I feel the need to point out the obvious:
-Suppose we singulated a board by using a pcb drill, drilling thousands of
overlapping holes. It's hard to keep drills sharp, so let's assume we just
swap out bits after the same number of "hits" that we deem the bit good for
via hole drilling. Now how 'bout we coat the exposed edge with plated metal,
just to make sure that if there's a problem with the drilling/routing process,
we have a real good chance of making a short.
Any reason that should be dis-allowed? Try calculating the exposed area on the
edge vs. all of the via circumferences you've got!
Therefore, it's ridiculous to specify coating of routed board edges, which if
anything, have less potential to damage fiber bundles than a drill. Snapped
areas are a different category because drilling/milling has a limited
capability to damage the laminate.
Wayne
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Monday, March 02, 2015 9:26 AM
To: [log in to unmask]
Subject: Re: [TN] Required Confromal Coating of Edges of PCB's
Rich,
In addition to what Dr. Pauls has detailed below, I also want to point out
that whether or not any specification "recommends" or "does not recommend"
coverage on certain areas of components and PWBs, it is the assembly drawing
which has historically taken precedence over all standards when it comes to
defining conformal coating coverage. This is because every assembly and PWB
has different design requirements and it would be too difficult to document
all of the exceptions to the different rules for each type.
Many circuit boards are simply blanked out on a press, leaving exposed
fiberglass edges, but these are also typically high-volume, low-reliability
PWBs used only for consumer electronics. All other PWBs are typically routed
or laser cut, and as Doug stated those are typically sealed by the singulation
process. Depending on the type of PWB material, the method of singulation, and
the application, there may be no need to coat the edges.
Or there might be, but then one would expect this to be detailed as part of
the assembly requirements on the drawing.
dean
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Douglas Pauls
Sent: Sunday, March 01, 2015 8:07 PM
To: [log in to unmask]
Subject: Re: [TN] Required Confromal Coating of Edges of PCB's
Rich,
MIL-I-46058 is simply a materials qualification document. It does not address
the coating of board edges. I would disagree with Graham and I do not believe
that conformal coating edges of boards is a value added process. Most boards
in high performance electronics have routed edges.
The routing process tends to smear the resin over the glass reinforcement,
sealing the edges. And since most design standards do not allow internal
circuitry closer than 25 mils from the edge of the boards, water or external
contaminants would have to penetrate 25 mils of epoxy resin to get to
circuitry. If the edges of the boards were sheared or snapped, where the
resin did not seal the ends, then perhaps the sealing would be justified. I
can say that Rockwell has coated some board edges and left other edges free.
We have no field failure, ever, that can be traced to lack of coating the
board edges.
Doug Pauls
Principal Materials and Process Engineer Rockwell Collins
On Fri, Feb 27, 2015 at 10:51 AM, Richard Kraszewski <
[log in to unmask]> wrote:
> Does anyone recall which MIL document calls out the requirement to
> cover the PCB edges of assemblies?
> I have been led to believe that one exists but that more than likely
> it is not MIL-I-46058C.
>
> Any thoughts??
>
> Rich Kraszewski
> Plexus
>
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