High-speed shear test for low alpha Sn-1.0%Ag-0.5%Cu (SAC-105) solder ball of sub-100-μm dimension for wafer level packaging

The issue of soft error in microelectronics packaging has necessitated the development of low alpha activity solders, as solders are found to be one of the major sources of radiation in electronic devices that causes soft error. Low alpha ray emitter Sn-1.0Ag-0.5Cu (SAC) solders were prepared and their alpha activity was measured using ultra-low background alpha particle counting systems. The solders are confirmed to be low alpha solders having activity less than 0.005 α h^-1 cm^-2. High-speed shear test is performed to assess the strength of low alpha SAC solder balls of sub-100-μm diameter for different pad finishes. Two types of fluxes, one rosin mildly activated (RMA) and the other water soluble (WS) types are used during soldering to investigate the effect of flux on solder joint strength. Fracture energy has proved to be better index than maximum shear force in interpreting the test results as the former correlates with the fracture mode better. The performance of WS flux is better than RMA flux across all pad finish and speed conditions and among the pad finish electroless nickel electroless palladium immersion gold (ENEPIG) is superior to electroless nickel immersion gold, which in turn is better than organic solder preservative. Clearly the best choice is ENEPIG pad finish with WS flux. Intermetallic compound composition and morphology has been found to have significant effect on strength of solder joint. The shear strength of low alpha SAC105 is found to be lower than the high-Ag containing SAC305 and SAC405 solders. However, the effect of removing radioactive impurities from SAC105 on its shear strength and fracture energy remains inconclusive and needs further investigations.