Conformal coating breakdown.

That coating is probably the silicone-based conformal coating, but I don't know what mechanism degrades the coating. During the late 70s through mid-90s, I serviced military equipment. Much of the stuff built in the 80s and later was sealed with the silicone-based coating, but I never saw any oily degradation. Exposure to salt water will cause the green corrosion that you're seeing. Could it be that the circuitry that you're experiencing the problem with was incompletely coated, or had the coating scuffed off the IC pins? That would have exposed the bare metal of the circuitry, leaving it unprotected and susceptible to corrosion. When the tip of the IC pin starts to corrode, it's just a matter of time before the corrosion starts to creep under the coating.

--
David
dgminala at mediacombb dot net

No surprise. *ALL* conformal coatings are somewhat penetrable by water (except maybe Parylene):

(Yech. The tech bull requires registration)

There are a mess of bulletins describing various failure mechanisms under:

However, they don't describe anything like you're experiencing. My guess(tm) is that you have a wax coating, not acrylic, silicon, or urethane. The wax has literally dissolved into mush from airborne solvent attack. It may also have melted due to overheating. Difficult to tell for sure without knowing the type of coating.

I used wax in the 1970's for marine radios. It worked well enough and was easy to apply. The best part was that I could solder right through the coating. If you want to remove the wax mess, just hit it with a hot air gun and watch the wax drip off the board. Solvents will also remove it. Then, just apply a new layer of wax and it's as good as new.

Do you have a long wave UV lamp (i.e. black light or UV LED flashlight)? Most PCB coatings have a phosphorescent die mixed in to make it easy to inspect for gaps. Light it up and see if there's a pattern. If you have large blank areas missing, it's a solvent problem.

(Duz anyone know what happened to Dr. Barry L. Ornitz, who would usually answer such chemistry questions? The last usenet posting I could find was Nov 26, 2009).

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

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Don't overlook the possibility of leaking electrolytic caps causing the copper leaching.

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breakdown

This is a previous encounter Digisonic 502 ultrasonic metal thickness meter The conformal coating may have been breaking down and causing an oil film on the board which could have seeped into the IC,

74C00 on the display board had one NAND gate o/p stuck high Only ever used indoors

Yesterday I broke into a new old stock , stored indoors (UK that is) Racal PIR unit to rob the RPY93 6.5 to 14 micron long-IR pyro to play around with, for remote temp monitoring and the pcb was covered in this waxy stuff now oily and green on some pins. Not powered up yet

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Translucent appearance, so can make out form but not detail of components under it, same when new. There is also iron rust brown colour as well as the green. If it was a mixture of paraffin wax or petroleum jelly and silica gel I could see the silica taking in water and the other breaking down to an oil.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Didn't work , I'd not noticed one of the pyro TO5 size leads was corroded through. So much for Racal RLC1/2 , opened up a used working Racal IR771 with a RPY96 (same spectral response) and LM324 to play with, no conformal around that one

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on