Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Table III. Concentrations of Methane Sulfonic Acid (MSA) and MSA Salts Used in Tin, Lead, and Tin-Lead Alloy Plating Product 50% MSA 70% MSA Stannous methane sulfonate Lead methane sulfonate Copper methane sulfonate Concentration (g/L) MSA 100% = 613 MSA 100% = 945 Sn = 120 Sn = 150 Sn = 200 Sn = 240 Sn = 300 Pb = 450 Pb = 500 Cu = 100 Specific Gravity (20O 1.225 1.350 1.230 1.285 1.390 1.450 1.560 1.630 1.700 1.260 C) organic additives must be added to tin, lead, and tin-lead baths to produce smooth, nontreeing deposits and to increase the throwing power of the plating bath. These additives are mandatory, as fluoborate baths cannot function without them. METHANE-SULFONIC-ACID-BASED PLATING Methane sulfonic acid (MSA)-based electroplating systems gained commercial acceptability during the early 1980s. Various plating baths and additives have been developed. Each MSA bath requires a custom-made additive for that specific content of tin, lead, and MSA in order to obtain a useful deposit. As with fluob- orates, metal salts of MSA are very soluble and various concentrations of MSA and its metal salts are available (see Tables III and IV). Troubleshooting guidelines for fluoborate- and MSA-based tin, lead, and tin- lead plating baths are given in Tables V to VII. The troubleshooting table for acid- tin-based baths (Table V) is also applicable to stannous sulfate baths. TIN PLATING FROM STANNOUS SULFATE BATHS The stannous sulfate bath contains stannous sulfate, sulfuric acid, and addi- tives (see Table VIII). Advantages of this bath are its good throwing power, high cathode efficiency, operation at room temperature, and ease of control. Addition agents are essential for the operation of this bath. They inhibit the oxi- dation of stannous tin, produce smooth, dense deposits, and prevent treeing. Among those in use are phenol- or cresol-sulfonic acid, gelatin, [gb]-naphthol, and resorcinol. Several proprietary additives are also available. Bright tin deposits are obtained through the addition of proprietary addi- tives. Solderability, corrosion resistance, and leveling characteristics are essen- tially the same in bright or matte plating. It is questionable whether bright tin plating is superior to matte in resistance to whisker growth and tin pest, as has been claimed. Refer to Table V for troubleshooting. TIN PLATING FROM STANNATE BATHS Alkaline baths are also used for tin plating, with sodium stannate and potassium stannate as the two available baths (see Table IX). Both will produce similar sat- isfactory results. The criteria for choosing one over the other are related to cost and speed of plating. 249

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