Effect of voids on thermomechanical cracking in lead-free Sn3Ag0.5Cu interconnections of power modules

In this article, the effect of voids on thermomechanically-induced failure in the lead-free solder interconnections of power amplifier (PA) modules is investigated. The interconnection of interest is between the module's flange and substrate (baseplate) initially having a large contact area, and whose cracking has strong detrimental effects on the RF performance and reliability of the PA. PA modules were attached onto baseplates with lead-free SAC305 solder and put to a thermal cycling test (TCT) in the 15 degrees C to 95 degrees C range. X-ray imaging was used to characterize the number of voids in interconnections after reflow soldering. The cross-sections of the pristine reflow-soldered and tested interconnections were inspected with cross-polarized light microscopy to reveal the shapes of the voids and crack paths. It was noted that the voids forming in interconnections during the final stages of the reflow process take an elliptical shape, leaning towards the module's outer edge, due to differences in the thermal expansions of the module and baseplate. During the TCT, high thermomechanical stresses caused localized recrystallization of the as-soldered SAC matrix in the vicinity of the elliptical voids. Finally, creep related intergranular cracks formed in these recrystallized areas. Anand's viscoplastic model was used to model the void formation and the effect of the formed voids on crack evolution in solder. Simulations of the high creep energy of the elliptical, tilted voids and their correlation with the recrystallization behavior of the SAC305 interconnections were consistent with the observed creep related failures. Confirmed by simulations, these voids are detrimental to the reliability of solder interconnection because the creep strain is strongly localized to the sharp curvatures of the elliptic void.