An investigation on Cu Wire Bond Corrosion and Mitigation Technique for Automotive Reliability

The increase of gold price had pushed industrial to develop copper wire in order to stay competitive. However, copper is not precious material like gold, there are numerous challenges in bonding & reliability risk associated with stringent automotive reliability requirements. Corrosion associated with copper wire bonding is considered one of the hard to solve reliability risk as the occurrence is in very low ppm and there was no specific pattern observed on the corroded bondpad location. This paper is focusing on the copper wire interconnect with Aluminum pad corrosion in humidity test including autoclave and temperature humidity bias test. The investigation begins with migration from gold wire to copper wire in automotive products for TSSOP. High temperature storage and Temperature cycling are commonly used in wire change qualification. The failure arise when the package qualification proceed with humidity test. A very low ppm corrosion observed in package qualification with the presence of chlorine. Traditionally mould compound is the first suspect of chlorine contamination source. However, investigation showed that mould compound A is considered the most copper wire friendly mould compound across the production platform (CI<10ppm). In further verification with mould compound supplier, mould compound A is modified to achieve for both aluminum and copper passivity/ immunity state toward corrosion based on Pourbaix diagram. The effect of modified mould compound A is insignificant as similar ppm level of corrosion still being detected. The investigation moves on to take away mould compound from the AC test that using un-moulded unit to check corrosion in shorter AC test duration. The method is found to be corresponding with moulded AC 96 hours result. In addition, un-moulded units with gold wire showed no corrosion. The investigation is then narrow down to differences between gold wire and copper wire bonding process. A thorough process mapping is done and found there are two prominent process differences between gold and copper wire bonding. The two processes are plasma prior to copper bonding and forming gas used in copper wire process. From these two processes, the un-moulded units gone through a surface analysis to check chlorine concentration. After plasma process, chlorine is found to be extremely high. Further study on plasma process is carried out in order to mitigate the chlorine contamination. The result showed that present plasma process is suspected to cause re-deposition of chlorine in plasma chamber and subsequently deposited again onto products. In order to minimize this effect, new plasma concept has been evaluated. The result from difference plasma concept shows significant reduction in corrosion rate during AC short duration test. As a conclusion, mould compound can be rule out as the source of chlorine contamination on aluminum bondpad. Secondly, plasma prior to copper wire bonding process has been the primary and most prominent factor for the aluminum bondpad corrosion. New plasma concept is able to reduce corrosion rate in AC short duration test. This is evidence with the surface analysis result as the chlorine contamination is reduced significantly after this new plasma concept is introduced.