Electrodeposition of Tin–Lead Alloys

Abstract

The deposition of tin-lead alloys has the most technical importance among all tin or lead alloys. Processes for the electrolytical deposition of tin-lead alloys are known since 1921 and were first developed for the coating of naval torpedos [1,2]. A second field of application evaluated was the deposition of tin-lead alloys for bearings [3][4][5]. Intensive research in the field of tin-lead plating has been stimulated by the different requirements in the production of electronic components such as PCB (printed circuit board), semiconductors, connectors, and the like. A further impulse has come since about 1980 with the development of fluoride-free tin-lead electrolytes.The standard potentials for tin and lead are very close together (Pb: À0.126 V; Sn: À0.136 V). Alloy deposition is therefore possible in all alloy compositions. Both metals have a high hydrogen overvoltage, so deposition of tin-lead alloys is possible from strong acid solutions without complexing agents with high current efficiencies. The current efficiency can be reduced by adding a range of different types of organic additives to such electrolytes as brightener systems. ELECTROLYTE SYSTEMSElectrolyte systems based on tetrafluoroboric or methanesulfonic acid (MSA) are mainly used for the deposition of tin-lead alloys. For special applications weakly acidic systems with special complexing agents are also on the market. Electrolytes can also be divided into those for the deposition of matte tin-lead coatings and for the deposition of bright deposits. A further division can then be made into processes for conventional plating such as rack-and-barrel plating and those for high-speed applications such as wire plating, reel-to-reel processes for connectors, and continuous processes for IC leadframe finishing. ALLOY COMPOSITIONSAlthough tin-lead alloys can form over the entire compositional range, they tend to fall into three classes in terms of technical applications:. High-lead alloys . Eutectic or near-eutectic alloys . Alloys with 5-15% PbIn the first category are alloys with around 93% Pb and 7% Sn that are used as coatings for fuel tanks and on various bearings. For bearing applications there is also usually a codeposition of about 2% copper. The deposition is in a form of a matte coating. In this category fluoroborate electrolytes are still the most frequently used systems.Eutectic alloys are those that possess the lowest melting point, which makes them suitable for solderable coatings. The eutectic alloy contains 63% tin. A considerable field of application has been the production of PCBs using the metalresist technique. When the electrodeposited tin-lead layer is used as a reflowed coating, which remains on the board, an alloy composition very close to the eutectic composition is important so that the boards are reflowed with the lowest possible temperature. The main function of the reflow process is to eliminate the overhangs of tin-lead, which are caused by underetching during the copper etching. By reflowing the electroplated tin-lead layer t...

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