Anyone familiar with potting material for automotive electronics?

[Ge0rge Marutz]

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

 

[Martin Riddle]

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

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

 

[Ge0rge Marutz]

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

 

[Jim Thompson]

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

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

 

[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

www.tough-seal.com
makes rubbery epoxy potting they claim prevents the thermal shearing
you mention.

 

[[email protected]]

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

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

greg

 

[Macgyver]

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

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.

 

[Martin Riddle]

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

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

Cheers

 

[Roy L. Fuchs]

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

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

 

[Ge0rge Marutz]

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

 

[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.

 

[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

 

[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

 

[Phil Hobbs]

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

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

 

[Roy L. Fuchs]

Another thing that might be ocurring is that when the potting absorbs
moisture it swells.

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.

http://www.ellsworth.com/display/productlisting.html?vendorID=135&Tab=Vendors

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.

 

[Roy L. Fuchs]

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?

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?

 

[Rich Grise]

Martin,

Dow Corning says primer is mixed right in with the material.

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.

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.

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

 

[Roy L. Fuchs]

- apparently, this NG isn't swamped with
auto stuff designers. ;-)

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.