Ken,
A few years ago, I became aware of a report covering the reliability of innerlayer
interconnect which attached to a PTH without annular rings, due to misregistration done
on purpose.  The results indicated two things.  1) That the annular ring interconnect
was better than the non-annular ring interconnect.  and 2) both interconnects met all
the requirements of the stiffest reliability criteria for boards, something like 300
thermal cycles from -10 to +60 C.  I believe the annular rings sampled lasted 600
cycles, and not non-annular ring sample lasted 450 cycles.  The round-robin (?) study
was sponcered (?) by IPC, and the results were leaked at an IPC meeting I attended.  No
one particularly liked the results, as it contradicts years of tradition.  Perhaps
someone more informed than I remembers this.  I think the IPC meeting was in Arlington
Va.

To answer your question, Designers route traces as close as possible to the innerlayer
pads.  If the drilled hole is outside the pad, you decrease the electrical spacing
between the drilled hole and any innerlayer trace that happens to be run near the
hole.  This may not be a short today, however one may well develop if conditions are
favorable.  So why 1 mil?  The cross section is probably not reflective of the worse
case condition.  If your design is going to allow the drill to wander outside the
innerlayer pads, you need to increase the innerlayer pad to trace spacing to allow for
this, preventing the minimum spacing from being violated.  This is not something
designers want to do.

In simple terms, you can think of those pads as innerlayer keepout areas, defining the
area that the drill may appear.  By requiring a 1 mil annular, you require to drill to
fall inside the pad, and greatly reduce the chance of a short due to a hole to trace
spacing violation.

My view is probably not one held by the majority, so take it with more than the
normally required amount of salt.

George Franck
"my Opinions are Just that".

Ken Patel wrote:

> I have the basic question:
> Why do we need annular ring on inner layers? If the answer is no then why
> IPC is asking for 1 mil annular ring on Class 3 products. The problem arisen
> due to microBGA application.
>
> re,
> ken patel
> ______________________________________________________
> Ken Patel                       Phone:  (408) 490-6804
> 1708 McCarthy Blvd.             Fax:    (408) 490-6859
> Milpitas, CA 95035              Beeper: (888) 769-1808
>
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