Conformal coating breakdown.
N_Cook
does the opposite. The waxy/jelly type coating (not the epoxy potting stuff)
breaks down to an oily film that is probably hygroscopic - plenty of green
copper carbonate and rotted IC pins. Gets inside chips capilliary-wise and
rots them internally as well. Anyone happen to know what this oily breakdown
product is chemically?
Dave M
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
Jeff Liebermann
wrote:
No surprise. *ALL* conformal coatings are somewhat penetrable by
water (except maybe Parylene):
<http://en.wikipedia.org/wiki/Conformal_coating>
<http://www.conformalcoating.com> (Yech. The tech bull requires
registration)
<http://www.conformalcoating.co.uk>
There are a mess of bulletins describing various failure mechanisms
under:
<http://www.conformalcoating.co.uk/technicalbulletins.php>
<http://www.conformalcoating.co.uk/FAQs.php>
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 je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
John-Del
Don't overlook the possibility of leaking electrolytic caps causing
the copper leaching.
N_Cook
news:8r5306p0ite891n7h...@4ax.com...
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
http://diverse.4mg.com/index.htm
N_Cook
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.
N_Cook
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