Removable water proof potting / encapsulant material?

[DTJ]

I'm trying to find out if an encapsulant material is available that has the following properties:

1) Removable - can easily be dug out / peeled / dissolved to remove and replace a sensor that would be soldered in place.
    The sensor would be in a well on the outer face of a larger solid housing.
2) Is soft and flexible - maybe around 50A shore. Soft like silicone rubber or soft neoprene.
3) Is suitable for water immersion over the range say 0°C to 50°C.

I started out looking at potting wax but found some silicones, Sylgard 182 and 183.

https://www.ellsworth.com/products/by-market/consumer-products/encapsulants/silicone/dow-corning-sylgard-182-silicone-encapsulant-clear-0.5-kg-kit/

https://www.ellsworth.com/products/by-market/consumer-products/encapsulants/silicone/dow-corning-sylgard-184-silicone-encapsulant-clear-0.5-kg-kit/


Has anyone actually used these materials - what are they like to remove?

Has anyone other suggestions?

Cheers.

[tecman]

I do not have a product name, but some of our products, made in germany, use a red, soft rubber compound for potting.  It can be dug out and easily cleared from the inner components, but is water/moisture blocking.

paul

[helius]

https://www.mgchemicals.com/products/conformal-coatings/silicone-conformal-coating-422b

[DTJ]

I do not have a product name, but some of our products, made in germany, use a red, soft rubber compound for potting.  It can be dug out and easily cleared from the inner components, but is water/moisture blocking.
paul

That sounds like what I'm looking for. At least I know it exists! Thanks.

[DTJ]

https://www.mgchemicals.com/products/conformal-coatings/silicone-conformal-coating-422b

Thanks Helius. I'm not sure if that will work as I need a thickness of 10 or 12mm. I can't see any data on thickness / curing etc.

[tautech]

I do not have a product name, but some of our products, made in germany, use a red, soft rubber compound for potting.  It can be dug out and easily cleared from the inner components, but is water/moisture blocking.
paul

That sounds like what I'm looking for. At least I know it exists! Thanks.

Sounds just like RTV sealant.

[Nauris]

Maybe just fill our product with grease? It is easy to rework and resists water well. Somewhat messy thou.

[helius]

Thanks Helius. I'm not sure if that will work as I need a thickness of 10 or 12mm. I can't see any data on thickness / curing etc.

For that thickness you are definitely in RTV potting territory and not conformal coating as the product I linked. I have used it and the thickness was more like 0.25mm.

[CopperCone]

Honestly I would recommend that you get a box that's IP rated and then pot the lid of the box (if its desired for extra reliability).

Unless something is under alot of pressure or needs shock resistance (research this well), potting may degrade the life expectancy due to mechanical issues, (and I mean extra one time shock resistance for artillery and missiles) for instance

http://www.m-coat.com/ICSR%20Paper.pdf
http://www.circuitinsight.com/pdf/effect_encapsulation_materials_tensile_stress_ipc.pdf


And repair is really fucking nasty. Based on what I read, I would consider it a bad solution.

Even for shock loads, I would recommend doing as much as you can with 3d printed precision supports for things like capacitors, and liberal use of mounting screws (not just in say, 4 corners). And of course following torque specifications for tightening, and using higher grade bolts and threads to ensure torque stability.

A precision made plastic or metal enclosure that actually mates with the components (well not perfectly, but reducing the amplitude of vibrations that can occur) I think would be better still, but I suspect this would be difficult to engineer.. automotive stuff uses these press fit connections that are more reliable then solder in vibration environments. Problem with machined enclosure might be that the height of the components is minutely different due to the soldering process.

I think also, encapsulants may encourage solder whisker growth in lead free solders, but I am not sure on this, and I would have to find statistics.

Also, I think that a very soft encapsulant may decrease reliability during vibration because it will want to deform and stretch in the middle like a trampoline. I am not sure on this, but it is my mechanical hunch here. If you have difficulty imagining this, then look at a non newtonian fluid on a speaker, or fat man homer simpson running, the lard/starch kinda moves as a group, growing in thickness in the middle and stretching on the edges, with the surface tension holding it together, so this may result in a larger then normal strain force in the middle of a potted PCB. I would recommend adding a screw in the center of mass (probably direct middle) if a soft potting must be used, to prevent bowing. I am not sure if the potting material meets the criteria for this effect. If a enclosure with a strong, tight lid is used, and there is zero air gap in the enclosure, then this could not occur, but it would have to be rigid.

If a flowable material is used, that is still kinda goopy, a leak could make this occur, as there will be extra mass that wants to be displaced under or above the PCB.

[DTJ]

Honestly I would recommend that you get a box that's IP rated and then pot the lid of the box (if its desired for extra reliability).

Unless something is under alot of pressure or needs shock resistance (research this well), potting may degrade the life expectancy due to mechanical issues, (and I mean extra one time shock resistance for artillery and missiles) for instance

http://www.m-coat.com/ICSR%20Paper.pdf
http://www.circuitinsight.com/pdf/effect_encapsulation_materials_tensile_stress_ipc.pdf

Unfortunately an off the shelf or custom enclosure is not practical. The entire assembly is already epoxy encapsulated and we want to fit a replaceable sensor on the side of the instrument.

It's in a shock free environment so thankfully we don't have to deal with those issues.

Thanks for the reference material links.

[Conrad Hoffman]

I've used the 2-part silicones and you want everything clean and probably want to use the primer they suggest. Even then, the weak point is the adhesion to surfaces where leakage can occur. Think about it- if the adhesion is great, removal will be difficult. It it isn't, it will likely leak. Lots of old electronic standards were covered with a plate, then the area above potted in hot rosin. Worked well and could be serviced, at least for some years. 50 years later the stuff can be very difficult to melt out. It's basically road tar!