Abstract:

Creep can have a significant impact on the reliability of Sn-based solder alloys even at room temperature because of their relatively low melting temperatures. SnAgCu (SAC) solder joints used in electronic packaging typically consist of only a few highly anisotropic grains, which consist of Sn dendrites embedded in a near-eutectic Sn-Ag component. The unique grain structure of each joint leads to stochastic variations in the thermomechanical response of such joint. Therefore, grain-scale testing and modeling are recommended to better characterize the anisotropic behavior and to estimate the influence of stochastic variability of grain structure on the viscoplastic response of the joint. This work aims to investigate the anisotropic steady-state creep behavior of single-crystal SAC solder joints. The orientation-dependent viscoplastic behavior of individual SAC grains is modeled with a multi-scale crystal-viscoplasticity approach for representing the relevant dislocation mechanics. The response predicted by the crystal viscoplasticity model is then captured in an equivalent homogenized continuous-scale constitutive model (Hill-Garofalo formulation). This modeling strategy was implemented in numerical simulations, as an illustrative example, to analyze the behavior of single-grain solder joints subjected to combined steady compression (from heat-sink clamping force) and thermal cyclic loading. The cyclic ratcheting in the presence of the steady compressive force causes: (i) transverse expansion of the solder ball, potentially leading to eventual short circuits; and (ii) cyclic fatigue damage leading to eventual failure of the solder joint. The proposed grain-scale modeling approach is shown to be able to address the stochastic variability of both these damage mechanisms, as a function of grain orientation. The present methodology can be used to predict the realistic behavior of solder joints, based on their microstructure, and provide valuable insights for their reliability analysis.

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