Development of constitutive equation parameters and reliability evaluations of tin-silver-copper interconnects

Impact of introducing lead-free solder due to the legislation restriction, generates numerous researches to characterize the lead-free alloys and evaluate the interconnect reliability. The objective of this dissertation is to characterize the SnAgCu alloy and also evaluation of SAC solder interconnects reliability under different loadings. Solder material properties were measured at different stress/strain rates, and temperatures. Bulk solder tensile test specimens were developed where controlled cooling rate gives microstructures similar to bulk solder joint. Also a modified Iosipescu solder joint lap specimen for uniaxial shear testing was introduced. Thermo-mechanical viscoplastic material constitutive parameters were derived using a unified creep-plasticity model and incorporated it into commercial finite element solver. Verification of unified model with a partitioned model, where the elastic behavior is separated from rate-independent and rate-dependent behavior. The parameters developed are in line with similar studies. Temperature dependent coefficient of thermal expansion (CTE) was measured for SAC alloys with a derived linear correlation.; The influence of Au addition in SAC solder was studied for 0.5 to 2.5 weight percentage of Au. Stress-strain measurements showed that significant strength increase (20 to 30%) with higher strain range was observed for Au addition in SAC alloy.; Solder microstructure was studied for bulk and solder interconnects and influence of surface finishes were also investigated. Joint interface was investigated at different thermal aging conditions. Both scallop and cellular structure of Cu6Sn5 were observed at the interface.; SAC solder joint reliability was investigated under mechanical and thermomechanical loadings using three PWB pad finishes (immersion Ag, OSP and Ni/Au) by employing standardized thermal cycling, cyclic bend, and drop tests. Failure mechanisms were identified that affect the products in typical user environments. SAC joints passed 1000 thermal cycles whereas early failures observed under bend and drop loadings. It has been shown that, PWB pad finish and associated reflow profile had impacts on the interconnect reliability. The failures under mechanical loading are not only solder joint failures, but also Cu pad pull-out and resin coated Cu (RCC) trace failure. Drop and bend reliability for lead-free assemblies will require optimizing the solder strength with the strength of the PWB materials.