Effects of β-Sn Crystal Orientation on the Deformation Behavior of SAC305 Solder Joints

Abstract

Solder joints provide mechanical support, and electrical and thermal interconnection among various packaging levels in microelectronics assemblies. Reliability of electronic packages and expected functionality of these interconnections predominantly depend on the micromechanical properties of the solder joints. Therefore, material characterization of reflowed area array solder joints is essential for predicting the deformation behavior and reliability of electronic components. The dimensions of solder joints are typically in the sub-millimeter range. Hence, size effects must be taken into account when evaluating constitutive properties of lead-free solder joints. However, it is difficult to evaluate constitutive behavior by using traditional uniaxial tests for material samples that are so small. Thus, instrumented indentation or nanoindentation techniques have proven to be convenient in investigating mechanical properties such as elastic moduli and hardness of different materials.In the first part of this study, samples of solder joints were collected from ball grid array packages. The samples were then cross-sectioned and prepared for electron back-scattered diffraction (EBSD) experiments to record the crystal orientations of the various single and multiple-grained solder balls. After characterization of the joint orientations, nanoindentation experiments were performed on individual grains with various crystal orientations to obtain the elastic moduli along different directions. Using the recorded experimental modulus data, calculations were performed to evaluate the elastic compliance and stiffness matrices, which incorporate the directional material properties.Solder joints are subjected to various kinds of mechanical and thermal loadings, and cyclic loading in variable thermal environments is a common loading type observed in practical applications. In such cases, the solder interconnects undergo cyclic stresses because of CTE mismatches between the various materials in an electronic package. In the second part of this study, a physics-based crystal plasticity model was used to explain the mesoscale deformation behavior of solder joints under the application of shear stress. A crystal plasticity theory-based subroutine was implemented in ABAQUS finite element (FE) software to predict the effects of the β-Sn crystal orientation on overall deformation behavior of the SAC BGA solder joints. Numerical models were developed for various material orientations of the single grain solder joints. Since the crystal c-axis is the strongest axis in terms of elastic modulus, a number of finite element models were developed and run with varying c-axis orientation in the x-y plane to assess the effects of various slip systems on the deformation patterns.