A moderate pH electrolyte bath for electroplating

Abstract

 An aqueous electrolyte bath for electroplating a tin-lead alloy onto a substrate contains a tin salt and a lead salt of alkanesulfonic acid of 1-4 carbons, a conductivity enhancer of an alkali, alkaline earth, or tetraalkylammonium salt of an alkanesulfonic acid of 1-4 carbons, and a grain refiner; the pH of said bath is adjusted between about 1 and the pH at which tin or lead hydroxide begins to precipitate out due to reaction with one or more bases of alkali, alkaline earth, or tetraalkylammonium hydroxide, oxide, or carbonate. A tin-lead alloy solder is electroplated onto a substrate, such as glass or ceramics, in an electrolytic process by impressing an electromotive force across such an aqueous electrolyte bath between electrodes until the substrate has been plated.

Description

BACKGROUND OF THE INVENTION

[0001] This invention relates to the electroplating of tin-lead alloy solder, particularly on acid-sensitive substrates such as glass or ceramics, in an electrolytic process using an electrolyte solution bath under moderate pH conditions.

[0002] Formulations in the prior art for plating tin/lead solder are based on the use of soluble tin and lead salts of strong acids. Acids used include fluroboric acid, methanesulfonic acid, ethanesulfonic acid, hydroxyethanesulfonic acid, and other alkane sulfonic acids containing up to 4 carbon atoms in the molecule. In order to improve the conductivity and thus the plating efficiency of these formulations, free acid concentrations of 10% or more of the total bath weight were employed; therefore, the solutions, were highly acidic. In applications involving acid-sensitive substrates, such as glass or ceramics, such a plating solution is unsuitable because the acid destroys the substrate. For such substrates, a plating solution of moderate pH (i.e., pH greater than 1 but less than 7) is required with pH of greater than or equal to 2 generally being preferred.

[0003] In attempting to formulate moderate pH solder-plating solutions, two unfavorable conditions are encountered in the prior art. The first is the precipitation of metal hydroxides as base is added to raise the pH of the solution. When this occurs, the tin/lead ratio in the solution will change, and effect the composition of the deposit. The other is that the quality of the deposit and the throwing power of the bath decreases as the pH is raised. Therefore, preparing a solution to meet all the requirements is difficult.

[0004] The addition of chelating agents to control metal hydroxide precipitation is well known. Maruta (U.S. patent 4,329,207 and the publication "New Tin-Lead Alloy Decomposition from Fluroborate-free Bath: New Mater. New Processes, 1983, 2-183-7) discloses a formulation using an organic carboxylic acid as the chelating agent. This solution has a pH of 3.3 to 4.5. The solution has the good performance features required of a moderate pH plating bath. However, because of the chelating agent either expensive waste-treatment procedures must be employed to remove residual heavy metals from the bath or the spent solution must be treated as a hazardous waste.

[0005] Proell (U.S. Patent 2,525, 942) discloses the use of metal salts of low alkanesulfonates without excess free acid as electrolytes; however, the addition of between 0.01 and 5% of alkanesulfonates of 10-30 carbons was required to obtain a good quality deposit.

[0006] Nobel et al ("High Speed Tin and Tin Lead Alloy Plating" proceedings: American electroplaters society symposium on strip and edge-board plating for electronics (April 16-17, 1985)) discloses that in the absence of chelating agents, residual heavy metals can be removed to parts per million levels from a solder plating bath based on alkanesulfonic acid by simply adjusting the pH; thus, this eliminates the need to employ complex and expensive waste treatment proceedures prior to disposal.

STATEMENT OF THE INVENTION

[0007] This invention is an electroplating bath for electroplating a tin-lead alloy solder onto a substrate which comprises an aqueous solution of (i) a tin salt and (ii) a lead salt of alkanesulfonic acid containing 1-4 carbons, wherein the combined concentration of said tin and lead salts is from about 0.1% by weight to the solubility limit of said salts in solution and the ratio of the tin salt to lead salt is 1:99 to 99:1, (iii) from about 1% to the solubility limit of a conductivity enhancer from the class of alkali, alkaline earth, or tetraalkylammonium salts of alkanesulfonic acid containing 1-4 carbons, and (iv) from about 0.1 t o about 2.0% of a grain refiner, the pH of said aqueous solution having been adjusted with a base from the class of alkali, alkaline earth, or tetraalkylammonium hydroxide, oxide, or carbonate to from 1 to the pH at which tin or lead hydroxide begins to precipitate.

[0008] This invention also comprends a process for electroplating using the above mentioned bath wherein the cathode is a substrate of glass or ceramics and the anode is selected from graphite, stainless steel, or soluble tin/lead solder, comprising immersing said substrate into the aqueous electrolytic bath and impressing a direct current voltage across said electrolytic bath between the electrodes until the substrate has been plated.

DETAILED DESCRIPTION OF THE INVENTION

[0009] This invention is a bath for electroplating solder, particularly suitable for plating onto acid-sensitive substrates such as glass or ceramics under moderate pH conditions. The bath employs an aqueous electrolyte solution of tin and lead salts of at least one substituted or unsubstituted alkanesulfonic acid having a carbon chain length of 1-4 carbons, a conductivity enhancer, and a grain refiner. Such sulfonic acids include, for example, methanesulfonic acid, ethanesulfonic acid, propanesulfonic acid, isopropanesulfonic acid, butanesulfonic acid, and isobutanesulfonic acid. The preferred sulfonic acid is methanesulfonic acid (MSA) because of its availability, low molecular weight, high inate solubility in water, and high water solubility of its metal salts. The combined concentration of the tin and lead salts in the bath can vary from about 0.1% by weight to the solubility limit of the salts in solution. The tin and lead salts ratio in the bath is set by the tin/lead composition desired in the deposit; the ratio of tin salt to lead salt can range from 1:99 to 99:1.

[0010] The conductivity enhancer improves the conductivity of the bath which, in turn, improves the quality of the plating out of the solder. The concentration of the conductivity enhancer in the bath can range from about 1% by weight to the solubility limit of the conductivity enhancer in the bath. The conductivity enhancer includes alkali metal (such as potassium or sodium), alkaline earth metal (such as calcium, strontium, barium, or magnesium), or tetraalkylammonium (where the alkyl has 1-4 carbons) salts of unsubstituted or substituted alkanesulfonic acids.

[0011] The grain refiners are surfactants (i.e., wetting or dispersing agents) capable of modifying the plating deposit particle sizes in such a manner that the deposit is a continuous adherant film. In tin or tin-lead alloy plating, without grain refiners, the deposit is grainy and loose and falls off the substrate. The grain refiners are ionic or non-ionic surfactants with certain molecular weight ranges. The most commonly used grain refiners are ethoxylated alkyl phenols having the structure

where n is an integer of 10-14 average; R is an alkyl of 8 to 20 carbons. A preferred grain refiner is the condensation product of nonylphenol condensed with about 10-14 moles of ethylene oxide. A commercially marketed grain refiner is sold under the trademark IGEPAL®710 by GAF Corporation which is nonylphenol condensed with 10-11 moles of ethylene oxide. The concentration of the grain refiner in the bath can range from about 0.1 to about 2.0% by weight. Normally, from about 1 gram to about 20 grams per liter, preferably from about 5 grams to 10 grams per liter, of the grain refiner is used in the bath.

[0012] The electromotive force impressed on the electrolytic system is a direct current (dc) voltage ranging from about 1 to about 10 volts, preferably 3 to 5 volts, resulting in the electroplating of the tin/lead alloy of the anode being plated out onto the substrate (i.e., the cathode). The optimum voltage for maximum current efficiency in this process ranges from about 1 to about 10 volts with the use of aqueous tin and lead methanesulf onates salts with an alkali, alkaline earth, or tetraalkylammonium salts of alkanesulfonic acid (the conductivity enhancer) and a grain refiner as the electrolyte.

[0013] The cathode useful with this invention is preferably an acid-sensitive substrate of glass or ceramics.

[0014] The anode useful with this invention may be any electroconductive material in any desired shape which is substantially insolube in the electrolyte under the conditions of the process or a soluble tin-lead alloy. Typical insoluble anode materials are stainless steel and graphite, preferably fabricated in a flat shape.

[0015] The temperature in which electroplating using the bath of this invention is operated is not critical. Preferably, it is carried out at ambient temperature although, due to a moderate exotherm during operation, the temperature may rise. The temperatures may generally range from ambient to about 100°C, normally 30 to 40°C, and external cooling is typically not required.

[0016] The pH of the bath is maintained between 1 and the pH at which tin or lead hydroxide begins to precipitate out. Preferably, the pH of the bath is between 2 and the precipitation pH of tin or lead hydroxide. More preferred is a pH between 2.0 and 2.5. The bases which are used to adjust the pH are selected from alkali, alkaline earth, or tetraalkylammonium hydroxides; oxides and carbonates can also be used to adjust the pH. The preferred base is potassium or sodium hydroxide.

[0017] The electrolytic cell useful in this invention includes a tank or vessel of a suitable size and shape in which the electrolytic process may be operated in the desired fashion. The material from which the tank is fabricated may be nonelectrically conductive or electrically conductive, if properly insulated, as is well known in this art.

EXAMPLE

[0018] An aqueous stannous methanesulfonate solution (22g) containing 21% tin and having a density of 1.60 at room temperature was mixed with 10 g of a nonionic surfactant (25% Igepal® 710), 2.5 g of PbO, 54 grams of potassium methanesulfonate and distilled water to make the total volume up to about 240ml. The resulting slightly turbid solution was back-titrated with 70% MSA until it became clear. The solution had a pH of 1.98. It was neutralized with 1.9g of 45% KOH solution to reach a pH of 2.34, and diluted to 267ml in a Hull Cell. A brass panel was plated for 10 min. at 4 volts, and 0.9 amp. An area of white deposit on the panel (indicative of a high quality deposit) ranged from current density 36 amp/ft² to 1.6 amp/ft².

[0019] The above bath is suitable for electroplating a lead-tin solder on surfaces of glass or ceramics when such surfaces constitute the cathode of the electroplating system. The bath is well suited for use in a barrel plating apparatus commonly used by plating shops for plating small articles, such as, for instance, micro-ceramic capacitor chips.

[0020] When the electroplating described above was repeated using substantially the same bath but in which the conductivity enhancer was omitted, a current density of 16 to 0.8 amp/ft² (i.e., a slower plating rate) was required for a matte white (high quality) finish and the remainder of the low current density area was gray (indicating a low quality deposit). Thus, the presence of the conductivity enhancer provided a faster plating rate, improved bath conductivity and improved bath efficiency.

Claims

1. An electroplating bath for electroplating a tin-lead alloy solder onto a substrate which comprises an aqueous solutuion of (i) a tin salt and (ii) a lead salt of alkanesulfonic acid containing 1-4 carbons wherein the combined concentration of said tin and lead salts is from about 0.1% by weight to the solubility limit of said salts in solution and the ratio of the tin salt to lead salt is 1:99 to 99:1, (iii) from about 1% to the solubility limit of a conductivity enhancer from the class alkali, alkaline earth, or tetraalkylammonium salts of alkanesulfonic acid containing 1-4 carbons, and (iv) from about 0.1 to about 2.0% of a grain refiner, the pH of said aqueous solution having been adjusted with a base from the class of alkali, alkaline earth, or tetraalkylammonium hydroxide, oxide, or carbonate to from 1 to the pH at which tin or lead hydroxide begins to precipitate.

2. The composition of Claim 1 wherein the tin and lead salts are tin and lead methanesulfonate.

3. The composition of Claim 2 wherein the conductivity enhancer is sodium or potassium methanesulfonate.

4. A process for electroplating using the bath of claim 1 wherein the cathode is an acid sensitive substrate and the anode is selected from graphite, stainless steel, or soluble tin/lead solder, comprising immersing said acid sensitive substrate into the aqueous electrolytic bath and impressing a direct current voltage across said electrolytic bath between the electrodes until the substrate has been plated.

5. The process of Claim 4 wherein said acid sensitive substrate is glass or ceramics.

6. The process of Claim 5 wherein said direct current voltage is in the range of from about 1 to about 10.

7. The process of Claim 6 wherein the conductivity enhancer is sodium or potassium methanesulfonate.