Sulfur Corrosion of Printed Circuit Board Surface Finishes in Three Different Sulfur-Rich Environments
Authors: Matthew D. Weeks, Ph.D., David J. Zueck Company: Western Digital Technologies, Inc. Date Published: 9/27/2015
Abstract: An array of conventional printed circuit board (PCB) surface finishes were tested in three different corrosive environments: (Test 1) conventional ISA/ASTM Class III mixed flowing gas (MFG) test, (Test 2) high hydrogen sulfide MFG test, and (Test 3) Chavant clay test with condensing humidity. Finish performance was found to be strongly dependent on environmental variables such as chemical air composition, temperature, humidity, and laminar air flow rates. The results of this study suggest that chlorine gas plays an important role in the degradation of noble metal finishes like immersion silver (ImAg) and electroless nickel immersion gold (ENIG) and may exacerbate lateral creep corrosion development of unprotected copper areas in a non-condensing MFG environment. Condensing humidity in the absence of chlorine (Test 3) favored oxidation over sulfidation of copper, nickel, and silver, with copper, unsurprisingly, being the most susceptible to the oxidation process. In the case of the Chavant clay test, corrosion deposition on solder mask regions adjacent to metal surfaces is attributed to redeposition of the product from the liquid upon precipitation of the insoluble corrosion product.
Different surface finishes were found to exhibit different forms of corrosion, which result in fundamentally different corrosion outcomes. Corrosion of ENIG proceeded by stress-corrosion cracking (SCC) of the nickel phosphorous plating, which was followed by pitting/crevice corrosion of the underlying copper pad in the vicinity of compromised NiP regions. ImAg finishes corroded by a combination of crevice and galvanic mechanisms, primarily in solder mask defined (SMD) pad regions and those directly underneath compromised ImAg plating. Corrosion of PCB finished with organic solderability preservative (OSP) proceeded by way of a uniform/pitting corrosion mechanism.