Encapsulating electronics in candle wax?!  - Page 1
Forum section: electronics / general

author: The Soulman
author_rank: Super Contributor
timestamp: November 21, 2020, 06:48:42 pm
content: For a low voltage dc project I'm looking for a medium to encapsulate a bunch of TO-92's.The goal is to shield the circuit from moisture and provide some thermal mass/bonding anything better than regular air is an improvement in that respect.Traditionally oil is used for this application but I don't like the idea of the mess that could make and potentially humiditygathering at the bottom of the bath.Alternatively a commercial potting compound could be used but is not only expensive but also very permanent.How about paraffin wax?Electrical and thermal parameters are great, cheap, no mess when cured etc.Looks ideal, apart from the fact that the electronics need to survive 80 degrees Celsius for longer period (preheat, poor, cool-down),should be fine for my application.Any obvious reason why it wouldn't work?

author: Ian.M
author_rank: Super Contributor
timestamp: November 21, 2020, 06:58:55 pm
content: Shrinkage?

author: The Soulman
author_rank: Super Contributor
timestamp: November 21, 2020, 07:12:04 pm
content: Quote from: Ian.M on November 21, 2020, 06:58:55 pmGood point, need to experiment see how much and why (rapid cool-down?).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.

author: Gyro
author_rank: Super Contributor
timestamp: November 21, 2020, 07:13:29 pm
content: Quote from: Ian.M on November 21, 2020, 06:58:55 pmYes, from my wife's candle making experiments, shrinkage is significant.

author: PartialDischarge
author_rank: Super Contributor
timestamp: November 21, 2020, 07:18:21 pm
content: Don't do that, you're trying to reinvent the wheel, when the wheel was invented long time ago and works fine. You'll have paraffin dripping and a mess.If you want more thermal mass add a small aluminum tab, very cheap.If you want moisture protection use a spray of dielectric lacquer

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 21, 2020, 07:19:11 pm
content: 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.

author: Gyro
author_rank: Super Contributor
timestamp: November 21, 2020, 07:23:32 pm
content: Quote from: The Soulman on November 21, 2020, 07:12:04 pmI'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?

author: The Soulman
author_rank: Super Contributor
timestamp: November 21, 2020, 07:31:19 pm
content: Quote from: MasterTech on November 21, 2020, 07:18:21 pmHow would the paraffin be dripping? when pouring you mean?Aluminium tap connecting 30 to-92's together is rather impractical to say the least.Lacquer spray or dip could be an option but can't see why that would be less messy.Thanks for the input tho.

author: The Soulman
author_rank: Super Contributor
timestamp: November 21, 2020, 07:40:40 pm
content: Quote from: CatalinaWOW on November 21, 2020, 07:19:11 pmFire 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."?

author: The Soulman
author_rank: Super Contributor
timestamp: November 21, 2020, 07:53:59 pm
content: Quote from: Gyro on November 21, 2020, 07:23:32 pmGood 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/changewith 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.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 21, 2020, 10:01:47 pm
content: Quote from: The Soulman on November 21, 2020, 07:40:40 pmAny 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.

author: Gyro
author_rank: Super Contributor
timestamp: November 21, 2020, 11:14:45 pm
content: Quote from: The Soulman on November 21, 2020, 07:53:59 pmAh, 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.

author: PlainName
author_rank: Super Contributor
timestamp: November 21, 2020, 11:33:17 pm
content: QuoteThat'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- 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 googlesuggests it might be ideal:Quote--- Probably not the right term here, but couldn't think of the correct term at this time

author: james_s
author_rank: Super Contributor
timestamp: November 21, 2020, 11:55:42 pm
content: 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.

author: PlainName
author_rank: Super Contributor
timestamp: November 22, 2020, 01:25:01 am
content: QuoteWell, 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.

author: Ian.M
author_rank: Super Contributor
timestamp: November 22, 2020, 03:25:11 am
content: It may be worth experimenting with adding a small proportion of medical grade liquid paraffin to the wax to soften it to reduce stress due to shrinkage and to make small cracks more or less self-healing.Back to the aluminum idea - its reasonably simple (albeit tedious) to drill a bunch of holes in an aluminum block to isothermally couple TO-92 devices. As package stress must be minimized for this application, they should be slip-fit in the holes with heatsink compound to improve thermal coupling, and should be soldered to the PCB after seating them in the block.

author: tautech
author_rank: Super Contributor
timestamp: November 22, 2020, 07:32:19 am
content: Quote from: james_s on November 21, 2020, 11:55:42 pmAnd earlier.I've seen PCB's almost covered in wax and if the stress from cooling paraffin was sufficient to damage a modern SMD design I'd eat my hat !

author: Siwastaja
author_rank: Super Contributor
timestamp: November 22, 2020, 08:50:50 am
content: Do not try to pot. It's more complex than you think. You'll spend a year getting it right. Before you get it right, it significantly reduces the reliability of your PCB assembly, cracking capacitors and so on.And when you are prototyping and building small batches, 99% likely you want to open the product up to measure something, even if you were completely sure that won't be needed.Just buy a waterproof case with proper silicone gaskets / o-rings and you're done.

author: magic
author_rank: Super Contributor
timestamp: November 22, 2020, 10:38:27 am
content: Quote from: The Soulman on November 21, 2020, 07:53:59 pmI wonder if wax is really less permeable to moisture than epoxy resin.You could try to seal the epoxy package in a TO35/TO3 metal can (2N2219/2N3055).Still, I think long term changes in temperature may slowly affect the distribution of trapped moisture between the epoxy and the air, unless everything is dehumidified before assembly.

author: David Hess
author_rank: Super Contributor
timestamp: November 22, 2020, 04:18:38 pm
content: A mixture of various things is commonly used with paraffin wax to improve its performance as potting compound. Paraffin wax by itself is not very good but adding 20% beeswax considerably improves it.

author: coppercone2
author_rank: Super Contributor
timestamp: November 22, 2020, 07:15:35 pm
content: I would recommend burning beeswax in the house to provide positive ions and to stick to reasonable solutions like liquid spray on electrical tape for electronicsIt is acutally good. You can make good quality cables terminations by wrapping them with self amalgamating silicone tape (all the rage in the hardware store glue section) then spraying on 3 layers of liquid electrical tape. Impressive product, works better then plasti-dip IMO. And I have successfully used it after 5 years storage.Warning, 1 layer will look bad. You need to add a few layers to make it look like a solid thing, the spray is not perfect enough to leave a nice deposition with a single pass, much worse then spray paint.

author: Potato
author_rank: Contributor
timestamp: June 22, 2021, 07:48:57 pm
content: Scented candles would be lovely

author: rsjsouza
author_rank: Super Contributor
timestamp: June 22, 2021, 08:15:45 pm
content: Quote from: tautech on November 22, 2020, 07:32:19 amSame here... I lost count of how many fully working 1960s radios I have seen full of wax holding parts together, especially the cores of IF coils and to isolate the resistors from the shields of these coils.

author: thm_w
author_rank: Super Contributor
timestamp: June 22, 2021, 08:49:31 pm
content: Paraffin thermal conductivity is ~0.15 W/mKBeeswax 0.25Air 0.024Thermal potting 1.0-4.0+Maybe find some non-conductive fillers to add in there:https://iopscience.iop.org/article/10.1088/1742-6596/1369/1/012022/pdfOne video mentioning they tried a "paraffin based wax" for selective PCB coating:https://www.youtube.com/watch?v=Zhj7zOzjh3UThe old RF stuff uses beeswax according to here:https://www.antiqueradios.com/forums/viewtopic.php?t=146657

author: TimFox
author_rank: Super Contributor
timestamp: June 22, 2021, 11:46:37 pm
content: Back in graduate school, we encapsulated high-voltage resistors inside acrylic tubes using radio-grade beeswax (which was a thing in my youth).The practical problem was, in fact, shrinkage. The shrinkage is worst right at the melting point, so one would leave the partially-filled object overnight, and find that the wax had shrunk away from the resistor when it cooled below the liquidus temperature. We had to spend a long time with partial pours to minimize this problem.