Journal of SMT Article


Authors: Randy Schueller, Ph.D.
Company: Dell, Inc.
Date Published: 1/31/2008   Volume: 21-1

Abstract: The material and process changes required to eliminate lead from electronics as required by Restriction of Hazardous Substance (RoHS) legislation was likely to result in new quality and reliability issues. The industry had over 50 years experience with tin-lead solders. Changing to different solder alloys, fluxes, termination plating materials, PCB surface finishes and soldering temperatures in a span of 1-2 years was a high risk undertaking. Although many potential issues were uncovered and resolved, one new failure mechanism was not foreseen by 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 when exposed to sulfur, but it was a surprise to find that it suffers extensive creep corrosion when the sulfur and humidity levels are high enough. Failures can occur in as little as a few weeks in industries such as rubber manufacturing, water treatment, paper mills or fertilizer production, among others. This paper uncovers the root cause of the creep corrosion mechanism and shows how to eliminate it. Corrosive environments were measured and the results of field testing of various surface finishes in these environments is shared. Also presented are some more effective corrosion test methods currently under development.

Keywords: immersion silver, creep, corrosion, Pb-free, corrosion testing.

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