The formation of Cu-Sn intermetallic compound and its effect on the fracture behavior of 80Sn-20Pb electrodeposits on Cu-based leadframe alloy

Abstract

Alloy deposits of 80Sn-20Pb, electroplated on a Cu-based leadframe alloy from an organic sulfonate bath, were aged at 150 degrees C to form Cu-Sn intermetallic compounds at the deposit-substrate interface. The effects of deposit morphology on the formation of intermetallic compounds and their influence on the fracture resistance of the 80Sn-20Pb deposits were examined. The growth rate of the intermetallic compound layer on aging exhibited a significant difference depending on the deposition conditions; it was fastest in an extremely fine deposit formed using a pulse current in a bath without grain refiner (additive), but slowest in the deposit formed using a d.c. current in a bath containing the additive, in spite of its equally fine structure. The additive incorporated at the grain boundaries of the electrodeposit appears to slow down the diffusion rate of Sn atoms across the intermetallic-deposit interface on aging, resulting in slow growth of the intermetallic layer in the thickness direction. The frequency of surface cracks, formed when the aged samples were subjected to the 90 degrees bending test, was found to increase with the growth rate of the intermetallic compound layer.