Pb-free external lead finishes for electronic components: Tin-bismuth and tin-silver

While research and development of both Pb-free solder pastes and Pb-free circuit board coatings has been substantial, to date minimal information has been published regarding Pb-free finishes on the external component leads. To create a truly environmentally safe, Pb-free electronic assembly, it is required that Pb-free materials be used throughout the interconnection. While the replacement of tin-lead by nickel/palladium and nickel/palladium/gold coatings does produce a Pb-free component finish which is being used commercially in certain applications at present, this technology is limited to copper alloy base materials. In Japan, an iron/nickel alloy (Alloy 42) is a very common base material for electronic components, and therefore layering systems based on palladium are not a viable alternative for the majority of electronic components in Japan. An external lead finish composed of a Pb-free solder is required which replaces existing tin-lead. To date, most research on Pb-free solder paste materials has identified two binary alloys of tin, tin-bismuth (Sn-Bi) and tin-silver (Sn-Ag), along with ternary and quaternary alloy variations of these, as the most promising for most electronics assembly operations. External lead finishes which are compatible with tin-bismuth and tin-silver solder pastes are therefore required. This paper introduces Sn-Bi and Sn-Ag electroplating processes for external lead finishing applications which satisfy Pb-free finish requirements for the electronics industry.