentioned above,
It may be an outer lead or the like. However, in the normal bump formation method, a gold bump electrode is formed on a semiconductor chip and this bump electrode is joined to a tin-plated lead, but in the transfer bump method, the mesa bump method, etc., a bump is formed on the lead side. Then, this is joined to the electrode of the semiconductor chip. Therefore, the fine pattern of copper and copper alloy according to the second aspect of the present invention is not limited to the lead in the normal bump formation method, but includes a lead in the transfer bump method or the like. That is, the present inventions 2 and 3 are widely applied to the step of tin plating the leads when the electrodes of the semiconductor chip and the leads of the film carrier are joined via the bumps, regardless of the difference in the bump forming method.

The composition of the low melting point metal in the tin deposit film of the present invention 2 is 0.1 to 3.0% by weight based on the total weight of the deposit plating film, whether used alone or in combination. When the composition ratio is less than 0.1% by weight, the effect of the low melting point metal is lowered, and whiskers cannot be effectively prevented. If the composition ratio is more than 3.0% by weight, the added low melting point metal hinders the formation of the eutectic alloy, and the bonding strength between the bump electrode and the tin plating becomes weak.

On the other hand, the tin plating film of the present invention 2 has a thickness of 0.1 to 1.0 .mu.m. If the thickness is less than 0.1 .mu.m, it is sufficiently formed between the bump electrodes of the semiconductor chip during bonding. No gold-tin eutectic alloy is formed, and the joint strength is also reduced. Also, as mentioned above, the plating film thickness
(In particular, the upper limit) and the effect of preventing whiskers by the low melting point metal are closely related. When the thickness is more than 1.0 μm, the action of adding the low melting point metal becomes difficult to appear, and whiskers cannot be sufficiently prevented. In addition, plating sagging may occur at the time of joining, a good fillet may not be formed, and there is a high possibility that a short circuit will occur between the bumps.

In addition to the specific low-melting-point metals such as bismuth, indium, lead and antimony, for example, when a trace amount of a metal such as silver, zinc, nickel, copper or palladium is contained in the tin plating film, tin plating There is a function to effectively prevent the occurrence of whiskers. However, silver,
A metal such as zinc is not the same as the low-melting point metal in that it also has the adverse effect of reducing the bonding strength between the tin plating and the gold bump electrode due to the mixture thereof.

[0026]

[Operations and Effects] (1) In the present inventions 2 and 3, when tin plating is applied on a fine pattern of copper or copper alloy foil such as TAB inner lead, a tin plating film containing a predetermined trace amount of metal at a low content rate is used. Since it is formed under a thin condition of 1.0 μm or less,
A good gold-tin eutectic alloy is formed between the tin plating and the gold bump electrode, and there is no elution of excess copper metal even when plating is performed by the electroless plating method, increasing the bonding strength between the tin plating and the gold bump electrode. Can hold

(2) In the present inventions 2 and 3, as described in (1) above, since a trace amount of a predetermined low melting point metal is contained and the thickness of the deposited film is formed to be a predetermined thickness or less, As described above, it is possible to effectively prevent the generation of whiskers in the tin-plated film without requiring special treatment such as annealing treatment. Therefore, the process of preventing whiskers is simplified, the plating time is shortened, and the productivity is increased.

(3) Generally, it is important to keep the bonding strength between the tin plating and the gold bump electrode of the semiconductor chip high and to effectively prevent the formation of whiskers in the tin plating film without the need for annealing treatment. Although not compatible with each other, the present invention 1 can newly provide a tin plating bath that can clear this difficult requirement. Moreover, the present plating bath can be implemented at low cost because only a trace amount of the soluble salt of the predetermined low melting point metal is added to the soluble salt of stannous.

[0029]

[Examples] Hereinafter, preparation of an electroless tin plating bath for preventing whiskers capable of plating on inner leads of TAB under predetermined conditions, and examples of electroless plating using this bath are listed, and whiskers of the deposited tin plating film are listed. The measurement results of the number of occurrences of the above and the test results of the joint strength of tin plating are also shown. However, the present invention is not limited to the following examples, and it goes without saying that many modifications can be made within the scope of the technical idea of the present invention.

Examples 1 to 7 First, electroless tin plating baths having the following compositions were successively formed. (1) Example 1 (a small amount of bismuth salt added to tin salt) Stannous methanesulfonate (as Sn ²⁺ ) 20 g / l Bismuth nitrate (as Bi ³⁺ ) 1.0 g / l Methanesulfonic acid 40.0 g / L Thiourea (complexing agent) 120g / l Sodium hypophosphite (reducing agent) 65g / l Nonionic surfactant 10g / l Amphoteric surfactant 2g / l

(2) Example 2 (indium salt added to tin salt) Stannous methanesulfonate (as Sn ²⁺ ) 20 g / l Indium chloride (as In ³⁺ ) 5.0 g / l Methanesulfonic acid 40 0.0 g / l Thiourea (complexing agent) 120 g / l Sodium hypophosphite (reducing agent) 65 g / l Nonionic surfactant 10 g / l Amphoteric surfactant 2 g / l

(3) Example 3 (lead salt added to tin salt) Stannous methanesulfonate (as Sn ²⁺ ) 20 g / l Lead methanesulfonate (as Pb ²⁺ ) 6.0 g / l methanesulfone Acid 40.0 g / l Thiourea 140 g / l Sodium hypophosphite 65 g / l Nonionic surfactant 10 g / l Amphoteric surfactant 2 g / l

(4) Example 4 (addition of antimony salt to tin salt) Stannous methanesulfonate (as Sn ²⁺ ) 20 g / l Antimony tartrate (as Sb ³⁺ ) 3.0 g / l methanesulfonic acid 40 0.0 g / l Thiourea 140 g / l Sodium hypophosphite 65 g / l Nonionic surfactant 10 g / l Amphoteric surfactant 2 g / l

(5) Example 5 (adding bismuth salt and lead salt to tin salt) Stannous methanesulfonate (as Sn ²⁺ ) 20 g / l Bismuth nitrate (as Bi ³⁺ ) 1.0 g / l methane Lead sulfonate (as Pb ²⁺ ) 4.0 g / l Methanesulfonic acid 40.0 g / l Thiourea 130 g / l Sodium hypophosphite 65 g / l Nonionic surfactant 10 g / l Amphoteric surfactant 2 g / l

(6) Example 6 (Adding indium salt and lead salt to tin salt) In this Example 6, a tin source, an indium source and a lead source, methanesulfonic acid, thiourea, and It is composed of sodium phosphite and two kinds of surfactants of nonionic type and amphoteric type, and the addition conditions of the metal source such as tin are as follows, but the components other than these and the addition conditions thereof are as described above. The same setting as in Example 5 was performed (hereinafter, referred to as Example 7
The same shall apply). Stannous methanesulfonate (as Sn ²⁺ ) 20 g / l Indium chloride (as In ³⁺ ) 5.0 g / l Lead methanesulfonate (as Pb ²⁺ ) 5.0 g / l

(7) Example 7 (adding lead salt and antimony salt to tin salt) Stannous methanesulfonate (as Sn ²⁺ ) 20 g / l Lead methanesulfonate (as Pb ²⁺ ) 5.0 g / l Antimony tartrate (as Sb ³⁺ ) 2.0 g / l

(8) Comparative Example 1 The minute addition of bismuth nitrate was omitted from Example 1 described above, only the soluble salt of tin was used as the metal source, and the external composition and addition conditions were set as in Example 1. Comparative Example 1
And

(9) Comparative Example 2 An electroless tin plating bath corresponding to the above-mentioned prior art 1 was prepared, and the bath concentration was adjusted so that the lead composition of the deposition plating film exceeded 13.0% by weight. Example 2 is given. Its composition is as follows. Stannous methanesulfonate (as Sn ²⁺ ) 20 g / l Lead methanesulfonate (as Pb ²⁺ ) 10.0 g / l Methanesulfonic acid 50.0 g / l Thiourea 150 g / l Sodium hypophosphite 65 g / l l Nonionic surfactant 10 g / l Amphoteric surfactant 2 g / l

Therefore, the above-mentioned Examples 1 to 7 were formed on the inner lead of TAB formed by VLP (a kind of electrolytic copper foil).
And, as a result of performing electroless plating under the treatment conditions of temperature and time shown in FIG. 1A using the plating baths of Comparative Examples 1 and 2, the deposition composition of each tin plating film is as shown in FIG. 1B. The thickness was as in Figure 1C.

<< Example of Whisker Measurement Test >> Using a scanning electron microscope, the entire inner lead was observed at a magnification of 750, and a whisker having a length of 5 μm or more immediately after plating, 7 days, 30 days, and 90 days later was observed. The number of occurrences was counted and converted as the average number of occurrences per inner lead. The left column of FIG. 2 shows the result.

<< Formation state of fillet and bonding strength test example >> Bonding machine (TCW-made by Avionics Co., Ltd.)
115A) was used to bond an inner lead of TAB to a copper plate plated with 0.5 μm of gold under the conditions of a load of 50 g / unit lead, a temperature of 450 ° C., and a time of 5 seconds. Then, the formation state of the fillet around the inner lead after bonding was observed with a magnifying glass from a bird's eye view. Also,
A simple test of the peeling strength (peeling strength) of the lead was performed by pulling one end of the inner lead after bonding until it broke in a direction perpendicular to the copper plate and examining the breaking mode. The middle column of FIG. 2 shows the fillet generation result, and the right column of FIG. 2 shows the fracture mode result.

<< Evaluation of the above test results >> In each of Examples 1 to 7, the number of whiskers of tin plating was 5 μm or more immediately after plating for 90 days (although normally, the number of whiskers of tin plating was 5 μm or more (usually, It is often the case that the lower limit of the whisker length is around 20 μm), and all were zero. Also,
In each of Examples 1 to 7, the fillet was in a uniform and continuous generation state over the entire circumference of the lead (evaluation was good), the lead was broken even in the peeling test (evaluation was good), and tin plating and gold plating of the copper plate were performed. It was confirmed that the strength between the two was stronger than the strength of the lead itself. That is, using the tin baths of Examples 1 to 7, the tin film electrolessly plated under the condition of containing a trace amount of metal in a predetermined ratio and forming a film having a predetermined thickness has the following properties. It was confirmed that both requirements can be satisfied. Prevents tin film whiskers. Maintains strong bond strength between the gold bump electrode of the semiconductor chip and the tin film.

On the other hand, in Comparative Example 1, since the tin plating film did not contain the above-mentioned trace amount metal, the fillet formation and the peeling test were evaluated as ◯, but of course, the generation of whiskers could be effectively prevented. There wasn't.

In Comparative Example 2 (corresponding to Prior Art 1),
Whiskers could be effectively prevented because the tin plating film contained lead, but the lead content was 13.0% by weight.
Since it is extremely high, there is no fillet formation (evaluation is x), and even in the peeling test, breakage occurred at the interface between the lead and gold plating, and it was confirmed that sufficient bonding strength could not be obtained between the tin plating film and gold plating. It was

[Brief description of drawings]

1A is a chart showing plating conditions of an electroless plating bath, FIG. 1B is a chart showing deposition composition of a tin plating film, and FIG. 1C is a chart showing film thickness of a tin plating film.

FIG. 2 is a table showing a test result of the number of whiskers generated, a generation state of fillets, and a result of a peeling test.

Claims (3)

[Claims] 1. Stannous salt, bismuth, indium,
It contains a mixture of soluble metal salts to which a salt of a low melting point metal selected from the group consisting of lead and antimony is added, and the total composition of the low melting point metal in the deposited film is 0.1 to 3.
A tin plating bath for preventing whiskers, characterized in that the bath concentration can be adjusted so as to be 0% by weight. 2. Using the tin plating bath according to claim 1, the composition of the low melting point metal in the deposited film is 0.1 to 3.0% by weight in total, and the thickness of the deposited film is 0.1 to 1. A tin-plated whisker prevention method characterized in that a tin-plated film is formed on a fine pattern of either copper or a copper alloy so as to have a thickness of 0.0 μm. 3. The whisker prevention method for tin plating according to claim 2, wherein the tin plating film is applied by an electroless plating method.