What is Causing Oxidation?

Pierce Pillon is the Laboratory Manager and lead formulations chemist at Techspray, a division of Illinois Tool Works (ITW) and a leading manufacturer of chemical products for the electronics industry.

Harold Hyman has been involved in metallurgical aspects of the electronics industry since the 1950's, and in semiconductor development and engineering for STL, Ediswan & RCA. He later joined HTC, a pioneer of vapor phase soldering and continued industry experience Dynapert, GenRad, Teradyne, SRT and VJ Electronics.

Jim Willimas is a PhD Chemist in Polymers and Materials Science. He specialize in printing, cleaning, inks, and coatings used in electronics manufacturng operations. Williams has more than 30 years experience.

Bryan Kerr has 35 years experience in providing technical support to PEC assembly manufacturing. His experience ranges from analysis of materials and components to troubleshooting and optimizing, selecting reflow, cleaning, coating and other associated processes.

Phil Kinner - Electrolube - Global Business Director conformal coatings division.

Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

While in the Navy as a computer tech we saw a blacking of silver soldered components in out Magnetic Tape Unit. After extensive lab analysis it was found to be silver sulfide. The source was the closed atmosphere in a submarine where we vented sanitary tanks into the ship through charcoal filters. This enriched the atmosphere with hydrogen sulfide. The cause of the problem was ATC capacitors that had leads silver soldered onto surface mount parts to meet a contract. They did not proper clean the silver bearing flux from the capacitors. The result was an additional plate was added to the capacitor, in a critical timing circuit, by the migration of silver sulfide across the faces of the capacitor. The cure was a replacement of all ATC capacitors in this equipment with epoxy sealed capacitors of another vendor. BTW this problem was never seen at a shore facility, only onboard the submarine. An interesting problem to troubleshoot.

As a number of responses have indicated the black material is silver sulfide and indicates that the environment has a source of sulfur which is most likely from the air. I know that one instance I was involved in we found that the area had metal smelters processing sulfide ores. the problem you have is that silver really likes to react with sulfur and so I doubt that even putting a conformal coating will prevent blacking in the long run. If possible you will probably need to go to a nickel plating instead of the silver.

We have seen this issue when the board was in an enclosure with a lead-acid battery. Hydrogen sulfide is emitted in the charging process. The customer thought it was a process issue, but it turn out their end users were replacing the sealed batteries provided with cheaper vented batteries

Working with Electronics Industry in Mexicali we saw thick film silver tarnish within hours. Mexicali sits on hot springs & volcanic "belt", "burping" out sulphur compounds. Attached link has good explanation of Ag Tarnish http://cdn.intechopen.com/pdfs-wm/37975.pdf

Look internally on the part - We have experienced similar issues on LED circuit boards - Silver on the dies were attacked and contaminated by sulfur. We had found an internal seal that used a sulfur curing process for the rubber.

I agree the source of the discoloration is sulphur. Another source of sulphur contamination is the food preparation of eggs or fish nearby in a enclosed environment like a building.