Growth Kinetics of Cu–Sn Intermetallic Compounds at Interface of 80Sn–20Pb Electrodeposits and Cu Based Leadframe Alloy, and Its Influence on the Fracture Resistance to 90°-Bending

Abstract

Electrodeposits of 80Sn–20Pb were formed on a Cu-based leadframe alloy from organic sulfonate solutions under different plating conditions, and were then aged at 100 to 180°C to examine the effects of the microstructure of electrodeposits on the growth kinetics of the Cu–Sn intermetallic layer formed at the electrodeposit/substrate interface, and the resultant influence on the fracture resistance of the electrodeposits under bending. The Cu–Sn intermetallic compound layers [ε (Cu3Sn) + η (Cu6Sn5)] increase linearly in thickness with the square root of aging time. However, the growth rate of the intermetallic compounds layer exhibits significant differences depending on the deposition condition; it is fastest in an extremely fine elecrodeposit formed using pulse current in a bath without any grain refiner, but is slowest in that formed using dc current in a bath with grain refiner, in spite of an equivalent grain size in both electrodeposits. This is due primarily to the inhibiting effects of the grain refiner remaining at grain boundaries of the electrodeposit on the diffusion of Sn across the intermetallic compounds/electrodeposit interface. The occurrence frequency and the degree of opening of surface cracks, formed when the aged samples are subjected to the 90°-bending test, are found to be associated with the growth rate of the intermetallic compound layer and the ductility of the electrodeposit. For samples aged under the same conditions, the sample electrodeposited from the solution containing grain refiner shows the highest resistance to surface cracking during the 90°-bending test due to the lower growth rate of its intermetallic layer and the higher ductility of the electrodeposit compared with those of other samples.