Tin coating of connectors - reliability risk for electrical equipment

Abstract

The EC directive 2002/95/ES (RoHS) limited (with few exceptions) the use of lead in electronic production, especially in solders and surface finishes. This brought significant changes in their reliability. The electroplated tin coating is currently among the most widely used surface finishes in the electronic industry, appearing everywhere from the coating of solder pads on SMD devices to corrosion prevention coating on large containers. The tin is usually deposited galvanically, in a layer 10mm thick, often with a yet thinner interlayer between the tin and base material. The most common base materials are copper, brass and bronze. Usually the connector or component lead is cut, bent or otherwise shaped either before the coating or after it. This combination of materials and mechanical stress can lead to appearance of one important risk factor: tin whiskers. Whiskers are very thin and very long crystals of pure metal (usually tin, but silver and other whiskers were observed as well), growing from the surface of the coating. The whiskers are, of course, electrically conductive, their rate of growth is very fast and they can easily breach even organic protective coatings. This naturally presents a significant risk of short-circuit formation, either by the whisker growing between two contacts or by the fragile and thin crystals being broken off the surface. All in all, tin whiskers can pose a serious problem from the point of view of long-term reliability of electronic devices [2].