Effects of Plating Condition on the Growth of Cu-Sn Intermetallic Compound of Sn-Pb Electrodeposits on Cu Based Leadframe Alloy

Alloy deposits of 80Sn-20Pb were electroplated on the Cu-based leadframe alloy from an organic sulfonate bath using dc or pulse currents, and aged at 150 .deg. C to form Cu-Sn intermetallic compounds at deposit/substrate interface. Influences of plating parameters on the microstructure of the 80Sn-20Pb deposits, effects of deposit morphology, deposit composition, and substrate on the formation of intermetallic compounds were investigated. The growth of Cu-Sn intermetallic compound layer were showed a parabolic time dependence, and the growth rate of that depended significantly on the microstructure of Sn-Pb electrodeposits which were influenced by deposition condition: it was fastest in an extremely fine deposit formed using pulse current in bath without additive, but slowest in deposit formed using dc current in bath containing the additive in spite of equally fine structure. The additive incorporated at grain boundary of electrodeposit appears to retard the diffusion rate of Sn atoms across the intermetallic/deposit interface on aging, resulting in slow growth of intermetallic layer in thickness direction but substantial growth in lateral one. The growth rate of the intermetallic compound at the substrate/Sn-Pb interface increased with increasing the Sn content in deposits. The growth rate of the intermeallic compound was also affected significantly by the microstructure of Cu-substrates. The growth rate of Cu-Sn intermetallic compound was faster when the substrate was solution treated than it was age-hardened