Does anyone have any recommendations for potting compounds or other sealants for weatherproofing electronics?
20 years ago I used transparent two part silicone with great success, but I did have access to a vacuum pump for getting rid of the air bubbles.
Today I don't have a vacuum pump therefore I need something that has a low viscosity, or a slow setting time so that air bubbles have a chance to escape before it sets.
Ideally it would be transparent enough to allow LEDs to be visible and soft enough to allow re-work.
Locally I can get polyurethane, epoxy 'with fillers' and various types of two part silicone rubber which are mainly designed for mold making, not electronics (as far as I can tell).
At the moment I'm using boat builders epoxy with solid glass beads (as used in reflective road markings) as a filler to reduce the volume required and increase thermal conductivity, but this is impossible to re-work.
you know... i dont know what it does when you get it hot... maybe I will drop a regulator or resistor or something into the next batch and run it up warm like and see if there is any negative effects.
Discussions on otherpower.com will provide some detail as to what happens to epoxy when it gets hot -- it's used to encapsulate the stator coils which as you can imagine get quite hot. More or less you'll burn up your electronic components before you burn up the epoxy.
If you want to thin a two part epoxy you will have good luck using denatured alcohol at up to 5% by volume.
I was more thinking about the components not being able to shed heat. Right now I am working on a project where a couple of components would way over heat with out air flow.
I am encasing shiftbrite modules with it. sets long enough to let bubbles out. using a candy mold to shape it
for a test I dropped an encased module in the pool and ran it for a few hours. no problems..
but like I said.. once its done its permanent
Is there much shrinkage?
As far as I am aware, polyester casting resins usually suffer from high shrinkage during cure which puts a mechanical stress on components.
I suppose a filler such as sand may reduce this to some degree, but it doesn't seem to be widely used in industry.
jackrae:
a vacuum cleaner may be good enough to get the air bubbles out. Also warming epoxy type fillers with a hair drier reduces their viscosity.
I found a video on YouTube, "How to make a simple Silicone Mold Part 2" where they show you how to de-bubble silicone by pouring it from a height so that the stream thins out. They call it the 'bombs away' method.
For weatherproofing it may be simpler to use a conformal coating, I use fine-l-kote SR by techspray for waterproofing motherboards. It works great.
Hideously toxic while spraying though, and not cheap ($17-$25/can).
As far as I am aware, polyester casting resins usually suffer from high shrinkage during cure which puts a mechanical stress on components.
I suppose a filler such as sand may reduce this to some degree, but it doesn't seem to be widely used in industry.
I havent noticed any shrinkage on the outside surface (don't know how I would test the inside).
when cured it is still very snug in the mold, I have to work to get it out even with release agent.
To remove tiny air bubbles on the surface you can fire up a blowtorch and sort of quickly brush it across the surface with the flame at a steep angle. I think that the heat expands the air inside and so pops the bubbles.
I have used this because it is very clear: Epoxy : Kleer Koat Table Top Epoxy
Note all of these products will yellow or cloud a bit when exposed to UV (sunlight for example) over several years.
I've started doing some tests with RHODORSIL RTV 3428 two part 'addition cure' silicone rubber.
It's very viscous compared to epoxy so you need to use a spoon or scales when measuring quantities.
The 'bombs away' de-bubble step still leaves tiny bubbles, but is probably good enough for my needs.
The working time is about an hour and there doesn't appear to be an exotherm so there's no need to rush what you are doing and large volume pours are OK.
The silica sand I add to increase the thermal conductivity sinks to the bottom and is well embedded.
The cured rubber is translucent, but not clear. It has the typical milky white appearance you see in other silicone products.
Once cured, it easily detaches from the plastic spoon and cups I use for mixing which allows them to be reused.
It's compatible with plasticine so you can make your own casting containers without worrying about the cure process being inhibited.
Conformal coating is technically just clear acrylic paint, factories use it with an added pigment that glows under black light so they can see to remove it from 'keep out' areas. Thats how military/medical grade electronics help protect against moisture and condensation. But clear acrylic can be had, you can find it in a spray can they use in arts supply as a 'fixative' or you can hand paint it on.
Your average 2 part thermal potting compound isn't cheap but I've used it and it works well.
