Optimization of Ultrasound and Bond Force to Reduce Pad Stress in Thermosonic Cu Ball Bonding

Ball bonding processes are optimized on Al pads with a 25.4 µm diameter Cu wire to obtain maximum average shear strengths of at least 120 MPa. To quantify the direct effect of bond force and ultrasound on the pad stress, ball bonding is performed on test pads with piezoresistive microsensors integrated next to the pad and the real-time ultrasonic signals are measured. By using a lower value of bond force combined with reduced ultrasound level, the pad stress can be reduced by 30%. An ultrasound/bond force process window for low-stress copper ball bonding is determined, which shows that a proper optimization of ultrasound and bond force leads to Cu ball bonds of high quality while transmitting lower stress to the pad during the bonding process.