While the standard gives the definitive answer to the annular ring question,
from the practical standpoint here is a useful rule-of-thumb for most
fabricators "standard process" for a 0.002" min. annular ring:

min. outer layer pad dia. = nominal finished hole dia. + 0.017"
min. inner layer pad dia. = nominal finished hole dia. + 0.032"

As a circuit and pcb designer who has worked with many fabricators over the
years, this is what most of them will do today without extra charges or
questions.  One caveat is that all of my work has been commercial, not
military.

Seth Goodman
Goodman & Associates
7687 Leta Way
Verona, WI 53593


-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of [log in to unmask]
Sent: Monday, November 09, 1998 10:59 AM
To: [log in to unmask]
Subject: Re: [TN] Annular Ring!- May we have an arbiter please...Paging
Mr. ...


All,

The IPC-2221, Generic Standard on Printed Board Design, and its predecessor
IPC-D-275 indicate to use the maximum finished hole size when calculating
land
size for outer layers and the maximum drilled hole size when calculating
inner
layers lands. One must include twice your annual ring requirement (one for
each size of the land) and a manufacturing allowance for process variation.
Both standards include a table to guide you when determining how much of an
allowance to provide. If you call fro etchback, then you must also add it to
your inner layer land calculation.

The referenced section in IPC-2221 is 9.1.1 Figure 9-2 and 9-3 illustrate
how
annula ring is determined.

Regards,

Gary Ferrari
Executive Director
IPC Designers Council
860-350-9300

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