Metallurgical changes in tin-lead platings due to heat aging

One-inch test samples of bright and matte tin-lead platings over both copper and nickel substrates were examined after heat aging for various times at 150°C in forming gas. The intermetallic compounds formed are somewhat harder and more brittle than the platings. Therefore, as the intermetallic growth approaches the outer surface of the platings, various surface properties 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. Samples aged to simulate a period of time for storage exhibited an increase in contact resistance when compared to as-received samples