CREEP CORROSION ON LEAD-FREE PRINTED CIRCUIT BOARDS IN HIGH SULFUR ENVIRONMENTS
Authors: Randy Schueller, Ph.D. Company: Dell Inc. Date Published: 10/11/2007
Abstract: The significant and rapid changes required to eliminate lead from electronics in the short period of time required by Restriction of Hazardous Substance (RoHS) legislation was likely to result in new quality and reliability issues. The industry had been using the same materials for over 50 years with much data and experience to guide in the creation of design rules, manufacturing processes, appropriate reliability test methods, and in predicting failure mechanisms. Changing the solder alloy (and its material properties), the fluxes, termination plating materials, PCB surface finishes and soldering temperatures in a span of 1-2 years was a high risk undertaking; many in the industry predicted dire consequences. Indeed many new failure mechanisms were detected in Dell’s reliability testing and actions were taken to prevent these failures from occurring in the field. Product quality data shows this was an overall success. However, one new failure mechanism was not foreseen by Dell or the industry. Immersion silver (ImAg) was widely adopted to replace hot air solder level (HASL) as the surface finish on PCBs. ImAg was known to tarnish but it was a surprise to find that it suffered creep corrosion when exposed to high sulfur and humid environments. Failures could occur in as little as a few weeks in industries such as rubber manufacturing, water treatment, paper mills or fertilizer production (to name a few). The failure mechanism has been well characterized. Acceptable surface finishes and design rules to mitigate the risk of this failure mechanism are identified and discussed. Finally, some new corrosion test methods currently under development, and their effectiveness, are presented.