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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Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
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