Water solubility and diffusivity in BCB resins used in microelectronic packaging and sensor applications

This paper presents a simple measurement method to estimate water solubility and diffusivity in dielectric polymers which have low sorption rates and high diffusivities. The polymer dielectric is sandwiched between electrodes to create a specific capacitive structure. It is then exposed to step-like humid air stimuli. The resulting transient capacitance increase is recorded and the sorption rate and the diffusivity of water in the polymer are numerically extracted from experiments. The extraction model is derived from the theory of gas diffusion in polymer coatings. After experimental validation on Cyclotene 4024 processed with hard cure, the method is used to compare the impact of different cure methods on the solubility and the diffusivity of this material and of Aqueous Developable PhotoBCB. Hence good candidates are determined for microelectronic, packaging and humidity sensor applications.