Effects of Intermetallics on the Reliability of Tin Coated Cu, Ag, and Ni Parts

The temperature dependence of the growth rates of intermetallic compounds in the Cu-Sn, Ag-Sn and Ni-Sn systems was determined between 100°C and 213°C for Sn-dipped and Sn-plated samples. Below 175°C the fastest growing intermetallic compound was Ag3Sn. The Ni-Sn compoknd, Ni3Sn, was the slowest growing phase below 150°C, but the fastest growing phase above 175°C. The two Cu-Sn intermetallic phases, Cu3Sn and Cu6Sn5, had a combined growth rate which increases more slowly with temperature than the single intermetallic phases observed in the Ni-Snand Ag-Sn systems. The growth rate data plotted against reciprocal temperature satisfies the Arrhenius relationship and yields a range of apparent activation energies between 14.8 Kcal/mole for Ag3Sn and 37.6 Kcal/mole for Ni3Sn4. The growth of intermetallic compounds can affect the reliability of electronic parts through a reduction in lead solderability or a decrease in the mechanical strength of soldered connections. Lap shear testing of metal strips bonded with Sn demonstrated that strengths of both bonded Ni and Cu strips decrease as the thickness of the intermetallic compounds increase during annealing at 213°C. In the case of Ni-Sn, a 50% reduction in joint strength occurred after only one day-at temperature. For Cu-Sn, the reduction in joint strength occurred at a slower rate, and only a slight decrease was observed in the Ag-Sn lap shear samples. The degradation observed in the Ni-Sn and Cu-Sn systems is related to the growth rate of brittle intermetallics forming at the interface (i.e. Ni3Sn4 and Cu3Sn).