Multi Chip Module (MCM) Design for Packaging of a MEMS Pressure Sensor

Abstract

This research describes design and development of an MCM MEMS pressure sensor package. The low profile (0-50 psia) packaged module contained the silicon MEMS pressure sensor die designed based upon piezoresistive principle and had footprint of 2.3 mm times 2.3 mm with a thickness of 0.5 mm. The sensor was integrated with an application specification integrated circuit (ASIC) chip (MAX 1452trade from MAXIMreg) in an MCM configuration designed on a 75 mum polymer substrate (Upilextrade). The flex substrate was first passivated with 4 mum layer of BCB followed by Ti-Cu-Ti sputtering and selective electroplating of Ni and Au to define the required conductor pattern. The electrical connections from the sensor and ASIC chips to the flex circuitry were performed by thermosonic wire bonding. The wire bonds were secured in place by a glob top encapsulant. Ribbon cables having conductor size of 36 AWG served as input-output (I/O) connections from the package to the outside environment. The I/O cables were solder attached on to the flex circuitry and eventually encapsulated with an electrically non-conducting epoxy, thereby ensuring a strong mechanical joint. The cable attached flex circuitry loaded with sensor and ASIC dies were attached to a custom machined metal base using a suitable epoxy. Finally, the whole package was adhesively capped using a meshed lid. The final package had an outside diameter of 8 mm and a total height of 1 mm. This package can be implemented in a range of applications from aerospace to biomedical environments. Reliability tests (thermal shock and temperature cycling) were performed on the packaged devices. The stressed sensor package revealed excellent robustness with no signs of physical damage. As part of initial qualification on the packaged module to test its electrical functionality characteristics, units packaged with only the 'sensor' device were subjected to controlled pressure conditions. The output result showed perfect linear response of voltage in accordance to change in pressure. These initial tests underlined the feasibility of an MCM approach for integrating a sensor with an ASIC chip for low profile package designs.