Reliability of Bonded Interfaces for Automotive Power Electronics

In automotive power electronics packages, conventional thermal interface materials such as greases, gels, and phase change materials pose bottlenecks to heat removal and are also associated with reliability concerns. There is an industry trend towards high thermal performance bonded interfaces. However, due to coefficient of thermal expansion mismatches between materials/layers and resultant thermomechanical stresses, adhesive and cohesive fractures could occur, posing a problem from a reliability standpoint. These defects manifest themselves in increased thermal resistance in the package. The objective of this research is to investigate and improve the thermal performance and reliability of emerging bonded interface materials for power electronics packaging applications. We present results for thermal performance and reliability of bonded interfaces based on thermoplastic (polyamide) adhesive, with embedded near-vertical aligned carbon fibers, as well as sintered silver material. The results for these two materials are compared to conventional lead-based (Sn63Pb37) bonded interfaces. These materials were bonded between 50.8-mm x 50.8-mm cross-sectional footprint silicon nitride substrates and copper base plate samples. Samples of the substrate/base plate bonded assembly underwent thermal cycling from -40°C to 150°C according to Joint Electron Devices Engineering Council standard Number 22-A104D for up to 2,000 cycles. The dwell time of the cycle was 10 minutes and the ramp rate was 5°C/minute. Damage was monitored every 100 cycles by acoustic microscopy as an indicator of an increase in thermal resistance of the interface layer. The acoustic microscopic images of the bonded interfaces after 2,000 thermal cycles showed that thermoplastics with embedded carbon fibers performed quite well with no defects, whereas interface delamination occurred in the case of sintered silver material. Both these materials showed a superior bond quality as compared to the lead-based solder interface even after 1,000 thermal cycles.