Sorption Physics
Kinetic transport mechanisms define the rate at which water molecules penetrate polymeric packaging materials. Testing identifies the solubility of water within the polymer matrix as a function of temperature and pressure. Fickian moisture diffusion assumes that the flow of water follows a concentration gradient from the high humidity exterior to the dry interior of the package.
The process continues until the component reaches saturation at the ambient temperature. Engineers use these laws to predict how long a part can sit on a factory floor before it becomes unsafe to reflow.
Rate Determinant
Material properties like the diffusion coefficient dictate how fast the moisture moves through the epoxy molding compound. Fickian moisture diffusion happens more rapidly as temperatures increase because the thermal energy excites the water molecules. Small packages reach their critical moisture level much faster than large ones due to the shorter distance to the die surface.
Saturation Profile
Steady state conditions occur when the internal concentration matches the external environmental humidity. Testing for moisture sensitivity involves weighing components at specific intervals to map the fickian moisture diffusion curve. Once the weight gain stops, the part is considered fully saturated.
Storage in a dry cabinet reverses this gradient and slowly removes the water.
