Conformal coating breakdown.
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From: N_Cook on 29 May 2010 15:03 Supposed to be there to keep damp/condensation out but over 20 years or so 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?
From: Dave M on 29 May 2010 16:57 N_Cook wrote: > Supposed to be there to keep damp/condensation out but over 20 years > or so 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? 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
From: Jeff Liebermann on 29 May 2010 19:02 On Sat, 29 May 2010 20:03:14 +0100, "N_Cook" <diverse(a)tcp.co.uk> wrote: >Supposed to be there to keep damp/condensation out but over 20 years or so >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? 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 jeffl(a)cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
From: John-Del on 29 May 2010 19:58 On May 29, 3:03 pm, "N_Cook" <dive...(a)tcp.co.uk> wrote: > Supposed to be there to keep damp/condensation out but over 20 years or so > 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? Don't overlook the possibility of leaking electrolytic caps causing the copper leaching.
From: N_Cook on 30 May 2010 05:14 Jeff Liebermann <jeffl(a)cruzio.com> wrote in message news:8r5306p0ite891n7h22dguduphcgnavinc(a)4ax.com... > On Sat, 29 May 2010 20:03:14 +0100, "N_Cook" <diverse(a)tcp.co.uk> > wrote: > > >Supposed to be there to keep damp/condensation out but over 20 years or so > >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? > > 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 jeffl(a)cruzio.com > 150 Felker St #D http://www.LearnByDestroying.com > Santa Cruz CA 95060 http://802.11junk.com > Skype: JeffLiebermann AE6KS 831-336-2558 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
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