conformal coating

[John Larkin]

Hi,

can anybody suggest a good conformal coating? I have a PCB that is
moisture sensitive; apparently the PCB itself absorbs enough moisture
to change the laminate dielectric constant enough to cause me trouble.
We coated the critical oscillator section with household polyurethane
varnish, and it helped a lot; absorption after 48 hours at 100%
humidity is down by about 3:1, and the recovery time constant is much
better (shallower moisture penetration?). But this stuff is thin, and
we'd like something gloppier that we can apply a bit thicker. We'll
brush it on by hand, to keep it exactly where we want it.

Any suggestions? We'd prefer something that sticks well, goes on
fairly thick (high viscoscity) and is reasonably easy to rework. Who
makes good stuff these days?

John

 

[Greg Neff]

On Mon, 08 Nov 2004 11:16:54 -0800, John Larkin
<jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

Hi,

can anybody suggest a good conformal coating? I have a PCB that is
moisture sensitive; apparently the PCB itself absorbs enough moisture
to change the laminate dielectric constant enough to cause me trouble.
We coated the critical oscillator section with household polyurethane
varnish, and it helped a lot; absorption after 48 hours at 100%
humidity is down by about 3:1, and the recovery time constant is much
better (shallower moisture penetration?). But this stuff is thin, and
we'd like something gloppier that we can apply a bit thicker. We'll
brush it on by hand, to keep it exactly where we want it.

Any suggestions? We'd prefer something that sticks well, goes on
fairly thick (high viscoscity) and is reasonably easy to rework. Who
makes good stuff these days?

John

Type AR (acrylic) is the easiest to work with by hand, and the easiest
to rework. In high volume type UR (urethane) is prevalent. For small
jobs you will probably need something that is solvent cured. You will
need good ventilation to work with it though. UV cure is best, since
you don't have the solvent to evaporate. We have UV curing equipment
but this is expensive. You might want to get something with a UV
tracer in it for inspection. Look for MIL-I-46058C compliant
coatings.

For AR hand-work we use Humiseal 1B73 in spray cans.

http://www.humiseal.com/protect/acrylic.htm

================================

Greg Neff
VP Engineering
*Microsym* Computers Inc.
greg@guesswhichwordgoeshere.com

 

[ChrisGibboGibson]

John Larkin wrote:

Hi,

can anybody suggest a good conformal coating? I have a PCB that is
moisture sensitive; apparently the PCB itself absorbs enough moisture
to change the laminate dielectric constant enough to cause me trouble.
We coated the critical oscillator section with household polyurethane
varnish, and it helped a lot; absorption after 48 hours at 100%
humidity is down by about 3:1, and the recovery time constant is much
better (shallower moisture penetration?). But this stuff is thin, and
we'd like something gloppier that we can apply a bit thicker. We'll
brush it on by hand, to keep it exactly where we want it.

I've often wondered about this.

Surely you must accept that you will *never* seal it against *some* form of
moisture ingress.

If that's the case then surely, once equilibrium with the outside world is
reached, conformal coating will hold the mositure *in* as well as it will hold
it *out*.

What have I missed?

Gibbo

 

[John Larkin]

On 09 Nov 2004 02:08:06 GMT, chrisgibbogibson@aol.com
(ChrisGibboGibson) wrote:

John Larkin wrote:


Hi,

can anybody suggest a good conformal coating? I have a PCB that is
moisture sensitive; apparently the PCB itself absorbs enough moisture
to change the laminate dielectric constant enough to cause me trouble.
We coated the critical oscillator section with household polyurethane
varnish, and it helped a lot; absorption after 48 hours at 100%
humidity is down by about 3:1, and the recovery time constant is much
better (shallower moisture penetration?). But this stuff is thin, and
we'd like something gloppier that we can apply a bit thicker. We'll
brush it on by hand, to keep it exactly where we want it.


I've often wondered about this.

Surely you must accept that you will *never* seal it against *some* form of
moisture ingress.

If that's the case then surely, once equilibrium with the outside world is
reached, conformal coating will hold the mositure *in* as well as it will hold
it *out*.

What have I missed?

Gibbo

Well, if I can get the diffusion time constant up to 30 years or so,
that would be OK.

John

 

[Robert Baer]

John Larkin wrote:

Hi,

can anybody suggest a good conformal coating? I have a PCB that is
moisture sensitive; apparently the PCB itself absorbs enough moisture
to change the laminate dielectric constant enough to cause me trouble.
We coated the critical oscillator section with household polyurethane
varnish, and it helped a lot; absorption after 48 hours at 100%
humidity is down by about 3:1, and the recovery time constant is much
better (shallower moisture penetration?). But this stuff is thin, and
we'd like something gloppier that we can apply a bit thicker. We'll
brush it on by hand, to keep it exactly where we want it.

Any suggestions? We'd prefer something that sticks well, goes on
fairly thick (high viscoscity) and is reasonably easy to rework. Who
makes good stuff these days?

John

How about changing the PCB type?
Try Megtron 5 from MEM: http://www.mem-or.com/
I have some sample material if you have need for immediate delivery,
but i would have to make a box for shipping and you would have to
specify the sheet size and thickness (20 mil and 40 mill, up to 18x22).

 

[Fred Bloggs]

Spehro Pefhany wrote:

Here's a good overview of the types and manufacturers of conformal
coatings:

http://www.thermospray.com/ccfaq.html

Most assembly houses should be able to do the coating for you. The
ones I've seen have separate rooms, with their own ventilation and a
robotic sprayer or something like that.


Best regards,
Spehro Pefhany

Good link- the process is *all* in the application- an inexperienced
user unaware of all the pitfalls can make the best product in the world
fail.

 

[Ken Smith]

In article <32hvo0hdriuocs3av3duldtcjok7a3pl0h@4ax.com>,
John Larkin <jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote:

Hi,

can anybody suggest a good conformal coating? I have a PCB that is
moisture sensitive; apparently the PCB itself absorbs enough moisture
to change the laminate dielectric constant enough to cause me trouble.
We coated the critical oscillator section with household polyurethane
varnish, and it helped a lot; absorption after 48 hours at 100%
humidity is down by about 3:1, and the recovery time constant is much
better (shallower moisture penetration?). But this stuff is thin, and
we'd like something gloppier that we can apply a bit thicker. We'll
brush it on by hand, to keep it exactly where we want it.

If it was me, I'd put the whole thing in a sealed housing. The coating
can increase the time until the board's vapor pressure matches the
external vapor pressure, from hours to weeks but not to years.
--
--
kensmith@rahul.net forging knowledge

 

[Product developer]

John Larkin <jjlarkin@highSNIPlandTHIStechPLEASEnology.com> wrote in message news:<32hvo0hdriuocs3av3duldtcjok7a3pl0h@4ax.com>...

Hi,

can anybody suggest a good conformal coating? I have a PCB that is
moisture sensitive; apparently the PCB itself absorbs enough moisture
to change the laminate dielectric constant enough to cause me trouble.
We coated the critical oscillator section with household polyurethane
varnish, and it helped a lot; absorption after 48 hours at 100%
humidity is down by about 3:1, and the recovery time constant is much
better (shallower moisture penetration?). But this stuff is thin, and
we'd like something gloppier that we can apply a bit thicker. We'll
brush it on by hand, to keep it exactly where we want it.

Any suggestions? We'd prefer something that sticks well, goes on
fairly thick (high viscoscity) and is reasonably easy to rework. Who
makes good stuff these days?

John

There are some clear serviceable potting compounds out there that
resemble silicon that could be considered for dipping. The stuff can
be picked out in chunks for servicing then re-sealed. I'll ask my QC
director for the MFGR's name tomorrow.

The thin conformal coatings will likely create more nightmares than if
left untreated. Nothing like leaking in moisture then sealing it in
over time!