Hi Richard - just an FYI but many companies do not consider the soldermask
as part of the dielectric question as Joyce indicated because its thickness
is not a controlled attribute.

Dave Hillman
Rockwell Collins
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On Mon, Nov 14, 2016 at 12:16 PM, Stadem, Richard D. <
[log in to unmask]> wrote:

> No, it is part of the dielectric and does have properties to be concerned
> with.
> It is not considered to be an insulator, however.
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo
> Sent: Monday, November 14, 2016 10:49 AM
> To: [log in to unmask]
> Subject: Re: [TN] Copper to edge distance
>
> solder mask should not consider as dielectric during design clearance
> calculation.  IMO..
>          jk
> > Food for thought,
> >
> >    I have seen Field Return PWB in which a short was created between the
> > PWB edge and the metal chassis /metal hardware.   Due to spacing
> > clearance and solder mask thickness
> >
> > Victor,
> >
> > -----Original Message-----
> > From: TechNet [mailto:[log in to unmask]] On Behalf Of Yehuda Weisz
> > Sent: Sunday, November 13, 2016 4:15 PM
> > To: [log in to unmask]
> > Subject: [TN] Copper to edge distance
> >
> > Hello  Technetters,
> > For the beginning of the week I have a question to you that occupies
> > me more and more lately, and it concerns one of the design guidelines
> > - "distance of copper to edge".
> >
> > IPC-2221 specifies the minimum distance by design as 20 mils and
> > raises this value as the voltage drop increases.
> > Well, as PCBs become more and more dense, it becomes a very
> > challenging task to convince designers to give up real-estate along
> > the edges of the board.
> > Their claim - if the manufacturer can work with a tolerance of
> > +/-0.1mm, why do I need to keep a clearance of 0.5 mm along the edges.
> > Most of the boards we are dealing with are for high reliability
> > customers (class 2, class 3 type guys) and from what I have heard from
> > other designers
> > - no one goes below 20 mil. Some even keep a minimum of 40 mil along
> > the edges.
> >
> > So - my question to you is simply - WHY ???
> > Why did the spec. call for 20 mil minimum clearance??
> > I do give reasons to the designers, involving reliability and so on
> > but I feel that I might be missing the real point (or the fundamental
> reason).
> >
> > Could any/some of you, knowledgeable people, help me out on this??
> >
> > Thanks,
> > Yehuda Weisz
> >
>