Anyone familiar with potting material for automotive electronics?

I am looking to bounce a few ideas off somebody with experience in potting materials for an automotive electronics application. That and some general physics and possibly thermodynamics.

Can you help?

Thanks,

Ge0rge

Reply to
Ge0rge Marutz
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Not an expert but I can forward some links on epoxies.

There are standard OT Shelf potting compounds for automotive, and most Thermal poxies can handle fairly high temps.

Anyone come across poxies with SG of 4 or greater? Like to know.

Cheers

Reply to
Martin Riddle

Hell, I use black silicone from the local hardware store. And yes, I actually sell my stuff.

Reply to
kell

I'm having problems with water intrusion through Silicon potting material. Mositure is getting in some how and becoming trapped against the PCB. Once trapped electrolysis and other nasty things start to occur.

We aren't considering epoxy since it is a rigid material. I'm affraid that parts would be sheared off the board or damaged due to the differences in expansion and contraction of materials.

Ge0rge

Reply to
Ge0rge Marutz

Been a VERY long time since I've done this, but I recall buying plastic boxes open only along one face.

Insert circuit board with connecting leads sticking out through the opening.

Partially fill with a soft encapsulation, covering all the electronics.

Top off/seal with epoxy??? I'm not sure what we used, but cured it was the consistency of a very hard rubber material.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Reply to
Jim Thompson

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makes rubbery epoxy potting they claim prevents the thermal shearing you mention.

Reply to
kell

I always understood silicone to be microporous letting moisture through. A layer of enamel is good to put on before the silicone.

greg

Reply to
szekeres

We are using an inexpensive 2-part liquid Polyurethane Prepolymer potting material (from Era Polymers in Aus) to pot a printed circuit board in a bulkhead style box (one end open) for harsh outdoor use up to temperatures of 60 deg C. This material has good electrical properties and has a Hardness/Shore A of 95 (similar to tap washers), meaning that it will not shatter like epoxy potting resins, or crack due to environmental stress and vibration (as it is slightly soft and has some "give"). For us this was the best option for both water protection and security (makes it much more difficult for people to remove the potting compound and see what's inside). Normally this stuff is transparent but we use a black colouring additive to maintain the mystery of what's inside.

Reply to
Macgyver

against

You may need to prime the surface. We had used SylGuard Potting compund and it required a primer to stick to certain surfaces.

Cheers

Reply to
Martin Riddle

On 22 Mar 2006 06:24:29 -0800, "Ge0rge Marutz" Gave us:

The best thing to do is to conformally coat the PCB before encapsulating it. Make sure also to use the specified PRIMER material for the potting to achieve and maintain good adhesion between the potting medium and the assembly.

For our HV PCB designs, we leave the mask OFF of the HV area, as the matte finish of the non masked PCB adheres to the potting media better. So we do not conformally coat first.

What you are experiencing is likely a delamination between the potting and the assembly, allowing air gaps (and hence water intrusion). The proper primer is essential to maintain the adhesion between these two.

The "black silicone" mentioned previously is a BAD solution. For one thing, it has to flow around the parts and into all the various nooks and crannies of the assembly, so the potting media has to be "potable". Potability is a measure of useability (usually rated in time increments) for potting media that consists of pourablity, flow characteristics, and the capacity to have the air bubbles removed by way of vacuum. The thick tube dispensed silicon rubbers he is likely (not assuredly though) using are too rigid from the get go to use and get good performance and repeatability. This is another key step in adhesion as the vacuum allows the media to work its way into the tiniest places on and in the PCB and the assembly. It also allows the last bits of water (PCBs are hygroscopic) to be gassed out before the potting "vulcanizes". An oven cure at a typical temp of 60C is usual for these products, though many cure in air over a much longer period.

General Electric makes some extremely good potting media, but they have to be bought through a distributor. BTW, REAL potting compounds are NOT cheap. If you product is a commercial product, expect it to be a major portion of the COM. RTV-11, and RTV-625 are good.

The most reliable (NASA approved) media is polyurethane encapsulants like "Epon" resins

Reply to
Roy L. Fuchs

Thanks for the input Greg. We don't coat the PCB prior to assembly into a housing and potting because there are too many keep out zones. We have to mask off too much of the board and componentry. We experimented with conformal coating but found it to be more expensive and higher risk than potting when all was said and done.

George

Reply to
Ge0rge Marutz

Macgyver,

I didn't add this earlier but should have. The material I use needs to be rated to 125C. We ruled out polyurethane because of this. It is only good up to around 90C.

Reply to
Ge0rge Marutz

Martin,

Dow Corning says primer is mixed right in with the material. We get excellent adhesion off the assembly line. The problem seems to occur out in the field after the assembly has been subjected to moisture, heat, and vibration for a few weeks. Moisture is finding its way in and working its way through the material to the board.

Ge0rge

Reply to
Ge0rge Marutz

Roy,

I prefer not to conformal coat the board prior to potting for reasons mentioned in one of my last messages. It requires too much masking. This is labor intensive and not very reliable. I also mentioned earlier that we obtain excellent adhesion off the line. The delamination and fluid ingress only seem to occur on a specific iteration of our product over time.

Here is what we think is happening. Our housing is constructed of glass filled Nylon. The housing has two chambers. One holds fluid, the other houses the PCB which is encapsulated in silicone potting material. The fluid chamber is heated. We believe hot fluid is absorbed by the Nylon housing then migrates through the Nylon to the potting. The potting absorbs this moisture and fluid migrates towards the cooler PCB. Once fluid hits the PCB it stops. It has nowhere to go so it slowly collects on and around the PCB.

Another thing that might be ocurring is that when the potting absorbs moisture it swells. The swelling action causes it to shear away from the PCB. Air pockets form. Water can get trapped in these air pockets.

We are in the process of considering various fixes now. The problem being we need to prove our hypothesis before proceeding. I don't want to wander off into the wrong direction. This would waste too much time and money. Does what I say make the least bit of sense?

Ge0rge

Reply to
Ge0rge Marutz

Yep. Most polymers are surprisingly poor moisture barriers, Nylon being among the very worst. A very thin layer of metal will dramatically reduce the water diffusion rate.

A friend of mine makes dust-free dessicant cakes by mixing zeolite with two-part silicone rubber--the moisture goes through the silicone and gets trapped in the zeolite. They work great.

(Cross-posted to sci.materials)

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

On 24 Mar 2006 13:46:36 -0800, "Ge0rge Marutz" Gave us:

Proving that you adhesion isn't as good as you think it is.

If you are using silicone based potting material, then you MUST contact your supplier and obtain the proper primer media. It also has to be applied right (dipped, not sprayed), then cured, before the potting is applied. Also, if you BAKE your potted assemblies, they will settle in at the temperature your expect them to experience in the field, or a median temperature between the most common cold temperature, and the most common hot temperature expected.

The PCB assemblies MUST be absolutely clean, as in hot alcohol bath. If water solubles are used, the assemblies MUST be baked out, if not placed under vacuum for a period of time to get ALL of the water out of them. If there is ANY sulfur or sulfur gassings occurring (such as with heat shrink tubing), it WILL inhibit silicone potting curing with many types of silicone based potting compounds.

I stated that conformal coating wasn't required. You DO need to prime the assemblies though. I would be willing to bet that is the issue. A properly primed assembly makes it through thermal cycling up to 70C in our lab and production environments, and some of our gear has a quart of potting in them each.

I now remember the only NASA approved potting compound for HV space applications. It will NEVER delaminate, even at space level thermal cycles (near absolute zero through 200+C) It is "CONAP". It is expensive. It is guaranteed not to fail, it will stop bullets, and you cannot even stab a knife through it more than a quarter inch with a hard strike. It has better adhesion without priming than ANY other material known to man that is not a firm solid. It is flexible. It is polyurethane.

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You should talk to Ellsworth about your problem. If you are close, they will send a technical rep to you to help you fix your problem, and they KNOW what takes place in such settings.

They could also likely get what you use for you cheaper! :-]

Good luck.

Reply to
Roy L. Fuchs

On 24 Mar 2006 13:46:36 -0800, "Ge0rge Marutz" Gave us:

Sure, but you NEED to vacuum your potted assemblies right after the pour to remove micro sized air bubbles which contribute to thermal expansion. That is a required step in potting. Priming is another. Talk to the guys at Ellsworth, they may be able to fix you aver the phone. What type of potting material are you using?

Reply to
Roy L. Fuchs

This must be some kind of miscommunication. "Primer" is to prime the surface, before the material is applied. See if you can get anybody at DC to verify this.

Either you're not using the right stuff, or you're not using it right. Call your DC guy and ask him what's wrong with his stuff - somebody has been potting auto stuff for a very long time, maybe decades, so somebody clearly knows how it's done - apparently, this NG isn't swamped with auto stuff designers. ;-)

Good Luck! Rich

Reply to
Rich Grise

On Sat, 25 Mar 2006 01:59:28 GMT, Rich Grise Gave us:

It isn't about an "auto" application. It is about an ENVIRONMENTAL application.

You spout some of the most retarded crap a technical newsgroup ever gets posted into it. You "rise to the top" like an airy turd, boy. You read other posts then make shit up as you go along.

Reply to
Roy L. Fuchs

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