The Effect Of Microalloy Additions On The Morphology And Growth Of Interfacial Intermetallic in Low-Ag and No-Ag Pb-Free Solders
Authors: Keith Sweatman, Jonathan Read, Tetsuro Nishimura, and Kazuhiro Nogita Company: Nihon Superior and University of Queensland Date Published: 10/16/2011
Abstract: It is widely accepted that the intermetallic layer that forms at the interface between a solder and a copper substrate has an effect on the properties of the solder joint, particularly under high strain rates such as occur in drop impact. The nature and extent of the effect is related to the thickness and morphology of this interfacial layer and the phases present in the layer. It has been recognized also that thickness, morphology and mix of phases changes over time as a result of diffusion across the interface in both directions as the solder/substrates system moves toward thermodynamic equilibrium and these process are accelerated at the elevated temperature to which some electronic circuitry is exposed. While the basic composition of the solder obviously has a major effect on the thickness, morphology and growth of the interfacial intermetallic microalloying additions have also been found to have an effect. In this paper the authors report a study of the thickness morphology and growth of intermetallics formed on a copper substrate by eight alloys representative of the range of low-Ag and no-Ag formulations currently used or proposed as Pb-free solders for electronic assembly. In addition to the basic constituents of Sn, Cu and Ag these alloys included additions of Ni, Bi, Ce, Ge, and P. Cross-sections were examined by SEM and the thickness and growth at 130, 140 and 150°C of both the Cu6Sn5 and Cu3Sn phases was monitored. While Ni was found to have a strong effect on all aspects of the interfacial intermetallics any effects of Bi were more subtle.