Investigation of nonlinear behaviors of packaging materials and its application to a flip-chip package

The creep behavior of a flip-chip package under thermal load is studied using finite element models and high density laser moire interferometry. FEA accuracy strongly depends on a reliable input material database, and thus a series of tests were first carried out using a 6-axis mini fatigue tester for eutectic 63Sn37Pb solder alloy and FP4526 underfill. From the test data, six FEA models are used to simulate creep under elastic, elastic-plastic, viscoelastic or viscoplastic behavior for both solder balls and underfill. The results show that solder and underfill nonlinear material behavior do not have a major effect on flip-chip warpage, but do affect the stress. In all visco FEA models, Von Mises stresses at the corner or center of the outermost solder ball greatly decrease. However, Von Mises stresses predicted by elastic-plastic and elastic models remain unchanged during the temperature holding time and are much higher than those from visco FEA models. Although solder ball stresses predicted by visco models have no major difference, this is not true for underfill. By comparing FEA results, it is suggested that the strain rate-dependent model is used to describe underfill creep behavior. However, the inelastic equivalent strain which is usually used as the fatigue life prediction parameter also shows big differences between FEA models, and thus a suitable model must be carefully chosen for accurate fatigue life prediction. Furthermore, flip-chip package creep is measured by real-time moire interferometry. The FEA model package deformation values are compared with the laser moire interferometry data and are in good agreement.