Intermetallic compound layer growth by solid state reactions between 58Bi-42Sn solder and copper

Solid state intermetallic compound layer growth was examined following thermal aging of the 58Bi-42Sn/Cu couple for a temperature range of 55-120 deg C and time periods of from 1-400 days. The intermetallic compound layer was comprised of sublayers that included the traditional Cu sub 6 Sn sub 5 stoichiometry and one or more complex Cu-Sn-Bi chemistries. The number of sublayers increased with aging temperature and time. Time-dependent layer thickness computations based upon the empirical expression, At exp n +B, revealed a time exponent, n, that decreased with increasing temperature from a maximum of 0.551 at 70 deg C to 0.417 at 120 deg C. The apparent activation energy for growth (at 100 days) was 55plus/minus7 kJ/mol. The Bi-Sn/Cu data, together with that from the other solder/copper systems, suggested that at a given homologous temperature, the quantity of Sn in the solder field determines the intermetallic compound layer thickness as a function of time.