Temperature characterization of flip-chip packaged piezoresistive barometric pressure sensors

In order to miniaturize piezoresistive barometric pressure sensors, a new flip-chip packaging technology has been developed. The thermal expansions of chip and package are different. So in a standard flip-chip package the strong mechanical coupling by the solder bumps would lead to stress in the sensor chip, which is unacceptable for piezoresistive pressure sensors. To solve this problem, in the new packaging technology the chip is flip-chip bonded on compliant springs to decouple chip and package. As the first step of the packaging process an under bump metallization (UBM) is patterned on the sensor wafer. Then solder bumps are printed. After wafer-dicing the chips are flip-chip bonded on copper springs within a ceramic cavity housing. Due to the compliance of the springs, packaging stress is induced into the sensor chip. As sources of residual stress the UBM and the solder bumps on the sensor chip were identified. Different coefficients of thermal expansion of the silicon chip, the UBM and the solder lead to plastic straining of the aluminum metallization between UBM and chip. As a consequence the measurement accuracy is limited by a temperature hysteresis. The influence of the chip geometry, e.g., the thickness of the chip or the depth of the cavity, on the hysteresis was investigated by simulation and measurements. As a result of this investigation a sensor chip was designed with very low residual stress and a temperature hysteresis which is only slightly larger than the noise of the sensor.