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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