I have some IC's exposed to moisture, and a conformal coating is in order.
In my imagination, I'd like an adherent water-white 2-part silicone but I'll consider anything. This will be under fairly high solar flux, so Silicone is a good bet.
But an epoxy loaded with carbon black a possibility.
I'd like it brush-on consistency.
Home RTV is almost good enough, except it's viscosity a bit high because it is loaded with silica particles. An RTV without the silica would be ideal.
? jb
I used to design marine radios. We did it mostly without any conformal coating. I suggest you review your design and look for high impedances that are sensitive to moisture.
The IC's are exposed to sunlight? A sun shade or enclosure might offer better UV protection. Is water proofing your problem, UV protection, or both?
Are you trying to hide the IC part numbers?
Epoxy is good for chemical resistance. Otherwise, the typical two part mixes are a PITA to handle, and the one part variety has to be refrigerated. If you're worried about chemical attack, by all means use epoxy. Otherwise, I suggest something that's easier to handle.
Start here: Note that Humiseal does not sell epoxy coatings. See MG Chem and Masterbond for those.
From your very limited description, and you want silicon, I find: One problem is that it requires oven thermal curing, which may not be convenient. It's also tricky to rework.
I personally prefer acrylics, because they are cheap, easy to handle, cheap, dry quickly, cheap, look good, cheap, store well, and offer some UV protection. Also, they're cheap.
Sand in the mix? I doubt it. Perhaps you mean silicone in the RTV, which is what RTV is made from.
Same old questions:
-- 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
@@@ No high impedances...but dirty ionic conditions...
@@@ Both.
@@@ C black protrcts against UV - auto tires
@@@ NO
@@@ Yes, acrylics good on UV resistance along w/ teflon, silicones.
@@@ I believe they put small particle silica in it to prevent flowing. See Cabot-sil.
@@@ H2O, UV resistance
@@@ My hands I guess :)
@@@ circuit ready 2 go - I haven't tested in situ, but a low viscosity acrylic sounds good - I wonder if an acrylic varnish available at Hware store? THANKS!
Most military type conformal coats are polyurethane. Hardware store polyurethane varnish works great on electronics. Just brush on a coat or two and bake gently.
Silicones are nasty, and not allowed in a lot of places.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
If there's any chance of dust and condensation, there's a risk of electrolytic corrosion and runaway, green snakes growing between PCB pads. Even a light conformal coat will protect against that.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
urethanes (orig. IBM 'parylene') are good, but crap on UV. However, I'm trying some "mini-wax Polycrylic varnish' from Lowes. Thanks John.
If you ever ripped apart a 10 million $ IBM360, you will see silicone gels everywhere on the RTL. I can get one from my brother in law for you for $25. :)
We're not allowed to use any silicones - adhesives, lubricants, heatsink stuff - in gear that goes into a semiconductor fab. It outgasses gunk. Epoxies and super-glues are mostly OK.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
How close to total immersion is the environment? The problem is that conformal coating is not the same as waterproof. You will have conduction paths between exposed connectors, test points, parts with sharp edges, and areas where flexing produces cracks. Components that get hot also cause problems.
There's not much dirtier and more ionic (conductive) that condensed salt water. In my admittedly ancient (1973-1983) experience, you're not going to fix an overly sensitive conduction path design problem by hiding the mistakes under a layer of insulating goo. Conformal can make it better, but if you're relying on the coating to remain consistent and intact across production runs and mixed environmental conditions, you're going to get wide variations in effectiveness. In other words, conformal coating might work for a while, but will probably crack or break at some point. I know that you said that there are no high impedances, but I'm skeptical. For example, got any unconnected floating inputs to gates or amps?
I have a little test running on my roof to check the effectiveness of Scotch 33 vinyl tape over PTFE pipe wrap for waterproofing RF connectors. The vinyl tape will usually self destruct after about a year due to UV exposure. However, if I coat it with clear acrylic (Krylon), it seems to last forever.
Black is certainly better than clear. The question is how much better. I did a radio that ended up on poles along the coast highway in the middle east. Hot with plenty of unshaded sun every day. There was no exposed electronics, but the mechanical parts and cosmetics had to be protected. Painting it with carbon black loaded paint would have probably melted the electronics. Titanium dioxide white was far more effective at reflecting the UV and sunlight. Clear over brushed aluminum would also have worked, but was rejected by the customer as too reflective which might blind motorists on the roadway.
Everyone (in China) does that.
Light reading:
Watch out for coatings that don't quite adhere to the PCB and components. Capillary action can pump water along the interface, and all over your PCB. I had that happen with acrylics, there the thermal expansion for the PCB was different from the acrylic, resulting in cracks in the coating. We eventually had to switch to a more flexible urethane coating.
Ok, a thickener. That's the last thing you would want in a conformal coating. What you want is a very fluid mix, that flows into all the cracks and cavities, not sits in a blob on top of these. A thickener might be useful for window and bathroom caulking to prevent dripping, but not as a coating.
Chemical resistance? You said ionic, which means that there's some kind of solvent or caustic involved.
I meant the equipment you're trying to protect. For example, if it's a flex PCB, the coatings are quite different.
What's missing are the phosphors needed to do a proper inspection. Buy a UV LED flashlight. You'll need it. These are not really made for conformal coatings. I would go with the real stuff: You're probably better off asking their support people for applications advice (or read through the available literature) for specific recommendations. If you brush on the stuff, be sure to get the correct thinner.
-- 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
I didn't see much of that because when I was doing radios, the spacing between traces was much larger than today's tightly spaced PCB's. The only times I saw that was when the PCB's were not properly cleaned. I also saw it when production forgot to flush out the final rinse water heater, and ended up coating the PCB's with a thin layer of calcium carbonate, which provided the necessary paths.
At one point, some manager decided that all the "marine" quality PCB's would be coated with some manner of conformal coating, leaving me to find the something. I decided that it was impossible to mask off all the trimmers, pots, adjustable caps, connectors, crystal sockets, etc. What I needed was something that would work over everything. After some experiments, I settled on a very thin coating of paraffin wax. I conjured a mix of phosphor dye, paraffin, and solvents that could be applied with either a dip or spray. The trick was to get it as thin as possible, or it would get dirty/dusty/filthy very quickly. At one point, I demonstrated the radio operating in a fish tank at a boat show. Never mind that it would also operate under fresh water without the wax coating.
As usual, there were surprises. The kerosene and naphtha solvent attacked unsealed film capacitors and "plastic" trimmer capacitors. It also removed all the inspection stickers and serial number tags. Production had some not so nice things to say about the smell and the hazardous chemicals. OSHA arrived and declared it unsafe. A few years later, kerosene and other VOC's were banned as environmentally disgusting. I'm not sure paraffin can be used without a VOC.
Ooops... I'm late. Good luck.
-- 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
Those don't have phosphors mixed in to allow for black light (UV) inspection. Otherwise, the composition of the hardware store stuff is the same as the commercial urethane coatings.
-- 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
Yep. The 360 was a raise floor machine that required air conditioning. There was no de-humidifier. I was told that when the hot machine was shut down for some reason, moisture would condense on everything, preventing a restart. So, the machine had to be water resistant using conformal coating.
-- 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
Since the 40's the military has used MFP. What is wrong with shellac or varnish?
You can use motor varnish. It comes in various colors, even clear.
Jamie
What's an MFP? Pick one:
11th commandment: Thou shalt not abrev.-- 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
I'm back on Silicones, even the bathroom caulk RTV. I realized that I may be dealing with some heat. This is for a 10W led used to light a wet terrarium. I have satisfied myself that I can spread the Silicone around the perimeter of the led chip, which is heat-sunk against an aluminum sheet. Even if the coating leaks, the perimeter coating will prevent the dirty water from forming a film, and therefore a serious short. So the design is failure tolerant. So the hydrophobic nature of Silicone is another plus:
-UV resistant
-high temperature resistant
-hydrophobic
-good hsat transmission
A serious coating would use silane surface pre-treatment. But I am confident of the Silicone to break up water films. I would prefer some RTV without the thickener, but RTV is thixotropic, so banging on the aluminum sheet will help.
I'm sure Dow Corning has a non-thickened version, but I'm reluctant to navigate their distributor channels. Perhaps I can get a sample direct from them.
Can leak and support a water film. jb
Ok. Make sure your silicone goo of choice does NOT contain an acetic acid curing agent (acetoxy silicone) , which will rot your traces and components. The right stuff will be labeled "electronics grade" or something similar. This eliminates many of the common hardware store silicone bathroom and roofing caulks which all reek from acetic acid. If you find something without acetic acid, try xylene, toluene, or naphtha (Coleman fuel) as a solvent. Those are the common solvents used to get the silicone thin enough (i.e. self leveling) to make molds. Ummm... wear gloves, use eye protection, and do it outdoors.
This might help decode the different types:
and this might give you some ideas: (I haven't tried it).
-- 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
Call Smooth-On and talk to them about their products. Among others, they make a non-acetic acid cure pourable silicone for potting solar cells:
I'm back on Silicones, even the bathroom caulk RTV. I realized that I may be dealing with some heat. This is for a 10W led used to light a wet terrarium. I have satisfied myself that I can spread the Silicone around the perimeter of the led chip, which is heat-sunk against an aluminum sheet. Even if the coating leaks, the perimeter coating will prevent the dirty water from forming a film, and therefore a serious short. So the design is failure tolerant. So the hydrophobic nature of Silicone is another plus:
-UV resistant
-high temperature resistant
-hydrophobic
-good hsat transmission
A serious coating would use silane surface pre-treatment. But I am confident of the Silicone to break up water films. I would prefer some RTV without the thickener, but RTV is thixotropic, so banging on the aluminum sheet will help.
I'm sure Dow Corning has a non-thickened version, but I'm reluctant to navigate their distributor channels. Perhaps I can get a sample direct from them.
snip May I pick a bone on the AcCOOH issue? The first reference you gave says that Acetic Acid is more harmful to health than acetone. Have you eaten a salad with vinegar recently? Acetone much worse.
Now, the proscription never to use acetic acid curing RTV I don't believe is true. If you have IC's potted in epoxy and do a gentle bake-out, that AcCOOH is history quickly. Silicone has the highest diffusion constant of any solid material, and the AcCOOH exits quickly, if you leave an exit path.
AcCOOH is a medium strength acid, not like HCl. AcCOOH needs water to ionize and form a corrosive solution. How corrosive? If you made a solution of AcCOOH, and wanted to eat your PCB traces, it will take 6 months for any noticeable corrosion. And a water environment is missing from the Silicone interior. AcCOOH evolved from the Silicone is a gas.
And a non-toxic one, see "salad dressing."
One thing to watch for with Silicone is not to get it on your clothes. Wear old clothes is my advice.
BTW, in that 2nd video, they warn against an acetic acid cure Silicone, but then he proceeds to demo a caulk for potting which is acetic cure! It seems to work OK after potting.
One way to make a mold for potting is to just use silicone. Be a good idea to use release and keep walls thin. Post bake after release from mold.
jb
snip I have some industrial chemistry/organic chemistry experience.
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