Sensitivity analysis of Pb free reflow profile parameters toward flip chip on silicon assembly yield, reliability and intermetallic compound characteristics

Flip chip process excels due to its low cost, fine pitch, small form factor and its ready-adaptation to the conventional Surface Mount Technology (SMT) process, in the fact that the reflow is often used to form the solder joint. As the use of Pb free solder is legislated today, it is vital to understand the impact of reflow process conditions on the formation of the flip chip solder joint, so that the assembly process of the flip chip can be better controlled. This paper introduces a comprehensive experimental study on the impact of Pb free reflow profile parameters towards flip chip on silicon assembly solder joint formation characteristics as well as the reliability performance. The reflow parameters studied include the soak time, peak temperature and time above liquidus. Three levels of each reflow parameter are investigated. The Response Surface Methodology (RSM) is used for Design of Experiment (DOE) to explore the quadratic effect of the investigated parameters. Results studied include the package assembly yield, package shear strength, intermetallic compound thickness as well as the package reliability performance. Study results show that the fine pitch flip chip on silicon package has a wide reflow process window to achieve 100% yield, if reflowed in a Nitrogen environment. Yield loss was found when the packages are reflowed in air. With the fifteen reflow profiles studied, it was found that the reflow parameters are not significant in terms of the package shear strength. For the intermetallic compound thickness, it was found that the time above liquidus is a significant factor, with a 99.9% confidence level. No statistical difference was found among packages assembled under different reflow conditions up to 2500 liquid to liquid thermal shock reliability testing.