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May 3, 2018

What is Causing Oxidation?

After 2-3 months of service in the field we are noticing the silver connectors on our circuit board assemblies have turned black, and appear to have heavy oxidation. We believe the new ROHS standards for our circuit boards, in combination with Barium (present globally in our city air), may be causing this problem. Is anyone familiar with this problem?

V. N.

Experts Comments

You didn't state what type of service the assemblies are in. I assume that they are not hermitically sealed.

In industrial areas, sulfur components (commonly hydrogen sulfide) are the main culprits. Look for sources of H2S -- high auto traffic, proximity to manufacturing emission stacks, and (here in Texas) gas well or acid gas flares -- all common sources of heavy sulfur contaminants.

As for the barium question, I think it would necessarily need to be a barium salt to force the reaction, ie barium sulfate. It also depends on the purity of the silver or the composition of the alloy. Silver alloys containing copper are notorious for the copper "bleeding" through and oxidizing in the air, but that doesn't seem to be the case since the tarnish is black. Copper/air/sulfur tarnish is generally a green or blue-green.

Sealing the connectors would be one way of rectifying the problem, perhaps with an encapsulant of some sort. This essentially blocks out the offending contaminant. The amount of access required for the connector would dictate whether this is a viable solution.

Pierce Pillon
Laboratory Mgr.
Techspray
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.

Initial observation is that the blackening of the silver is more likely due to the creation of Sulphide rather than Oxide.

This could be caused by a high Sulphur content in the atmosphere. Is this the case?

If the "Tarnish" is a field problem, perhaps a Conformal Coating as a final process step could be a solution.

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Harold Hyman
Consultant
VJ Electronix
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.

I would also be very suspicious of sulfur. For example from a coal fired power plant nearby, and even automobile emissions with catalytic converter. Rubber tubing that is heated may also emit traces of sulfur.

Jim Williams
Chairman
Polyonics, Inc.
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.

Black tarnishing on silver is most likely caused by some form of sulphur contamination creating silver sulphide often seen as crystalline needles under a microscope.

The origin of the sulphur could be from the local atmosphere or even from a local source such as rubber materials which often contain it. This can be released from rubber through time and is encouraged by increasing temperature.

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Bryan Kerr
Principal Engineer - CMA Lab
BAE Systems
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.

It sounds like creep corrosion, well known phenomenon with silver, usually associated with high levels of sulfurous compounds to me.

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

Couldnt be due to Nitric Acid residues used to plate the Silver onto the Copper in the first place being left on due to improper rinsing.

Most Silver Solutions are dissolved in Nitric as the carrier system.I would have thought this was a very likely candidate OR Silver Brightener being left on in large volumes due to improper rinsing after plating.

Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has twenty two years of service in Electronics industry. York has installed over 350 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.
Reader Comment
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.
Glenn Little, Amateur Radio WB4UIV, USA
Reader Comment
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.
George Brutchen, PE, EnerDel, Inc., USA
Reader Comment
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
Don D. Vogel, Siemens MFG, USA
Reader Comment
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
Dave Tolino, Trans Tech
Reader Comment
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.
Randy Eadie, Truck-Lite Co. LLC
Reader Comment
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.
Randy Reed, R.Reed Consultancy LLC
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