Evaluation of Electroless Nickel Electroless Palladium (ENEP) finish as a replacement for Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) for Gold Wirebonding and SAC305 Solderability

摘要

In this paper, we compared the wirebonding and solder joint reliability of Electroless Nickel Electroless Palladium (ENEP) and Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) under bump metallurgy (UBM) on Aluminum bond pads formed by redistribution layer (RDL) process. Since the immersion gold bath operates at a higher temperature and the final layer is extremely thin, the risk of corrosion to the underlying pad due to galvanic potential difference and discoloration due to pin holes is always a concern. In that aspect removing the immersion gold layer from the UBM and reducing the finish to ENEP only can mitigate this issue. Both Chip on board (COB) and Flip Chip packages were prepared and side by side comparison is shown for each finish's interaction with gold wire and 96.5%Tin-3%Silver-0.5%Copper (SAC 305) solder. Mechanical (wire pull, ball shear) and high temperature storage testing was performed to determine the reliability of each finish over time. Micro structural study over high temperature aging was also performed using scanning electron microscopy (SEM/EDS) and Transmission electron microscopy (TEM). The intermetallic compound (IMC) formation is very well in agreement with the literature. Overall, results show that ENEP finish has a high potential to replace ENEPIG finish.