Evaluating the Effect of Conformal Coatings in Reducing the Rate of Conductive Anodic Filament Formation

Abstract

Abstract : Conductive anodic filament (CAF) formation is a failure mode in printed wiring boards (PWB) which occurs when the circuit is operated under a high voltage gradient. Humidity also plays a factor since the boards must be stored, or operated in a humid environment. This failure mode is enhanced by certain water soluble fluxes or hot air solder leveling (HASL) fluids. The objective of this research was to examine the effect of three different conformal coatings in reducing the incidence of CAF associated with a variety of water-soluble flux formulations. To achieve this, a series of fluxes were evaluated for their propensity to enhance CAF. Several of these were chosen for further examination with acrylic, silicone and parylene C conformal coatings. Of the three conformal coatings tested only parylene C was effective in preventing CAF and dendrite formation in all cases. Silicone coatings showed bubbles present on some of the samples aged at 85 deg C for the 28-day test. Localized debonding occurred on some of the acrylic and the parylene C boards. Also, some boards exhibited localized debonding of epoxy/glass after during aging of the coating, suggested chemical interactions with flux residues creating mechanical stresses. One set of boards was subjected to higher processing temperatures similar to those that would be associated with lead-free soldering. When compared to boards processed with the same fluxes using the lower processing temperatures, the high temperature boards showed significant increase in CAF formation.