Non-destructive monitoring of Au ball bond stress during high-temperature aging

A real-time in situ ball bond stress signal is recorded without destructing the sample, using a piezoresistive integrated CMOS microsensor located next to a test pad on a testchip. The sensor is sensitive to in-plane shear stress tau_xy that arises due to changes of the principal stress components at the test pad. Without the ball bond, the signal remains almost unchanged during 400 h high temperature storage (HTS) at 200degC. With a ball bond at the contact zone, significant stress changes are observed during HTS. For comparison, the contact resistance of the bond was measured with a four-wire method. Two connection paths lead to the test pad, and a second wire was bonded on top of the test ball bond. Constant current was introduced via the first ball bond and the first pad contact, and the voltage drop was sensed using the second ball bond and the second pad contact. The contact resistance values measured at room temperature (25degC) before and after HTS are 2.1 mOmega to 6.1 mOmega, respectively. Effects influencing the microsensor signal during HTS include the temperature coefficient of the signal offset and bond degradation by the growth of intermetallics and cracks. The first effect is accounted for by using the signal from reference pads without ball bond. An increasing stress signal means an increase in tensile stress as caused by the formation of IMCs expanding in volume compared to the base material. The initial two phases of tensile stress growth observed might correspond to IMC growth without the presence of interfacial cracks, resulting in a volume shrinkage. The subsequent phase of signal drop indicates the presence of different mechanisms partly reducing the tensile stress built up before.