Material Interaction
Internal pressure exerted by soft encapsulation compounds on sensitive electronic components results from thermal expansion and chemical shrinkage during the curing process. While gel potting stress is lower than that produced by rigid epoxies, it still impacts the performance of delicate sensors such as MEMS or precision resistors. The material surrounds the component to provide protection against moisture and vibration, but its physical presence creates a mechanical load on the silicon die or wire bonds.
This load can shift the calibration of the device as temperatures fluctuate.
Thermal Coefficient
Mismatch between the expansion rates of the gel and the substrate leads to variable forces as the operating environment changes. Most silicon components have a low coefficient of thermal expansion, whereas the organic polymers used in gels expand significantly more when heated. This difference causes the gel to pull or push against the surface of the chip, inducing strain that appears as an offset drift.
Choosing a material with a lower modulus of elasticity helps to decouple these movements from the active sensing elements.
Curing Shrinkage
Volume reduction during the transition from liquid to solid phase creates a residual tension within the assembly. Even at a constant temperature, this initial gel potting stress can be high enough to fracture fragile wire bonds.
Protective Boundary
Resilience of the gel allows it to absorb external mechanical shocks that would otherwise damage the internal circuitry. It acts as a dampening medium, reducing the acceleration seen by the components when the device is dropped or vibrated. However, this protective layer must be thick enough to provide isolation without becoming so heavy that its own mass creates problems.
The geometry of the potting cavity is designed to distribute the gel potting stress evenly across all surfaces to prevent localized failures.
