David has some great points. Also, the testing should be done prior to
coating, especially Silicone due to the difficulting in removing it.
Lastly, if you are concerned with falloff or want the coating thicker. You
have two choices. Change to a different coating configuration that wet's
out differently and/or coat the pcb with the same material twice.
I think what you are seeing on the components is still adequate coverage.
Of course I cannot be sure, but from your note, it seems to be a new process
for you. When specifying thickness, that is the coating on a flat surface.
Even mil specs, allow for deviation from that callout on component edges
etc. If that coating needs to be consistent thickeness across all features,
you will need a Parylene coating, which is very expensive, but very
consistent across different features.
We have been applying Silicone coating for years. There is a learning
curve, but controlling it and the thickness can be done fairly easily, and
once controls are in place, you can get even better with controlling it with
little PCB design changes to control surface tension.
Rodney
-----Original Message-----
From: David Douthit [mailto:[log in to unmask]]
Sent: Thursday, September 25, 2003 12:19 PM
Subject: Re: conformal coating help needed
Pete,
1. How much "leakage current"/resistance are you measuring?
2. What is the voltage level being used by the probes?
3. What is the distance/gap between the probes?
4. What kind/type of flux are you using?
5. Are you "baking" the boards after cleaning and before coating?
There is a source of conductive contamination somewhere.
Have you tried to "peel/remove" the coating from an affected area
then examine/test the area (leakage current and or contamination testing).
If all else fails send some samples to a lab. There are several who
specialize
in
locating contamination sources. By the way a RH >50% can create problems
with
leakage
currents.
David A. Douthit
Manager
LoCan LLC
Peter Zosimadis wrote:
> Hi David,
>
> Thank you for your response. Answering your questions,
>
> 1. Yes the coating is completely cured.
> 2. No shadow areas.
> 3. Humidity was at standard indoor levels, nothing out of the ordinary.
> Also, we processed boards on different days with similar results.
> 4. We thought of contaminated surface as well. We cleaned the boards,
then
> coated them again, same result. Even tried extra thick coating, same
> results.
> 5. Yes, complete coverage over entire board. Used a UV light chamber to
> check distribution of conformal coating.
> 6. Not sure about this one, seems the solder mask is not being effected by
> the UV light.
>
> Any other ideas?
>
> Thanks,
>
> Pete
> ----- Original Message -----
> From: "David Douthit" <[log in to unmask]>
> To: <[log in to unmask]>
> Sent: Thursday, September 25, 2003 1:26 AM
> Subject: Re: [TN] conformal coating help needed
>
> > Pete,
> >
> > 1. Are you sure the coating is completely cured??
> > 2. Does this coating also have a secondary curing agent
> > for "shadow" areas that UV light can not reach?
> > 3. How high was the humidity during application and testing?
> > 4. Are you coating over contaminated surfaces?
> > 5. Is there complete coverage of edges, back sides, beneath, and points?
> > 6. Is the solder mask completely cured and
> > compatible with the coating and coating cure process
> > (intense UV can damage certain materials)?
> >
> > These make good starting places for your answer.
> > I am sure others will add to this list!
> >
> > David A. Douthit
> > Manager
> > LoCan LLC
> >
> > Peter Z wrote:
> >
> > > Hi Everyone,
> > >
> > > Having trouble with conformal coating. Currently using a UV cure
> silicone
> > > based spray on a PCB with gold plating.
> > >
> > > The problem we are experiencing is after the conformal coating is
> applied
> > > and curred (0.002" thick), using simple probes with very little
pressure
> on
> > > the probes, we obtain conductivity between certain traces. In other
> words,
> > > the conformal coating is not preventing electrical conduction between
> points.
> > >
> > > Specificially, we are having difficulty coating vias and SMT
> > > resistor/capacitors.
> > >
> > > We tried increasing the thickness of the conformal coating sprayed,
but
> no luck.
> > >
> > > Does anyone have any ideas?
> > >
> > > Thanks,
> > >
> > > Pete
> > >
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