JPH07252665A - Electroless plating solution generating no pseudo-whisker - Google Patents

Abstract

PURPOSE:To obtain the electroless plating solution with which the pseudo whisker is prevented from being generated and accordingly, no short-circuit is caused even when the line width and spacing of the wiring are very narrow and which has a long life as a plating solution when the solution is used for the tin plating at the time of mounting an IC on a printed circuit board, TAB tape, etc. CONSTITUTION:This tin plating solution contains as the essential components an alkanesulfonic acid, tin borofluoride, hypophosphorous acid, thiourea and polyoxyethylene or its derivative, or further, in addition to these essential components, phenolsulfonic acid.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electroless plating solution which does not generate pseudo whiskers when mounting an IC.

[0002]

2. Description of the Related Art As the integration density of ICs is improved, miniaturized, and improved in performance, the carrier on which the ICs are mounted, for example, TAB (Tape Automated Bonding) tapes and printed circuit wiring boards also have extremely high wiring densities. A high one is required, and accordingly, a wiring line width and a pitch interval thereof are extremely narrow. Generally, to mount an IC, a wire bonding method, a flip chip method,
Although there is a TAB tape method, particularly in the TAB tape, a method using tin plating is used for connecting the IC bumps to the TAB inner leads and the outer leads. In the case of tin plating, whiskers are the biggest problem at a narrow pitch, and even a whisker having a length of several microns has a risk of short circuit.

There is an electroless tin plating method as a typical tin plating method, and many electroless tin plating methods such as substitution type and reduction type have been proposed. From the viewpoint of preventing whiskers,
In particular, none of the conditions are satisfactory because the mechanism of whisker generation has not been clarified.

The present inventors have diligently conducted research to prevent the formation of whiskers, which is a problem during tin plating as described above, and in pursuit of the whisker generation process, whiskers were formed during plating. It was found that there are two types, one that grows and grows and does not grow after the end of plating, and the other that grows and grows several days after plating. this house,
The one that was generated and grew during plating was named a pseudo-whisker, and the one that was known and that was generated and grew several days later was named a genuine whisker. Fig. 1 shows a genuine whisker (7000 times), and Fig. 2 shows a pseudo whisker (200 times).
00X) micrographs are shown. The true whiskers include copper in their crystals, but the pseudo whiskers are made of pure tin. The present inventors have studied the preventive measures for each whisker, but for the genuine whiskers, it is possible to prevent their generation and growth by annealing at high temperature immediately after plating.
Assuming that the pseudo whiskers can be prevented, it will be completed as a measure for preventing the whiskers. As a result of various studies for the purpose of providing a tin plating solution in which the pseudo whiskers are not generated, the present invention has been accomplished.

[0005]

The tin plating solution of the present invention contains alkanesulfonic acid, tin borofluoride, hypophosphorous acid, thiourea and polyoxyethylene or its derivatives as essential components. The plating solution may further contain phenolsulfonic acid.

From the observation of pseudo-whiskers, it was found that the composition component most greatly involved in the generation of pseudo-whiskers among the components constituting the plating solution was an organic compound.
Typical of these organic compounds are phenol sulfonic acids and surfactants. Instead, we selected organic compounds that had the least effect on the generation of pseudo-whiskers and improved the characteristics of the plating solution. As a result, it was found that the combination of alkane sulfonic acid and polyethylene glycol or its derivative, or the combination of alkane sulfonic acid, phenol sulfonic acid and polyethylene glycol or its derivative has the least occurrence of pseudo-whiskers and has excellent plating film properties. It was

As the divalent tin source, there are many tin salts such as tin borofluoride, tin sulfate, tin chloride, and organic acid tin. Among them, the plating solution is stable (the divalent tin is oxidized. It became tetravalent tin, and because of its low solubility, white precipitates were formed and suspended in the liquid, but this was the least. It was tin borofluoride. Copper is more noble than tin and generally does not undergo a substitution reaction. Complexing agents make this possible, and thiourea is well known. Also known are phenolsulfonic acid as a regenerant of thiourea, antioxidation of divalent tin or a hypophosphite as a stabilizer of liquid, and a surfactant as a solution of uneven plating. As the tin source, tin alkane sulfonate may be used instead of tin borofluoride, and borofluoric acid may be used instead of tin borofluoride.

The appropriate amount of tin in the plating solution is 10 to 60 grams per liter of the plating solution in terms of metal. When the amount is 10 g or less, a plated film with many pinholes is formed, and when the amount is 60 g or more, a large number of particulate tin crystals are formed on the plated film, and uneven plating appears on the appearance. Thiourea reacts with copper to reverse its ionization tendency. If this amount is too large (240 g or more), the generation of pseudo whiskers will increase, and if it is too small (80 g or less), the plating rate will be slow, and many crystal grains will be generated on the tin-plated surface, and the smoothness of the plated surface, The uniformity is lost. Hypophosphite is necessary for preventing oxidation of tin ions, and the higher the amount, the better, but the solubility is limited, and the solubility in the plating solution at 60 ° C to 70 ° C is 50 to 60 g. From the viewpoint of preventing the generation of pseudo whiskers, it is better that no organic compound is present, but it is essential for improving the characteristics of the plating film. A combination of an acid and polyethylene glycol or its derivative, or a combination of alkane sulfonic acid, phenol sulfonic acid and polyethylene glycol or its derivative, but if the amount of alkane sulfonic acid is too much (240 g or more), the plating solution Permeability becomes too strong, and the plating solution penetrates between the layers of the resist and sizing agent, which tends to cause product quality problems. When the amount is too small (80 grams or less), deterioration of the characteristics of the plating film is observed. . Examples of alkane sulfonic acids include methane sulfonic acid, ethane sulfonic acid, propane sulfonic acid, 2-propane sulfonic acid, butane sulfonic acid, 2-butane sulfonic acid, pentane sulfonic acid, chloropropane sulfonic acid, and the like. Of these, methanesulfonic acid is most preferable.

If the amount of polyethylene glycol or its derivative corresponding to the amount of the alkanesulfonic acid is too small (less than 0.1 gram), uneven plating or a difference in crystal size depending on the part of the plated surface may occur, resulting in uneven gloss. Or a difference in plating thickness occurs. If too much (30 grams or more), generation of pseudo whiskers is recognized. Polyethylene glycol having a molecular weight of 4000 or less has the effect of not inducing the generation of pseudo-whiskers, and the lower the amount, the better the solubility, but on the contrary, the thickening effect of the plating solution is low and the molecular weight is 40%.
The optimum value is 0. Examples of the polyethylene glycol derivative include polyoxyethylene phenyl phenyl ether and polyoxyethylene octyl phenyl ether which are generally used as surfactants.

The preferred combinations of the composition components of the plating solution of the present invention described above are as follows. Plating solution Tin borofluoride (converted to metallic tin) 10 to 60 grams Sodium hypophosphite 20 to 50 grams Thiourea 80 to 240 grams Alkane sulfonic acid 80 to 240 grams Polyethylene glycol or its derivative 0.1 to 30 grams is there.

[0011]

Example 1 As plating solution A tin borofluoride (calculated as metal) 35 g / liter thiourea 210 g / liter sodium hypophosphite 40 g / liter methanesulfonic acid 160 g / liter polyethylene glycol or its derivative By changing the type and concentration of polyethylene glycol or its derivatives in the liquid, the occurrence of pseudo whiskers on the tin plating pattern on the copper pattern of TAB, the properties of the plating surface, the permeability to the resist or adhesive surface, the liquid life, etc. Tested The results obtained are shown in Table 1. In Table 1,
No. 1-4 are PEG # as polyethylene glycol
No. 400, the concentration of which was changed, 5-
No. 8 was obtained by using polyoxyethylene nonylphenyl ether as a derivative of polyethylene glycol and changing the concentration of methanesulfonic acid. Nos. 9 to 12 respectively show polyoxyethylene octylphenyl ether as a derivative of polyethylene glycol, the concentration of which was changed and the concentration of methanesulfonic acid was changed.

[0012]

[Table 1]

From Table 1, 70 g of methanesulfonic acid /
It can be seen that the pseudo-whiskers do not occur at all in the liquid containing more than the amount between liter and 80 g / liter. Also a surfactant (including polyethylene glycol)
It can be seen that when the amount is extremely small, the plated surface becomes uneven and the gloss differs depending on the part. However, when the amount of the surfactant is too large, pseudo whiskers are generated, but it is shown that the appropriate amount range varies depending on the type of the surfactant. In addition, the occurrence of whiskers varies greatly, and varies depending on the temperature, the degree of deterioration of the liquid, and the variation of composition components, and the tendency cannot be clearly understood without statistical processing. There are cases where the surfactants are used alone, not as a mixture. Therefore, in the present invention, taking the greatest common divisor, 0.1 g or more 3
It was set to 0 gram or less. The biggest drawback of this liquid is that the liquid easily penetrates, and special care is required when selecting a resist or a sizing agent.

[0014]

Example 2 Next, an example using a plating solution in which methanesulfonic acid and phenylsulfonic acid are used in combination is shown. Plating solution B Tin borofluoride (metal conversion) 40 g / l Thiourea 210 g / l Sodium hypophosphite 40 g / l PEG # 400 25 g / l Methanesulfonic acid 80 g / l Phenolsulfonic acid 80-240 g / 1 The results of plating with this solution are shown in Table 2.

[0015]

[Table 2]

From Table 3, it is shown that the combined use of methanesulfonic acid and phenolsulfonic acid suppresses the permeability of methanesulfonic acid and results in a harmonious plating solution. If the amount of phenolsulfonic acid is too much (240 g / l or more), generation of pseudo whiskers is recognized.

[0017]

Comparative Example Next, a comparative example using a plating solution that does not use alkanesulfonic acid will be shown. Plating solution C Tin borofluoride (converted to metal) 35 g / l Thiourea 210 g / l Sodium hypophosphite 40 g / l Phenolsulfonic acid 210 g / l Polyethylene glycol # 400 15 ml / l

The results obtained are shown in Table 3. With this liquid, pseudo-whiskers with a maximum length of 10 μm or more occasionally occurred, and generation of whiskers with a maximum length of 5 μm or more could not be prevented.

[0019]

[Table 3]

[0020]

As described above, according to the present invention, when used for tin plating during IC mounting of a printed circuit wiring board, a TAB tape or the like, even if the wiring has an extremely narrow wiring line width or pitch interval, the pseudo whisker can be used. Since it can be prevented from being generated, an electroless plating solution that has a long life and does not cause a short circuit can be obtained.

[Brief description of drawings]

FIG. 1 is a micrograph showing a genuine whisker at a magnification of 7,000.

FIG. 2 is a micrograph showing a pseudo whisker at a magnification of 20,000.

Claims (2)

[Claims] 1. An electroless plating solution containing alkanesulfonic acid, tin borofluoride, hypophosphorous acid, thiourea and polyethylene glycol or a derivative thereof as essential components. 2. An electroless plating solution containing alkane sulfonic acid, tin borofluoride, hypophosphorous acid, thiourea, phenol sulfonic acid and polyethylene glycol or a derivative thereof as essential components.