Conformal coating on PCBs of drives and starters

[jraef]

This may have been better posted in the more general electronics forum, but I would have to explain how VFDs and solid state starters are used over there, so I'll solicit responses from this group instead.

I get frequent requests for conformal coating of the PC boards in our drives and soft starters. We used to do that as standard years ago, but with the advent of CFC issues and as we became more dependant on mP technology and SMPSs, we went away from it. Now I am seeing this come up in specifications and user requests again. Has something changed in that arena that makes it viable once again as a method of corrosion protection? I am no longer doing board design, but it would seem to me that heat issues on surface mount devices would mean having to mask or cut out so much of it that it would be pointless.

"Venditori de oleum-vipera non vigere excordis populi"

 

[ozmosis]

jraef
we're in just the same position as you. We have been looking at this issue for a while. One theory on why this increase in demand is that product price is driving a lot of people to put, sometimes, inappropriate IP(NEMA) ratings in places they shouldn't be. This creates a reduction of life of components mounted on boards, and compare the higher voltage components now mounted on boards to a few years back and you have a higher risk of contamination causing damage. High volume production does not sit well with CC boards and the repair/service of coated boards is also pretty specialised and does generally not provide a cost effect process. However, more devices on a board means greater cooling, which means more air. More air opens the door for greater contamination.

 

[Marke]

Hello jraef

I have always been an advocate of conformal coating of pcbs. It reaaly does result in a great extension of the long term reliability of the product in many less savory environments.
My pcbs have traditionally had three phase voltages on them, and by using an RTV type conformal coating, we have been able to halve the creepage distance on the board and still comply with UL/CSA requirements. This is very usefull in reducing the size required for the power circuitry.

In some regions, the atmosphere is quite damp for parts of the year resulting in condensation on the boards. Conformal coating protects against the growth of mould, and tracking across the board.
I have seen may pcbs in pumping environments where there is chlorine around, whenre the uncoated boards just get eaten away. Replaced many 24VDC fans in my day where the electronics have suffered.
I always specify conformal coated electronics in fans.
I suspect that you are seeing the results of non conformal coated boards in less desirable conditions.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[buzzp]

There seems to be two types, acrylic and silicon based. The prices are comparable to one another (depends on how you want to apply it). I think the silicon is easier to rework than the acrylic but does not cure as fast (air dry type). Marke summarized it pretty well and I too, like to use conformal coating in certain environments.
The key here is if your customer really wants it, they will find a drive that has it, whether it be yours or not. Good luck.

 

[ScottyUK]

Hi buzzp,

I think you might have the two confused. In my experience the silicone material is a nightmare to remove, while the acrylic coatings can be stripped using a standard flux-removing solvent (not the aqueous ones though - proper ozone-killing solvents!).

We had an interesting attempt to remove Electrolube's SCC3 silicone coating from a PCB assembly by soaking it in methylene chloride overnight, having failed to shift it with anything more gentle like trichoroethylene. This particularly aggressive solvent attacked the epoxy resin in the FR4 board, leaving a gelatinous green mush full of glass mat and copper tracks. The silicone coating also started to break down, but it didn't really matter by this point!

Methylene chloride was useful for removing conformal from thick-film hybrids: nothing there for an organic solvent to attack.




------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[Marke]

Hi Buzzp

I agree with ScottyUK.
The Silcon coating sticks to any exposed metalwork, solder joints etc and is very difficult to shift. Hence is is given an improved voltage rating by UL CSA. The other coatings can often be soldered through when they are relatively new.
The silicon coating will often peel off the solder masking and PVC capacitor packages, but try to expose a metal or solder surface and it is a problem. A soldering iron can be used to melt the solder under the coating and peel away from the joint itself. Messy, but the boards will carry voltage under water if necessary!
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[buzzp]

You guys are right, I had the two mixed up. Silicon is harder to rework.
At least with the brands I used, both had the same voltage withstand rating per mil and tested using the same set of standards.

 

[jraef]

Well slap me silly...

As it turns out, we do still coat our boards, but because of the bad rep that cropped up with the term "comformal coating" when the EPA got nasty about chemicals in assembly plants, we started referring to it by its trade name of HumiSeal. I went to HumiSeal's website and found out that it IS a conformal coating! I thought it was just a post wave-solder treatment to get rid of left over moisture from cleaning, like WD-40.

semantics...

"Venditori de oleum-vipera non vigere excordis populi"

 

[ozmosis]

I've just had a look on the humiseal website. It is one of the most informative sites on cc I've come across. Thanks jraef!