Fillers to reduce potting compound usage? - Page 1
Forum section: electronics / beginners

author: e100
author_rank: Frequent Contributor
timestamp: October 19, 2016, 08:48:07 am
content: How do people minimise the amount of potting compound used when dealing with circuit boards with tall components which therefore has a large amount of void space?I don't see this mentioned in any of the manufacturer's application notes.I would guess that in a commercial environment people don't worry about wastage, whereas for a hobbyist spending $50 for 500ml of resin is a major cost.In the past I've used "pool filter sand" which has 0.5mm grains, but it's not exactly the ideal material as it mostly sinks to the bottom.Is there any granular material that is cheap, and of a similar density to potting compound, so that it doesn't all sink to the bottom or float on the top? Can you buy small quantities of recycled FR4 fragments, or recycled silicone rubber fragments or something like that?Mike

author: BradC
author_rank: Super Contributor
timestamp: October 19, 2016, 08:49:55 am
content: I've seen granite used before. Available from landscaping and garden centres.

author: e100
author_rank: Frequent Contributor
timestamp: October 19, 2016, 09:18:14 am
content: Quote from: BradC on October 19, 2016, 08:49:55 amYou got me thinking, I searched for "crushed gravel", "decomposed granite" then "fish tank gravel" which lead to what appears to be recycled glasshttps://www.petbarn.com.au/fish/aquarium-ornaments-plants-gravel-decor/aqua-one-betta-gravel-glass-blue-350gAs this is for aquariums, I would guess that it has been flame treated to dull the edges and remove the dust.Granite has a density of 2.75 and glass has a density of 2.5 which are both a little high compared to potting compound which is around about 1.1 - 1.5 depending on the type.

author: Ian.M
author_rank: Super Contributor
timestamp: October 19, 2016, 09:47:31 am
content: Any fibreglass lamination/mouldings materials supplier will sell you glass microballoons, in quantities ranging from a litre to a 200L drum. They are subminature hollow glass bubbles, with a consistency like extra fine smooth 'powder' sand, but a fraction of the weight. Shelf life in a sealed container is indefinite - there's nothing to deteriorate if you protect them from contamination. They are inert to any chemicals and environmental conditions the potting compound can tolerate, don't increase its tendency to absorb moisture and are non-conductive. However they will decrease its thermal conductivity considerably. Here's one of3M's glass bubble ranges, though for 99% of applications the brand is totally unimportant.Any additive may compromise the properties of your potting compound and unless you are equipped to vacuum de-gas it after mixing vastly increase the risk of bubbles due to air entrainment while mixing, so unless you can de-gas it, don't use any for high voltage potting applications, and finish with a final layer of un-'extended' potting compound if it is likely to be directly exposed to moisture.

author: RogerRowland
author_rank: Regular Contributor
timestamp: October 19, 2016, 10:37:14 am
content: Reminds me of this recent post on SE.EE:http://electronics.stackexchange.com/q/262104/52562

author: BurningTantalum
author_rank: Frequent Contributor
timestamp: October 19, 2016, 10:46:32 am
content: I have seen quite coarse grained sand and other minerals used in recent Chinese products.Many years ago I used to use slate powder obtained from a local stone mason (gravestones) but have heard of talcum also being used. The potting compound needs to be of a very fluid type as it thickens up with addition of the powder.The cost needs to be low or else there is no point in using it.BT

author: Ian.M
author_rank: Super Contributor
timestamp: October 19, 2016, 10:50:48 am
content: <LOL> It couldn't get much cheaper & nastier unless they conformally coated the board, poured in concrete then gave it an epoxy dip for appearances sake once cured!You can easily buy shotblasting glass beads for as little as $0.30/Kg without committing to Tonne quantities, which will be relatively pure clean recycled glass with objectionable fines and large fragments removed so you know the manufacturers of the module shown on SS.EE are cheapskate asshats and the other parts used are almost certainly gutter-recycled and river-washed.

author: SimonWasAnEngineer
author_rank: Contributor
timestamp: October 19, 2016, 10:52:21 am
content: Quote from: Ian.M on October 19, 2016, 09:47:31 am+1I know they used this stuff in the company I worked for, but I cant remember in which ratio resin to filler it was.

author: Ian.M
author_rank: Super Contributor
timestamp: October 19, 2016, 11:13:04 am
content: You just add the stuff till you have whatever consistency you need, ranging from 'ketchup' to 'peanut butter'. It doesn't have the same thixotropic effect as colloidial silica, and the bubbles are smooth and readily slide over each other. A light pour of pure compound, followed by tilting in every direction to distribute it would be a good idea to prevent voids under components and between closely spaced ones, then add the 'extended' compound while its still wet.It may be worth talking to a specialist epoxy resin supplier's technical support department - there may be cheaper alternatives in their laminating resin range to whatever specialist potting compound you are using. Some epoxies retail for as little as $30/L and if you conformally coat the board first to provide a compliant layer to reduce stress due to shrinkage, and are prepared to pour it in layers, allowing each to harden to a gel consistency between pours to allow the heat produced by curing to dissipate, (as in a sufficient volume it can even self-ignite!) with fillers added you could probably get the total cost down to $15/L

author: e100
author_rank: Frequent Contributor
timestamp: October 19, 2016, 12:58:31 pm
content: Quote from: Ian.M on October 19, 2016, 11:13:04 amI'm using the cheapest renterable potting compound I could find which is the Raytech "Magic Gel" (http://www.raytech.it/product/low-voltage/magic-gel?lang=en). It's a two part silicone. The cure is endothermic and takes about 20-30 minutes.The marketing materials shows it being used for mains voltage electrical splices, but my guess is that it's basically the same as the potting compound sold at higher prices for potting PCBs which is what I'm using it for. The current price is about $AU130 per litre.The problem with micro sized filler particles is that the high surface area increases the viscosity which in turn makes it harder for bubbles to find their way to the surface before it sets. The current project I working on is too big to fit inside my vacuum degasser. I can degas before pouring, but not after.I see ~1mm solid glass beads on Ebay for about $AU10 per kg which equates to about 400ml in volume. These will be semi-transparent inside the resin which is an advantage for post inspection.

author: Neilm
author_rank: Super Contributor
timestamp: October 19, 2016, 06:29:57 pm
content: Quote from: e100 on October 19, 2016, 08:48:07 amYou are kidding - if you have to pot something you want to ensure it is as efficient as possible. Any wastage comes straight out of your profit. Especially true if working in volume.What is cheap for a commercial unit would be to make some special boxes (vac formed) profiled to put enough of the potting compound on the PCB while using the minimum amount of compound.