THE FORMATION OF CU-SN INTERMETALLIC COMPOUND AND ITS EFFECT ON THE FRACTURE-BEHAVIOR OF 80SN-20PB ELECTRODEPOSITS ON CU-BASED LEADFRAME ALLOY

Alloy deposits of 80Sn-20Pb, electroplated on a Cu-based leadframe all oy 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 compou nds and their influence on the fracture resistance of the 80Sn-20Pb de posits were examined. The growth rate of the intermetallic compound la yer on aging exhibited a significant difference depending on the depos ition conditions; it was fastest in an extremely fine deposit formed u sing a pulse current in a bath without grain refiner (additive), but s lowest in the deposit formed using a d.c. current in a bath containing the additive, in spite of its equally fine structure. The additive in corporated at the grain boundaries of the electrodeposit appears to sl ow down the diffusion rate of Sn atoms across the intermetallic-deposi t interface on aging, resulting in slow growth of the intermetallic la yer in the thickness direction. The frequency of surface cracks, forme d when the aged samples were subjected to the 90 degrees bending test, was found to increase with the growth rate of the intermetallic compo und layer.