Isothermal and thermal cycling aging on IMC growth rate in Pb-free and Pb-based solder interfaces

The growth of interfacial intermetallic compounds (IMC) between Pb-free and Pb-based solders with different surface finish (Cu, Ni, Au metallizations) is one of the major concerns in long-term solder joint reliability performance in electronic assemblies. The growth rate of the IMC layer plays a major role on the life-time of the solder joints. As the diffusion and the reaction are thermally activated processes, the growth rate of the IMC layer is sensitive to temperature changes. Critical review and analysis of solid-state diffusion mechanism for the growth of IMC between solder-to-substrate interface for Pb-free and Pb-based solders subject to isothermal and thermal cycling aging is conducted. Through isothermal annealing the growth rate of the IMC layer at a given temperature and its general dependence on temperature can be determined by plotting the measured IMC layer thickness against time at iso-temperatures. However, real solder joints in electronic assemblies undergo thermal cycling due to environmental temperature changes and/or power on/off cycles. Therefore, data obtained by thermal cycling (TC) aging exposure is more practical in understanding IMC growth behavior in solder joints. However, in TC aging tests, the IMC growth rate is often determined also by plotting the measured IMC thickness against the accumulated time exposure (corresponding to the number of cycles of TC aging). This can lead to incorrect physics-of-failure characterization of IMC growth kinetics for TC aging effects, as IMC growth is temperature and time dependent and is expected to have thermally activated thresholds below which at a certain temperature no IMC growth is expected. Therefore, it is obvious that the whole TC cycle does not contribute to IMC growth and hence using accumulated time corresponding to TC cycle time is not justified. Hence, in this study, comparison between IMC layer thickness data by isothermal aging versus thermal cycling aging are used to develop a framework for correlating IMC layer growth behavior between isothermal and thermal cycling effects.