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Time-independent plastic behaviour of solders and its effect on FEM simulations for electronics packages

The time-independent behaviour of SnPb37, SnAg3.5 and SnAg4Cu0.5 has been investigated on flip chip solder joints. The test specimen consisted of two silicon chips (3.3 mm in square) bonded by 4 flip chip joints (one at each corner). The Dresden University Micro Shear Tester has been used for the experiments with this type of specimen. In contrast to similar set-ups, it is actively compensated for its finite stiffness. Therefore, it is able to record force displacement hysteresis with a resolution of better than 1 mN and 20 nm, respectively. Based on these measurements, the parameters of the constitutive equations have been evaluated by FEM analysis. This way, the complex stress state within the sample during the test has been considered precisely providing for high accuracy of the parameter extracted. As a typical application, a three point bending experiment has been simulated by FEM applying different constitutive models for the solder material. Comparing the results, it becomes clear: All the three ingredients, i.e., the elastic, the creep, and the time-independent plastic data, are required in the model. Otherwise it would be incomplete and hence insufficient for assisting in the design of today's electronics packages even with respect to the most frequent load cases.