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March 2015

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From:
"Stadem, Richard D." <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Stadem, Richard D.
Date:
Mon, 2 Mar 2015 14:25:34 +0000
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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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