Effect of Copper Grain Size on the Interfacial Microstructure of a Sn/Cu Joint

For pursuing a more comprehensive mechanism of void formation at the Sn/Cu interface, different grain size Cu samples with and without twinned boundaries are electrodeposited for the Sn/Cu joint and the IMC analysis. The formation of voids in the Sn/Cu joint is divided into three parts for discussion: Cu diffusion, Sn diffusion, and impurity diffusion. For the void formation at the Cu/Cu3Sn interface, the concept of the mechanism is from the Kirkendall effect, which is specially focused on the speed difference of Cu diffusion between at the Cu grain boundary and in the copper lattice in this study. The speed difference caused the vacancy formation. Impurities also play an important role in promoting the vacancy aggregation from a cluster to a void. Whether it is a diffusion effect or an impurity effect, it occurs at a Cu grain boundary. The voids would be formed when the density of the Cu grain boundary is high. The grain size of Cu is inversely proportional to the density of the Cu grain boundary; as a result, Cu grain size strongly affects the interfacial microstructure of a Sn/Cu joint, that is, the larger the Cu grain size is, the less the void at the Sn/Cu interface will be formed