Investigation of Non-Pressure Sinter Silver in Power Module with High Accelerated Stress Aging Test

Abstract

Traditional lead-free solder are widely employed for die attachment owing to low cost and easy process. The melting points of lead-free solder are under the tolerable levels for silicon semiconductor devices. However, wide band gap semiconductors such as Gallium nitride (GaN) and Silicon carbide (SiC) with excellent electrical characteristics can be utilized for power electronics application at high power and high junction temperatures beyond 300°C [1], [2]. Therefore, suitable die-attachment materials which could bear the high junction temperatures play an important role for these wide band gap devices. However, the traditional techniques to interconnect (such as the solder alloys) are inappropriate due to the limits of operating temperatures and lower thermal conductivity. Silver sinter joining is well known for die-bond material because of outstanding heat conductivity as well as excellent high temperature stability [3], [4], [5]. In the study, a high thermal conductivity was obtained from the micro-sized silver particles sintered at 280°C for 90 min without pressure [6]. Besides, shear strength of bulk silver joint on bare copper substrate larger than 35 MPa were successfully achieved, which was produced a lower porosity less than 8.5%.In order to investigate the stability of pressure-less sintered silver joints after high aging tests, we have carried out high temperature storage (HTS) at 175°C for 2000 hours. The power device characters with sinter material including the die shear strength (DSS), ratio of porosity, I-V curve, surge current ability and thermal conductivity were verified in comparison to the initial state. In this study, mechanical shear over 28 MPa has been remained after aging at 175°C for 2000 hours. Furthermore, we have proven the electrical and thermal stability of non-press sintered silver when environmental tests of high temperature storage. Cross-section microstructure of the interface between sintered silver and copper substrate was examined by Scanning Electron Microscopy (SEM). The SEM images show that the formation of metallic bonds between the sintered joints and bare copper. Then, the sintered density of the joints were higher after high-heat reliability test because of the micro-silver particles composed complex porous networks as well as thick connection. In addition, it is carried out high thermal storage at 250°C for 500 hours, as well as the surge current ability still kept stable after the aging heating tests. To sum up, there is no doubt that the novel sintered silver paste without applied pressure has been one of the most promising candidates for high temperature and high power electronic applications in the future.