Water ingress into potted electronics via the cabling - Page 1
Forum section: electronics / projects

author: DTJ
author_rank: Super Contributor
timestamp: April 16, 2023, 05:23:09 am
content: I've got some issues with water ingress into some potted electronics.The immersion depth is only 30cm or so.The electronics is potted in epoxy with the cable sheath and then the insulated cable conductors inside the epoxy.I trust the epoxy as it has been used hundreds of times in a very similar application.I'm still in the early stages of this but I suspect the water is getting in between the epoxy and the outside of the cable insulation.Has anyone come up with a simple effective means to fix this?I know fancy ozone (or something like that) etching of the cable insulation can be done to improve the epoxy bond but I'm hoping there's a simpler primer / sealer that can be used.Cheers.

author: mzzj
author_rank: Super Contributor
timestamp: April 16, 2023, 11:12:52 am
content: This would really depend on the cable sheath material. PTFE vs PVC needs totally different approach. Epoxy in general bonds poorly to plastics.

author: redkitedesign
author_rank: Regular Contributor
timestamp: April 16, 2023, 11:38:09 am
content: Water ingress in submerged assemblies is really hard to prevent, especially along openings in the assembly.Whenever possible, the easiest way to deal with it is to let it happen and remove the water in an other way (pump it out). Its what boats do :-)However, these people have posted some experiences you might find usefull:https://thecavepearlproject.org/2023/03/17/waterproofing-your-electronics-project/

author: PwrElectronics
author_rank: Regular Contributor
timestamp: April 16, 2023, 03:31:39 pm
content: Even if the cable jacket is not the source of leak, fluids can wick on the inside along the wire as well. So, the other end of the cable must also be sealed.

author: jonpaul
author_rank: Super Contributor
timestamp: April 16, 2023, 06:39:28 pm
content: we use Gardner Bender conduit putty.Also RTV 100% Silicon rubber calkj

author: 2N3055
author_rank: Super Contributor
timestamp: April 16, 2023, 07:04:09 pm
content: Quote from: PwrElectronics on April 16, 2023, 03:31:39 pmYep.I had water ingress into sealed sensor box through 10m length of cable via capillary action because box on the other side was damaged and water went in...

author: John B
author_rank: Frequent Contributor
timestamp: April 17, 2023, 12:02:32 am
content: If you've ever used untinned copper wire outdoors, you'll probably have seen the creeping green death inside stranded wire.I would think you'd need a solid core wire bulkhead on the enclosure to ensure water can't penetrate into the enclosure.

author: MisterHeadache
author_rank: Regular Contributor
timestamp: April 17, 2023, 01:13:00 am
content: I work in the OEM auto sensor industry. There is noeconomicaladhesive method or material that makes a reliable bond to low surface energy materials, which most wire insulation and jackets are made from. So the common solution is to use a silicone rubber gasket, either an individual one for each insulated wire (a sealing 'donut') or a single-piece with holes for each wire (a 'manifold'). Look under hood in the engine compartment - all the sealed connectors use one of these approaches.

author: ajb
author_rank: Super Contributor
timestamp: April 17, 2023, 03:29:01 am
content: So the cable jacket is embedded in the encapsulant? Could you fit a sealing bulkhead gland to the box before potting? Or address the issue at the other end of the cable?Quote from: MisterHeadache on April 17, 2023, 01:13:00 amI've been looking at automated wire processing equipment lately, and was initially surprised how big a deal seal handling was. Makes a lot of sense for the automotive market!

author: DTJ
author_rank: Super Contributor
timestamp: April 18, 2023, 12:24:58 am
content: Quote from: MisterHeadache on April 17, 2023, 01:13:00 amThanks - that's interesting.I doubt I'll find an off the shelf seal to suit. Would a single small tight fitting O ring on each of the wires work?I assume this works by the epoxy bonding to the O ring with the O ring squeeze on the wire providing the real seal?Cheers

author: DTJ
author_rank: Super Contributor
timestamp: April 18, 2023, 12:26:41 am
content: Quote from: ajb on April 17, 2023, 03:29:01 amYep. The sheath penetrates about 15mm then there's about 20mm of wire where the cable sheath is stripped.There's no opportunity for a gland or anything like that.

author: dmendesf
author_rank: Frequent Contributor
timestamp: April 18, 2023, 12:58:36 am
content: Seems that a cold plasma pen can activate surfaces and improve bonding. Problem is that they are expensive. Someday I will try to make one.

author: tooki
author_rank: Super Contributor
timestamp: April 18, 2023, 07:02:55 am
content: If cost is no factor, how about commercially available vacuum feedthroughs? For high vacuum and ultra high vacuum applications, there are companies that make feedthroughs — metal flanges with integrated wires. I have no idea whether these would work to keep water out at pressure, but I imagine that water pressures can be significantly larger. But ultra high vacuum requires essentially no leaking no matter what, so maybe it’s robust enough anyway.Or maybe there are equivalent manufacturers of submarine feedthroughs.

author: AndyBeez
author_rank: Frequent Contributor
timestamp: April 18, 2023, 08:44:59 am
content: I have no idea about the mechanical design of your enclosure but, one extra barrier to ingress is to place the component inside chandler's or maritime grease. This is the salt water resistant and repellant grease as used on prop shafts. It's sometimes branded as "stern grease" or "K99". About $20 a kilo.

author: TZ400T3B
author_rank: Contributor
timestamp: April 18, 2023, 09:20:22 am
content: Search for dielectric gel used to secure cable connections in boxes buried in ground. It sells in tubes same as silicone. It adheres to everything and is rubbery, exactly opposite to very unsuitable epoxy. Long ago it was plain asphalt used for this, heated and poured over. Worked too but the gel is much better.

author: dmills
author_rank: Super Contributor
timestamp: April 18, 2023, 12:41:07 pm
content: This is a well known issue in the sonar industry, but I am a little surprised to see it at such shallow depths.The cure is generally something like:Encapsulate the end of the jacket using something like a Pur resin that will bond properly to both the jacket AND the insulation of the individual conductors, this may involve some chemical/mechanical fuckery if PTFE wiring is in play.Then encapsulate the ends of the individual conductors, we used to do this by designing the boards with a solder pad and strain relied hole, conductor end is prepared, bought thru the strain relief hole, soldered to the terminal, inspected and then the end of the insulation encapsulated with a drop of epoxy resin.Finally pot the whole thing in something that will bond to the PUR encapsulation and the epoxy.Yea, it is a faff, but particularly cables not made for underwater service tend to leak.

author: DTJ
author_rank: Super Contributor
timestamp: April 19, 2023, 01:30:51 pm
content: Quote from: dmills on April 18, 2023, 12:41:07 pmI too cannot understand why its an issue in such low pressure situations.I like that approach, I've been searching to find a compound that will bond well to PVC cable sheath AND to epoxy. Then I could paint a little onto the wire insulation and it would be ready to pot.

author: thermistor-guy
author_rank: Frequent Contributor
timestamp: April 20, 2023, 02:55:22 am
content: Quote from: DTJ on April 16, 2023, 05:23:09 amHas the same cable been used to the same extent, in a similar application? Has the cable changed? Has something elsein the assembly or assembly process changed?Two thoughts come to mind re the cable. I don't know if they are relevant here.First, there is a phenomenon in watercooling PCs, where water gradually diffuses (permeates) out through some types oftubing. To the uninitiated, it looks like a problem with seals and couplings. But no, it's just a property of the tubing material.Second, it reminds me of a problem I've seen in a ship-to-shore power cable, carrying 100s of amps. The cable jacket wassupposed to be impermeable to water, but it wasn't. The problem was not in the synthetic rubber material itself, but in the catalystused to cure it.The catalyst was hydrophilic, and there were small amounts of it left in the cable jacket, enough to compromise insulation resistanceover time. Nowhere was this catalyst property mentioned in the cable data sheet.

author: ajb
author_rank: Super Contributor
timestamp: April 20, 2023, 02:33:08 pm
content: Quote from: DTJ on April 19, 2023, 01:30:51 pmIs there perhaps some air space enclosed by the encapsulation? As the temperature of an enclosed air volume changes so does its pressure, which could contribute to the problem. The air space wouldn't necessarily need to be connected to the cable entry if the encapsulant is soft enough.

author: redkitedesign
author_rank: Regular Contributor
timestamp: April 20, 2023, 03:47:16 pm
content: Quote from: DTJ on April 19, 2023, 01:30:51 pmGiven enough time, water will (slowly) leak through every material discontinuity. Tightening the seal will only limit the rate. Increasing the water pressure will increase the rate.Air inside the product will expand/contract with temperature changed. That will give pressure changes that can influence leakage.And very few materials are really impenetrable for water. I think only metals and glass are really impenetrable, the rest are just more-or-less leaky. It's just that above the water, the leakage is small compared to evaporation, so it is never an issue.

author: dmendesf
author_rank: Frequent Contributor
timestamp: April 21, 2023, 06:18:51 pm
content: Some pointers about vacuum seals... Just remember how tubes are made:https://en.wikipedia.org/wiki/Glass-to-metal_sealI can find a bunch for RF:https://www.alibaba.com/product-detail/Glass-to-Metal-Hermetic-Seals-RF_60807284004.htmlBut nothing cheap related to water. I'm also interested in products in this field. Maybe it's time to melt some glass...