Brian, thank you for the response. See my questions in red below.
Doug Pauls
From: Brian Ellis <[log in to unmask]>
To: <[log in to unmask]>
Date: 03/23/2013 02:24 AM
Subject: Re: [TN] Conformal Coat and Bubbles
Sent by: TechNet <[log in to unmask]>
IMHO, it depends what you mean by bubbles/voids. If they are a result of
humidity being absorbed by hygroscopic contaminants through the coating
(vesication or mealing) then the answer is a resounding 'not acceptable'
under all conditions.
**OK, so Brian, if you had to provide a definition of a bubble or void,
what would that be?
If they are the result of poor application of a liquid conformal coating
or solder mask, I would say that they would not be acceptable if any of:
1. the substrate is not wetted under any bubble
**What if the area under the substrate contained only solder mask or other
unreactive surface?
2. a bubble bridges two bare conductors
**I am assuming that the assumption is that the bubble exposes the metal
of the two conductors. What if there is still some coating on the
substrate under the bubble? Would this still be a defect if I could show
at least 1 mil of coating still covered the conductors?
3. a bubble occupies more than 30% of the space between two bare
conductors with a potential difference of >10 V
**OK. Is there a study that you can point to upon which such values are
based?
4. there are more than 2 bubbles per square decimetre on either side,
not counting the edges of a board where there are no conductors, up to 5
mm from the edge
**Oh hell, now I have to go look up decimetre. Hang on...... Why 2
bubbles in that space? Would it not depend on the size of the bubble?
Second question - do you feel that the sides of an assembly need to be
coated? Would it depend on whether the edges of the board were
routed/sealed vs. punched/unsealed?
5. a void or bubble can be seen by 10 X visual inspection under a
component
Again, would it not depend on whether or not the void or bubble left
anything open? I can see operators trying to underfill components with
conformal coating to get rid of things they perceive as voids under
components, which would be a big reliability risk.
6. a paraxylylene coating displays any void
Here we probably agree.
Some of the numbers in 3., 4. and 5. are arbitrary; common sense should
be applied, along with the coating.
Ahhh, but common sense is not all that common.
My 2 eurocents.
Which are always appreciated. Glad your banks got their bailout loans.
Brian
On 21.03.2013 17:18, Douglas Pauls wrote:
> OK, Minions, your next question in the quest to improve J-STD-001 and
> A-610 relates to bubbles in the conformal coating.
>
> I think we can all agree that the ideal conformal coating layer contains
> no bubbles or voids and is "purdy". BUT:
>
> 1. Are there bubbles in coatings in areas where they will not impact
> reliability and their presence should be viewed as a "Process
Indicator"?
> If so, where and what limits would you use?
>
> 2. What would you classify as a bubble requiring disposition, i.e.
> Becomes an actionable defect? If so, where and why?
>
> 3. Would any of your answers change depending on what KIND of coating
it
> is, i.e. Acrylic vs. silicone vs. Parylene?
>
> Well, OK, that is three questions. I'm on a roll.
>
>
> Doug Pauls
> Chairman, IPC Cleaning and Coating Committees
> Galactic Emporer
>
>
>
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