The formation and growth of intermetallic compounds between Sn-3.5Ag lead-free solder and Cu substrate

A new method was used to investigate the formation and growth of intermetallic compound (IMC hereafter) formed at the interface between Sn-3.5Ag lead free solder and the Cu substrate. Solder joints were prepared by Memisco (wetting balance, model ST50) at 250/spl deg/C with soldering time of 10sec, 30sec, 60sec and 90sec. Experimental data for IMC growth between copper and Sn-3.5Ag solder during soldering exhibit a t/sup 1/3/ dependence on time. The results indicate that during soldering up to 90 sec, grain boundary diffusion is the predominant mechanism for transport through the layer. During aging at 170/spl deg/C up to 1000h, the columnar morphology of IMCs changed to a more planer type. The growth behavior of IMC layers obeys t/sup 1/2/ equation /spl Delta/y/sub i/ (t) = k/sub i/t/sup 1/2/. Here, /spl Delta/y/sub i/ is the growth thickness of IMC during aging; k/sub i/ is the coefficient standing for the square root of the diffusivity at the aging temperature and t is the aging time. This equation reveals a diffusion-controlled mechanism during aging.