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Thu, 21 Oct 2010 15:39:58 +0200
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Yes to near-edge. However, one must see the design engineer's situation. If 
given a fixed space  for the board, and you have a lot, lot of objects to 
press into that space, I think  you bless every millimeter that can be used. 
We have a thumb rule that says 'no closer than 0.5 mm to the edge' for inner 
traces and 'no closer than 1.0 mm from any metal for the top traces' , which 
points at rack mechanisms of course. If there is plenty of space, then there 
is no reason for running close to the edge.

Inge



----- Original Message ----- 
From: "Glidden, Kevin" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, October 21, 2010 2:31 PM
Subject: Re: [TN] Conformal Coating Board Edges


Hallelujah Brother Doug....

Thanks for that post.  Not to make light of your problem, but it was GREAT 
for a morning laugh.  I feel your pain.  LEAN can be a double-edged sword.

And to provide just one man's opinion on some your questions:

I agree with your reasoning around NOT coating board edges that are routed 
vs coating edges of boards that are V-scored or mouse-bite.  One caveat to 
the routed edges is that different designs can have widely varying minimum 
edge spacing to inner conductors.  Much of the reliability, I would imagine, 
would depend on that edge spacing, the dielectric strength of the laminate, 
and the stack-up/resin content, and lastly the quality of the PCB 
lamination.  Also, and I'm not certain how common this is, but I've seen 
PCBs where the designers require the ground layers extend all the way to the 
edge for EMI purposes.  That sounds like a great candidate for edge coating.

Re. "how harsh an environment has to be in order to coat the edges?"...is it 
really a limit?  Is there a magic number?  5= good, 4.5 = bad?  I think it's 
relative.  Any humid or condensing environment deserves the consideration.


-----Original Message-----
From: Douglas Pauls [mailto:[log in to unmask]]
Sent: Wednesday, October 20, 2010 5:40 PM
To: [log in to unmask]
Subject: Re: [TN] Conformal Coating Board Edges

Thank you all for your responses.  As indicated earlier, how I wish I
would have had this kind of resource back in the 80s.  Here are my
thoughts on the matter.

At Rockwell, home of truth justice and the American way, we do a great
deal of coating by hand.  A classic case of LEAN principles applied wrong,
though I am told it made sense at the time.  I am working on bringing back
spray coating, but it is tough to change an entrenced culture.

We have often required coating the board edges on our drawings, generally
because "we have always done it that way".  We may have done it for some
of the reasons you all have given, but the technical rationale is probably
buried somewhere in our archives.  However, this edge coat requirement is
selective in application.  When the edges of the board have metal clad
areas on top and bottom, such as with card guides, the edges are not
coated, because it is very difficult to coat just the edge and not get any
on the top or bottom card guide.  Similarly, if we have a large high
density connector on the board edge, that is flush mounted to the board,
we do not coat the edge under that connector as it is next to impossible
to do without flooding the connector surfaces.  It is not unusual to have
three out of four sides of an assembly uncoated (two card guides, one
connector, one coated edge).  So our apparent philosophy is to coat board
edges unless it is too hard.

One of the things that having a LEAN philosophy as a driver does for you
is that you are constantly evaluating processes asking "is this really
necessary?", or "is this value added?".  For us, coating the edges of the
board takes additional time and effort.  It can also lead to rework or
touch up activities as well.  I cannot say we have ever had field failures
or problems from board edges that are uncoated.  Since we deal with some
pretty harsh environments, if this had truly been a failure mechanism,
such as for moisture ingress, we would have seen it by now.

One of the key points for me, is that we are dealing with board edges that
are routed, which has a tendency to seal the board edge, as opposed to
punching or V-scoring, which can leave exposed fibers and has a greater
risk of moisture intrusion.  If I had punched or scored/snapped edges, I
would consider edge coating to be necessary.  But since I have routed
edges, I ask myself the question of whether edge coating is value added.

Would you agree with my reasoning?

Inge, when you say board that are sensitive to water ingress, what do you
mean?  Is there a particular laminate material, such as Teflon or the high
speed laminates, that is a consideration?  Second, how do you determine
how harsh an environment  has to be in order to coat the edges?

As for Parylene, remember, a nightmare is also a dream.............

Now, to be perfectly honest, I can't really pass all this questioning off
as high minded LEAN driven noble pondering.  It's actually because I
messed up last week.  I used this reasoning on one, ONE mind you, program
to relax the edge coat requirement for  a tough application.  Now I have a
hundred requests, many along the lines of "Why do I have to coat my board
edges if Donna does not have to coat hers, huh, huh?".  There are some
days here I REALLY don't want to be the coating expert.........

Doug Pauls



Inge <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
10/20/2010 03:46 PM
Please respond to
Inge <[log in to unmask]>


To
[log in to unmask]
cc

Subject
Re: [TN] Conformal Coating Board Edges






Well, it depends..

example 1: boards sensitive to water ingress via edges: we used
multilayers
of cc along edges
example 2: boards with rough edges damaged the rack slides: we used cc
along
edges
example 3: boards with smooth edges, no harsh environment: we did not use
cc
along edges
there are more examples of course

Conformal coating the edges was tricky, because of the sharp edges. The
viscosity was therefore very important. Someone mentioned Parylene, that
one
has no such problem. Parylene is a dream stuff.

Lately, we have skipped edge coating, except when the customer wants it.


I had a look into MoonMan's POD, which covers most about PCB
manufacturing.
Strange enough, nothing about edge coating.

Inge

----- Original Message ----- 
From: "Douglas Pauls" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, October 20, 2010 8:20 PM
Subject: [TN] Conformal Coating Board Edges


> Good afternoon all,
>
> I am glad we all appreciate Inge and the time he puts into making this a
> fun and interesting forum.  To sum up: Ditto.
>
> I have a question related to conformal coating, a sort of survey.  For
> those of you that conformally coat your assemblies, do you coat the
edges
> of the boards?  Why or why not?
>
> There is an internal debate here and I wanted some other viewpoints
before
> offering my own.
>
> Doug Pauls
> Rockwell Collins
>
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