Evolution of the cyclic stress-strain behavior of doped SAC solder materials subjected to isothermal aging

Abstract

Solder joints in electronic packages are frequently exposed to thermal cycling. Such exposures can occur in real life applications as well as in accelerated thermal cycling tests used for the fatigue behavior characterization. Due to the CTE mismatches of the assembly materials, cyclic temperature leads the solder joints to be subjected to cyclic (tensile/compressive) mechanical loading. On the other hand, isothermal aging causes the evolution of the microstructure, mechanical response, and failure behavior of lead free solder joints. Aging also affects the cyclic stress-strain behavior of solder materials. Our prior studies have examined the changes in the cyclic stress-strain behavior occurring in Sn-Ag-Cu (SAC) lead free solders including SAC105, SAC305, and SAC405. Aging effects can be partially mitigated by the incorporation of dopants to SAC alloy. In this study, we have investigated the aging induced evolution of the cyclic stress-strain behavior of different doped SAC solder materials. The studied materials include SAC_R (Ecolloy) consisting of Sn, Cu, and Bi; SAC_Q (CYCLOMAX) which is a Bi-doped SAC alloy; and Innolot, which is an engineered combination of six elements. Cylindrical uniaxial test specimens of the doped SAC solder alloys have been formed in circular cross-section glass tubes using a vacuum suction process. The specimens were then aged at 125 °C for different durations (06 months). After aging, the specimens were subjected to cyclic mechanical loading at room temperature under strain-controlled conditions. Using the recorded cyclic stress-strain data, the evolution of the hysteresis loop area, peak load, and plastic strain range have been characterized in terms of the alloy and prior aging conditions. The results for the doped solders have also been compared with those of the standard lead free alloys. Innolot has shown much better cyclic properties than SAC305 for all aging durations. In addition, SAC_Q shows slightly higher cyclic properties than Innolot. Finally, the effects of aging on the cyclic stress-strain behavior were much less for all three doped alloys. Thus, the doped alloys have a high potential for use in thermal cycling conditions because of their improved cyclic properties and for their improved resistance to aging-induced evolutions.