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Encapsulating electronics in candle wax?!
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CatalinaWOW:

--- Quote from: The Soulman on November 21, 2020, 07:40:40 pm ---
--- Quote from: CatalinaWOW on November 21, 2020, 07:19:11 pm ---Paraffin will be fabulous from a thermal control standpoint (up to a point).  It melts pretty low, at a temperature well below those commonly reached by electronic components.   The kinds sold here in the US have a melting point somewhere in the 37 C to 66 C range and it is often not specified.  You pay your money and take your chances.   The heat capacity is huge so you will have to pump a lot of heat to get your components above that melting point.

But there is your mess potential, and also the problem of having a solvent fluid moving contamination around.  If you don't totally melt it it may self contain, but it is easy for a melt hole to develop.  And if something gets really hot there is fire hazard.  Flash point is around 200-250 C, not very high.

Shrinkage can be dealt with in a number of ways, but is a real issue.

--- End quote ---

Fire hazard is slim to none, as the (aprox.) 100cc of wax only needs to dissipate 100mW of power at room temperature.
On top of that it would be encapsulated in a almost airtight cast aluminum housing.

Could you explain: " and also the problem of having a solvent fluid moving contamination around.  If you don't totally melt it it may self contain, but it is easy for a melt hole to develop."?

--- End quote ---

Any liquid wax generated by melting can transport what it come in contact with throughout the liquified zone.  Something that wasn't an issue in its original location may become one elsewhere.

If your board undergoes thermal cycles it may become fractionated.  Lower melting point parts of the wax separating from higher ones.  Think zone refining.  This could result in the melt zone propogating to an edge or joint where a leak could occur.  So this is better than oil fill, but not absolutely rock solid.

People doing voltage references are usually concerned about mechanical stresses on the reference.  For this application I would be more than usually concerned about the shrinkage.
Gyro:

--- Quote from: The Soulman on November 21, 2020, 07:53:59 pm ---
--- Quote from: Gyro on November 21, 2020, 07:23:32 pm ---
--- Quote from: The Soulman on November 21, 2020, 07:12:04 pm ---
--- Quote from: Ian.M on November 21, 2020, 06:58:55 pm ---Shrinkage?

--- End quote ---
In my application the pcb itself doesn't have to be encapsulated, just the to-92's is enough, and could solder the leads afterwards to avoid strain.

--- End quote ---

I'm curious that one of the reasons for doing this is moisture protection, but you're more concerned about the TO92 packages than the PCB?

--- End quote ---

Good question, I should have mentioned its for voltage reference build from a bunch of LM329 in to-92 housing for lack of availability
(discontinued) of the hermetically shielded metal can version.
It is believed the voltage over time is more stable for a hermetically shielded device because of the lack of moisture ingress/change
with seasonal changes.
Alternatively I could use a LM399 but also isn't available at the moment (and costs 4 times more).
Btw, just a one-off hobby project.

--- End quote ---

Ah, ok that makes sense (I think). Just a thought though, Paraffin contains a lot of water, I've no idea if Paraffin Wax does too.

Ideally you would go for a hermetic enclosure - soldered copper sheet or foil with hermetic feedthroughs (ebay, former Soviet) for humidity level stability.
PlainName:

--- Quote ---If you want more thermal mass add a small aluminum tab, very cheap.
--- End quote ---

That's not quite the same thing. An ally tab would just take the heat off to somewhere else and would thus make this part track the temperature of wherever the heat is going. The candlewax, though, would tend to keep the temperature the same - it would have thermal inertia[1] - since the latent heat of fusion could be quite significant. Obviously, it's not a continuous thing but if the product is being cycled then a sold-liquid transformation (and back) could be just the thing required to keep the temperature constant.

A quick google suggests it might be ideal:


--- Quote ---Paraffin wax is an excellent electrical insulator, with a resistivity of between 1013 and 1017 ohm metre.[11] This is better than nearly all other materials except some plastics (notably Teflon). It is an effective neutron moderator and was used in James Chadwick's 1932 experiments to identify the neutron.[12][13]

Paraffin wax is an excellent material for storing heat, with a specific heat capacity of 2.14–2.9 J g−1 K−1 (joules per gram kelvin) and a heat of fusion of 200–220 J g−1.[14] Paraffin wax phase-change cooling coupled with retractable radiators was used to cool the electronics of the Lunar Roving Vehicle during the manned missions to the Moon in the early 1970s.
--- End quote ---

---

[1] Probably not the right term here, but couldn't think of the correct term at this time :)

james_s:
I've seen some kind of wax used in a lot of radios from the 70s-90s to cover things like coils and such to keep them from being disturbed. I don't know if it's paraffin though, and I haven't seen it used to pot an entire circuit.
PlainName:

--- Quote ---The OP does not want to use oils but paraffin instead, which to me are more or less the same,
--- End quote ---

Well, again, not quite. The specific advantage of candle wax is that it's a solid, which means it will take significantly more energy to raise the temperature past the phase change to a liquid than it would to raise liquid oil the same amount. The same could be said of ice/water or water/steam, etc, but candle wax just happens to do its phase change about the right temperature for electronics.

Edit: Something similar might be chocolate. Don't know the electrical properties but I suspect it might mysteriously evaporate once its presence is known.
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