Effect of compositional changes and impurities on wetting properties of eutectic Sn–Bi alloy used as solder

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Abstract
As the number of layers on multilayer printed-circuit boards increases, the interlayer bonding requirements become progressively more stringent. One way of easing interlayer stresses is to reduce the preheat-soldering temperature interval. To do this by raising the preheat temperature is damaging to both board and components, and one alternative is to choose a solder alloy with a lower melting point than conventional Sn-Pb solders. In this paper, a study is described of the wetting characteristics of a range of near-eutectic Sn-Bi alloys, whose melting points fall near 139°C, ~46 K lower than Sn-Pb solders, making possible a reduction of 55 K in soldering temperature. The effect of divergence of composition from the 42Sn-58Bi eutectic composition on wetting, wetting speed, and spread area is described for two flux systems on plain copper, immersiontinned copper, and palladium-coated substrates. Additionally, the effects of various impurity levels of Sb, As, Cu, Fe, Pb, Ni, and Pd on solderability are investigated, the type of defects which each produces being listed. Some difficulties involved in contamination investigations on Al and S are also discussed. Finally, the effects of these impurities on Sn-Pb and Sn-Bi eutectic solder alloys are compared. MST/88

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