Intermetallic Compounds and Their Effects on the Mechanical Performance of Micro Scale Solder Bonds

Interconnects in microelectronic packages and devices serve as the mechanical and electrical connections as well as thermal paths for heat dissipation. Miniaturization of electronic devices demands very high density interconnects and solder bonds with only a few microns of stand-off height. Although Intermetallic compounds (IMCs) are essential to form a reliable joint, large volume ratios of IMCs can be degrading to long term reliability. At very small joints, the volume of IMCs becomes significant, and in some cases, joints may completely transform to IMCs. Furthermore, small joints experience anisotropy due to the fact that all compositions may only contain a few grains. However, very few studies have been conducted to analyze the effect of the IMCs thickness and anisotropy on the mechanical behavior of solder bonds. In this work, these effects are studied through a combination of experiments and finite element simulations. Traditionally, regular finite element (FE) modeling techniques have been used to simulate the solder joints. However, in order to evaluate joint with only a few grains, a more sophisticated modeling of elastic and plastic behavior of grains is needed through crystal plasticity finite element (CPFE) modeling. In this study, CPFE is used to model all materials including solder, IMCs, and Cu in joints with different IMC thicknesses. Nanoindentation experiment on single grains of IMCs and CPFE simulation of the same were combined to obtain slip system parameters of IMCs that are necessary constants for CPFE modeling. Furthermore, the electron backscatter diffraction (EBSD) analysis was used to determine the preferred grain growth orientation of Cu6Sn5 IMC on polycrystalline Cu substrate. A lap-shear experiment was designed and conducted to investigate the effects of the different volume fraction of IMCs on the shear behavior of micro-scale solder joints with a 50µm stand-off height. The local strain was measured using a micro-scale Digital Image Correlation (DIC) technique. This experiment was used to determine the local and global stress-strain behavior of these joints. The joints were tested to failure, and fractography was conducted to determine the failure modes and failure sites. Comparison between experiment and modeling shows that the CPFE models are successful in capturing the local mechanical behavior of the solder bonds. CPFE models are observed to be more efficient in predicting the local plastic deformation behavior of micro-scale bonds with few grains than the regular FE analysis. Simulation results show that the overall stress distribution and shear deformation changes as the IMC thickness increases. Stiffer response and higher shear yield strength are seen as the IMC thickness increases for both simulation and experiment results. Also, the stress-strain distributions observed in the CPFE analysis performed to mimic the experiment gave a clear idea of the locations of the possible failure sites. Fractography shows failure mode changing from ductile to brittle where crack propagation path is modulated by the different volume fraction of IMCs. A significant influence of Cu3Sn/Cu6Sn5 interfacial morphology on the ultimate shear strength at a higher volume fraction of IMC samples was observed during the lap-shear experiment where planar interfacial morphology promoted lower ultimate shear strength. The effect of different interfacial morphologies on the shear behavior was verified using CPFE simulation. Calculation of the fatigue life indicates that high volume fractions of IMCs could degrade fatigue life of the solder bonds