The correlation between sintered silver joint reliability and pressure assisted sintering parameters

Nano-particle sintering silver (Ag) paste has recently become a topic of much discussion in commercial and academic circles and has been the subject of many papers at international technical conferences. Sintered Ag is one of the candidates for the replacement of high-lead (Pb) alloys such as Pb-5Sn and Pb-5Sn-2.5Ag in semiconductors, an application that is still exempt from the RoHS restrictions on Pb in electrical and electronic equipment. Although the EU Restriction on Hazardous Substances (RoHS) exemption has been extended to 2021 there is still a lot of interest in the use of sintered Ag in semiconductor die attach because it can survive the higher operating temperatures of more advanced SiC and GaAs power semiconductors. The demand for these advanced higher efficiency power semiconductors is expected to increase as electric and hybrid vehicles, renewable energy and smart electricity grids are more widely adopted and this provides further justification for the development of sintered Ag bonding technology. One of the advantages of sintered Ag is that it can form a joint at temperatures comparable with those used in die attach with high-Pb solder but once the process is completed will not melt until the temperature exceeds the 961°C melting point of Ag. In this paper, the authors report the results of power cycling of insulated-gate bipolar transistor (IGBT) semiconductors built using two variants of Ag sintering paste with a range of process parameters with the intention of determining the sensitivity of the bond life to such variations. Included in the experimental program as benchmarks are semiconductors built with conventional Pb-5Sn alloy and Pb-free alloy, SAC305. The semiconductor subjected to power cycling were subsequently subjected to examination by confocal scanning acoustic microscopy (CSAM) and cross-sectioning and scanning electron microscopy to determine the failure mechanisms. No fatigue cracks were observed in the sintered Ag joint regardless type of Ag sintering material used and the processing parameters. The dominant failure mechanism in the sintered Ag bonds was delamination. In the benchmark bonding materials, Pb5Sn and SAC305, classic solder fatigue failure appeared to be the dominant failure mechanism.