All of this is correct, but if you're really designing that close to the minimums,
a small manufacturing allowance for wicking or other hole wall irregularities
should also be added to the other allowances.
Don Vischulis
>I'm probably saying the same thing as George. The critical part of your
>question is that your asking if you can remove the pads to increase wiring
>density during layout. The answer is absolutely not. Remember what the pad
>is for, to make an interconnect to the drilled hole. The reason it's larger
>than the hole is to make sure the hole is still in the pad when all the
>manufacturing tolerances are taken into account. The non-functional pad
>means there's no interconnect on that hole on that layer, but the drilled
>hole will still be there. The non-functional pads are friendly reminders to
>keep traces out of that area.
>
>If you need more room, you need to ask two question of your board fabricator
>and one of yourself. For the fabricator, the first question is "how much
>larger than the drilled hole must a pad be and still capture the hole"? The
>second is "what's the minimum pad to trace space they can etch?" For you,
>"from a reliability viewpoint, what's the minimum conductor-to-conductor
>spacing I want in the finished board?" Take the larger of the answers to
>the last two questions and add it half the answer to the first. If you a
>run a trace closer to a hole than that, your fabricator will be throwing
>them away for shorts at electrical test or you'll have boards which don't
>meet your reliability expectations.
>
>For example, your fabricator says I need a pad 0.010" larger than the
>drilled hole and I can etch a 0.004" space. You follow IPC and want a
>minimum 0.0035" spacing. You should put in the desired pad size (while your
>at it, if you're cutting it this close, you might as well find out the exact
>drill bit diameter they will use). Then rout traces with a minimum 0.004"
>spacing.
>
>
>> ----------
>> From: George Franck[SMTP:[log in to unmask]]
>> Reply To: TechNet E-Mail Forum.;George Franck
>> Sent: Tuesday, January 09, 2001 2:00 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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