Wire Bond Qualification Challenges and Development of First Silver (Ag) Alloy Wire on Clip Bond Surface Mount Package

Abstract

Since the acquisition of Fairchild (FSC) in Q4 2016, onsemi has become one of the largest power electronics suppliers in the world. To further strengthen our market competitiveness, a new technology was recently developed for production to meet the customer demand for lower Rsp (Specific on-State Resistance) devices of increased power density. These devices are also assembled in a low-cost package with enhanced thermal performance as reflected in its lower Rdson. The newly developed product is an industry leading MOSFET technology platform tailored to the 40 V motor drive and battery protection market. Its new case outline integrates a new 50 um thick MOSFET die with an optimally designed lead frame and copper clip. To attain the overall solution of low Rdson (Drain-Source on Resistance) and a robust Clip Bond Surface Mount (CBSM) package, silver (Ag) alloy wire was developed for the gate pad connection. The current older technology production devices use 50 um diameter bare copper (Cu) wire as the die gate pad interconnect. However, the reduction in die thickness to 50 um challenges the wire bond process adversely, where chip cracking during the ball bonding process was seen. This is caused by the Cu wire's inherent material hardness and the resultant higher bonding parameter settings required. An alternative cost-effective wire that has a lower hardness than the Cu wire must be qualified to resolve the die cracking issue and achieve the lower cost target. This paper elaborates on the wire bond development and qualification challenges of the silver (Ag) alloy wire and the corresponding improvements executed to address the issues encountered. One of the key achievements was the elimination of the gap or crack between the Bond-Stitch-On-Ball (BSOB) and lead post, observed post TC1000 cycles during the Custom Destructive Physical Analysis (CDPA) cross-section test. A comprehensive root cause analysis and BSOB bond parameter optimization were conducted to resolve the defect. Reliability, and robustness tests, as well as thermal modeling to analyze the thermal-mechanical stress on the Ag wire were conducted. All the studies successfully established this new wire bond capability for onsemi's first Ag alloy wire in a mass production qualified product.