Effect of Ramp Rate on Microstructure and Properties of Thermomechanically-Fatigued Sn-3.5Ag Based Composite Solder Joints

Solder joints experience repeated reverse thermal stress in service, caused by the mismatch in the coefficient of thermal expansion (CTE) between solder and Cu substrate. Lead-free composite solder was fabricated by mechanically mixing Sn-3.5Ag solder paste with Cu reinforcement particles to improve service capability of the base solder alloy. Thermomechanical fatigue (TMF) tests were carried out in environmental chambers. Two temperature profiles, with the same extreme temperature of -40 degC to 125 degC and a 10-minute dwell time at temperature extremes, were applied in the tests. One was a 5-minute ramp time and another was a 20-minute ramp time. Microstructural characteristics and residual mechanical strengths of the solder joints were investigated after 0, 100, 250 and 500 TMF cycles. Composite solder joints showed better TMF performance than the Sn-3.5Ag eutectic solder joints. The results proved that ramp rate was one of the key issues for the damages in solder joints