How to protect connectors when potting - Page 1
Forum section: electronics / projects

author: mrkev
author_rank: Regular Contributor
timestamp: December 19, 2017, 10:51:41 am
content: Hi. I've made a module with microprocessor that I am using in other projects. I used PCIe mini connector, since it is small with many pins.The problem is, that the next project should be completelly potted in some sort of polyurethane and there are possible issues if it gets into the connector.What do you think? Can I somehow stop the resin/urethane from getting into the connector or is it a bad idea by itself and I should add some soldering pads to the module (forget that connector for this application).

author: tszaboo
author_rank: Super Contributor
timestamp: December 19, 2017, 10:58:04 am
content: If it is not a series production, I would give this a try.https://en.wikipedia.org/wiki/Blu_Tack

author: mrkev
author_rank: Regular Contributor
timestamp: December 19, 2017, 11:01:39 am
content: About a hundered pieces per year or two... Not serious production, but not exactly a homemade line either.

author: nicalitz
author_rank: Regular Contributor
timestamp: December 19, 2017, 04:28:35 pm
content: may i ask what issues you anticipate if the potting compound gets into the connector, specifically? might make it easier to suggest options. for many applications, it is desirable for the compound to do just that, seep into every small hole of a connector

author: mrkev
author_rank: Regular Contributor
timestamp: December 19, 2017, 07:09:23 pm
content: The worry is, that the polyurethane fixes the spring loaded contacts in place and there might be some issues with that connection. (because of difference in thermal expansion coeficient, expansion/shrinkage during setting, etc.)

author: ez24
author_rank: Super Contributor
timestamp: December 19, 2017, 07:17:25 pm
content: Quote from: NANDBlog on December 19, 2017, 10:58:04 amQuoteLast night I had trouble installing some 2 mm screws so I ordered some of this to try. Thanks

author: calexanian
author_rank: Super Contributor
timestamp: December 19, 2017, 11:40:43 pm
content: We had short production runs of a dozen or so units at a time. We used two solutions. For flatter surfaces we used peelable solder maskhttp://www.all-spec.com/Catalog/Soldering-Rework/Soldering-Chemicals/Solder-Masks/CM8-9700?gclid=EAIaIQobChMIrrjKl5-X2AIVB9VkCh3TSQueEAYYBCABEgI0i_D_BwEFor things with more of a 3D nature, such as filling connectors and such we used regular old silly putty.We would pot with a high grade epoxy, let it cure, then peel out the filler from wherever we had put it. It is important to use an epoxy that does not get too hot when using the silly putty.

author: Cerebus
author_rank: Super Contributor
timestamp: December 20, 2017, 01:23:19 am
content: If you're doing short run production then an alternative might be to make a small set or reusable silicone rubber plugs that you could push into the connector before potting. Get some of the 2 part silicone rubber putty*, mix it up, push it into the connectors on a small batch of boards. Once it's cured you'll have a plug that you can pull out, and more importantly use again on the next batch. Epoxy doesn't stick to silicone rubber, it'll be soft enough when cured to minimise the risk of damage to connectors and won't risk leaving bits behind stuck in the connector which I think might be an issue with blutack.*Widely available in small quantities on ebay and similar - people use it for craft moulding projects.

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 20, 2017, 02:35:05 am
content: Quote from: mrkev on December 19, 2017, 07:09:23 pmThat's right. Something to avoid at all costs. Spring loaded contacts are "springy" for a reason. I remember trying that in a project and we had a lot of connection failures.You may mitigate this by using gel-like compounds instead of hard ones. Silicone potting compounds for instance, which remain flexible once cured.(Take a look at:https://acc-silicones.com/products/encapsulants)The safer option is probably to mask the connectors during the potting process, like it's been suggested, though.

author: ez24
author_rank: Super Contributor
timestamp: December 20, 2017, 03:58:26 am
content: Quote from: Cerebus on December 20, 2017, 01:23:19 amIs this what you mean:https://www.amazon.com/GC-ELECTRONICS-10-8880-VOLTAGE-SILICON/dp/B00DECQFFK/ref=sr_1_15?s=industrial&ie=UTF8&qid=1513742026&sr=1-15&keywords=silicone+rubber+putty

author: ajb
author_rank: Super Contributor
timestamp: December 20, 2017, 07:01:47 am
content: Quote from: Cerebus on December 20, 2017, 01:23:19 amBut some potting materials might! It's important to verify compatibility against the actual material that will be used.Another option may be to use a viscous material, or a gel or putty, around the connector and then once that's cured fill the rest with a more conventional low viscosity material. Of course you'll need to verify compatibility between the two materials.Note that when it comes to fluids seeping into small spaces, there's a time component as well. The longer it takes to firm up, the farther a given material will make it into the nooks and crannies. Normally you want a reasonably slow-curing material for potting for that exact reason, but if you're concerned about it intruding into places you don't want a, then shorter gelling time will help with that.

author: Cerebus
author_rank: Super Contributor
timestamp: December 20, 2017, 08:00:46 am
content: Quote from: ez24 on December 20, 2017, 03:58:26 amProbably, it's a bit difficult to tell from that listing. I'd go for something that's specifically targetted at mould making. Here's an example:http://www.ebay.co.uk/itm/Mouldcraft-Fast-Sil-16g-1kg-Mould-Making-Silicone-Putty-RTV-Food-safe-Sugarcraft/252214469250?hash=item3ab9276282:m:m_8ZGOTub1YVEL3CMNzim3Q

author: Cerebus
author_rank: Super Contributor
timestamp: December 20, 2017, 08:11:25 am
content: Quote from: ajb on December 20, 2017, 07:01:47 amAgreed, but I've yet to find anything that will stick to mould making silicone rubber. Specifically I (or someone I know) have used all the following successfully with silicone rubber: 2 part epoxy, 2 part polyester resin, 2 part polyurethane resin and foam, sugar paste (fondant), chocolate, jelly, plaster, wax, low melting point metal alloys and silicone rubber itself. As mould materials go it's probably the best. Its only disadvantages are cost and the need for a rigid backing for larger moulds.

author: calexanian
author_rank: Super Contributor
timestamp: December 20, 2017, 04:05:07 pm
content: Yes. Material compatibilities should be investigated. Many postings are silicone these days. That is why we used latex masks. For potting these days we use a J Greer product. Their 21 style epoxy. You can find it from Aero Marine Company here in the States. It’s very reasonably priced and is a 1:2 mix. It cures to a glass like finish but has just enough give for things like wires and cables to exit. You don’t want a resin that is too hard or wires and cables may break at the point where they meet the resin over time.

author: Koen
author_rank: Frequent Contributor
timestamp: December 20, 2017, 08:11:23 pm
content: I use vaseline or custom-made silicone plugs. As said above, the gel time of the resin is an important factor.

author: babysitter
author_rank: Frequent Contributor
timestamp: December 20, 2017, 08:25:24 pm
content: Let me point at WIROSIL, used by dentists, its a moulding silicone that already has the form release agent already inside.

author: mrkev
author_rank: Regular Contributor
timestamp: December 20, 2017, 09:43:40 pm
content: These are great ideas, thank you. The problem is, that the final product should be potted fully (that means whole connector submerged).I tried to google this stuff, but there is not much info on reliability, but people mostly sudgest using only soldered joints. Another thing is, that if I pot everything but the insides of this connector, there is a possibility of condensing moisture in the air bubble at low temperatures.Now I don't know, it seemed to be a great solution - for everything else...

author: daqq
author_rank: Super Contributor
timestamp: December 20, 2017, 09:54:44 pm
content: Well, if it will be used in only one specific product you could simply solder the board onto the other board - avoid the connector all together. Not an elegant solution, but worth a try. I would try to avoid connections like this if possible.I don't suppose you could add some more solderable points to the MCU module? Make it a "dual-use" device, one assembly variation would include a small board to board connection such as a 1.27mm pitch dual row connector?

author: mrkev
author_rank: Regular Contributor
timestamp: December 20, 2017, 10:10:15 pm
content: That is probably what I will do. The point of doing it with the connector is in easy automatic debuging and programming of the device. I also wanted to use only SMD parts, so we won't have to use manual labour - which is exactly what any tru-hole header will force us to do.

author: Nusa
author_rank: Super Contributor
timestamp: December 21, 2017, 12:27:54 am
content: Quote from: mrkev on December 20, 2017, 10:10:15 pmYou can get machined pin headers in smd form, both male and female. So long as they're actually connected when you pot, I doubt you'd have a problem with the connector.

author: mrkev
author_rank: Regular Contributor
timestamp: December 21, 2017, 01:16:13 am
content: Female pin headers are just another form of spring loaded contact. That seems to be the same problem as the one I am having currently. So SMD headers will be OK, but you still have to solder them tru-hole on the other side.

author: Nusa
author_rank: Super Contributor
timestamp: December 21, 2017, 03:23:44 am
content: Quote from: mrkev on December 21, 2017, 01:16:13 amWhen I said SMD "machine pin", I meant something like this:https://www.mill-max.com/assets/pdfs/metric/037M.pdfWhen the pin is snugly in the matching socket, I don't see how any potting compound will get in where the spring clip is.

author: daqq
author_rank: Super Contributor
timestamp: December 21, 2017, 08:52:23 am
content: Well, there's another option - if one side of your board is not populated, you could probably use it as a module - solder the board directly onto the board (just like you'd solder some of the GSM or GPS modules), while retaining your PCIE connector. Not pretty, but could still work.