Kinetics of Intermetallic Compound Formation at the Interface Between Sn-3.0Ag-0.5Cu Solder and Cu-Zn Alloy Substrates

The growth kinetics of an intermetallic compound (IMC) layer formed between Sn-3.5Ag-0.5Cu (SAC) solders and Cu-Zn alloy substrates was investigated for samples aged at different temperatures. Scallop-shaped Cu(6)Sn(5) formed after soldering by dipping Cu or Cu-10 wt.%Zn wires into the molten solder at 260A degrees C. Isothermal aging was performed at 120A degrees C, 150A degrees C, and 180A degrees C for up to 2000 h. During the aging process, the morphology of Cu(6)Sn(5) changed to a planar type in both specimens. Typical bilayer of Cu(6)Sn(5) and Cu(3)Sn and numerous microvoids were formed at the SAC/Cu interfaces after aging, while Cu(3)Sn and microvoids were not observed at the SAC/Cu-Zn interfaces. IMC growth on the Cu substrate was controlled by volume diffusion in all conditions. In contrast, IMC growth on Cu-Zn specimens was controlled by interfacial reaction for a short aging time and volume diffusion kinetics for a long aging time. The growth rate of IMCs on Cu-Zn substrates was much slower due to the larger activation energy and the lower layer growth coefficient for the growth of Cu-Sn IMCs. This effect was more prominent at higher aging temperatures