1. Purpose and scope
Most chip packages keep the environment out. A sensor package has to let part of it in. A microphone needs an acoustic port and a back volume of the right size, a pressure sensor needs a path for pressure while being protected from water and particles, often with a gel or membrane, a humidity or gas sensor needs an open cavity with a filter, a light or image sensor needs an optical window, and an inertial sensor needs a sealed cavity that keeps out moisture while transferring motion faithfully.
At the same time the package must not disturb the measurement. Die attach, mould compound, lids and the customer's board all put stress on the MEMS die, and that stress changes offset and sensitivity, often with temperature and over time as materials creep or absorb moisture. Stress isolation, soft die attach, pedestals or isolation structures in the MEMS die are chosen here using thermo-mechanical models correlated with prototype assemblies. The package also sets the footprint customers design their boards around, so its freeze is a commitment to the market as well as to engineering.
In scope: the package design, the port, seal or window, die attach and stress isolation, the package stress sensitivity study, the assembly process flow, early reliability results on prototypes and materials that support environmental and materials compliance. Out of scope: qualification builds and their stresses, which run in Reliability & Qualification.
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