> From: "Jay DeKing (EMA)" <[log in to unmask]>
>
> I agree with John, shouldn't the external pads be larger (in my opinion, for soldering purposes)?
> Jay
>
> -----Original Message-----
>
>   John..
>          I'm still surprised! We have been designing 12-14 layer (standard)
>    smt designs for about a zillion years, maybe not quite a zillion, but
>    a looonnng time and no vendor has ever requested that the inner layer
>    pads be larger. In fact, unless I am looking at the formula/chart in
>   the Mil and IPC specs incorrectly, the minimum annular ring requirement
>   for internal is .001/.002 mils vs .002/.004 mils for external layers.
>   Therefore, by definition, your external pads should be larger. What
>   am I missing???
>                                        Regards,
>                                                   Bob

John, Bob, and others interested,

I've had this exact issue come up when I was designing a board. The issue of
<0.0135" diameter vias was discussed with my fabrication house. There seems
to be much more concern with the movement of the internal layers than the
movement (misalignment) of the external layers. AND so it was explained to
me that the fab house will look at the drill size to pad size of the internal
layers with greater concern than that of the external layers.

The formulas I was given were (and remember this is for SMALL vias sizes, I was
going to use .0135" vias, possibly .010" ):

External layer pads:  drill size + .012"= preferred MINIMUM size
Internal layers    :  drill size + .015"= preferred MINIMUM size
Internal antipads  :  drill size + .020"= preferred MINIMUM size

And what I used for the .0135" vias were .030 outers/.033 internal pads. The
fab house was comfortable with this and the boards worked great!

If you need more information I would recommend you contact Terry Wright at
Details, Inc in Anaheim, California (www.detailsinc.com) and he (or Anhil)
would gladly discuss this with you.

Hope this help,  :)

Mitch Morey
Cadence Design Supervisor
Praegitzer Design
San Diego, CA 92121
Phone: (619)450-0994 x-17
Pager: (619)982-7529
Fax:   (619)450-0529
EMAIL: [log in to unmask]
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