silicone potting woes

[ft]

Hi,
I am wondering if anyone has had some bad experiences with two part
silicone potting compounds, especially in the area of poor adhesion
and various growths on the board (FR4) and components. The PCB is
cleaned and baked after assembly.

Thanks

 

[Ken Smith]

Hi,
I am wondering if anyone has had some bad experiences with two part
silicone potting compounds, especially in the area of poor adhesion
and various growths on the board (FR4) and components. The PCB is
cleaned and baked after assembly.

I've used silicone to pot things and have found the following things
through experience:

1) Don't use plastics with Chlorine in them such as PVC heat shrink.
There is something funny about these plastics that the silicone doesn't
seem to like.

2) Don't use "no wash" fluxes. You can't leave them on and anything you
use to remove them is illegal to even think about having.

3) Inspect the boards after cleaning to look for evidence of not really
clean. If you can see anything that looks like runs and drips on the
surfaces, chances are the boards are very dirty. Clean boards look
spotless.

4) Don't rely on the silicone sticking to things for mechanical strength.
Even at its best silicone doesn't stick very well to some surfaces. When
stressed it will pull off. If left alone it is ok.

 

[Martin Riddle]

There are primers availbe that work well. (Its pink forget the name)
And yes the board must be clean, even finger oil will screw things up so wear gloves.

Cheers

 

[ft]

There are primers availbe that work well. (Its pink forget the name)
And yes the board must be clean, even finger oil will screw things up so wear gloves.

Cheers

Thanks for the replies and its all good stuff.
We are using no clean flux and washing it with an approved method (and
chemicals) and the potting gel manufacturer has approved it for use as
well. (after tests). The use of primer has been thought of but again
after tests the gel people said it wasn't necessary. We are using
clean handling with gloves.
But after all this we are getting a crystalline dendritic growth
(fairly low resistance) along a ceramic capacitor, and across some
close spaced pads on the board. The gel manufacturer is saying that
some components can do that because some materials cause the gel not
to cure in localised spots. It doesn't like some types of solder
either.
I was therefore wondering if this is a common problem with silicone
potting, are we unlucky with our materials, is this stuff just too
particular about what it coexists with, and should we make the change
to epoxy or polyurethane at some considerable cost and inconvenience?

Thanks

 

[Marc H.Popek]

Silicones for the most part has that acid catalyst. the two part is more
for molding than for sticking. Silicone having almost no adhesion
properties is a bad potting or encapsulate. Plus some times you can have
the encapsulate provide a relatively high degree of thermal conductivity, if
you have some hot spots.

 

[Marc H.Popek]

see its that chemistry business! there must have been a part of the catalyst
or the reaction products that is acid (or base) enough to eat lead frame!!

 

[Tony Williams]

Thanks Tony, that was especially useful as we are now looking at
epoxies and poyurethanes so its interesting to see the failure
modes in these. I never thought there would be so much chemistry
in electronic engineering!

I think a remark made by another poster was probably on the mark.
The initial attack starts when the chemicals in the potting
compound are still their raw liquid state.

 

[Marc H.Popek]

hello Tony,

it was me. I have a new found respect for chemistry. Formally trained as a
EE, I eschewed anything that would not accept solder

As time goes on you realize those same chemical reactions and bond electrons
are the same ones traveling in our circuits! Fortunately they have this
real handy chemistry chart

MP

 

[Tony Williams]

hello Tony,

it was me. I have a new found respect for chemistry. Formally
trained as a EE, I eschewed anything that would not accept solder

I suspect you are right though. ISTR that some catalysts
can be very reactive chemicals in their raw state.