Abstract:

The combined effects of elastic and plastic strains on solder joint reliability is investigated.  Experimental data from high-cycle lap-shear fatigue tests of solder is combined with data from low-cycle fatigue tests to obtain a plot of total strain amplitude against cycles to failure on a log-log scale.  The dependence of fatigue life on strain amplitude is found to obey the classical generalized Coffin-Manson relationship.  The plot of total strain versus life is also found to agree well with experimental results in the literature form displacement controlled tests.  The transition fatigue life of approximately 7,000 cycles indicates that elastic strains play a significant role in the fatigue damage of solders at a life of 103 cycles or higher.  These results suggest that the commonly adopted approach of relating plastic strains to fatigue life 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 combined large amplitude thermal loads and relatively lower amplitude vibrational loads.

A methodology is developed to evaluate the combined effects of simultaneous vibrational and thermal cycling of solder joints.  Superposing the effects of the vibrational and thermal loads simulates combined thermal and vibrational loading situations.  The damage due to each load-type acting individually is determined based on the generalized Coffin-Manson total-strain versus life relationship.  The damage from the two load-types is then superposed to assess the overall effective fatigue reliability of the joint.  As a first order approximation, linear superposition rules such as Miner’s rule are utilized.  Reliability predictions from this simple model are compared to thermal low cycle fatigue models for three sample combined loading problems.
 

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