Water sorption in epoxy thin films

Moisture uptake and desorption experiments performed on both thick (order of millimeters) and thin (order of 10 μm) free films of epoxies are presented. Supported thin films were prepared by spin coating and curing epoxies directly on cleaned glass slides. These latter samples were tested in a similar manner. Distinctive nonFickian behavior was observed. Equilibrium uptake is achieved via a two-stage process, and in certain supported films, weight gain overshoots are indicated. Diffusivities of water for sorption runs are usually greater than those for desorption, indicating matrix plasticization. However, diffusion coefficients are of the same order of magnitude for both free and supported films. Once sufficient water reaches the interface, sudden catastrophic delamination can occur. This event is always accompanied by a rapid weight gain immediately preceding film release from the substrate. The time required for debonding to occur decreases with increasing film thickness, strongly indicating the major role of thermoelastic stress in promoting replacement of glass/epoxy bonds by glass/water and epoxy/water interactions. Detailed analyses of the effects of film thickness and postcure thermal history on water ingression are presented. The relevance of this work to microelectronics encapsulation is elucidated