Characterization of the elasto-plastic behavior of actual tin silver copper solder joints using DIC

The solder joint reliability under thermal and mechanical loading is a great concern in electronic packaging. Reliability tests, such as thermal cycling and drop tests, are common methods for the reliability assessment. Modeling is also a powerful method which can provide insight into the thermal mechanical behaviors of the BGA packages. Since solder alloys deform plastically at high stress, the elasto-plastic properties of the solder alloys are needed to support the modeling. Therefore, it is important to characterize the elasto-plastic properties accurately in order to achieve a reliable drop test model. This dissertation presents the characterization of the elastoplastic behavior of actual Sn-1.0wt.%Ag-0.5wt.%Cu (SAC105), Sn-3.0wt.%Ag-0.5wt.%Cu (SAC305) and Sn-4.0wt.%Ag-0.5wt.%Cu (SAC405) solder joints. Several actual ChipArrayRTM BGA (CABGA) packages were cross-sectioned, polished and used as the test vehicles. Several testers were specially designed and fabricated for the characterization of the material properties using the customized test vehicle. Compressive and drop tests were performed to generate elastic and inelastic deformations, respectively, in the solder joints. Images of the cross-sectioned solder joints were recorded using microscope during the tests. The recorded images were then processed by using a digital image correlation (DIC) program to calculate the displacement and strain fields on the solder joints. Finite element method (FEM) modeling along with an iteration process was used to extract the material properties. The values of the Poisson's ratio, Young's modulus, coefficient of thermal expansion (CTE), and coefficients of the Ramberg-Osgood model for the solder alloys were obtained over the temperature range from 25°C to 105°C. The methodology developed in this work enables to characterize the elasto-plastic behavior of the actual solder joints with high accuracy, where other existing methods usually fail. The developed elasto-plastic model is recommended to be used for drop test modeling of electronic packaging.