Chip failure and air corrossion products

Silver tarnish appearance of that sort of granular/cellular/mottling effect of silver tarnish rather than the very black soot-like appearance of CuS.

Most silver plating is "fine" silver. And proper plating is quite complex s tarting with a copper layer, then the silver layer. Back in the day when co al was the main fuel of choice, and there was no differentiation between hi gh and low sulphur coal, plate-silver used to tarnish (not corrode) quickly due to high levels of SO2 in the air. Hence the references in Victorian ti mes to "polishing the silver" as a full time job. Also back in the day, the underlying materials were often brass or bronze, so quite stable in itself . Further, electroplating was not yet an established science so "plate" was made using a dipping process with sterling silver. Sterling silver is far more prone to tarnish than fine silver, and, why the 'very best' plate used fine-silver for surfaces exposed to food or drink. It does not tarnish und er normal conditions.

Today, the underlying materials might be anything from steel to brass to co pper to many other things. The copper layer is still necessary for the silv er to adhere, and the silver coating will be the minimum amount absolutely necessary. The silver will be fine silver from this method and very resista nt to tarnish.

So, here is what happens. As the plating ages, especially if there are heat /cool cycles, the silver coating will get tiny holes - right through the co pper to the base material. Now, electrolysis can occur as O2 and moisture g ets between the copper and the base material. This causes tiny flaking (gra nular) such as you describe. At this point, the silver actually makes thing s worse by using the copper as a sacrificial electrode.

At home, take a common nail and a bit of copper wire. Wrap the wire around the nail and drop both into a glass of tap water. Give it a week.

If you have a bit of silver wire (must be fine silver), do the same with bo th wrapped around the nail.

Peter Wieck Melrose Park, PA

Silver oxide conducts. So, it could have been enough to clean to body of the ttl chip.

a) Silver Oxide will not form in this case, as copper is sacrificial to sil ver. b) Copper Oxide (Cu0) is a semi-conductor, and is also black as described, and under the condition described would have looked like soot. But, as a se mi-conductor would have led to all kinds of erratic behavior.

But, that does not take away from your correct conclusion that it would hav e been enough (for now) just to clean. However, as long as there was now-ex posed copper, the process would have continued until all the copper was con sumed. By then, all the silver would have been dissipated as well.

Peter Wieck Melrose Park, PA

All this is one the external sections of the pins? how would these effects progress into the active die or at least weld-spots ? This piece of kit was not stored i na damp environment, so no chance of capilliary action.

This is all happening at the chemical level by electrolysis. So the corrosi on would follow the path as the copper starts being exposed. Clearly the si lver-over-copper extends into the body of the die - and the same oxides wou ld form within. Once the process is established, no moisture is really nece ssary, just oxygen and time. You have to understand that the galvanic actio n is made much worse by the presence of silver making copper act as a sacri ficial electrode to the silver reacting with anything attacking it and what ever the base metal might be.

The thickness of the coating - at least several molecules - is plenty wide enough, if you will excuse the expression.

Peter Wieck Melrose Park, PA

There have been problems in the past where gold plated IC pins have developed a microscopic insulating layer - but I don't know about silver plated.

Silver plating develops black silver sulphide after long exposure to air, with NOS parts you'd just clean it off and solder as normal. Silver is a component of several types of solder alloys. So I don't think it should cause any problems.

Mpfffff..... Some clarification required.

FINE silver plating (most of it) *will not* tarnish from exposure to air, nor will it form sulphur compounds - without other contributing factors. Sterling silver (.925 silver) is an entirely different matter.

What is going on here electro-galvanic (galvanic action) *corrosion* of the copper plating under the silver - and most things electro-silver-plated other than brass are first copper-plated to guarantee proper adhesion.

The black material noted is CuO, not tarnished silver.

Peter Wieck Melrose Park, PA

Thanks, we've had many an Eprom with corroded legs that are from the

70s, in some cases the legs break off inside the IC sockets when the EPROM is removed. Also this is common on some other ICs used in the early 80s, such as Galaga which has Namco custom ICs, where again the legs are corroding right off them.

Is there any way to arrest the process? Be nice to have something to dip the legs into to cancel the corrosion, similar to using white vinegar and water (50/50) to neutralize alkaline battery leakage corrosion.

John :-#)#

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The best way I have found - if at all possible - is a coat of flexible lacquer over the entirety. I admit to having a bottle of flexible fingernail polish hidden away amongst my other solvents for this sort of thing. Stop the air, stop the corrosion.

Peter Wieck Melrose Park, PA

No idea whether its related - but when I serviced VGA monitors, I often encountered brown glue that was probably applied to prevent components from fatiguing their leads.

This stuff eventually became brittle and crumbly - and highly corrosive. It frequently etched away component leads.

Once brittle it was fairly easy to chip away, but in the early stages it was elastic like contact adhesive, and lighter brown like Evo-stik.

Very often a component like a resistor would come out encased in this stuff and the leads completely eaten away.

Yep. "Elephant Snot" turned out to be a very bad idea over time. Much like foam speaker surrounds of recent memory.

I use an acid-free neutral-formula flexible hot-melt for the purpose. I source from AdTech.

Peter Wieck Melrose Park, PA

I've quite often found problems with conformal coating, over decades, it seems to trap moisture underneath as though hygroscopic/delequescent or whatever its called

sorry wrong followup. You failed to mention it going conductive

I've quite often found problems with conformal coating, over decades, it seems to trap moisture underneath as though hygroscopic/delequescent or whatever its called , an oily/ jelly type composition beneath, complete with green copper carbonate corrossion product

** You got proof of that ?

In any case, the black tarnish on silver exposed to air is Silver Sulphate - which is an insulator.

.... Phil

** That should be "Silver Sulphide" = Ag2S

.... Phil

True for sterling (.925) silver. Fine silver - what is deposited by electroplating - does not form compounds with sulphur at anything near normal room temperatures.

Peter Wieck Melrose Park, PA

Parylene is about the most impervious common coating, but any plastic has orders of magnitude higher gas diffusion rates than any metal. In a humid environment, water will penetrate until the net flux is zero, so if you have nasty hygroscopic corrosive crud underneath, your board is still doomed.

Cheers

Phil Hobbs

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