Conformal Coating a Gen3 Device

[jgskarda]

For those that use conformal coating… is there any tips/tricks on conformal Coating a Gen3 device such as a Boron? I had a few specific questions. There are several different mentions of conformal Coating in old topics but typically was a side comment. Rather than replying to several of them I thought I’d start a new topic:

1. Is it easier/better to spray on or brush on? If one or the other, what product have you used?
2. Can I conformally coat the reset/mode pushbutton and have the buttons still function or does that need to be masked out somehow?
3. Do I need to try and get conformal coating under the “tin”? The one on the top I’d assume no, but the tin on the underside of the Boron has a decent gap all the way around it. Should I use some sort of electronics RTV Silicon to try and fill the larger gap in the tin or don’t worry about it?
4. Are there any tips/tricks to applying conformal coating?

As of now, I plan on the following steps:

1. Use spare headers to plug the Boron into and protect the pins
2. Use a spare USB cable OR possibly a micro USB dust cover to protect the USB port
3. Use a dap of Hot Glue over the JTAG/SWD connector. Should be removeable if I ever need to in the future.
4. I do not use an external SIM so I’d likely spray right over that.
5. I’d insert a spare JST-PH connector into the battery connector. Although the battery connector is on the carrier board, you never know in the future.
6. Mask the Cellular Antenna connector with tape or just clip on the final antenna with the intent to not remove it again after conformal coating.
7. Spray MG Chemicals Modified Conformal Coating on via Aerosol can. Let dry, Flip it over and repeat.

As of now, these boards go into a weatherproof Polycase enclosure with connectors being IP67. These enclosures are then in the outdoors and so I have some concerns of condensation and humidity entering. I’ve used the Goretex vent in the past with pretty good success but still would like the added protection of Conformal Coating to maximize longevity.

For anyone who’s conformally coated a Gen3 Device your guidance is appreciated!
Tagging a few community members who made reference to conformal coating in prior posts:
@Rftop
@mikemoy
@hwestbrook

[Rftop]

The only product that I have experience with is:

MG Chemicals 422B Silicone Modified Conformal Coating, 55mL Glass Bottle with...

Think of it as a bottle of nail polish.
I don’t attempt to be very precise when applying with the little brush.

I cant recommend the liquid over the spay because I’ve never used the spray.
But I don’t mask off any parts when using the liquid.

My theory was “Anything metal that’s not coated will corrode and try to kill the device”, so I’m very liberal with the application.

I have Electrons still in outdoor service after many years with no apparent problems (haven’t seen them, but they are still operating). A few are still on 0.8 firmware.

As for your Gen3 question specifically, I haven’t deployed any Borons for long-term/permanent outdoor service due to the firmware issues. Maybe it’s time for another Validation Trial attempt ?

[Mjones]

I’ve used clear nail polish when needed, I live on the Florida gulf coast so corrosion is a big issue. If you need to do a lot of them I don’t think I would use it, but it’s been perfect for my use.

[jgskarda]

Thanks @Rftop and @Mjones I have both the MG Chemicals brush on as well as the spray on. I also have two dead borons due to early firmware issues causing them to brick (Particle was nice enough to send me replacements). So I’ll use these two as my test dummies one for each method. I’ll try and take some pictures on the process I use and report back here on my observations, recommendation and any tricks I learn along the way.

[jgskarda]

Alright, so I finally had a chance to test the conformal coating process and thought I’d share here my learnings here.

First, I had both the brush on as well as the spray on:

vs.

I tried both methods on my host PCB. I initially thought I was going to like the spray on method. The spray on can had lots of warnings regarding requiring adequate ventilation and other warnings that turned me off right from the start. I still thought I’d give it a try in my garage with the main garage door open. I didn’t try it very long however, it seemed to not go on as even as I’d like and it was hard to know where exactly it was going. It also was hard to know how thick it was going on. On a second host PCB, I tried the brush on Conformal coating and like others said, this brushes on similar consistency as nail polish. Overall, I personally liked the bush on better than the spray on. If I had a true spray fume extraction system then maybe I’d go with the spray on but all in all it seemed the brush on worked quite well.

So now… moving on to the Gen 3 Devices:

Method 1: Plug the Boron into a header to attempt to protect the pins and then tape off/plug the USB port. Then liberally apply conformal coating to the top, wait about 5 minutes, flip over and repeat.

Results: First, taping off the USB port and using the little plug thing is a pain. It’s easy to do but cutting the tape that size and dealing with the tiny strip is a pain. Second, I must of been a little too liberal with the Conformal Coating when I was applying it to the top. What ended up happening is the conformal coating leaked into the bare featherwing header and basically got all over the header pins. My guess is it leaked down the side when I was doing the top side. This was not ideal. So between the pain of taping off the USB port and Conformal coating getting in the header pins I figured I need to find a better way.

Method 2: Awhile back I got a PanaVise 350 This turned out to be a great tool for many things and now conformal coating as well. For the top side of the Gen 3 device, I put the Gen3 device vertical with the USB port at the top and just carefully brushed on the conformal coating. Since it’s vertical none of it got on the header pins this time and you can be precise with the placement using the brush so avoiding the USB port was fairly easy. For the back side, this is where the panavise really came in handy. I put the Gen 3 device at a 45 degree angle, started applying at the top of the Gen 3. To get under the “tin” I just put a dab/drop of it on the top side of it until it started coming out the bottom side of the tin. I could then easily rotate the device 360 to flow the conformal coating all on the underside of that tin. Worked pretty slick.

Here’s a quick video. Conformal Coating - Particle Boron - YouTube

Overall, I think method 2 will work well. Maybe I can make up a 3D printed jig to hold 4-5 devices at a time within the PanaVise. It goes quick but the 5-10 minutes handling time would take a long time one by one.

Going forward, I will likely also conformally coat the USB port and Battery Port (Battery and USB VIN come from the host PCB via header pins so really don’t need these). I’m thinking I would load the device OS and Firmware via SWD instead of USB and then once done, I could put a tiny amount of hot glue over the SWD pins to keep those available. If I ever need them again, I should be able to pull the hot glue right off.

I’m waiting a full 2 days for full cure time before testing the Reset and Mode buttons. It says it cures to a durable, flexible and smooth finish. So I think they would still function but time will tell.
Won’t really know how successful this is until a few years but I’ll have more confidence with devices being as robust as possible in the field this way.

If you have any other tips/tricks let me know.