Sensitivity of grain structure and fatigue reliability of Pb-free solder joint on the position in PBGA packaging assembly

This paper reports experimental evidences suggesting that fracture resistance of solder joints in packaging assembly is not necessarily uniform but can vary significantly with their locations in the assembly. Specifically, it is found that the solder joints located at the chip corner is much more prone to the fracture under thermo-mechanical or mechanical loads such as vibration and shock. This, the variation in fracture resistance, is found from the isothermal cyclic shear fatigue testing of 60Pb-40Sn, SAC305 and SAC105 solder alloys assembled in PBGA configuration. Under pure shear fatigue situation, the location of the joint failure should ideally be random because all joints are subjected to an identical shear strain. However, it is found that the solder joint that fails first is always the ones located at the corner site of chip. The inner joint, close to the center of the chip, shows much higher resistance against failure. Further study that investigates the joint microstructure with aging, produces evidences indicating that the corner is the place of high residual thermal stress or stored strain energy (by plastic deformation during cooling or heating). In an extreme case, where stored energy by plastic deformation is not released well by competing process, the stored energy triggers recrystallization, leading to polygranular structure of the corner joint solder. These results strongly suggest that the joints in package assembly have different reliability and microstructural evolution path depending on their location in the package and thermal history.