Prediction of Stress-Strain Behavior of Thermally Cycled SAC305 Solder Joints by Finite Element Modeling and Nanoindentation Characterization

Abstract

Plastic ball grid array (PBGA) packages are one of the widely used electronic packages in semiconductor industry. In PBGA packages, solder joint is the most vulnerable component due to continuous exposure of cycling temperatures in real applications. In our previous study, it was found that cyclic temperature causes the solder joints to evolve their mechanical properties. The continuous shear fatigue on solder joints due to CTE mismatch between die and PCB material causes damage on the solder material which eventually leads to crack propagation and failure. Hence, it is important to understand the change in stress-strain behavior in real solder joints and to identify the critical region of failure during this temperature cycling phenomena. In our previous paper, we have analyzed evolution of mechanical behavior of SAC305 solder joints as a function of cycling using nanoindentation technique.In this study, we have used FEA modeling technique to understand change in stress-strain behavior of as reflowed and thermally cycled solder joints and identify critical failure location under shear loading. A simplified three dimensional model of solder joint was generated and simulated using ANSYS software. Most of the prior studies predicted the stress-strain behavior of solder joints using viscoplastic model based on bulk solder material characterization and not on real joints. In this study, we have utilized nanoindentation technique to extract material properties of SAC305 solder joints to incorporate into ANSYS software. At first, test specimen were prepared by cross sectioning a PBGA package to reveal single grain SAC305 solder joints by surface polishing to facilitate SEM imaging and nanoindentation testing. After preparation, the package sample were thermally cycled from T = -40 to 125 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C in an environmental chamber. At various points in the cycling (e.g. after 0, 50, 100, 250, 500,750 and 1000 cycles), the package was taken out from the thermal cycling chamber. Nanoindentation testing was then performed at room temperature (25 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">o</sup> C) on above single grain solder joints to obtain equivalent mechanical behavior. Elastic-viscoplastic material properties were extracted as a function of no. of thermal cycles which was used for FEA analysis.