If we are allowed to remove non-functional pads, our hole to copper
spacing check will catch any violations of traces too close to holes.
Won't your design package error in this case also (given that the copper
is less than the minimum distance to the hole)?

:-) -----Original Message-----
:-) From: TechNet [mailto:[log in to unmask]]On Behalf Of George Franck
:-) Sent: 09 January, 2001 5:01 PM
:-) To: [log in to unmask]
:-) Subject: Re: [TN] Unused Inner Layer Pads: Remove or Not?
:-)
:-)
:-) Good afternoon,
:-)
:-) I have to provide another perspective on this.
:-)
:-) I assume that your pad stack is designed with the smallest
:-) pads possible, which allows for
:-) layer-to-layer registration errors, material shrinkage,
:-) drilling accuracy, annular rings,
:-) etchback, and all the other gremlins in the PWB Fabrication
:-) process that keep the drilled
:-) holes from being drilled exactly in the center of the
:-) padstack.  Lets look at what removing
:-) the Non-functional pads can get you.
:-)
:-) Lets use an example,  Your hole size in set up for a .0135
:-) drill.  You want 1 mil annular
:-) rings (internal), and you are allowing the PWB house .010
:-) for their tolerance build ups.  So
:-) the minimum pad size is going to be .0255.  Lets steal the
:-) half mil from the Fab house.....
:-) and use .025 pads.
:-)
:-) Now, You, or the supplier, is going to put the .0135 hole
:-) and .025 pad in their test coupon.
:-) And they will use this coupon and make a cross section to
:-) verify that the drilled hole is
:-) where is should be, ie at least .001 from the edge of the
:-) pad (annular ring).
:-)
:-) The Cross section demonstrates that the edge of the hole
:-) stays inside a .023 diameter area, at
:-) least in that cross section.
:-)
:-) Lets go back to the design process.  You are routing the
:-) board, and your design rules say you
:-) must maintain a .003 minimum space.  If you run a trace too
:-) close to a pad, your CAD tool will
:-) register a "line to pad" spacing error.  So all traces are
:-) kept .003 from the edge of the pad,
:-) and .004 from the edge of the hole.
:-)
:-) Now, lets remove the Non-functional pads, and rout traces.
:-) Some routers will now allow you to
:-) rout traces in the area once occupied by the non-functional
:-) pad.  (This is the reason you
:-) wanted to remove the non-functional pads, i.e., to squeeze
:-) traces into places they dont
:-) fit..)  If a trace is now routed .020 from the center of
:-) the hole, a spacing error is not
:-) generated.
:-)
:-) Remember the Allowances we gave the supplier.  The hole can
:-) be anywhere within a .023 diameter
:-) area.  The hole could be drilled thru this trace, and be
:-) within its mechanical tolerances.
:-) These dead shorts will be identified at electrical test.
:-) The reliability problems are the
:-) holes that are very close to the traces.  They are not
:-) shorting out today, but given some
:-) humidity and some time.... ZAP!
:-)
:-) In my ever so humble opinion,  (IMESHO)  the non-functional
:-) pads can be reduced to a minimum
:-) size, (0 annular ring requirement) during the layout and
:-) rout design process.  In this case,
:-) the pads are effectively a "keep out" area, representing
:-) the area where the drilled/plated
:-) hole may occur.  The pads will help your CAD tool maintain
:-) your minimum spacing requirements.
:-) You should never rout a board without non-functional pads
:-) present.  I suppose you could set up
:-) keep outs in your pad stack to do the same thing as
:-) non-functional pads, in which case, my
:-) concerns go away.
:-)
:-) After routing is complete, and all Design Rule Checks are
:-) completed, there may be some
:-) advantages to removing these pads.
:-)
:-) In my experiences as a PWB fabricator, we usually left the
:-) Non-functional pads in.  There are
:-) agruments either way about non-functional pads in the
:-) fabrication process, and I am 100%
:-) behind any position my current boss takes.  Before routing
:-) your traces, however, keep the pads
:-) IN.
:-)
:-)
:-) George Franck
:-)
:-)
:-) The reason fractions prevail and the Dewey Decimal system
:-) never caught on in America?
:-) Well...... that is because Truman beat Dewey... remember?
:-)
:-)
:-) Franklin D Asbell wrote:
:-)
:-) > Remove them all...provided of course your customer has no
:-) problem with
:-) > that.
:-) >
:-) > What you might learn later in fabrication is that the handful you
:-) > thought were not an issue has just bit your butt.
:-) Besides, if they're
:-) > unused, why use them ~grin~
:-) >
:-) > Franklin
:-) >
:-) >
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