2.0mils Au Wire Wedge Bonding Process Characterization to Eliminate Lift Bond on Accelerometer Device

Au wire thermosonic wedge bonding is applied for die to die interconnect on accelerometer device as traditional Al thermo-compression wedge bonding is not suitable for this application due to the constraint of fine pitch silicon bond pad and fragile bond pad structure of MEMS devices. With thermosonic wedge bonding, a few of lifted (wedge) bonds were reported during end customer application, the lifted bond causes malfunction of the device. Wedge bond failure mode was analyzed and mapped (including IMC - Inter-metallic compound coverage, mold cavity / leadframe index positional variation), there is a relative fixed failure pattern found, eventually, the lifted bond was narrowed down to unit to unit variation within one wedge bond index pitch. Digital modeling on wedge bonding was carried out focusing on wedge bond tooling and leadframe design. The modeling reveals that under the imbalanced leadframe tie bar design, the mismatch of wedge bond heater block cavity depth and leadframe downset fluctuation becomes a key contributor to the inconsistent wedge bond. This mismatch brings gradient of wedge bond deformation thickness, which causes the inconsistent bond. With the identified problem heater block and specially programmed wedge bond parameter, the weak bond was successfully turned-on and turned-off. Based on the modeling result as well as experimental run and statistical analysis, wedge bond tooling was redesigned, leadframe downset tolerance was tightened to eliminate the mismatch. In the same time, enhanced statistical process control on wedge bond quality was implemented in production to provide timely feedback for wedge bond process as well as catch positional variation of the leadframe and wedge bond tooling. The methods employed during wedge bond failure study provide an effective and systematic approach on root cause exploration, the characterized wedge bond process effectively eliminates wedge bond lift bond incident and guarantees the smoothness and health of mass production.