METALLURGICAL CHANGES IN TIN LEAD PLATINGS DUE TO HEAT AGING

Tin or tin-lead platings form intermetallic compounds with some routinely used substrate materials such as copper or nickel. These intermetallics form at the plating substrate interface, changing in both composition and thickness with time and temperature. The intermetallic compounds are somewhat harder and more brittle than either the tin or tin-lead platings; therefore, as the intermetallic growth approaches the outer surface of the platings, various surface properties such as contact resistance, friction and wear, alloy composition, and corrosion resistance may be altered. Generally, the coefficient of friction decreases with a corresponding increase in wearability and contact resistance. The formation of the intermetallic continues as long as there are sufficient amounts of the alloying constituents present. The lead in the tin-lead platings is rejected by the formation of the copper-tin and nickel-tin intermetallic, resulting in the lead being pushed towards the plated surface, thus becoming a lead rich surface. Samples that were aged to simulate a period of time for storage exhibited an increase in contact resistance when compared to as received samples.