Alloy deposits of 80Sn-20Pb were electroplated on the Cu-based lead frame alloy (PMC-102M) from an organic sulfonate bath using dc or pulse currents, and aged at 150℃ 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 and their influence on the fracture resistance of the electrodeposits were investigated.
Addition of grain refiner to the bath increased the cathodic polarization 640 mV greater than that in the bath without the grain refiner(additive) at cathodic current density of 10 A/d㎡, thereby producing an extremely fine and round deposit. The grain morphology of electrodeposit produced by pulse currents was also extremely fine at low duty cycle (10 %) and low frequency (25 Hz), but became coarser with increasing the duty cycle and/or the frequency, approaching that of electrodeposit formed by dc current. The electrodeposits of 80Sn-20Pb was identified a mixture of Pb and β-Sn crystals by X-ray diffraction. The Pb crystals were randomly oriented, but β-Sn crystals exhibited {200} preferred orientation in deposit formed from the bath without additive and {220} preferred orientation in that formed from the bath with additive. In pulse plating, β-Sn crystals exhibited {200} preferred orientation which was gradually transformed to {220} one with decreasing the duty cycle and/or the frequency.
When alloy deposits of 80Sn-20Pb were aged at 150℃, the growth of Cu-Sn intermetallic compound layer [$\varepsilon(Cu_3Sn)$ +\eta'(Cu_6Sn_5)$] were showed a parabolic time dependence, and the growth rate of that depended significantly on the microstructure of Sn-Pb electrodeposits which was 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 delay 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 apparent activation energy of intermetallic compound growth in deposit formed using dc current in bath containing the additive was 43.32kJ/mol, that in deposit formed using dc current in bath without additive was 34.36 kJ/mol, and that in deposit formed using pulse current was 30.35 kJ/mol.
The occurrence frequency of surface crack, formed when the aged samples were subjected to the 90-bending test, were found to be associated with the growth rate of the intermetallic compound layer and ductility of deposit layer. For the same aged samples, the occurrence frequency of surface crack was the least in sample deposited in bath containing the additive because of the slowest growth rate of the intermetallic compound layer and the highest ductility, and the most in that formed using pulse current in bath without additive because of the fastest growth rate of the intermetallic compound layer and the lowest ductility of deposit layer.
When the alloy deposits of 90Sn-10Pb, 80Sn-20Pb, and 60Sn-40Pb were electroplated using dc, and aged at 150℃, the composition of deposit affected the growth rate of the intermetallic compound, The growth of the intermetallic compound for the 90Sn-10Pb system was fastest due to its higher content of Sn. The growth rate of the intermeallic compound was also affected significantly by the precipitate of Cu-substrates. It was slow in Sn-Pb system electrodeposited on as-received PMC-102M having fine precipitation, and fast in Sn-Pb system electrodeposited on solution treated PMC-102M
