Effect of Co Addition on Interface Reaction Between Sn–Ag–Cu Solder and Cu Substrate

Sn-Ag-Cu (SAC) alloys are considered as the most promising Pb-free solders in electronic industry.The solidification microstructure and interface reaction behaviors of SAC alloys are therefore of fundamental importance for the service reliability of electronic devices.This is particularly true for these SAC alloys with low silver contents, partially because the coarsened interfacial intermetallics (IMCs) of these low-Ag SAC alloys with higher surface tension than conventional near-eutectic SAC alloys.As a result, it is desirable to refine the grain size of interfacial IMCs between low-Ag alloys and common substrates such as Cu.In this work, the effects of addition of trace amount of Co on the interface reaction between both conventional SAC305 and low-Ag SAC107 alloys on Cu substrate have been studied by reflowing experiments at temperature close to 260 o C. In addition, effects of Co additions on the solid state growth of interfacial IMCs have been studied at 150 o C after ultra-long annealing treatment for 384 h, 768, and 1536 h.Both top-view and cross-section micro-graphs have been obtained using electron microscopes.It has been found that addition of trace amount of Co can significantly refine the interfacial Cu 6 Sn 5 IMCs grains after reflow process and impede the growth of Cu 3 Sn after annealing treatment.This attributed to the replacement of Cu atoms by Co atoms in Cu 6 Sn 5 crystals, which in turn depresses the diffusion of Cu and impedes the transformation from Cu 6 Sn 5 to Cu 3 Sn during aging.8 Ref., 1 Tabl., 9 Fig.