Interfacial Hygroscopic Swelling Stress Analysis in Surface Mount Encapsulants
Interfacial hygroscopic swelling stress arises from differential moisture expansion, compounding thermal mismatch and driving package delamination during reflow.
Moisture Vapor Drive
Pressure Potential
The difference in partial pressure of water vapor across a membrane creates a force that pushes moisture through the material. A strong moisture vapor drive exists when a high humidity environment is separated from a dry interior by a thin wall. This force is the primary reason why hermetic seals are required for long term reliability in harsh conditions.
It represents a physical pressure measured in pascals that actively drives molecules into the enclosure.
Thermal Gradient
Temperature differences between the inside and outside of an enclosure significantly alter the local vapor pressure. Even if the relative humidity is the same the moisture vapor drive will push water toward the colder side if a large temperature gap exists.
Barrier Selection
Materials are chosen based on their ability to resist the force of incoming gas molecules. The effectiveness of a shield against moisture vapor drive is measured in terms of grams per square meter per day under specific test conditions.
Corrosion Risk
Internal condensation occurs when the drive pushes enough water into a space to reach the dew point. Managing the moisture vapor drive is essential for preventing the liquid water formation that leads to electrical shorts or metal oxidation.