Electroless Pb-Sn alloy plating bath composition

Abstract: An electroless Pb-Sn alloy plating bath composition is obtained by mixing at least one of methane sulfonic acid, 1-hydroxyethane-1, 1-diphosphoric acid and a salt of 1-hydroxyethane-1, 1-diphosphoric acid, compounds for forming Pb-Sn alloy and a reducing agent.

Description: This invention relates to an electroless Pb-Sn alloy plating bath composition. Solder plating is used on electronic components, printed circuit boards and the like for corrosion prevention and the attachment of components. The steady increase in the integration density of such electronic components and printed circuits has been accompanied by a tendency toward discrete conductor portions and finer circuit line features. Known methods of solder plating include electrolytic plating and molten solder plating. Both have drawbacks. Specifically, depending on the shape of the component or object to be plated, the former methods cannot always produce uniform plating. Moreover, it is not possible to plate portions that are not in electrical continuity. The latter method is incapable of controlling, or ensuring the uniformity of plating thickness and thus may result in the formation of electrical shorts, particularly in the case of fine patterns. It also entails heat-related problems such as heat-induced deformation of the substrate of the article being plated. As a result of research conducted to overcome the shortcomings of the conventional methods, a number of electroless plating methods have been developed. The first plating bath compositions developed for these methods typically consisted of a mixture of stannous chloride, lead chloride, thiourea and hydrochloric acid, having a reducing agent, or the like blended therewith. In a bath of such a composition, reaction between the sparingly soluble metal chlorides and the thiourea produces addition compound precipitates. The bath has to be maintained at a temperature of at least 70°C to ensure that the precipitates dissolve completely and the liquid remains transparent. Even then, the aforesaid addition compounds tend to precipitate and separate from the liquid when the temperature of the liquid surrounding it decreases as the article to be plated is immersed therein. Moreover, when electroless plating is conducted at this temperature, the plating rate is of the order of only 1 µm per 10 minutes. While the plating rate can be increased by raising the temperature of the bath, when this is done the surface of the plating film tends to become uneven and to be degraded by the deposition of coarse crystal grains. What is more, the plating film obtained reaches a thickness of no more than 3 µm. For overcoming the defects of the aforementioned chloride bath, it has been proposed to use a borofluoride bath such as the bath disclosed in Japanese Patent Publication No. 62(1987)-2630 comprising tin borofluoride, lead borofluoride, thiourea, borofluoric acid and a reducing agent or such as that disclosed in Japanese Patent Publication 62(1987)-17202 comprising tin borofluoride, lead borofluoride, thiourea and a reducing agent whose concentration is varied between at least two levels during the electroless plating process. Being composed of borofluorides, the baths used are toxic and pose a danger to those who work with them. They also require special treatment of the waste liquid and involve other problems regarding their practical application. In either of the methods using a chloride or borofluoride bath, the thickness of the film obtained when plating at 80°C for 15 minutes is of the order of only about 3 µm. The percentage of tin in the Pb-Sn alloy film obtained is too low to meet any of the S . A and B standards prescribed by JIS-Z3282. Nor does the film do well in fusing tests. DE-A-4001876 discloses a halogen free bath composition including as components alkanesulfonic acid or alkanolsulfonic acid along with the tin and lead salts thereof. JP-A-1-184279 also discloses the use of organic sulfonic acids and their tin and lead salts to form a tin-lead alloy plating bath. The inventors conducted research for developing a different plating bath composition that is free of the aforesaid defects of the prior art, specifically for developing a bath composition that does not require any special waste liquid treatment and which is capable of easily forming an electroless solder plating film on a copper surface. This invention achieves these aims by providing an electroless plating bath composition obtained from a liquid which contains as its essential main components (a) at least one member selected from among the group consisting pf 1-hydroxyethane-1,1-diphosphoric acid and salts thereof, (hereinafter referred to as "HEPA and/or salts thereof"), (b) methane sulfonic acid, (c) a tin compound for solder plating film formation, (d) a lead compound for solder plating film formation, and (e) a reducing agent. In addition to the aforesaid main components, the liquid is required to contain thiourea and a surface active agent. The tin compound is preferably tin (II) oxide. The lead compound is preferably lead oxide or lead acetate. Unlike when a chloride is used, the use of tin oxide results in virtually no low-solubility products. As a result, no precipitation is caused by the temperature drop occurring when the article to be plated is immersed in the bath and the likelihood of an inferior plating being formed as a result is eliminated. Furthermore, the concentration of metal in the liquid can be maintained at a high level. Thus, the area of the plating film formed per unit volume of the plating bath is therefore increased and the life of the bath and film forming rate of the bath are greatly improved. The concentration of the tin compound is preferably 18 g/ℓ - 36 g/ℓ as tin, while the concentration of the lead compound is preferably 15 g/ℓ - 35 g/ℓ as lead. The methane sulfonic acid content is preferably 125 g/ℓ - 240 g/ℓ and the content of HEPA and/or salts thereof is preferably 4 g/ℓ - 20 g/ℓ. When the methane sulfonic acid content is too low, the bath becomes milky and hydrolysis occurs. If it is too high, the alloy film redissolves which slowing the rate of film formation and increasing the tin content of the film. When the tin compound (tin oxide) concentration is below the range mentioned above, the tin content of the alloy becomes low, the film surface becomes coarse, and prevention of corrosion of the copper substrate, which is one of the reasons for the solder alloy plating process, cannot be realized. On the other hand, when the concentration is too high, the tin content of the coating film become excessive. The lead salt is preferably lead oxide or lead acetate and, as mentioned earlier, is preferably contained at 15 g/ℓ - 35 g/ℓ as lead. When its concentration is too low, the tin content of the alloy film becomes large. On the other hand, when its concentration is too high, the lead content of the alloy film becomes large, which is undesirable because it causes the coating film surface to become coarse, reduces the adherence of the film to the substrate and, by raising the melting point of the alloy film, markedly degrades its solder attachment property thus making it impossible for the film to achieve one of its main purposes. The thiourea concentration is preferably 70 g/ℓ - 140 g/ℓ. When its concentration is too low, the plating rate is low, and when too high the tin content of the alloy film is high and the plating rate decreases markedly. As stated above, the concentration of HEPA and/or salts thereof is 4 g/ℓ - 20 g/ℓ. At concentrations within this range, the surface of the deposited film is fine grained and exhibits a dull lustre. In short, the results are excellent. Lower concentrations do not contribute to obtaining a fine-grained texture nor to realizing a finish with a dull lustre. Above this range, deposition of tin does not increase in proportion to the increased amount of tin present, with the result that the tin content of the alloy film decreases. The concentration of the reducing agent is preferably 44 g/ℓ - 150 g/ℓ. Below these concentrations, a large amount of metal remains in the bath after completion of the reduction reaction, while at higher concentrations, gas is generated as the temperature increases and causes pits to form in the deposited film. An anionic surface active agent is preferably used. For example, it is possible to use ligunosulfonic acid at a concentration of 0.1 g/ℓ - 0.2 g/ℓ or polyacrylic acid at a concentration of 5 mℓ/ℓ - 0.2 mℓ/ℓ. The amount of either of these added is determined in light of the concentration of the composition. In the solder plating composition according to the invention, the tin and lead are present as salts of methane sulfonic acid and HEPA. The effect of the invention will now be explained with respect to examples of the invention and comparative examples. The substrates for plating and the processing steps used in the examples and comparative examples were as follows: 1) Substrates for plating a) 50 x 50 x 0.4 mm copper sheet b) 60 mm x 10 mm ⊘ copper tube c) Double-sided printed circuit board having a discrete copper circuit constituted of 50 [500 mm (L) x 0.1 mm (W)] lines with a line spacing of 0.05 mm 2) Processing steps Each of the substrates of 1) were treated as follows: a) Pretreatment Acid degreasing (40 °C) --> water rinsing --> soft etching --> water rinsing --> acid soaking --> water rinsing b) Posttreatment Cold water rinsing --> hot water rinsing --> drying in hot air stream 3) The resulting substrates were subjected to electroless plating in the baths indicated as examples of the invention in Table 1 or in the baths indicated as comparative examples in Tables 2 and 3, under the conditions shown in the respective tables. The plating film obtained in each of the examples and comparative examples was then subjected to the following tests. 4) Plating film test a) Measurement of film thickness and alloying ratio by x-ray fluorescence b) Measurement of state of fusing following holding at 190 °C - 195 °C for 40 sec in an infrared furnace. G (Good) = total fusing F (Fair) = 40 - 60 % fusing NG (No Good) = total lack of fusing c) Testing for short-circuiting of printed circuit board. G = No shorting NG = Shorting d) Bending test (copper sheet and copper tube) Observation of abnormalities after bending at 180 °C. G = No abnormalities NG = Abnormalities found e) Visual observation of plating film surface G = Lustrous F = Rough NG = Very uneven f) Plating film-to-substrate adhesion test (by adhesive tape) G = Pass The results of the plating film tests with respect to the examples and comparative examples are shown in the tables. Table 1 relates to examples according to the invention. Table 2 relates to comparative examples using baths containing the same materials as the baths in the examples but in amounts outside the range of the invention. Figure 3 relates to comparative examples using conventional electroless plating baths. From the results indicated in Tables 1, 2 and 3 it is clear that the electroless Pb-Sn alloy plating bath compositions according to the present invention produces excellent effects in practical applications.

Claims: An electroless plating bath composition for the formation of a Pb-Sn alloy film, characterized by comprising (a) at least one of 1-hydroxyethane-1,1-diphosphoric acid and salts thereof, (b) methane sulfonic acid, (c) a tin compound, (d) a lead compound, (e) a reducing agent, (f) thiourea and (g) a surfactant. A bath composition according to claim 1, characterised in that the tin compound is tin (II) oxide. A bath composition as previously claimed characterised in that lead compound is lead oxide or lead acetate. A bath composition according to any preceding claim characterised in that the surfactant is one of ligunosulfonic acid or polyacrylic acid. A bath composition according to any preceding claim wherein there are 4 to 20 g/ℓ of 1-hydroxyethane-1,1-diphosphoric acid, or 4 - 20 g/ℓ of a salt thereof. A bath composition according to any preceding claim wherein there are 125 - 240 g/ℓ of methane sulfonic acid. A bath composition according to any preceding claim wherein there are 18 - 36 g/ℓ as tin of a tin compound. A bath composition according to any preceding claim wherein there are 15 - 35 g/ℓ as lead of a lead compound. A bath composition as claimed in any preceding claim wherein there are 44 - 150 g/ℓ of reducing agent. A bath composition as claimed in any preceding claim wherein there are 70 -140 g/ℓ of thiourea.

Inventors: FUKUOKA KAZUHIRO, KIYOHARA KEIKI, KONISHI HARUO, MIYATAKE ISAO, YONEKURA SABURO, FUKUOKA, KAZUHIRO, KIYOHARA, KEIKI, KONISHI, HARUO, MIYATAKE, ISAO, YONEKURA, SABURO
Applicants: KOSAKU KK, KOSAKU & CO. LTD.
Application year: 1991
Main classification: C23C18
IPC codes: H05K3, C23C18