Any downsides to be aware of with soft silicone conformal coat/potting compound - Page 1
Forum section: electronics / projects

author: Psi
author_rank: Super Contributor
timestamp: January 10, 2023, 07:04:09 am
content: I have ordered some 2part soft silicone conformal coat/potting compound for PCBs from china to test.Are there any traps or downside or just general things to be aware of with the soft (squishy) silicone conformal coating on a PCB?I plan to just pour it over the board in specific areas of the PCB that contain the electronics.I've considered dripping the entire PCB but then I'd then have to deal with the press fit terminals. Those need to have exposed metal for the ring terminals to connect onto, so would require some sort of masking which would probably make it more trouble than just pouring the coating on manually and avoiding the terminals.(Id be doing 40 boards every month or so, probably not worth setting up a 3D printer type table to automate the pour)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 10, 2023, 07:17:57 am
content: Just the usual, I think? Clean thoroughly, remove contaminants before sealing, and also to improve bonding.I guess I wonder how much value there would be in vacuum impregnation, since just pouring it flat will leave many air bubbles and voids around and underneath components?It could still be confined to an area (by free meniscus or by masking, or building up walls of a mold proper), but just the fact that bubbles are expanded and popped, and resin forced into the gaps, seems like it would be valuable.Silicone is very permeable to air and moisture, so such voids WILL experience condensation under some combination of conditions (namely long exposure (~weeks?) to some level of humidity, then a relatively rapid (<days?) drop in temperature below the previous dewpoint). TSSOP/QFP pins would seem the most vulnerable here, being dense enough the resin likely doesn't seep through them, leaving an airspace with exposed metal around its edges (and maybe vias underneath the part too, but they can be tented at least).I think conformal coating is usually sprayed, not sure if that involves a solvent as well, or to what extent if it does, but the thinner application layer, and lower viscosity if solvated, probably gloms around leads more easily?On the upside, the pressure ratio (vacuum to ambient) might not need to be much; enough to bubble some air out, then hope the collapsing airspace draws just enough resin in to surround most of the pins. Mind, a shop vac isn't going to have nearly enough suction to do that, you still need a vacuum pump as such, just not a very good one: an air or water venturi type pump would probably be enough, no need for a whole-ass vane pump.Temperature cycling might help as well, also reducing viscosity on heating (maybe?), but only buys you so much as the curing reaction is accelerated on heating too. This definitely works for epoxy (viscosity drops nicely with temp), not sure about silicone...Certainly something you can test, just mix up a small batch and pour it over some SMTs, then try and peel it off and see how well it bonded and spread.Tim

author: Psi
author_rank: Super Contributor
timestamp: January 10, 2023, 07:46:54 am
content: Quote from: T3sl4co1l on January 10, 2023, 07:17:57 amConformal coating yes, I think this stuff is more potting compound and usually poured into a box.I'm trying to get away from spraying as it is messy and the fumes are not good. It also gets on the terminals so they need masking.

author: SeanB
author_rank: Super Contributor
timestamp: January 10, 2023, 11:15:57 am
content: A lot of conformal coatings are brushable, so easy enough to buy a few cans of it, and then use a small brush to apply it. Cheap disposable brushes probably are best, buying in packs, and use one per batch to apply, after first taking the brush and working the bristles to remove the loose ones, then use it, and when finished put in a plastic bag and toss in the bin. If you want to use for a few days old painters trick for solvent based paints in a large glass jar, with a hook to suspend the brush off the bottom, and the appropriate solvent in the jar to just touch the bristles. Caveat for this is you need to use "blonde" brushes, that is unpainted or plastic handled, as otherwise if you use solvent the handle paint will peel, and long term the epoxy used to hold the bristles will degrade.Once the conformal coat is on then the potting will adhere better after the conformal is cured, typically a day in warm conditions.

author: Psi
author_rank: Super Contributor
timestamp: January 10, 2023, 11:32:03 am
content: I'm not sure i want to conformal coat and then apply potting compound.The conformal coat spray step is what I'm trying to replace with something easier.

author: jonpaul
author_rank: Super Contributor
timestamp: January 10, 2023, 11:51:24 am
content: motivé to coat?Vehicle use, outdoors, underwater?Generally conformal is a pain to use and rework, used in military boards if requiredSilicone rubber and similar potting use reactive catalysts which can damage delicate ICS and componentsMy General experience is plotting reduces reliability.If you decide to use silicone compound be sure that the Chinese Communists material will be the cheapest and lowest qualityWe use genuine USA or EU made potting materialsJon

author: Psi
author_rank: Super Contributor
timestamp: January 10, 2023, 12:17:27 pm
content: Quote from: jonpaul on January 10, 2023, 11:51:24 amVehicle use. On a car trailer. PCB is in a box and that box is inside another bigger box which is exposed to the environment.

author: jonpaul
author_rank: Super Contributor
timestamp: January 10, 2023, 01:52:28 pm
content: suggest you use a waterproof plastic case with rubber gasket and wiring through a waterproof packing gland fitting. Try Home Dépôt or electrical distributor.jon

author: Ian.M
author_rank: Super Contributor
timestamp: January 10, 2023, 02:12:32 pm
content: Short of a hermetically sealed enclosure, there is no such thing as waterproof in an environment subject to severe temperature cycling. The pressure differential across the gaskets and glands on cold nights tends to suck in any condensation present, but only a tiny fraction of it escapes on the next hot day. Add a Gore-tex vent to your proposed case, and it will at least prevent a pressure differential building up making it water resistant. However as its in a double enclosure, Gore-tex may not be necessary as a simple vent hole/drain at the lowest point of the inner enclosure, and a similar vent/drain on the outer one protected by internal and external baffles for resistance to pressure-washing should do for anything short of tail lights on a boat trailer! Keep the holes small enough that they don't attract mud dauber wasps, mason bees etc. and surround them with a larger diameter exterior drip lip.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: January 10, 2023, 05:33:55 pm
content: No idea what you'll actually get, but be aware that many 2-part silicones have terrible adhesion unless you use a primer on the parts. Bad adhesion also means bad sealing against moisture and such.

author: David Hess
author_rank: Super Contributor
timestamp: January 10, 2023, 05:43:36 pm
content: Do not underestimate the mixing process. Get a small paint mixer attachment and use it on your hand drill to mix the components thoroughly.It is well worth using a vacuum to remove air from the just mixed potting compound. This will make it clear for easier visual inspection, and seems to improve the performance once hardened.Personally I think better results are gained with a coating rather than potting the entire assembly, but the coating must be applied correctly.

author: Jackster
author_rank: Frequent Contributor
timestamp: January 10, 2023, 06:38:28 pm
content: Personally, I would go with brush on conformal coating but potting compound will work fine. Just make sure it is moisture proof and ideally suck out all the air.Good luck repairing the PCB if something goes wrong 😅AFAIK most car stuff is conformal over potted to save weight. Typically you see potted in HV applications and where they don't want people to open up the item to modify.Brush on conformal coating only requires a clean board and a somewhat dry humidity level to get good results. It is easy to remove and replace if you need to repair the PCB as well.Note that not all conformal coatings are moisture proof. Be sure to read the PDFs.They are also available in air, UV and heat curing.TLDR; conformal coating is easier and probably more than what you need. But if you want to mix up some 2 part potto compound and pour it into a box with some connectors, that will work.

author: Psi
author_rank: Super Contributor
timestamp: January 11, 2023, 01:12:40 am
content: Thanks.Yeah, i need to check the exact details for the one i'm getting a sample of.It's soft stuff but it also says epoxy and it have seen reference to silicone.So I need to confirm it blocks moisture.