Influence of moisture variation on SAC405 solder ball oxidation characteristic in WLCSP

Solder bump oxidation has been one of the concerns of poor wettability during FC bonding or SMT process. It is known that high temperature and/or high humidity environment will continuously accelerate solder bump oxidation over long storage/staging time. The result of solder bump oxidation is discoloration which can be observed via naked eye or by optical microscope. In order to understand the severity of bump discoloration, various level of discolored bump samples were sent for surface analysis to measure the solder oxidation thickness. These samples were subjected to XPS for oxide depth profiling. This analysis is trying to create a bump oxidation severity table based on the degree of bump discoloration. By referring to bump oxidation severity table, a study was conducted to analyze the main factors that contribute to discolored bump. In this study, factors such as numbers of wafer in FOSB, moisture-barrier bag (MBB) packing method and wafer drying time were investigated. Due to the fact that bump oxidation was never found at bumping line before MBB packing, bump oxidation is suspected to occur during transportation and/or during storage at warehouse. Our experiment result indicates that the moisture trapped inside wafer re-passivation material can be easily released and condensed inside the FOSB container under low temperature condition. The condensed moisture inside FOSB will accelerate bump oxidation process even after the environment is returned to room temperature.