Hi Doug,
Does the handbook cover conductive traces beneath devices on boards
without solder mask.....?
In my young engineer days, I had to apply thinned Type UR conformal
coating beneath DIPs (that tells you how long ago this was) using a
syringe. We were then spraying from all four sides in order to "seal"
the edges of the CCA (aka PCBA these days). It was always interesting
to watch the techniques the operators used to get the proper coverage
without having edge build-up that violated the thickness requirements.
I always wonder about designs that require edge masking, especially when
there is a de-panel tab involved.
I have the greatest admiration for clock makers (brother-in-law) as they
are extremely self sufficient and always know what time it should be.
Phil
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas Pauls
Sent: Friday, October 22, 2010 7:35 AM
To: [log in to unmask]
Subject: Re: [TN] Conformal Coating Board Edges, reflexion
Yes, this brings back similar memories from when I was a young engineer
and knew everything. I had a few painful "education sessions" with some
of the graybeards who had been doing avionics for the Navy for decades.
I
learned a great deal about the gap between industry consensus standards
and application specific needs. Since that time, I usually enter into
discussions with the "OK, educate me" attitude. I often learn more that
way. As my grandfather used to say, always listen, even a broken clock
is
right twice a day. And he should know, he was a clock maker.
Thank you everyone for your responses. They are greatly appreciated and
will probably go into the revision of the IPC Coating Handbook.
Doug Pauls
Inge <[log in to unmask]>
Sent by: TechNet <[log in to unmask]>
10/21/2010 09:49 AM
Please respond to
Inge <[log in to unmask]>
To
[log in to unmask]
cc
Subject
Re: [TN] Conformal Coating Board Edges, reflexion
And so we have the CREATIVE engineer, who frowns at all these rules and
standards. As a young and unexperienced guy, I had the task to control
that
the MIL or DIN or BS or ASTM or other standards were fulfilled. Had the
shelters filled with standards, meters of standards! One day I visited
an
old CAD guy for checking a board under design, with the new NAFI
backplane
connectors. The man had a list of changes that he wanted, which I
thought
was unnecessary because there were standards created after lots of
testing
by the manufacturer, and furthermore, it would be very expensive. I had
some
heavy books with standards which I placed on his desk, and I was ready
to
correct the elder. He listened with a stone face for a while, then
waved
his hands and said 'My friend, close those books. And sit down beside me
and
you listen. You see, my friend, I THINK when I create a board. And he
began
to ask me (my memory may slide after all these years):
- What are these boards for?
- An aircraft
-What aircraft?
-A fighter.
- Have you been in a fighter?
-No
-Do you know who is the customer?
-Yes it's xxxxx
-Do you know any guys there?
-No
-Do you know where these boards will be placed?
- Not exactly
- How can you insist all your remarks when you have not a clue
where/what
the boards are for?
- Erh..hu..
-Are you familiar with calculation of random vibration's impact on
boards
with two-point axes mount?
- Not much..
- Are you familiar with the kinetics behind fretting mechanisms?
- What?
- Are you familiar with punching connector forks from beryllium bronze
sheets.
- No
etc
And after a couple of hours, I went back to my office with headache, but
with lots of notes on my drawings. A few days later I was on my way to
Teradyne for discussions. The responsible design team were sitting on
the
opposite side of the table. After some hours, the manager was so angry
and
upset, that he left the room. The team was frustrated. 'You come to tell
us
how a Nafi connector ought to be made! Do you know to whom we ship
thousands
of these connectors?
To make a long story short, they accepted, we payed a lot, and we had no
more problems. And I learned to be careful whenever I meet the Creative
design engineer....
Inge
PS. No, you can't change such a guy. If you try, he leaves, because
others
want him.
>
>
> ----- 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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