Another 2 cents' worth from me - what is the surface pad diameter for the
holes in question? Our normal routing procedure is that the surface pad
diameter is a "keep-out" diameter right through the board, and the surface
pad diameter is maintained on all the other layers. Unless you're really
short of real estate, I would suggest you follow this same "rule", as it's
less trouble that specifying a different diameter for internal layers.

10 mils over finished hole diameter gives you a 5 mils pad around the hole.
Depending on the plating thickness, that will give you less than 4 mils
annular ring, which  (though not so important on internal layers) could be
a bit tight if registration/drill placement accuracy isn't tightly
controlled. Again how does the 10 mils diameter you propose compare with
the surface pad diameter?

Best regards

Peter




                    Patrick Lam
                    <Patrick_Lam@S        To:     [log in to unmask]
                    ELINC.COM>            cc:     (bcc: DUNCAN Peter/Asst Prin Engr/ST
                    Sent by:              Aero/ST Group)
                    TechNet               Subject:     Re: [TN] Remove unused "dead" pads on
                    <[log in to unmask]        internal layers
                    RG>


                    01/12/02 07:01
                    AM
                    Please respond
                    to "TechNet
                    E-Mail Forum."








Thanks Steve,

Would 0.010" over the finished hole size be good pad size to be used on
internal
layers?

Thanks,
Pat




Stephen Gregory <[log in to unmask]> on 01/11/2002 02:33:46 PM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>

To:   [log in to unmask]
cc:    (bcc: Patrick Lam/SEL)
Subject:  Re: [TN] Remove unused "dead" pads on internal layers



Hi Pat!

I don't think it's a good idea, we got burned by it once. The internal pads
will provide support during drilling, and will help prevent rough walls
during drilling.

In our case, the unused pads were removed (this was a fairly thick ~.093"
or
so polyimide PCB) and plating chemicals were absorbed into the fibers from
the rough drilling. We built the boards, did ICT and a low power functional
test, everything was fine. The boards were shipped to our customer and they
plugged it into their system, powered it up, and then the board died after
about 20-minutes, internal short in the fab. They RMA'd the board back to
us,
we sent it out for cross sectioning and found the problem.

Had the used pads put back in and haven't had a problem since.

-Steve Gregory-


> Hi TechNetters,
>
> For class 3 boards, is it acceptable to have unused pads removed on
> interanal
> layers.
>
> Thanks,
> Pat
>



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