Paul's comments are essentially accurate if you are not required to do 
breakdown testing which is common in telecom. In that case you will need a 
keepout margin to account for the possibility of arcing between the PTH and 
the adjacent conductive feature.

Best regards

Lee
J. Lee Parker, Ph.D.
JLP Consultants LLC
804 779 3389



----- Original Message ----- 
From: "Paul Reid" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, June 11, 2008 2:42 PM
Subject: Re: [TN] Annular Rings on Via holes


In support of Dwight and Paul's comments.

From a PWB reliability point of view misregistration in itself is not a 
reliability liability until it produces "break-in". This is were the hole is 
breaking into a trace to a point where there is no connection between the 
pad and the trace. We have seen significant reductions in reliability where 
the hole breaks into the incoming trace producing an internal 
interconnection that is limited to the trace width.

For example, a well registered hole with have a 360° internal 
interconnection on inner layers. So if the hole is .010" the interconnection 
will be pie times diameter or 3.14 X.010" or .0314". If you have 50% 
breakout then there is still .015" of internal interconnection! If the hole 
breaks out of the pad and into the trace, and given the trace is .005" the 
interconnection could be as low as .005". A .005" post sticking into the 
side of a barrel is not reliable.

Here is the key. If the misregistration is too large the board fails 
electrical test (ET) with opens or shorts. If the breakout is not into a 
trace and does not short to adjacent structures, there should be no 
significant reliability issue. If the breakout is into a trace, isolating 
the trace from the pad, there can be a significant reliability liability.

So all that being said, as far as registration's effect on reliability ... 
"it depends". There you go "Déjà vu all over again."

Paul Reid

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Paul Edwards
Sent: Wednesday, June 11, 2008 1:09 PM
To: [log in to unmask]
Subject: Re: [TN] Annular Rings on Via holes

In addition to Dwight's comments I would like to add a warning...

If you are looking at breakout...

Look need to look at the PCB internal pads where the vias pass with x-rays..
If it is out on the visible outer layers it can be off the pad or cutting 
through the pad traces on the internal layers...

I have had PCBs with breakout that have missed the internal pad, just 
touched the internal pad and have cut through the trace to pad joint...
These gave opens, opens after processing and intermittents in flexure...

Breakout is a process indicator that tells you about the alignment quality 
of the PCB... and that you need to look deeper...

Paul

Paul Edwards
Surface Art Engineering

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Dwight Mattix
Sent: Wednesday, June 11, 2008 9:31 AM
To: [log in to unmask]
Subject: Re: [TN] Annular Rings on Via holes

At 07:06 AM 6/11/2008, Paul Baine wrote:
>Hello all,
>
>We received some PCBs from a Vendor where the Via holes have an
>incomplete annular ring.  See attached photo.

In an of itself that may not be a problem.
2 issues in play:
1) Spec compliance/acceptability
    * What class (1, 2, 3) is specified for the pwb?
2) MRB disposition: (Reliability/technical/end use performance requirements 
i.e. is it safe to use the pwb) Breakout has little to do w/ actual PTH 
reliability. The mere presence of an innerlayer pad is the 1st order 
contributor to reliability.
The bigger concern is whether or not the drill breakout from the pad reduces 
drill to copper feature (i.e. trace or plane) spacing below acceptable 
minimums required to provide adequate isolation and prevent CAF concerns.


>We have looked at Section 2.10.3 & 2.10.4 of IPC-A-600E (we don't have
>the most recent Revision yet) and we are not sure whether these
>Sections of the Standard apply to via holes.
>
>Any comments?  Many thanks.
>
>Paul Baine
>Q.A. Manager
>C-Tech Ltd.
>
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