Dough,
Yes, high speed laminates.
harsh: when risk of water condensation
Various microscopic salts can be accumulated along the rough edges. These salts will sooner or later absorb water and start migration on a nanoscale level. That will, on long term conditions, have an impact on the dielectric constant, and finally cause changes of high speed signals for traces close to the edges.
I'll send to you one page of a report I wrote many years ago. There is a table with 70 of the most frequent salts, with surface tension, critical humidity level and temperature.
(On a level that most people are happy to be spared from, except MIL ones.)
Inge
----- Original Message -----
From: [log in to unmask]
To: Inge
Cc: [log in to unmask]
Sent: Wednesday, October 20, 2010 11:40 PM
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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