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