Reliability of lead-free solder interconnects - A review

This paper discusses reliability issues related to lead-free printed circuit board interconnects. A comprehensive review of the published literature was carried out, and complemented further by reliability experiments that have been conducted to understand the behavior of some of the lead-free alternatives. The failure mechanisms for lead-free flip chips during accelerated testing are discussed. Some of the metallurgical effects due to the change in soldering system are also reviewed. In the case of assembly of lead-free flip chips, solder joint properties were still found to depend on the reflow profile (peak temperature and time above the liquidus temperature). The ultimate shear strength of Sn/Ag/Cu (SAC) joints on Ni/Au-coated substrate pads proved very sensitive to small changes in peak temperature and time above liquidus. A similar effect was not observed on Cu/OSP pads. The reflow profile with a lower peak temperature and a lower time above the liquidus temperature gave slightly lower shear strength of joints between two Ni-pads, while the profile with a higher peak temperature and a higher time above the liquidus gave a considerably higher strength. Not surprisingly, the fatigue resistance of both encapsulated and non-encapsulated Sn/Ag/Cu joints was significantly lower on Ni/Au-pads than on Cu/OSP.