How to add conformal coating to a BMS?

[Syonyk]

If one wanted to add a nice, water blocking conformal coating to a BMS that came without one, at a scale of "ones" or "tens" - how would one do so?

Can you buy generic conformal coating that sprays onto a BMS?

And, if so, how do you go about prepping the surface for it?

Thanks!

 

[amberwolf]

When we cc'd boards for aircraft computers and displays at Honeywell cFsG, we first cleaned them with a citrus-solvent (I don't know the names anymore, as that was almost a quarter century ago now), to remove any flux or other contaminants, then they were spray-coated with a clear acetone-dissolvable CC, which also had a UV dye in it so you could use a blacklight to verify it was correctly coated. You could also then use acetone on a qtip to remove cc over areas that needed repair or rework, and you could brush on the coating by hand to re-cover those areas. Took about 10-15 minutes to completely dry the cc.


This cc could also be dipped, but we werent' allowed to do that due to something in the FAA rules or something, about how boards had to be coated (or so I was told), something about saving weight I think, or "gloppiness" of the finished parts. Can't remember.


Sorry it's probably not helpful wihtout a product name, though it's likely whatever we used then isnt' even made anymore. :/

 

[Mad Huey]

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[methods]

Assuming you have some sort of connector on the board... no matter how good of a job you do conformal coating the PCB you will still have weakness at that point. If I wanted to rain-proof a bms at the cost of serviceability - I would do it like this:

* Remove all foam or plastics leaving only the PCB and wire harness
* Clean it like crazy with a tooth brush. If you can afford it MG Flux Remover otherwise 90% alcohol from the store, Acetone, or whatever you can get your hands on with low water content.
* Dry it well and clean again if needed - remove all oils and fluxes. Dry it with a hair dryer or heat gun.

* Get some epoxy. Slower setting the better. 5 minute may peel up on you later.
* Be patient and paint the PCB, connector, wires, inside the connector, basically get expoxy everywhere you can. Do one side at a time as it will be runny.
* Flip it over and repeat.
* Touch up anything that was missed

* If your PCB had balancing resistors get them to turn on and make sure its not overheating now.

* An alternative is to find a small plastic box (or make a small plastic box). Suspend the PCB inside the box and slowly fill it with epoxy. Do a "Long Pour" - meaning you pour from 24" up above the box causing the stream of epoxy to get very thin thereby allowing any bubbles to pop.

* An alternative would be to "Bag" the BMS... then someplace on the wire harness (raw wires, loom pulled back) pack a bunch of dialectic grease and zip-tie the bag shut. Do this 2X or 3X with grease each time. Then mount the BMS such that the bag opening(s) face down. Water will drip away. To get agro you could pack the entire bag full of grease. You could pack the BMS in grease, then have the second bag packed with a desiccant, then service every few months.

* An alternative would be to use a silicone - like the stuff they use for jelly dongs. Just completely encapsulate your BMS and wires under the jelly (sorta expensive tho...)

* An alternative: cut all the BMS wires (CAREFULLY). Solder them up to an IP67 connector pair, one bulkhead and one standard. Stuff your BMS inside of anything water tight that can take a bulkhead connector. 100% serviceable.

* Alternative: Just carefully wrap the BMS in layers of heat shrink facing different directions. Encourage the water to flow away. Like a thatch roof... guiding water goes a long way.

Experiment and let us know what you learn

-methods

 

[dmwahl]

https://www.google.com/search?q=conformal+coating+spray&tbm=shop

You'll need to clean and dry well beforehand, then don't handle the board with bare hands until it's coated and dry. Read the application instructions and follow them. Personally I'd use a more flexible silicone option.

Packages with all the pins hidden (ie BGAs) are a problem as some of the coatings expand as they cure, and can pop the BGA right off the board or break traces as the pads pull up. I doubt your BMS has any, but double check to be sure. Connectors can also cause trouble, as the coating tends to wick up the connector housing and get onto the contacts. Typically you would either use a barrier such as silicone around the connector and mask, or just mask to keep the coating away from the connector housing by at least 1-2mm.

Why do you want to coat your BMS?

 

[Syonyk]

Why do you want to coat your BMS?

I rebuild I2C BionX packs, and the BMS doesn't have any sort of coating. The later BionX BMS boards have a nice coating.

If it's reasonably affordable to do, it would be a nice upgrade to offer when I'm doing pack rebuilds for people in wet climates.

 

[dmwahl]

You might want to look into having it done professionally, it may not be as expensive as you think. At a prior job we had some boards coated, 100x160mm and as I recall it was only in the $10-15 range for relatively small quantities (10-30 boards). The supplier can suggest a coating, will likely have it in stock, and you get professional results.

 

[methods]

I have seen automated processes where a cnc type machine applies coating in a precise manner. This is pretty low cost and the way it is done for some of the highest fidelity hardware.

Really the ultimate end goal is to stop electrolysis before it starts so any method or combination of methods that accomplishes this over the prescribed environment for the length of time needed works.

As to the aerosol spray on coatings... my findings have been that they go on a bit thin and are prone to being scratched off but to the first order they sure as hell protect orders of magnitude better than a raw PCB. I used a silicone conformal coating for a long time that I was happy with. The ability to rework is critical in selection... as there is an inverse relationship.

The Chinese solve it through clever wrapping and an understanding of how water and moisture behaves.
Dont invite it in
Dont trap it if it does get in
Encourage it to leave

Desiccant is the most looked over alternative to a lot of moisture problems (obviously because it requires service)

How about a magic potting material that absorbs water and converts it into a dialectic?

-methods

 

[dmwahl]

Or you could just use some spray polyurethane like this guy http://www.digitalunderpants.com/polyurethane-for-protecting-one-off-pcbs/

Not sure if it's a good idea but it seems to work OK.

 

[methods]

MG Chemicals