Combined Vibrational and Thermal Solder Joint Fatigue—A Generalized Strain Versus Life Approach

The combined effects of elastic and inelastic strains on solder joint reliability are investigated. Experimental data from high-cycle fatigue tests of solder are combined with data from low-cycle fatigue tests to obtain a plot of total strain amplitude against cycles to failure. The generalized Coffin-Manson fatigue equation is used to describe this relationship. The transition fatigue life of approximately 7000 cycles indicates that elastic strains play a significant role in the fatigue damage of solders at a life of 10³ cycles or higher. The results suggest that the commonly adopted approach of relating only inelastic strain, or the total strain, to fatigue life with a single power law relationship may be inadequate when predicting solder joint reliability. Instead, both elastic strains and plastic strains should be considered, especially when the electronic assembly is subjected to a combination of large amplitude thermal loads and relatively lower amplitude vibrational loads. A methodology is presented to evaluate the combined effects of simultaneous vibration and thermal cycling of solder joints. This combined loading situation is simulated by superposing the effects of the vibrational and thermal loads. The damage due to each load-type acting individually is determined and then superposed to assess the overall effective fatigue life of the joint. As a first order approximation, a linear superposition rule is utilized, Miner’s rule. Reliability predictions from this simple superposition model are then compared to standard low cycle thermal fatigue models.