Diffusion Coefficient
Physical models describe the rate at which atoms or molecules move through a solid or liquid medium based on concentration gradients. In electronics, fickian kinetics govern the absorption of moisture into plastic packages and the diffusion of dopants in silicon. The relationship states that the flux is proportional to the steepness of the concentration change.
Concentration Gradient
Movement always occurs from areas of high density to areas of low density until equilibrium is reached. If the external humidity is high, water vapor enters the package until the internal concentration matches the environment. The geometry of the component dictates the path length and the time required for this process.
Saturation Limit
Every material has a maximum capacity for a secondary substance at a specific pressure and temperature. Reaching this limit stops the net flux even if a concentration difference remains. This boundary is critical for determining the storage requirements of moisture sensitive components.
Mass Transport
A material property known as the diffusivity determines how quickly a substance spreads at a given temperature. This value follows an Arrhenius relationship where the rate increases exponentially as heat is applied. Engineers use these constants to calculate how long a component can remain in a humid environment before saturation.
The total mass of absorbed water follows a square root of time progression in most polymers.
