Conformal Coating / Potting

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RustyH

Senior Member
Goooooooood Evening everyone, Hope all are well.

I was wondering if there are many experienced potters here (not the stuff you smoke, or plant! ;)).

I want to potting a couple of circuit boards, to protect from humidity moisture, etc. The boards carry quite a few video signaling.

So I was wondering what would be the best stuff you use?

I was thinking of standard potting resin, and just adding a layer to each side of the board with a bead of silicone or something as a boundary for the liquid (similar to what they are doing on the second section of this video - http://www.youtube.com/watch?v=D35ui3Ab4hw). I was looking at this which seems a really good price - Conformal Coating EPCRYL 100 Acrylic

However, I have read the epoxy resins are prone to cracking if they are slightly flexed??

So I was then wondering about using RTV Neutral cure Silicone, but I wondering if these would be thin enough to spread easily over a reasonable size surface?

Any advice would be hugely grateful

Thanks

Russ
 
J

john2051

New Member
Hi Russ,
going off at a slight tangent. I've been told by two electronics engineers in the military that lead free solder over time, grows conductive filaments.
Obviously not a good thing in high density equipment. I was therefore wondering if conformal coatings would overcome this.
As for your potting compound, the stuff we used to use contracting to the raf several years ago was just a non-descript black resin designed for
use in missile guidance control. But, we were never told exactly what it was.
Sorry to be of no help, but was worried over this lead free solder concern.

regards john
 
R

RustyH

Senior Member
Hi John,

Im not sure on that one, but I think the PCB Lacquer might offer protection of that issue. Unfortunately, I need more environmental protection that what the Lacquer would offer.

Do you work in the Military now then?
 
fernando_g

fernando_g

Senior Member
Don't use the general purpose RTV that smells like vinegar. That WILL corrode your electronic components.

Use only an electronics-grade RTV, google it for suitable brands.
 
J

Jeremy Harris

Senior Member
Potting compounds have to be able to cope with more than just sealing things up well, as I found out once when I potted a circuit in epoxy resin. They need to to be able to accommodate the stresses induced by things warming up and cooling down, so have to have a little bit of flex. They also have to be non-corrosive and contain no solvents likely to damage things like the rubber seals on electrolytic capacitors. Ideally they need to be reasonably good conductors of heat, to prevent components overheating from being unable to shed heat normally. Finally, there is a lot to be said for using translucent, soft, potting compounds, like two part silicone rubber or perhaps solvent free polyurethane. The advantage of soft, translucent potting compounds is that they sometimes allow surgical repair if necessary, by cutting away the potting to get at and replace a component.
 
M

Markbishop16

Member
Raytech Ray gel.

Mix it like epoxy but sets to a gel like feel. Totally waterproof, each time we use it, and if it all goes wrong easy to dig back out.

Expensive but you get what you pay for.
 
G

geoff07

Senior Member
There is a NASA website that deals with the tin filament issue, which is indeed real and why many old tin-plated-can-encapsulated transistors, such as AF117, stop working.

http://nepp.nasa.gov/whisker/background/index.htm

There is some advice there regarding conformal coating as a preventative, though it takes years for the effect to be a problem so may not be relevant to many Picaxe projects.
 
J

john2051

New Member
Hi Russ, No I wasnt in the military, we used to do contract work for them, which was many years ago. I havent
been able to do all that much since a heart attack, and at my age theres not a huge point.
Thanks to geoff about the filaments, that explains the dead AF117s, and also the OC171s!
Was it the tin itself that was growing these whiskers?
regards john
 
rq3

rq3

Senior Member
RustyH said:
Goooooooood Evening everyone, Hope all are well.

I was wondering if there are many experienced potters here (not the stuff you smoke, or plant! ;)).

I want to potting a couple of circuit boards, to protect from humidity moisture, etc. The boards carry quite a few video signaling.

So I was wondering what would be the best stuff you use?

I was thinking of standard potting resin, and just adding a layer to each side of the board with a bead of silicone or something as a boundary for the liquid (similar to what they are doing on the second section of this video - http://www.youtube.com/watch?v=D35ui3Ab4hw). I was looking at this which seems a really good price - Conformal Coating EPCRYL 100 Acrylic

However, I have read the epoxy resins are prone to cracking if they are slightly flexed??

So I was then wondering about using RTV Neutral cure Silicone, but I wondering if these would be thin enough to spread easily over a reasonable size surface?

Any advice would be hugely grateful

Thanks

Russ
Click to expand...
There are basically four choices:
1) silicone
2) acrylic
3) epoxy
4) urethane

You will need to define the environment that you are protecting against. Any of the above four materials can be suitable, given the right conditions. Any of the above can be formulated to be rock hard or jelly soft. They can be flammable or not. They can be opaque or optically clear. They can be gas or water permeable or (relatively) impermeable.

Platinum cured one part silicones are easy to use, easy to remove for repair, but gas and water permeable. Epoxies are generally too stiff, and have a glass transition temperature too high for extended temperature range use, and are UV sensitive. Acrylics are (usually) nasty two part mixtures with the same glass transition issues as epoxies, although there are exceptions.

You're right if you think I am steering you towards urethanes. They are available with Tg of -70C or better, and in a wide range of durometers. Unless you need to physically touch the product while in use, stick with durometers under Shore 60A (relatively soft), to prevent stressing components due to thermal expansion and contraction cycles. The only drawback to urethanes is their flammability, although there are urethanes specifically designed for potting applications that include additives to suppress burning.
 
G

geoff07

Senior Member
I believe it was the tin, a concern with lead-free (i.e. 95-99% tin) solder. If you are building a communications satellite.
 
R

RustyH

Senior Member
Thank you for your replies so far

Are Silicones water permeable??

I cant seem to find any Polyurethanes that have a shore of 60 or below, the only one I can find are around the 70-80 mark?

One thing I didnt mention clearly, was that the PCB wont be exposed directly to outdoor elements, will will be under some form of cover, but I want to ensure its as protected as it can be from airbourne moisture, maybe the odd drip seeping through, cold weather / frost, etc

Many thanks

Russ
 
G

geoff07

Senior Member
I believe it was the tin, a concern with lead-free (i.e. 95-99% tin) solder. If you are building a communications satellite.

(I could swear I already posted this but it didn't appear in the item listing, so trying again) Ah, it repaginated between showing it to me and displaying it in the list.
 
R

RustyH

Senior Member
The environment is is simply under shelter outside, all year round, in the UK
Standard shelter to be built around the PCBs, but worse case may allow a tiny amount of water ingress.

Protection required to prevet PCB, Components and Soldered Joints degrading from the natural environment such as, Air Moisture, Dampness, Morning Dew, Frost, worse case the a drip of water.

Populated Board has, ICs, SMT devices, lots of radial caps, etc

Cheers

Russ
 
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R

RustyH

Senior Member
I cant use a food container, or project box, as the PCBs have to mount to a frame and there is mimimal space. Hence me.looking for a suggestion on Conformal Coating / Encapsulation
 
Technoman

Technoman

Senior Member
Some years ago, I was working in the aeronautics field. We were using a vacuum chamber for the degassing of epoxy before heating. Conformal coating was also very common.
 
rq3

rq3

Senior Member
RustyH said:
I cant use a food container, or project box, as the PCBs have to mount to a frame and there is mimimal space. Hence me.looking for a suggestion on Conformal Coating / Encapsulation
Click to expand...
I'm not suggesting you buy from them, but one excellent source of tech info across the range of possible compounds is: www.epoxies.com
 
R

RustyH

Senior Member
If I go down the route of a Polyurethane option, which from what I understand its a good choice for SMT populations (Potting Compound Types. I have found some from Farnell that start from a pretty good price, and then gradually increase.

Are the cheaper ones from inferior to the more expensive ones. Ive listed a few at Farnel, start with number 1 being the cheapest

1) PRO POWER - PPC125

2) ROBNOR - EL171C/566

3) ROBNOR - EL420C/NC/100

4) ELECTROLUBE - UR5041RP250G

5) ELECTROLUBE - EUR5562RP250G

6) ELECTROLUBE - EUR5083RP250G


Are the cheaper ones from inferior to the more expensive ones?
Any comments or recommendations?
 
rq3

rq3

Senior Member
RustyH said:
If I go down the route of a Polyurethane option, which from what I understand its a good choice for SMT populations (Potting Compound Types. I have found some from Farnell that start from a pretty good price, and then gradually increase.

Are the cheaper ones from inferior to the more expensive ones. Ive listed a few at Farnel, start with number 1 being the cheapest

1) PRO POWER - PPC125

2) ROBNOR - EL171C/566

3) ROBNOR - EL420C/NC/100

4) ELECTROLUBE - UR5041RP250G

5) ELECTROLUBE - EUR5562RP250G

6) ELECTROLUBE - EUR5083RP250G


Are the cheaper ones from inferior to the more expensive ones?
Any comments or recommendations?
Click to expand...
Inferior? No, just different products with different purposes. For instance, you'll tend to notice that black encapsulants have lower volume and surface resistivities because they tend to be tinted with carbon black. Not good for electronics potting. Now that you're down to the nitty-gritty, look at things like mix ratio (urethanes are sensitive to errors in mix ratio, and it's tough to split milligrams of accelerant in a small pour), viscosity (do you want to trowel on a paste, or pour a liquid), and transparency (it's easier to repair a bad connection if you can see it through the encapsulant). Some urethanes also require a high temp cure. A quick look at the data sheets you link would steer me to the EL420C. It's a pourable, translucent material with a 2:1 mix ratio, although the link goes to a data sheet with a different part number(?) It has high moisture resistance and excellent electrical properties. If you can, contact the manufacturer directly. They are often more than eager to provide fairly generous samples. Be aware that most urethanes will cure with ANY exposure to moisture, even atmospheric, with or without a catalyst or accelerant, so if they send you a half kilogram sample don't expect to keep it for more than 6 months or so!
 
rq3

rq3

Senior Member
Haku said:
This is pretty good stuff - http://www.plastidip.co.uk/

'Liquid rubber' in a spraycan or paint tin so you can (single/multiple) dip your object you want coated, available in lots of colours including transparent.
Click to expand...
Plastidip can be a good, inexpensive conformal coat. But you need to test it on your particular boards. Because it's solvent based there is a risk of dissolving solder masks and component designators.
 
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