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November 2016

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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:
Tue, 15 Nov 2016 12:27:35 +0000
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The other reason for copper edge clearance is shorting caused by copper smear from layer to layer due to depaneling. If the copper is exposed at the edges of the PWB due to misregistration or routing too close to the edges, the copper can smear.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Yehuda Weisz
Sent: Monday, November 14, 2016 3:51 PM
To: [log in to unmask]
Subject: Re: [TN] Copper to edge distance

Thanks for the example Victor,
This is just one of those things that concern me while referring to this parameter.
Is there a chance that you remember what was the clearance in THAT failed board?

Best regards,
Yehuda

-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Monday, November 14, 2016 3:30 PM
To: [log in to unmask]; [log in to unmask]
Subject: RE: [TN] Copper to edge distance

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

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