BATHS OR ORGANIC SULFONATE SOLUTION FOR BISMUTH AND BISMUTH ALLOY PLATING

Abstract: A bismuth or bismuth alloy plating bath comprises as essential ingredients an organic sulfonic acid of the general formula (I) (X₁) n -R-SO₃H (I) wherein R is a C₁₋₅ alkyl radical, X₁ is a halogen atom or hydroxyl, aryl, alkylaryl, carboxyl, or sulfonyl radical which may be in any optional position of the alkyl radical, and n is an integer of 0 to 3, or of the formula (II) wherein X₂ is a halogen atom or hydroxyl, alkyl, aryl, alkyl­ aryl, aldehyde, carboxyl, nitro, mercapto, sulfonyl, or amino radical, or two X₂'s which may combine with a benzene ring to form a naphthalene ring, and m is an integer of 0 to 3, and a bismuth salt of the acid or a mixture of the bismuth salt of the acid and one or more other metal salts of the acid.

Description: BACKGROUND OF THE INVENTION This invention relates to bismuth and bismuth alloy plating baths. More particularly, it is concerned with bismuth and bismuth alloy electroplating baths which use an organic sulfonic acid as a co-soluble complex salt of bismuth and the metal other than bismuth and give smooth electroplated deposits without the emission of highly polluting matter. Bismuth is extensively used as a lubricant in nuclear reactor construction as well as in such newly opened fields as the manufacture of rectifier and ohmic contacts. These applications require relatively thick plates, approximately from 30 to 100 µm in thickness. With conventional complex salt baths such as perchlorate, glycerate-tartrate, and Trilon baths, however, the resulting deposits free of trees dendrite are at best 10 µm thick. Ordinary baths using no complex salt provide only coarse-crystal films. Fast progress of the electronic industry in recent years has created demands for various bismuth-base alloys of low-melting points. Increasing importance is being attached to these low-melting solders because of the introduction of semiconductors with more and more delicate thermal properties. Ordinary solders, typified by the Sn-Pb alloy, have the disadvantage of so-called low-temperature brittleness. Component parts soldered with them become brittle when placed in environments below ordinary temperature, as are encountered by the soldered parts of devices and apparatus for use in superconductive and space development projects. For services in such severe environments, therefore, low-temperature resistant solders are desirable. Examples are Bi-Sn, Bi-In, Bi-Pb, Bi-Co, Bi-Ni, Bi-Sb, Bi-In-Sn. and Bi-Sn-Pb alloys. Since the standard electrode potential of bismuth is far nobler than those of tin, indium, and lead, there occurs practically no codeposition of these alloying elements from their simple solution. To make their deposition potentials as close to one another as possible, a complex salt bath must be used instead. Varied Bi-Sn alloy plating baths, for example, in the form of sulfate, chloride, perchlorate, borofluoride, and alkali baths, have hitherto been reported. Nevertheless, because of the electric potential difference, it is difficult to codeposit a relatively large proportion of bismuth to tin, and an electrodeposit of the low-melting alloy can hardly be obtained. As for the Bi-In alloy plating from a perchlorate bath, the limitation to the current density range over which the codeposit is formed makes it rarely possible to obtain thick, good alloy plates. SUMMARY OF THE INVENTION The present invention has resulted from investigations made in view of the foregoing. It is aimed at providing a plating bath which consists essentially of an organic sulfonic acid that seldom poses a pollution problem and salts of bismuth and a metal other than bismuth, and gives substantially better electrodeposits than those from conventional inorganic acid baths, and permits easy control of the bath in alloy plating, inasmuch as the electrodeposit composition is close to the metal proportion in the bath. Thus the investigations have led to the discovery of useful acids for the above purpose. In the case of Bi-Sn alloy plating, a major disadvantage is that, where the electric potential difference between tin and bismuth is too wide for the passage of current, bismuth can be lost by its contact deposition on the anode tin surface. It has now been found that the plating bath of the invention characteristically inhibits to a large measure the unwanted bismuth deposition on the anode tin surface. DETAILED DESCRIPTION OF THE INVENTION The organic sulfonic acids to be used in accordance with the invention are sulfonic acids as aliphatic or nonbenzene alicyclic compounds of the general formula (X₁) n -R-SO₃H wherein R is a C₁₋₅ alkyl radical, X₁ is a halogen atom or hydroxyl, aryl, alkylaryl, carboxyl, or sulfonyl radical which may be in any optional position of the alkyl radical, and n is an integer of 0 to 3. Examples of these organic sulfonic acids are methanesulfonic, ethanesulfonic, propanesulfonic, 2-propanesulfonic, butanesulfonic, 2-butanesulfonic, pentanesulfonic, chloropropanesulfonic, 2-hydroxyethane-1-sulfonic, 2-hydroxypropane-1-sulfonic, 2-hydroxybutane-1-sulfonic, 2-hydroxypentanesulfonic, allylsulfonic, 2-sulfoacetic, 2- or 3-sulfopropionic, sulfosuccinic, sulfomaleic, sulfofumaric acids. These acids are used alone or as a mixture of two or more. Their bismuth salts and salts of metals other than bismuth are prepared in the usual manner. A bismuth plating bath contains such an organic sulfonic acid and its bismuth salt. In the case of a bismuth alloy plating bath, such an organic sulfonic acid, its bismuth salt, and another or two or more metal salts are contained. The total concentration of bismuth and another metal or metals is, in terms of the metals, from 0.5 to 200 g/ℓ, preferably from 10 to 100 g/ℓ. The concentration of the free organic sulfonic acid to be present in the plating bath is stoichiometrically at least an equivalent to the bismuth or other metal ions in the bath. Preferably, the concentration of the free organic sulfonic acid is 30-400 g/ℓ. The plating bath of the invention may contain peptone, gelatin, nonionic surfactant, or other additive effective in avoiding burnt deposits due to excessive current intensity and in inhibiting dendritic growth. The concentration of the additive usually ranges from 0.01 to 50 g/ℓ, preferably from 0.05 to 20 g/ℓ. Among typical nonionic surfactants for this use are "Evan 740", "Liponox N-105", and "Neugen EN" (all trade names). EXAMPLES While the present invention is illustrated by the following several examples in which certain plating bath compositions and operating conditions are used, it should be noted that the invention is not limited thereto but may be variously embodied with changes in the compositions and conditions to realize the objects of obtaining uniform, smooth plated deposits. Throughout the examples, the outward appearance of the plate obtained was evaluated by the Hull cell test. Example 1 Example 2 Example 3 With the plating baths of Examples 1 to 3, Hull cell tests were conducted under the conditions of 1 A and 5 min. Each example gave a grayish white smooth electrodeposit. Example 4 Example 5 The plating baths of Examples 4 and 5 were subjected to Hull cell tests under the conditions of 1 A and 5 min. All gave grayish white smooth electrodeposits. In order to confirm the composition of the electrodeposit formed by the bismuth alloy plating, plating was carried out by 600-coulomb constant current electrolysis, using a stainless steel plate (0.3 x 3 x 5 cm) as the cathode, with cathode rocking at a rate of 2 m/min, while varying the current density over a range of 0.5 - 3 A/dm². After each run the electrodeposit formed was scraped off by a knife and dissolved in 6N-HNO₃ with the application of heat. Tin was separated and the bismuth in the solution was determined by the atomic absorption-spectroscopy. The results are given in Table 1. Dk (A/dm²) 0.5 1.0 2.0 3.0 Bi in electrodeposit, % 26.0 20.4 13.8 15.3 Current efficiency, % 89.2 90.3 73.2 56.4

Claims: A bismuth or bismuth alloy plating bath comprising as essential ingredients an organic sulfonic acid of the general formula (X₁) n -R-SO₃H wherein R is a C₁₋₅ alkyl radical, X₁ is a halogen atom or hydroxyl, carboxyl, or sulfonyl radical which may be in any optional position of the alkyl radical, and n is an integer of 0 to 3, and a bismuth salt of the acid or a mature of the bismuth salt of the acid and one or more other metal salts of the acid, said bismuth salt or salts of bismuth and one or more other metal of the organic acid being used at a concentration, in terms of the respective metallic elements, of 0.5 to 200 g per liter of the bath, and the concentration of the free organic sulfonic acid in the plating bath being 30 to 400 g per liter of the bath and being stoichiometrically at least an equivalent to the bismuth or other metal ions in the bath.

Inventors: AKUTSU, TOSHIJI, MASAKI, SEISHI, OKUHAMA, YOSHIAKI, TSUJI, KIYOTAKA
Applicants: DAIWA FINE CHEMICALS CO., LTD., ISHIHARA CHEMICAL CO., LTD.
Application year: 1986
Main classification: C25D3
IPC codes: C25D3