Metallurgical changes in tin-lead platings due to heat aging

Heat aging to simulate long-term storage or operation at elevated ambient temperatures resulted in the formation of an intermetallic compound between the copper or nickel substrate and the tin in tin-lead plated samples. Two intermetallics, Cu₆Sn adjacent to the substrate followed by Cu₆Sn₅, were formed on the aged copper samples, while only one nickel-tin, a Ni₃Sn₄ intermetallic, was found on the nickel samples. Increasing the aging duration resulted in increased thickness of the intermetallic compounds. As the intermetallic compound adjacent to the substrate, either the Cu₃Sn or Ni₃Sn₄, increases in thickness the lead from the tin-lead plating is rejected, although there is some evidence that not all the lead is rejected from the Cu₆Sn₅ phase. The rejected lead agglomerates result in an enriched lead surface. The copper-tin intermetallics tend to grow faster than the nickel-tin intermetallic compounds, suggesting that usage of a nickel barrier plating may retard the growth rate of the copper-tin intermetallics. Contact resistance vs. force curves exhibit small increases in resistance as long as there is still some unreacted tin-lead on the plated surfaces