Roles of service parameters on the mechanical behavior of lead-free solder joints

Lead-based solders have been extensively used as interconnects in various electronic applications due to their low cost and suitable material properties. However, in view of environmental and health concerns, the electronics industry is forced to develop lead-free alternative solders. Eutectic Sn-3.5Ag based solders are being considered as suitable substitutes due to their non-toxicity, tolerable melting temperatures, and comparable mechanical as well as electrical properties. Smaller electronic packaging and emerging new technologies impose several constraints on the solder interconnect that require better inherent properties in the solder to resist failure during operation. Hence, it is important to develop a clear understanding of the deformation behavior of eutectic Sn-Ag solder joints.; Mechanical characterization was performed to investigate the behavior of eutectic Sn-Ag solder joints. Peak shear stress and flow stress decreased with increasing testing temperature and with decreasing simple shear-strain rate. The effect of simple shear-strain rate on the peak shear stress was found to be more significant at temperature regimes less than 125°C. The deformation structure of specimens deformed at higher temperatures was dominated by grain boundary deformation, while at lower temperatures it was dominated by shear banding.; Stress relaxation studies on eutectic Sn-Ag solder joints were carried out to provide a better understanding of various parameters contributing to thermomechanical damage accumulation. Monotonic stress relaxation tests at various pre-strain conditions and testing temperatures can provide information relevant to the effects of ramp rates during heating and cooling excursions experienced during thermomechanical fatigue. Peak shear stress and residual shear stress, resulting from stress relaxation period, decreased with increasing testing temperature for a given pre-strain condition. A faster ramp rate was found to cause higher resultant residual stress as compared to slower ramp rates.; In order to understand the roles of service related parameters, such as pre-strain and pre-strain rate imposed prior to the repeated strain cycling, cyclic strain amplitude and cyclic strain rate, and testing temperature, cyclic shear straining with associated stress relaxation was carried out on pre-strained eutectic Sn-Ag solder joints. These parameters may play significant roles during actual thermomechanical fatigue cycles of the solder joints in electronic devices. Cyclic straining may be able to provide more stress relaxation in specimens deformed to higher pre-strain/cyclic-strain at a higher pre-strain rate/cyclic-strain rate under low testing temperature. In specimens deformed at higher temperatures, residual stress build-up is less significant and as a consequence cyclic straining has no significant influence on the stress relaxation behavior. At high temperature, processes such as crack healing (or recovery) can take place, whereas microstructural damage may accumulate easier at lower temperatures.