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

摘要

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 Cu₂(OH)₃Cl, 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 85°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. Keywords Printed wiring board - solder flux - conductive anodic filament