X-Ray Microtomography of Thermal Cycling Damage in Sintered Nano-Silver Solder Joints

Novel high operating temperature thermal interface materials (TIMs) in power electronics are required to realize performance gains from the use of wide band-gap (WBG) semiconductor devices, such as Silicon Carbide (SiC) or Gallium Nitride (GaN). Additionally, the anticipated operating temperature of these devices is higher than 250 degrees C, preventing use of traditional solder material for packaging. The thermomechanical stresses induced inside the electronic package can severely degrade the reliability and life of the device. In this light, a new non-destructive approach is needed to understand damage mechanisms when subjected to reliability tests such as power and thermal cycling. In this work, a sintered nano-silver TIM is identified as a promising high temperature bonding candidate. Sintered nano-silver samples are fabricated and their shear strength values are reported. Thermal cycling tests are conducted and damage evolution is characterized using a lab scale three-dimensional (3D) X-ray system to periodically assess changes in the microstructure such as cracks, voids, and porosity in the TIM layer. The evolution of the microstructure and the effect of the cycling temperature profile during thermal cycling is discussed.