Evolution of the Mechanical Properties of Lead Free Solder Joints Subjected to Mechanical Cycling

Solder joint failure is one of the primary reasons for the failure of electronic packages. When electronic components are subjected to environments with temperature gradients, the solder joints present in them undergo a fatigue thermomechanical state due to differences in the Thermal Expansion Coefficients (CTEs)of the different assembly materials. Understanding the mechanical cyclic properties of these lead-free solder joints has thus become a necessity to improve the reliability of these packages. In this study, we have characterized the mechanical cyclic induced microstructural evolution and consequent changes in mechanical properties of individual solder joints. The testing of individual joints is important because there are significant variations of properties between actual joints and conventional bulk samples (e.g. dog-bone, rectangular, or lap shear samples). The test assemblies in this study were (3⨯3)BGA packages composed of total nine 0.75 mm diameter lead free solder joints that were formed by reflowing solder spheres soldered onto 0.55 mm diameter Cu pads on FR4 coupons. The solder joints were cycled using a Micromechanical tester along with a newly designed fixture which facilitates the tester to cycle a solder joint individually. The solder joints tested were of different alloys thus also giving an idea of the effect of the addition of various dopants on the mechanical cyclic properties. The joints were cycled for various durations including to failure. Nanoindentation tests were performed on the specimens to study the evolution in mechanical properties (e.g. elastic modulus, hardness and creep strain rate)of the solder joints as a function of duration of cycling. The prime objective of this study was to better understand the damage accumulation and evolution of a specific solder joint due to mechanical cyclic fatigue loading.