Measurement of water evaporation rate from epoxy

Epoxy based materials are used as encapsulants to enhance the reliability of electronic packages. Epoxy is hygroscopic in nature and absorbs water when exposed to humidity. During solder reflow, the absorbed moisture in the package can generate steam pressure at interfacial defect voids and drive interfacial delamination, leading to popcorn failure of the package. Due to the lack of data, the steam pressure in the defect void has been assumed to be a single valued parameter, normally some fraction of the steam saturation pressure at a convenient temperature in popcorning models. A new experimental and analytical methodology to determine the engineering rate of water evaporation from polymer is described in this paper. The evaporation rate from water saturated epoxy is measured by thermogravimetric analysis (TGA). The experimental results indicate that the conventional estimate of water evaporation was found to overestimate the measured rate by nearly two orders of magnitude. To prevent steam driven delamination and popcorning in electronic packaging, the actual evaporation rate should be used in process design as well as polymer material selection.