Electroless Nickel and Immersion Gold deposition on Single Chips for Flip Chip Assembly of Pixel Detectors

Single chip handling has received significant attention for the diced chip without an Under Bump Metallization (UBM). In many cases, the Electroless Nickel Immersion Gold (ENIG) process is a suitable method to deposit UBM layer. Especially creating a solderable surface for the subsequent Flip Chip Bonding (FCB) process is attractive for pixel detector applications. Thus, ENIG process development was set as a priority motivation in this study. The test chips with Al bump pads (Al, Si -2 %, Cu -1 %) 20 µm in diameter and pitch of 110 µm were prepared. The ENIG UBM layer was deposited on the test chips. The ENIG process encompasses four key steps: surface activation, double-zincation, Electroless Nickel (EN) deposition and Immersion Gold (IG) deposition. The Ar plasma surface activation was found to be a key for successful double-zincation process. In the current investigation, the EN deposition that employs both sodium hypophosphite acidic and alkaline bath solutions has been used. After more than 5 µm of EN was deposition, a 60 nm IG layer was deposited to improve the wettability of the bond pads for the FCB process. In order to study the FCB behaviors, ENIG UBM and eutectic PbSn solder bumps were tack bonded using a thermocompression cycle followed by assembly reflow. Not only the Scanning Electron Microscope (SEM) image with energydispersive x-ray (EDX) analysis showed that the solder bump connections had been achieved but also the electrical measurement confirmed electrically functional connections. Also, the first trial of the Reverse Re-Work (RRW) process was successfully demonstrated.