Online methods to measure breaking force of bonding wire using a CMOS stress sensor and a proximity sensor

Two real-time, in situ methods to measure the breaking force of fine bonding wires while on the wire bonder are reported and compared. The first method uses a special test chip with a piezoresistive microsensor integrated next to the bonding pad. A 25μm diameter Au wire piece is attached with a ball bond to the test pad of the microsensor. The wire piece between the ball bond and the lower edge of the wire clamps is 15mm in length. The clamps tear the wire at a speed of 2mm/s. The wire breaks at the heat-affected zone (HAZ) next to the ball bond. The microsensor is calibrated using FE models. The numerical results show that the microsensor signal is highly sensitive to ball and pad geometry, values of the piezoresistive coefficients, and the z-location of the microsensor under the bonding pad. This results in a high estimated error of about 46% for the calibration factor of the microsensor.The second method uses a proximity sensor attached to the wire clamp of the bonding machine for which an accurate calibration is available. The proximity sensor is calibrated by hanging a weight of 22.2g to the wire clamp. The average breaking force at the HAZ is measured to be 98.6±1.67mN. This value is approximately 77% of the breaking load of the non-heat affected wire as measured with a standard tensile tester. Using the proximity sensor method, an accurate calibration of the microsensor is found. The signal precisions of the microsensor and the proximity sensor are found to be approximately 1%. These methods are ideally suited for the automatic measurement of tail breaking force (TBF) as observed in thermosonic wire bonding processes.