Intermetallic compound layer growth at the interface between Sn-Cu-Ni solder and Cu substrate

Phase analysis and growth kinetics of intermetallic compound (IMC) layers formed between low cost Sn-Cu-Ni solder for wave soldering and Cu substrate by solid state isothermal aging were examined at temperatures between 80 and 150°C for 0 to 60 days. The IMC layer was composed of two phases; Cu ₆Sn₅ (also (Cu,Ni)₆Sn₅) adjacent to the solder, Cu₃Sn adjacent to the copper. The Cu₃Sn layer was absent at all time periods for the aging temperatures of 80 and 100°C. After isothermal aging at 120°C for 30 days and at 150°C for all time periods, the solder/Cu interface exhibited a duplex structure of Cu ₆Sn₅ and Cu₃Sn intermetallics. From the EPMA analysis, a comparatively high Ni concentration is found between the Cu ₆Sn₅ and Cu₃Sn layer. The Ni concentration in the (Cu,Ni)₆Sn₅ intermetallic was about 4.3at.%. The total IMC layer thickness was approximately 11.5μm after 60 days of aging at 150°C. The apparent activation energies for growth of total and Cu ₆Sn₅ IMC layer were 76.17 and 66.35kJ/mol, respectively.