Understanding the Cyclic Mechanical Behavior of Lead/Tin Solder

Abstract

For lead/tin solders, room temperature is hot enough that deformation is controlled by creep. Creep deformation is thermally activated and is a function of time. Therefore, the appropriate parameters which can describe cyclic behavior are the ones which are functions of temperature and time, such as strain rates, ramp rates, and hold times. The traditional cyclic loading parameters of strain amplitude and stress amplitude are inadequate and can even be misleading as they do not take into consideration the rate at which strain or stress was applied. In addition to permanent creep strains, anelastic strains are also stored as a function of time during cyclic loading. Anelastic strains are recoverable and thus can be nondamaging. During cyclic loading, anelastic strains are stored along with creep strains during the time spent on load. If the hold times in accelerated cycling are shortened to the extent that a significant portion of the total strain stored during on-load is anelastic (around 3 seconds) then the damage storage rate decreases rather than accelerating. These factors need to be considered both when accelerated testing is contemplated and in the development of life prediction equations.