Conductive Anodic Filament (CAF) Formation Part I: the Influence of Water-Soluble Flux on Its Formation

Conductive anodic filament (CAF) is a failure mode in printed wiring boards (PWBs) which occurs under high humidity and high voltage gradient conditions. The filament, a copper salt, grows from anode to cathode along the epoxy-glass interface. Ready and Turbini identified this copper salt as Cu-2(OH)(3)Cl, atacamite. In the present work polyethylene glycol (PEG) and polyethylene propylene glycol (PEPG) fluxes were used to process hole-to-hole test coupons. The coupons were exposed to temperature, humidity, and bias testing of 85A degrees C, 85% relative humidity (RH), and 200 V bias for 28 days. The aged coupons were then cross-sectioned, and scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) revealed that, in addition to CAF formation at the epoxy-glass interface, a copper-chloride-containing compound was present in the polymer matrix for coupons processed with PEPG. This compound was characterized using x-ray photoelectron spectroscopy (XPS) as CuCl. Coupons processed with PEG created CAF only at the epoxy-glass interface. Based on previous publications on copper electrodeposition in the presence of PEG, it appears that a PEG-Cu-Cl complex forms, binding the available Cu and acting as a barrier to the formation of CuCl in the polymer matrix.