HI Alan,
Sounds like the only thing you can do is reduce via drill size from .010 to .008.
That would give 5.5mils per side annular ring on .019 pad. With the board being .064 thick that is
only 8:1 aspect ratio.
A good fabrication shop should be able to handle this.
I take it this is an 8 layer board with external foils and micro vias on top only? This should be no
problem for experienced fabrication shop doing this kind of stackup process. It always helps if this
is your tightest drilling tolerance requirements on board that it is located
as close to center of board as possible.
Regards,
Greg Scott
Cray Inc.
Alan Groves wrote:
> Hi Technetters, any help that could be given on following problem would be
> greatly appreciated...
>
> I am currently routing a PCB which has 532 pin uBGAs on it, these have a
> pitch of 0,8mm (31,5 thou). I have followed manufacturer's design guidelines
> in the past but am still receiving complaints from fabricators. I have
> resolved vertically by having a 6 thou microvia drilled thru pad centres to
> layer 2, then routed on layer 2 to a buried via (layers 2-7) of 10 thou
> drill, 19 thou land. The track and gap allowance is 4 thou minimum. I am
> using thermal connection pads of 17 (internal) and 26 (external) thou, and
> isolating pads of 24 thou - all only on buried vias, I cannot increase these
> pad sizes (much) or I could block GND and PWR from getting to destinations.
> Fabricators complain of copper features too close to drill hole, are they
> talking skew, or 'tear out' by that much (8 thou 'ish)?
> The board is 1,6mm thick FR4, 8 conducting layers, no controlled impedance.
>
> Can anyone suggest any tricks or tweaks to sizes that would assist?
>
> Thanks in advance,
>
> Alan Groves,
> Senior PCB Designer CID
> Spectel Head Office, 21 Stillorgan Ind. Park, Stillorgan,
> Co. Dublin. Ireland.
> Phone: 353 907 62803
> Fax: 353 1 2953740
> Email: [log in to unmask]
> Internet: www.spectel.com
>
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