Re-Evaluating Radiated Emissions and Exposure for Potted Transmitter Housings
Encapsulating transmitters alters dielectric loading, detuning antennas and shifting RF exposure boundaries, demanding re-evaluation of emissions and SAR compliance.
Shrinkage Strain
Meaning
Internal tension develops when a liquid or gel material loses volume during the transition to a solid state. The presence of shrinkage strain is a common challenge in the curing of adhesives and potting compounds. This physical change is caused by the closer packing of molecules as they form chemical bonds.
Dimensional Change
Linear and volumetric measurements track the extent of the contraction during the hardening process. While shrinkage strain is inherent to most thermosetting resins, the magnitude varies with the chemistry of the polymer. Fillers can be added to the resin to reduce the total volume of the reactive material.
This reduces the movement and helps to maintain the tolerances of the assembly. Testing at different temperatures confirms the coefficient of thermal expansion.
Stress Concentration
Pressure on sensitive components like glass diodes can lead to cracks or shifts in electrical values. High levels of shrinkage strain may pull the resin away from the walls of the enclosure and create paths for moisture ingress. Corner geometries often experience the highest forces as the material pulls inward.
Mechanical decoupling through a soft buffer layer can prevent damage to delicate parts.
Material Hardening
Rate of reaction influences how the stress is distributed within the bulk of the part. Fast curing cycles often result in higher shrinkage strain because the material cannot relax as it solidifies. Managing the temperature profile during the bake cycle allows for a more gradual transition.
Final properties are reached once the chemical conversion is complete and the internal forces stabilize.