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

摘要

Conductive anodic filament (CAF) formation is a failure mode associated with electronic circuits which operate at high voltage gradients and which are stored under high humidity conditions. Certain soldering fluxes and hot air solder leveling (HASL) fluids enhance this failure mechanism. Research was conducted to examine the effect of three different conformal coatings in reducing the incidence of CAF associated with a variety of water-soluble flux formulations. The fluxes chosen contained a water-soluble vehicle as 20 wt% in isopropanol. Some formulations contained 2wt% HCl, HBr, and/or monoethanolamine. The conformal coatings tested were acrylic (Humiseal 1B73), silicone (Humiseal 1C55) and parylene C. IPC B-24 test boards were coated with the flux, reflowed to create the thermal cycle, and then cleaned. Some boards were conformally coated and others were not. All test boards were exposed to SIR testing at 85°C, 85% RH and 100V bias for 28 days. This paper will report on the electrical results, as well as the number of CAF observed with and without conformal coating. It will also discuss visual observations of dendrites on boards with/without conformal coating.