Measurement of water evaporation rate for popcorning

Popcorning during solder reflow is driven by evaporation of absorbed water into interfacial defect void. Due to lack of data, the steam pressure during popcorning has been assumed as a single value, normally some fraction of the steam saturation pressure at a convenient temperature in popcorning models. A new experimental and analysis methodology to determine the engineering rate of water evaporation from polymer is described in this paper. Using the proposed method, evaporation rate from immersed epoxy is measured. Comparison of the measured rate with a conventional approximation of water evaporation, the conventional approach was found to overestimate the measured rate by nearly two orders of magnitude. To prevent popcorning, the actual evaporation rate can be used in process design as well as material selection of polymer materials.