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 Absorption Rate
Kinetic Coefficient
The mass of water vapor entering a material per unit of time indicates the vulnerability of a component to environmental humidity. A high moisture absorption rate leads to rapid changes in mechanical properties and dielectric constants in electronic substrates. Measurements are typically taken by weighing samples at regular intervals under controlled temperature and humidity conditions.
Diffusion Path
Water molecules travel from the surface into the bulk material through microscopic gaps and intermolecular spaces. The moisture absorption rate depends on the porosity of the material and the chemical affinity between the polymer chains and water.
Ambient Influence
Elevated temperatures increase the kinetic energy of molecules and accelerate the intake process. A moisture absorption rate can double for every ten degree rise in temperature according to the Arrhenius relationship (which governs many chemical and physical reactions).
Material Stability
Polymers and resins used in packaging must maintain low levels of water intake to prevent internal pressure buildup during soldering. If the moisture absorption rate is too high the material may suffer from delamination or internal cracking when the trapped water expands into steam. This phenomenon is known as the popcorn effect and is a major cause of failure during surface mount assembly.