Conformal coating advice

[mikeselectricstuff]

I'm looking for advice on conformal coatings to protect PCBs in a semi-outdoor environment.
This is a large PCB (480x250mm), with LEDs and light sensors one one side, and nothing apart from an IP68 circular connector on the rear. There won't be any serious hotspots
This will be living outdoors, inside a stone enclosure which although nominally closed on all sides, will not be fully sealed. This will be in Canada, so temp range could be +50 to -40 deg.C
We need reasonable optical clarity, but more important is good consistency - things like lumpy bits on the edge of LEDs will be a problem. The PCB will not be directly visible, but behind thick diffused glass. (so at least UV won't be an issue!)
This is for a 1-off production run of approx 100 PCBs, which need to be right first time.
I'm struggling to find information on the best type of coating and process to protect against moisture, frost and possibly insect damage.

I'm certainly not comfortable with a thin sprayed coating, and am visualising either a thick gloopy dipped coat (like you see on some automotive assemblies) , or a potting process using a mould (maybe a teflon plate) to give a flat optical finish.
The former is going to be the cheapest and easiest, if it can provide the optical properties. The latter will probably give a good optical finish, but use more material and cost more due to need to make a mould, then the time it needs to sit in the mould for curing.
We can put a cap on the connector to seal it, so that's not a problem.

Even some advice on choice of compounds would be good - I'm happy to experiment, but an issue is many are only readily available in fairly large pack sizes. 
From what I've found so far, it  looks like a silicone compound is probably the way to go, but there are a lot to choose from.

I will make some enquiries with manufacturers, but wonder of anyone here has already been though this and has good/bad experience to share.


BTW this is what it's for....

[Fsck]

Just one nip to your request, I'm assuming southern Canada?
Because -40C isn't enough in the north...

[mikeselectricstuff]

Just one nip to your request, I'm assuming southern Canada?
Because -40C isn't enough in the north...

Yes, very South. We will also have some background heat from PSUs etc. We're not too bothered at working under extreme temps, just surviving.

[Fsck]

the lazy answer is parylene, probably the C variant
it should easily satisfy consistency and optical properties

the cost may not be what you're looking for though..

[Mechatrommer]

car clear coat paint is a proven comformal coat  not sure what you are trying to do coat all the components and solder joints?

[bookaboo]

Most potting compounds will flow nicely around LEDs if you have the board nice and flat and are patient with a thin nozzle.
I've done it myself for a few projects, not that difficult.

Its exactly how the "big screen" led scoreboards are constructed too, if your up close you can touch the actual led with your fingers and the potting compound keeps it waterproof. Usually theres a plastic mask over the compound too.

[krivx]

I have to say, this kind of installation design work looks like a lot of fun. It looks like it would be much more rewarding than the usual commercial work. Cheers for posting about it.

[mikeselectricstuff]

I have to say, this kind of installation design work looks like a lot of fun. It looks like it would be much more rewarding than the usual commercial work. Cheers for posting about it.

yes but the flipside is you need to design something to last for years halfway across the world with a single production batch that has to be right first time - unusually for this project we will get the chance to have a prototype unit on-site for a few months, but that is pretty rare.

[Len]

But I bet working for artists pays really well.

[robrenz]

IMO your cleaning materials/methods on the boards before coating will be just as critical as the coating itself.  Including a bake to remove all moisture so it is not trapped to cause problems later.

[Huluvu]

We have very good experiences with Elpeguard coating from "Peters Lacke"
We use it in industrial and medicine environment since nearly 20 Years

[Towger]

Silicone compounds absorb moisture over time.  Years ago I learned the hard way after using silicone sealant in aerial connectors.

[electronupdate]

You may wish to consider adding a heater (and associated control loop) to keep the temperature inside the product more constant.  This may also help limit moisture which can also affect the visual look of your installation?  Canadian weather can be extreme.   I fondly remember flying through Edmonton a few years ago in January.... it gets so cold that they need to wrap the parking lot payment machines with thermal blankets to keep them operating

[fcb]

Not really the advice your looking for Mike, BUT I would be concerned about the risk of insect/fungal/bacterial damage - I certainly would be nervous unless someone could give you similar example.

My solution would (probably) be to create a simple perspex sandwich (with greased o-ring to seal the lid), and use some big silica-gel packs, if you free float the PCB it should stand-up to expansion/contraction very well.

Project Plastics in Colchester (if you don't already deal with Janson) could advise.

[mikeselectricstuff]

Not really the advice your looking for Mike, BUT I would be concerned about the risk of insect/fungal/bacterial damage - I certainly would be nervous unless someone could give you similar example.

My solution would (probably) be to create a simple perspex sandwich (with greased o-ring to seal the lid), and use some big silica-gel packs, if you free float the PCB it should stand-up to expansion/contraction very well.

Project Plastics in Colchester (if you don't already deal with Janson) could advise.

As regards insects I've already told them they should be putting fine mesh where necessary as anything crawling around inside may become visible. The potting option would effectively turn the thing into a block of plastic, but that seems like overkill.
Not sure about fungus/bacteria as I don't think it would have any food source to grow. Sealing  a lit on a 450x250mm sandwich would be a non-trivial operation, unless maybe ultrasonic welded.

[SeanB]

There are quite a few fungi and bacteria that will happily live of almost any plastic or epoxy if the humidity is right.  Potting or coating with Parylene will likely be the best, though the cost will make you wince a little, but it does protect well. We had 2 ways to remove it, and either sat with a fine burr in a dremel or used an ultrasonic vapour reflow unit that would destroy any PCB within 2 minutes of full immersion, but if you went and did it in 5 second dips you could come to a compromise where the coating was soft enough to remove with a knife blade or the board would dissolve into fibre. With the shelf life of the opened cans we did waste a fair amount as we could never use a 1l can in the six months shelf life, generally we used 50ml at most before it went solid.

Another option would be an UV cure clear epoxy which at least gives the ability to fix all errors before you place under the UV source to cure it. Most do have a UV degrading pale blue dye to enable you to see imperfections in the uncured resin, but they cure water clear. Loctite make some that are available in 1l packages.

[BravoV]

I have friend working in shipyard, he told me their company uses Electrolube HPA conformal coating -> http://www.electrolube.com/docs/conformalmain.asp?id=77 , datasheet -> http://www.electrolube.com/tds/044/044HPA.pdf

Can be sprayed, dipped or brushed, so thickness can be controlled.

List few of it's properties :

Temp range -55 C to +130 C
Can be soldered through 
Resistant to mould growth
Excellent dielectric properties .. and etc

[Smokey]

That's one hell of a project.  Looks super cool!

I have no idea how to protect a PCB in those conditions, but without a solid solution I would probably take some small boards and try a bunch of different coatings.  Beat the hell out of them like submerging them, and putting them in the freezer/oven. 

Most of all, let us know what worked best!  Good luck!

[george graves]

What your asking for remind me of just epoxy - maybe even a bar top epoxy

....but then I think you have to worry about shrinkage of the epoxy.  I'm sure a small shrinkage on a large enough board could cause havoc.

I have had really good results with hardware store Polyurethane.  I just put up a blog post about a home-etched board I've had aging for ~7 years now.

http://www.digitalunderpants.com/polyurethane-for-protecting-one-off-pcbs/



Otherwise check out MGChemicals.  http://www.mgchemicals.com/products/protective-coatings/conformal/

[jerry507]

We've made a couple LED indicators for agricultural machines. We apply four coats acrylic from different angles and it holds up really well. Dust doesn't even adhere