Piezoresistive accelerometers for MCM-package - Part II:The packaging

This paper is the second part of a twofold work related to the full integration of piezoresistive accelerometers in a multichip modules-deposited (MCM-D) type with flip-chip interconnection package. This part is fully devoted to the packaging of an accelerometer with movable parts in an MCM so a description of the package fabrication is given. The accelerometer was designed taking into account its final package and, therefore, some modifications to the standard piezoresistive accelerometer were done. In particular, a sealing ring was defined around the movable parts of the sensor in order to protect them from the underfill used during the final packaging process. This underfill is being studied at the moment so the tests performed have been without taking into account its effects. After the manufacturing step the MCM packaged accelerometers are attached to a PLCC44 substrate and characterized. The results show an improvement on the sensors performance when using the MCM solution in comparison to the accelerometers packaged in standard TO-8. The results show that the influence of the packaging technique on the final stress of the sensors is negligible. They also show an extra damping due to the confined air volume around the swinging proof mass of the accelerometers. The quality factor of the TO-8 packaged accelerometers is of 12 whereas for the MCM packaged devices is of 1, 2. This low-quality factor is of great interest for applications where the devices can suffer large acceleration changes. Finally, the results are in good agreement with the FEM simulations previously done and they demonstrate the viability of this technology.