Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 247 of 707

electroplating solutions TIN, LEAD, AND TIN-LEAD PLATING BY STANLEY HIRSCH LEEAM CONSULTANTS LTD., NEW ROCHELLE, N.Y. AND CHARLES ROSENSTEIN TESSERA-ISRAEL, LTD., JERUSALEM, ISRAEL Tin and tin-lead alloys are solderable and, therefore, are used extensively in the electronics industry to bond electronic components. This precludes the need for strong fluxes to wet the deposit. Tin and lead can be codeposited easily because of the closeness of their standard electrode potentials. Tin and tin-lead deposits must possess all of the following characteristics: good solderability and reflowability; low porosity; good corrosion resistance; and uniformity of alloy composition, thickness, smoothness, and appearance, over a wide current density range. ADDITIVES Organic additives are required in all tin, lead, and tin-lead electroplating solutions to produce a useful deposit. In the absence of additives, treed, nonadherent, and nodular deposits result; therefore, additives are absolutely essential in order to yield smooth, uniform deposits and to impart good throwing power. Additives are depleted during plating and must be routinely replenished. Electrolysis causes some decomposition of the organics, resulting in their occlu- sion in the deposit. If present in sufficiently large amounts in the deposit, organ- ics can cause solderability and reflow problems. Accelerated aging tests are per- formed on tin and tin-lead deposits to help predict their shelf life, as components are sometimes used long after they are plated. TIN, LEAD, AND TIN-LEAD PLATING BATHS Tin, lead, and tin-lead electroplating solutions are used to plate components in numerous engineering, communications, military, and consumer product appli- cations. Tin Barrel, Still, and High-Speed Baths These baths are well suited for electrolytic tin plate and are used to plate transis- tors, semiconductors, various electronic parts, refrigerator parts, and kitchenware. Higher temperatures permit higher current densities and wire speeds. Lead Barrel and Still Baths Lead baths are used in the plating of bearings, connectors, internal and con- forming anodes for chromium plating, valves, seals, and parts for storage batteries. Because lead deposits are soft, slow barrel speeds are recommended in order to pre- vent heavy parts from bonding to one another. 60 Tin/40 Lead Solder Barrel, Still, and High-Speed Baths The high throw of the baths and the excellent solderability and shelf life of the deposit make these baths suitable for use on printed circuit boards, connectors, and other specialized electrical devices. The high-speed bath is used for reel-to-reel applications, such as wire and strip plating. 246

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