The Creep-Fatigue Interaction in Solders and Solder Joints

Two models are proposed for relating the metallurgy of the solder to the growth of fatigue cracks through solder joints. These models illustrate how different aspects of the creep behavior can contribute to the so-called “creep-fatigue interaction”. The first model treats fatigue crack growth through the solder, far from the interface between solder and substrate. Either an intragranular or intergranular path may be taken depending upon conditions of loading. Intragranular fatigue dominates when the cycle frequency is high, in which case failure life is governed by the Coffin-Manson law. Intergranular failure occurs at low frequencies because grain boundary sliding at low frequencies allows the grain boundaries to become exposed to the atmosphere, which in turn causes oxidation. This model predicts the effects of frequency, strain amplitude, and grain size on fatigue life. In the second model, the fatigue crack travels within the interface region between solder and substrate. Here, the strain introduced in the solder joint during fatigue is not relevant; instead, the stress transferred to the interface is more important. The second model considers the effect of solid solution concentration on fatigue life. The predictions of both models agree reasonably well with published fatigue data from solders and solder joints.