Potting PCBs with SMT components - what compound to use? - Page 1
Forum section: electronics / general

author: DTJ
author_rank: Super Contributor
timestamp: June 17, 2017, 12:07:48 pm
content: There's lots of threads here about potting compounds but I couldn't find what I wanted.I want to pot some PCBs (all SMT parts) that will be used submerged in water.The PCB is around 10mm x 100mm.It will be inside an open ended stainless steel tube.Potting compound needs to be reasonably 'runny' to get into nooks and crannies and expel all the air.Will be done in batches of 10 at a time (total 150CC of compound).Macromelt / Low Temp over-molding would be nice but is too expensive for this job.I tried one unit using a rigid black "electronics" epoxy with a CTE=75. The unit worked fine until I rapidly thermally cycled it from 0°C to 90°C.I think it died due to surface mount parts being fractured from the PCB.I'm thinking of perhaps a two part silicone or polyurethane with a bit more flex (even though some seem to have a higher CTE than epoxy).Maybe something with a Shore of around 40A-50A??Maybe something like Chemtools PCB-7000:http://www.altronics.com.au/p/h1621-chemtools-potting-compound-silicon-1kg-pct-7000y/I'd prefer an Aussie supply (RS / Element14 etc) as there can be shipping hassles with this sort of stuff.Has anyone any experience successfully potting boards with SMT parts on them?What did you use and where did you get it?Cheers.

author: DTJ
author_rank: Super Contributor
timestamp: June 18, 2017, 11:09:57 am
content: No one potting boards with SMD parts?

author: HighVoltage
author_rank: Super Contributor
timestamp: June 18, 2017, 11:16:27 am
content: For protection against water and humidity, I am using a 2 component silicone gel, made by Elantas, calledBectron SK 7504. It runs in to every little corner of the PCB and seals very well.http://www.elantas.de/europe/products/casting-potting/electronic-applications/applications.html

author: thtc
author_rank: Newbie
timestamp: June 18, 2017, 12:51:21 pm
content: For a few month, I also wanted to buy potting compound to protect a PCB (but sorry, they wasn't SMD components on the PCB)It have to be cheap and I found this product, which satisfied me very wellhttp://fr.farnell.com/pro-power/ppc125/potting-resin-pu-500g/dp/1776513?ost=1776513&searchView=table&iscrfnonsku=false&ddkey=http%3Afr-FR%2FElement14_France%2FsearchMay be, you find it at element 14

author: DTJ
author_rank: Super Contributor
timestamp: June 18, 2017, 01:27:06 pm
content: Quote from: HighVoltage on June 18, 2017, 11:16:27 amThanks HV. That looks promising. I'll hunt around and see if there's a Bectron supplier in Australia.Quote from: thtc on June 18, 2017, 12:51:21 pmThanks THTC. I can get that from Element14, the price looks ok too, $20/500g.I'll check out the data sheet for it in the morning.http://au.element14.com/pro-power/ppc125/encapsulant-polyurethane-resin/dp/1776513?ost=+PPC125&categoryIdBox=&selectedCategoryId=&searchView=table&iscrfnonsku=false

author: ahbushnell
author_rank: Frequent Contributor
timestamp: June 18, 2017, 03:31:47 pm
content: If you want to avoid voids, I suggest using vacuum degassing.

author: Koen
author_rank: Frequent Contributor
timestamp: June 18, 2017, 04:25:31 pm
content: Twinpacks will be degassed already. If you buy bulk or re-use twinpacks, degassing is a must. If you need a bit of volume, don't base your idea of prices on twinpacks. A 250g twinpack can be 25 EUR yet 5kg bulk will be 100 EUR only. Contact manufacturers, they'll help you choose from their extensive catalog.

author: floobydust
author_rank: Super Contributor
timestamp: June 18, 2017, 04:38:08 pm
content: Potting PCB's is such a hassle.Production ran into all the problems including what OP is seeing - expansion/contraction shearing off SMT parts with the hard epoxy potting compounds.During curing, the potting compound shrinks. Then there's thermal expansion coefficient. Two reasons for trouble.The thermal expansion coefficient of "electronic" potting compounds is low, but the two-part oven cured at say 60C and it's still enough to break parts, we were finding with the Henkel hard epoxy types.We resorted to using a soft silicone RTV (dip) coating first on the PCB and then encapsulate that in the hard potting compound. It's double the hassle and labour.I don't know what (water) pressure rating you need. The softer silicone encapsulants likeDow Sylgard 170are quite popular, good chemical safety etc. Problems with that stuff is air bubbles from aggressive mixing, so vacuum potting works best- that gets rid of the air bubbles/foam from mixing. You can also get trapped air bubbles pouring. Underneath a large part, unless you have extra flow holes on your PCB to allow the encapsulant to get in and bubble to escape. You may be OK with air bubbles, if no high voltage on the PCB etc.

author: DTJ
author_rank: Super Contributor
timestamp: June 21, 2017, 06:04:44 am
content: Thanks Koen, Floobydust & AHBushnell.Lots of good info for me to use.I think I will try some vacuum degassing. I've got the vacuum pump I just need to make up a quick vac chamber.It looks like Silicone is the route to take based on your comments and a chat with a company in Perth that sells all sorts of resins. They assure me a customer is using a silicone called Ultrasil to pot circuit boards for use in water immersed situations.Its about $60/kg and is a 2 part sytem. Its 25A Shore which is a little softer than I would like but if it works, who cares!I just hope its viscosity is low enough to fully flow around the PCB when its poured into the assembly.I'm not getting it from Barnes but here's the data sheet:http://www.barnes.com.au/condensation-curing/ultrasil-pro-silicone-1632

author: Koen
author_rank: Frequent Contributor
timestamp: June 21, 2017, 11:17:10 am
content: Quote from: DTJ on June 21, 2017, 06:04:44 am30.000 mPas is very viscous !

author: Kean
author_rank: Unknown
timestamp: June 21, 2017, 01:35:36 pm
content: I've been using Raytech Magic Gel. It is a re-enterable gel, so maybe not quite what you're after if you need rigidity or a pressure rating, but could be worth a look.It is available from element 14 - get a quote as you will probably get a better price than list. I recently bought 4L of it for a temporary outdoor/semi-immersed installation (Vivid).

author: DTJ
author_rank: Super Contributor
timestamp: June 21, 2017, 02:10:21 pm
content: Quote from: Koen on June 21, 2017, 11:17:10 amYep that's whats worrying me. They had another type with a lower viscosity, 20000 from memory.Quote from: Kean on June 21, 2017, 01:35:36 pmThanks, I'll bookmark that stuff for future use. It's too soft for the current job.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 21, 2017, 02:53:56 pm
content: In the factory I work for, we pot or conformal coat everything we do. Over 500 different products.We use several brands and types, based on board size and intended end-use application.The very best, from the sealing point of view, is the hard epoxy. But as others have correctly mentioned, it is very tough on SMT components. We use it exclusively for boards no larger than 1.5 X 3 cm. Yet, sometimes we do have sheared solder joints.Anyways, the best overall potting compound we use is Tyco Electronic's Guronic.It is expensive though, but according to its website, it is available in the ANZPAC region.http://www.te.com/usa-en/plp/guronic/X25xxT4B.html

author: HighVoltage
author_rank: Super Contributor
timestamp: June 21, 2017, 05:05:03 pm
content: When potting PCB's with epoxy, we always apply a thin layer of soft, rubber like PolyUrethan on all SMD components first. Otherwise the mechanical stress is far too high on the components and can cause big problems down the road.

author: plazma
author_rank: Frequent Contributor
timestamp: June 21, 2017, 05:36:23 pm
content: Quote from: HighVoltage on June 21, 2017, 05:05:03 pmCan you tell the exact products you use for potting?

author: floobydust
author_rank: Super Contributor
timestamp: June 21, 2017, 05:47:33 pm
content: Regulatory requirements are another constraint for potting compounds.Flammability UL-94 and high-voltage tracking CTI rating is not always provided by the manufacturer.The environmental hazardous chemical/electronic recycling/RoHS is another pile of paperwork.Just take extra time to select your compound, depending on where you are selling your products. I spent 4 months finding one with global approvals for hazloc use.

author: Koen
author_rank: Frequent Contributor
timestamp: June 21, 2017, 11:24:46 pm
content: Quote from: DTJ on June 21, 2017, 02:10:21 pmYou'd need something below 3000 to have an easy task. Below 1000 to not worry at all. If you wish an easy test and an easy supply, pick up a 250g twinpack from Electrolube in stock at Element14 AU. If it's successful, ask your local distributor for a quote on bulk. The price will drop drastically. I use their products but the hard type so I can't really help you on softer products.

author: Kean
author_rank: Unknown
timestamp: June 22, 2017, 02:12:43 am
content: Yeah, in Australia it is worth a chat with Oritech or Mektronics. They carry MG Chemicals, Electrolube, various resins, coatings, etc.

author: BravoV
author_rank: Super Contributor
timestamp: June 22, 2017, 03:44:24 am
content: Quote from: HighVoltage on June 21, 2017, 05:05:03 pm+1 , have a friend used this Urethane spray as coating, once dried, then he just dip the whole board into epoxy resin which will be hardened like a rock, it works quite well for years through high span thermal cycle and he manufactures electronics for food processing machinery.For example he used this product below, it is quite reliable which can endures longterm up to 100C and downto -70C, with breakdown voltage at 20 KiloVolt/mm.Just google for it's datasheet, its quite concise what its capable of.

author: DTJ
author_rank: Super Contributor
timestamp: June 23, 2017, 02:43:47 am
content: I thought I should update what I've tried.I purchased some RTV 3428 silicon potting compound.Colour - translucent.Cure time > 6 hours.Durometer - Shore A28.Viscosity < 28000 - its like really thick honey but even small bubbles seem to work their way out to the surface. It seems to flow well into every where. I'm going to set up a clear housing to check it's really getting in everywhere. I'm going to vacuum degas the next test.Adhesion - weak, it doesn't really stick at all to PCBs, stainless or PVC. It readily peels off. I'm not sure if this is an issue or not.I need to do a submerged pressure test to check for water ingress.Data sheet:http://www.kirkside.com.au/Uploads/Images/rhodorsil-rtv-3428.pdf

author: mrpackethead
author_rank: Super Contributor
timestamp: June 23, 2017, 07:09:07 pm
content: Im a huge fan of macromelt.. Unforutnatly the distribution in NZ/OZ is very broken.. But bypassable making it much more affordable.

author: jimmeh11
author_rank: Newbie
timestamp: June 26, 2017, 03:04:14 am
content: +1 for MG Chemicals. Had lots of luck with their 832-C clear epoxy and complex PCBs. Reasonably cheap, decent dielectric strength, good datasheets (my favourite) and easy to get in Aus.

author: DTJ
author_rank: Super Contributor
timestamp: June 26, 2017, 04:39:08 am
content: I've purchased some of the CRC-2049 as recommended by HighVoltage & BravoV. I'll probably try it in conjunction with some epoxy.The RTV-3428 seems to seal well and despite its high viscosity (~20000) it .definitely gets in everywhere and has no bubbles.The only problem is that it does not adhere to anything at all - PCBs, stainless, PVC - it very easily peels straight off.I'll look at MG Chemicals 832C. A bit pricey at RS, $44 / 375mL. Specs and shelf life look good.http://au.rs-online.com/web/p/potting-compounds/9184986/Do potting silicones usually bond well to the boards & housings (plastic/metal)?

author: jimmeh11
author_rank: Newbie
timestamp: June 26, 2017, 07:51:00 am
content: There is a pretty good comparison of the advantages of silicones vs epoxys vs urethanes for potting purposes here:https://lordelectronicmaterials.wordpress.com/2013/10/29/what-type-of-material-should-i-use-epoxy-urethane-or-silicone/One thing I have found is that plenty of manufacturers are keen to show off the specs of their many products, but whether those products actually exist to be able to purchase is another thing, and anything not stocked in Australia is likely 3 months lead time away.

author: noras
author_rank: Contributor
timestamp: June 26, 2017, 10:15:40 am
content: My preference:Neutral cure silicone rubber as the long term anti-vibration and sealing agent. Expect 20+ year lifetimes.If initial mechanical location is required, hot-glue (hot melt glue) only to hold things in place while the silicone rubber sets. [Hot glue is a completely unsatisfactory long term solution !!!].https://electronics.stackexchange.com/questions/18525/what-kind-of-glue-should-i-use-for-pcb-mounted-components-to-avoid-vibrations

author: CJay
author_rank: Super Contributor
timestamp: June 26, 2017, 10:27:26 am
content: Dow Corning make some lovely silicone potting compounds for electronics in harsh environments and have/had a nicely responsive tech team who can help you, they'll even send out samples for you to test.Worth an email if you've a company email address.

author: DigitalDeath
author_rank: Regular Contributor
timestamp: June 27, 2017, 08:45:25 am
content: 832FX - BLACKFLEXIBLEEPOXY ENCAPSULATING AND POTTING COMPOUNDMG Chemicals.

author: DTJ
author_rank: Super Contributor
timestamp: June 27, 2017, 09:57:34 am
content: Quote from: DigitalDeath on June 27, 2017, 08:45:25 amThe 832FX looks good but I can't find it at any Australian suppliers.http://www.mgchemicals.com/products/potting-compounds/epoxy-potting-compounds/black-flexible-epoxy-encapsulating-and-potting-compound

author: DigitalDeath
author_rank: Regular Contributor
timestamp: June 27, 2017, 07:08:35 pm
content: You can also go to a local industrial epoxy resin supplier and ask for encapsulating epoxy and a compatible toughener additive. The additive makes the resin resistant to impact by making it flexible. It adds some sort of elastomer process to the mix.

author: sdouble
author_rank: Frequent Contributor
timestamp: July 14, 2017, 12:21:20 pm
content: I'm hacking that thread since I have a similar need.For a high impedance circuit (a charge integrator) which needs to be included into a detector, I need a potting which exhibits the following characteristics :-Low outgassing-very high resistivity-high voltage capable ( 1kV / 1mm)-moderates cure temperature if applicable-SMD compliant.I'm currently using an epoxy bought from RS, but not a big fan : outgasssing is not specified and obviously quiet strong, even after 1 week of cure.Money is not a major issue : I don't need kilos.thanks for you help !

author: DTJ
author_rank: Super Contributor
timestamp: July 14, 2017, 01:04:10 pm
content: Not sure if it helps you Sdouble (I initiated this thread).........I've ended up using a two pronged solution. I needed waterproof potting for SMD parts.I'm spraying the boards with the polyurethane coating first, then potting in a grey PUR from RS.So far it seems ok. I'm part way through testing the water proofing an will do the temp cycling tests in the next week or so.https://www.crcindustries.com.au/products/corrosion-protection-and-inhibitors/urethane-seal-coathttp://au.rs-online.com/web/p/potting-compounds/1991395/The RS potting compound sets quite firmly - probably comparable to a firm jogging shoe sole hardness.I've been cold curing, the data sheet says its a 34 hour cure but I found it was pretty much fully set after 12 hours at less than 20°.