2N Au wire bonding for ultra fine picth BGA

Abstract

In this study, ultra fine pitch wire bonding on a BGA device with Al bond pad was performed to compare 2N wire with conventional 3N and 4N wires for reliability performance after thermal aging. Strip level thermal aging at 175 degree C was carried out. Wire pull test was performed after different thermal aging read point of 100hrs, 250hrs, 500hrs, 750hrs, 1000hrs, 1250hrs, 1500hrs, 1750hrs and 2000hrs to check for pull strength and lifted ball failure mode. The result had proven that 2N wire demonstrated more superior thermal aging reliability performance as compared to 3N and 4N wire. In the 2nd portion of this study, 2N wire was successfully implemented in mass production. However, several challenges were encountered due to the fact that 2N wire is much harder than conventional 3N and 4N wire which resulted in narrower process window and poorer manufacturability. This study is also aimed to share the major problems encountered and the methods to overcome those challenges during high volume manufacturing. First major challenge was lower 2nd bond peel strength due to higher sensitivity towards 2nd bond surface condition. It was found that substrate bond finger surface roughness variation is a critical factor to 2N wire bonding process. This can be improved through substrate manufacturing process optimization to reduce bond fingers surface roughness. At the same time, less optimized bonding input parameters was found to induce low wire peel strength while certain bonding input parameters was found to cause higher missing ball and short tail problem. Hence bonding parameters optimization has to be done carefully through thorough DOE and RSM studies in order to achieve higher peel strength and improve process robustness against 2nd bond surface variation. The 2nd major challenge was off-bond-pad due to harder wire that resulted in higher aluminum squeeze-out upon bonding on aluminum bond pad. It was found that proper process characterization to determine the optimum length of time needed for device expansion on heater block plus further improvement on the wire bonder and bonding parameters could help resolve this issue. The 3rd problem was higher wire stickiness due to higher sensitivity towards moisture from the environment. The main impact of wire stickiness issue was found to be higher machine stoppages due to inconsistent tail, hence resulted in off-center-ball defect. 2nd impact was higher capillary clogging which caused wire sagging and wire loop collapsing. Such wire stickiness problem could be overcome by using different wire quenching solution. In summary, 2N wire is able to improve thermal aging performance of ultra fine pitch wire bonding to meet reliability requirement as stringent as for automotive application. However, to improve mass production friendliness, careful characterization and process optimization need to be done on several areas, namely bonding surface condition, wire bonding process, as well as wire manufacturing process.