The influence of hold time and fatigue cycle wave shape on the-low cycle fatigue of 60/40 solder

Previous experiments have shown that fatigue life is reduced when the cycling frequency is reduced. These tests utilized symmetric ramp loading with equal loading and unloading rates. While this sort of cycling is useful in identifying the influence of cycling frequency, it does not reproduce the type of loading actual solder joints experience. Duty cycles generally consist of a relatively rapid change in temperature, then a relatively long hold at a more or less constant temperature, followed by a relatively rapid temperature change back to the starting point and another hold time. To investigate this sort of cycling, isothermal experiments were performed with varying hold times, and failure was induced by the application of mechanical strains, at a constant temperature, rather than thermal strains. It was found that the frequency dependence of the fatigue life, induced by varying the hold time, was similar to that observed when the frequency was varied by changing the ramp loading and unloading rates. The frequency effect is actually somewhat less for the hold time tests than for the ramp loading tests. This difference is due to the load relaxation which takes place during the hold times. This relaxation is described and relaxation rate curves are provided