Fickian Diffusion Kinetics and Desorption Times for Molded Packages
Fickian desorption calculations use temperature-dependent diffusivity to determine exact package bake times, preventing reflow popcorning and structural delamination.
Moisture Concentration Gradient
Distribution Profile
Spatial variation in moisture concentration within a material layer determines the direction and rate of water vapor diffusion. A steep moisture concentration gradient exists when the outer surface of a polymer package is exposed to humid air while the interior remains dry. This gradient acts as the driving force for vapor diffusion, forcing moisture to migrate inward until equilibrium is reached.
Ingress Dynamic
Sorption begins at the exposed boundaries of the package and progresses inward over time. As diffusion occurs, the moisture concentration gradient becomes shallower as the internal polymer regions absorb water and approach saturation. Finite element models track these gradient shifts to estimate the time required for moisture to reach critical internal interfaces, such as the die surface or lead frame bonds.
Understanding these dynamic profiles helps in determining when a component’s internal moisture has reached a hazardous level that requires baking.
Desiccation Path
Baking reverses the direction of the concentration slope to drive moisture out of the package. The moisture concentration gradient then runs from the wet center to the dry exterior of the component, allowing desiccation to occur. Controlling this profile through managed thermal cycles ensures that moisture is extracted efficiently without inducing thermal or physical damage to the delicate package structure.