PRINTED WIRING BOARDS

Ionic migration on printed wiring boards (PWBs) involves the transport of metal ions between two copper tracks under bias through an aqueous electrolyte, which results in deposition at the cathode. Dendrite-like deposits are then formed, leading to a short circuit and subsequent failure. Standardised test 85"C/85%RH (relative humidity) is typically used for modelling and predicting ionic migration failure, however, the possibility of moisture condensation - a prerequisite for ionic migration - at such a relatively high temperature and low relative humidity is unlikely. In order to assess the effects of moisture condensation, this work compares two extreme tests for ionic migration on PWBs. The two tests arc water droplet (WD) and cyclic temperature - relative humidity (20°C to 35"C, 95%RH maximum, with 21 hr cycle) conditions under a bias of 5V DC. The cyclic test was conducted over a 21 day period with continuous in-situ monitoring of dendritic growth. Water drop tests were performed in order to draw correlatjon with cyclic tests. Investigative techniques were conducted to evaluate the migration development on the PWBs after testing using optical microscopy, scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDS).