Determination of Fatigue Model Constants for Bi-Based SAC305 Hybrid Solder Joint

Fatigue model constants of a board level reliability model are obtained for Bi-based SAC305 hybrid solder joints. Three tasks are performed to achieve the goal. In the first task, the constitutive properties (the Anand model constants) of hybrid solders are obtained as a function of Bi concentration from the constant strain rate tests at various temperatures. In the second task, ideal package assemblies containing hybrid solder joints are assembled using a metal-core PCB on the top and an FR-4 PCB at the bottom. Two assemblies are formed under different reflow conditions. They are subsequently subject to a thermal cycling test, and the characteristic life of each assembly is determined. Thermal cycles of 0°C and 125°C are used for testing: rapid heating to 75°C followed by a 5-minute ramp to 125°C and 5-minute dwell, then rapid cooling to 40°C, 5-minute ramp to 0°C, and final 5-minute dwell. In the final task, a numerical modeling scheme is developed and implemented to determine the accumulated energy density per cycle over an area where solder fatigue cracks are observed during the thermal cycling test. The modeling results and the characteristic lives are used to determine the fatigue constants of a strain energy-based fatigue model. The fatigue constants obtained in the study can be used for any Bi-based SAC305 hybrid solder joints since detailed diffused Bi-concentrations are taken into consideration during the calculation.