A Modified Energy-based Low Cycle Fatigue Model for Eutectic Solder Alloy

Surface mount technology (SMT) is increasingly used in microelectronics to mount components by soldering onto the printed circuit board (PCB). The solder alloys are used as the electrical and mechanical connections between the component and the board. Fatigue failure of solder joints is recognized as a major cause of failure in electronic devices. An approach to this problem is to determine the fatigue behaviors of solder alloy by accelerated fatigue testing at different temperatures. In the present research, smooth specimens made entirely of a 63Sn/37Pb solder alloy were tested over a wide temperature range at various low frequencies to study its low cycle fatigue properties. Strain-based models, notably the Coffin-Manson model (1–2), have been widely used to characterize low cycle fatigue behaviors of engineering materials including solder alloys (3–5). However, it is practically very difficult to obtain a single plastic strain value in a solder joint because of the complex stress state (6). In contrast, it is much easier to calculate energy density from the low cycle hysteresis loops for any types of solder joints under test (7). Therefore, in recent years, energy-based low cycle fatigue models have been increasingly used for solder alloys (7–9). These models are not so well established as the strain-based ones, and some researchers (10) even expressed doubt on whether energy density is a true parameter governing fatigue life of solder alloy. Therefore, in the study special effort was made to examine the existing energy-based models. In the end, a flow stress modified energy-based model is proposed based upon the examination and the experimental results. It is demonstrated in the paper that the new fatigue model can be used to predict the fatigue life of solder alloy at different frequencies or temperatures.