A damage model for SnAgCu solder under thermal cycling

As the failure of solder joints under thermal cycling is as a result of creep-fatigue damage evolution, the failure mechanism of SnAgCu solder was studied by using the theory of continuum damage mechanics (CDM) and a new damage model was proposed here. A special bimetallic load frame with single joint-shear sample was designed to simulate actual joints in electronic packages. Thermo-mechanical cycling and thermal cycling tests were conducted to determine material parameters in the creep-fatigue interaction damage model, in which the damage variable D showed a power-dependence upon thermal cycles. The damage variable D = 1-R₀/R was selected and measured every dozens of cycles during thermal cycling tests to verify the model. The results showed that the experimental damage data can be fitted reasonably well by the relationship of the damage model proposed here for SnAgCu solder. And the evolution of solder microstructure during thermal cycling was observed by using metallographic sectioning and optical microscopy analysis, which gave the microscopic explanation for creep-fatigue damage evolution law of SnAgCu solder.