This sounds like you need to seal the box. "Seal" doesn't mean, "fill".
I kept humidity out of some lasers (which were inside boxes) without any potting at all.

Yeah, I have that option as the last resource I have: RTV silicone + rubber gasket. It's a damn hard work to find all the o-rings/gaskets of the right measure and thickness of your screws and holes. A magic fluid like polyurethane simplify your life: you mount everything, flood the box, screw the lid and you're done. But maybe this is just a dream.

I used "dry nitrogen" as a filler because it did not contain humidity that would condense on the lenses at low temperatures.

nitro... whaaaaat?
Everyday I discover something new on this forum. This forum is like drug!

 

There are waxes that melt as high at 500°F or higher. They are used to measure the temperature of steel during tempering or preheating for welding. If course, I wouldn't suggest using such a high melting one, but you could probably find one that melts higher than the highest motorcycle exposure and lower than solder melts.

Interaction of a sulfide with copper could be a problem that needs to be answered. Aircraft aluminum alloys do contain copper. For example, the common 2024 has 3.8 to 4.9% copper.

John

 

I know nothing about waxes for potting electronics. Why they are not broadly offered by potting-compund vendors? Maybe there is some hidden drawback? For example, may I make the wax fluid to flow inside a box around a PCB?

 

Another english vendor of polyurethane for molding:

...water, ozone and UV resistant as well as being widely resistant to oil, grease and chemicals.

but

...INCOMPATIBLE MATERIALS: Amines, strong bases, alcohols, water, copper and copper alloys.



edit: somebody can explain me how can be a material water-resistant and, at the same time, incompatible with water???

 

Those incompatibilities are possibly related more to the polymerizing agent/catalyst, not the cured compound. However, that is probably a moot point in your application.

As for the waxes, a lot of potting is done to prevent copying, and waxes are easily removed.

John

 

Those incompatibilities are possibly related more to the polymerizing agent/catalyst, not the cured compound.John

What you write could be true. Many of the technical sheets that I have checked (the ones reporting the copper-incompatibility) are describing just the part-A of the the two compound mix (part A + part B), not the mixed and cured compund.
Maybe this commercial/cheap polyurethanes could be really used for pcb-potting??? mmmmh... still unsure on this....

Time to go bed for me!!!

 

Here is one polysulfide that is even recommended for potting: http://www.masterbond.com/tds/ep21tplv

Some elastomers are extremely expensive. I would assemble a list of those that are not too expensive for my application and then confirm with the manufacturer suitability for use in potting copper PCB's. If you go at it from the other direction, you might find a great product that costs $500/250 mL.

John

 

I saw master bond prods before, unfortunately they doesn't sell online and/or in Europe.

John if you will find any data about copper please post here what you discovered. In the meanwhile I will send some emails to some cheap polyurethane producers.

Little Off Topic--------------------
I've found dozens and dozens vendors that does not publish prices on line. I don't like them, I hate this "big" vendors with corporate websites. You have to contact their service, provide data of your application spend time with them and bla bla bla. At the end you discover that they will sell you 25 mL of loctite glue at 500$ (+ 75$ for freighting cost).
------------------ end off topic

 

By reading this old (no more working?) website I think I've got the keys to the polyurethane-compounds world.
As I understand, the key concept is the combination of two components that give to a compound particular properties: POLYOL and ISOCYNATE. In other words:

POLYOL X + ISOCYNATE Y = compound Z

so different combinations of X and Y will produce different results Z. Polyol and isocynates are identified by numbers, for example

POLYOL30008 POLYOL30020,...and so on
ISOCYNATE40002, ISOCYNATE40020,... and so on

EFI Polymers is the sole manufacturer of compounds for electronic potting listing whar components are combined:


I strongly suspect that behind bombastic names used by famous potting companies (e.g. 3M, Loctite and other) there is just a particular combination of polyol and isocynate that they keep hidden.

My final consideration: I think that if you can discover what components are used in a commercial/cheap compound for molding/casting then you can have an high quality potting without killing your wallet (or avoiding that your wife kills you).

 

What you say is generally true with plastics/polymers. It is quite common to have one component and various second components to change its properties and visa versa. Sometimes a plasticizer is added that can have significant effects on properties. The plasticizer(s) may not be listed or may be listed in some generic way because they are a low percentage of the total gemisch. As you point out, the problem is finding exactly what is in the mixture, not just the generic labels.

It looks like you are well on the way to getting an economical solution. I have found that Henkel, which owns Loctite, is hard to get detailed information from. Before its purchase, Loctite, which used to be just a few miles from where I lived in Cleveland, was easier to get through to knowledgeable technical service representatives.

John

 

Throughout this thread I haven't seen anyone mention two of the more important properties of potting compound: thermal conductivity and the thermal expansion coefficient. Maybe conductivity is not so critical in this application, because the power dissipation is low, but on a motorcycle I think the device will see temperature swings up to 120°F or so. If the potting compound expands more or less than the Arduino, it will eventually tear it apart with a sort of freeze-thaw stress.

 

Throughout this thread I haven't seen anyone mention two of the more important properties of potting compound: thermal conductivity and the thermal expansion coefficient. Maybe conductivity is not so critical in this application, because the power dissipation is low, but on a motorcycle I think the device will see temperature swings up to 120°F or so. If the potting compound expands more or less than the Arduino, it will eventually tear it apart with a sort of freeze-thaw stress.

In my first post I said that I've selected polyurethane as tradeoff between shrinkage problems and costs. It is not the best material, but it's fair better than epoxy resin (for digital electronics).

About thermal conductivity: I know that it would be better to have a good heat conduction, but I saw only some professional (high costing) stuff offering that. Do you know any cheap filler that can be used?
Anyway the thermal-cycle problem should be minimized by using a combination of polyol and isocynate producing a "soft" compound.

 

Of course, synthetic diamond powder would work great. As a cheaper alternative, ordinary glass beads or sand might be better and cheaper than the pure resin.

https://upload.wikimedia.org/wikipedia/commons/1/1e/Thermal_conductivity.svg

John

 

Thanks a lot John, instructive chart!
I will go for the cheaper alternative; I've found 5 liter of small beads for 6,95 Euro.

 

In my first post I said that I've selected polyurethane as tradeoff between shrinkage problems and costs. It is not the best material, but it's fair better than epoxy resin (for digital electronics).

About thermal conductivity: I know that it would be better to have a good heat conduction, but I saw only some professional (high costing) stuff offering that. Do you know any cheap filler that can be used?
Anyway the thermal-cycle problem should be minimized by using a combination of polyol and isocynate producing a "soft" compound.

Polyurethane is a great material because there are countless combinations of polyols and isocyanates and various additives and blowing agents allow you to get materials ranging from skateboard wheels to seat cushions. You should be looking for polyurethane elastomers (skateboard wheels) instead of foam if you want to keep the elements out and entomb your part until the end of time. They are going to come at a bit higher cost than the foam. But, then again, you are not buying air, you are buying only polyurethane.

 

- UPDATE -
Some feedbacks from manufacturers of "low-cost" polyurethane - the one used for molding - about electronic potting and copper corrosion problem.

Polytech:

I will check with our manufacturing folks but
I do not believe it would be an issue. I have
attached some tech info for your review.

Stan Chickey
Technical Sales
610-559-8620 x 106 Office
610-559-8626 Fax

S u. K Hock GmbH (a little german manufacturer)

Hello,
the use of the resin with a copper mold or box
is no problem. We don´t see any complications
with the combination.

Mit den besten Grüßen aus Regen/ kind regards
Florian Stadler
S u. K Hock GmbH

Straßfeld 12 | D- 94209 Regen

So, first inputs seems positive. I will keep you informed.

 

This thread is kind of old, but what the heck. I work for a DOD contractor. What you need to look for is a material conforming to MIL-I-46058 / IPC-CC-830 (this is the conformal coating spec). There are several favors or types (TY); AR - acrylic, SR - silicone, UR - urethane, ER epoxy, XR - paraxylyene (paralyene, vacuum deposited).

Go to the government website "ASSIST", and search for the qualified parts listing for MIL-I-46058 (QPL-46058). Within this document you will find everything that conforms to the specification, which by the way is equivalent to the IPC spec. Along with each conforming material you will find a cage code, and at the end of the document cage codes are defined.

There are lots of materials that could be used which are not qualified to this spec, but, it is a benchmark with respect to resistivity, temperatures, curing methods, etc.

 

This thread is kind of old, but what the heck. I work for a DOD contractor. What you need to look for is a material conforming to MIL-I-46058 / IPC-CC-830 (this is the conformal coating spec). There are several favors or types (TY); AR - acrylic, SR - silicone, UR - urethane, ER epoxy, XR - paraxylyene (paralyene, vacuum deposited).

Go to the government website "ASSIST", and search for the qualified parts listing for MIL-I-46058 (QPL-46058). Within this document you will find everything that conforms to the specification, which by the way is equivalent to the IPC spec. Along with each conforming material you will find a cage code, and at the end of the document cage codes are defined.

There are lots of materials that could be used which are not qualified to this spec, but, it is a benchmark with respect to resistivity, temperatures, curing methods, etc.

Oh, and by the way, this should satisfy all of the sparkys (EEs)...and if not, you are pointed in a very specific direction.

 

Well... I will check the documents that you indicate, anyway the polyurethane resin is working well until now.
I've potted together a TFT display and related electronics (arduino included) with zero problems. It's solid like a rock and I cannot see any appreciable degradation of the material until now.

 

..the polyurethane resin is working well until now.

Which one did you end up using?