Experimental Observation of Diffusion Reaction in the (Sn-Ag)/Cu System at Solid-State Temperatures

Abstract

The diffusion reaction kinetics for Sn-Ag alloys with pure Cu has been examined experimentally to determine the effects of adding Ag to Sn on the growth behavior of compounds at a heated interface between a Sn-base solder and a Cu-base conductor. A solid-state diffusion bonding technique was utilized to make sandwich (Sn-Ag)/Cu/(Sn-Ag) diffusion couples with Ag mol fraction of 0.011 to 0.033. The diffusion couples were isothermally annealed at 433 K to 473 K for up to 1944 h. During annealing, Cu6Sn5 and Cu3Sn layers formed at the bonded (Sn-Ag)/Cu interface in the diffusion couple. The layer thickness of Cu3Sn (l3) was less than that of Cu6Sn5 (l6), and the concentration of Ag was negligible in both Cu6Sn5 and Cu3Sn. The total thickness (l = l3 + l6) was proportional to a power function of the annealing time with an exponent n of 0.30 to 0.40. Because n is less than 0.5, boundary diffusion as well as volume diffusion must contribute to the layer growth. The ratios of l3 and l6 to l (r3 and r6, respectively) were insensitive to the annealing time and the Ag concentration. Addition of Ag to Sn barely affects the layer growth. The value of r3 was 0.34, 0.34, and 0.30 at temperature of 433 K, 453 K, and 473 K, respectively, with corresponding value of r6 of 0.66, 0.66, and 0.70, respectively.