Improvements in Au wire bondability of rigid and flexible substrates using plasma cleaning

Thermosonic Au wire bonding is the most widely used interconnection technology in the BGA packages. Compare with Au ball bonding, the lower wire loop and finer bond pitch of Au wedge bond makes its ideal for low profile and high I/O applications, which is the subject of the current study. As bond pad surface cleanliness, substrate materials and wire bonding process parameters play a critical role on the yield of wire bonding, this study compare Au wedge wire bondability of rigid FR-4 and flexible polyimide (PI) substrate and examine the bonding improvements using plasma cleaning by the concept of process windows. Process windows demonstrate the bondability of the substrates at a combinations of varies bonding conditions, such as bonding temperature, bond power and with and without subjected to plasma cleaning. The plasma cleaning condition is optimized based on the process windows. Results indicated that a low plasma cleaning power with a short cleaning time resulted in the widest process window. Significant improvements of bondability were noted after plasma treatment with a substantial reduction in minimum bonding temperature for both substrates. The process window for the flexible substrate was much wider than the rigid substrates in the bonding conditions tested.