John:
               Most quality boards shops nowadays can handle .025" via
          pads on .062" thick boards using a drilled via.  There are a
          few caveats, however:
              You should stipulate that vias may have no bottom
               tolerance (i.e., they may plug shut).  In the case of
               your callout, the size/tolerance should be .012"
               (+.003", -.012").  This will allow the fabricator to
               fit the drill size used to the pad given, allowing for
               their manufacturing process and how much over the drill
               size the pad must be.
              You should allow tangency on vias.  There is no need
               for an annulus on vias, and bridging is usually reduced
               when vias have a minimal or no annular ring.
              These pads (all pads, actually) should be
               "teardropped".  If your CAD platform does not support
               this, ask your board fabricator to do it for you.  This
               helps avoid "break-in" of the hole to any incoming
               trace.
              Remember that smaller drills usually carry a premium.
               Drills smaller than .012" - .0135" can be 2x TO 3X the
               cost of standard drills.  Stack height under the N/C
               drills is also affected by these small drills, lowering
               throughput and raising the cost of the board.  Drilling
               is the second biggest contributor to board cost.
              You did not indicate whether your parts are low or high
               in volume.  High volume runs would be affected by more
               fallout than small quantities.
              My company uses the following formulae for calculating
               the minimum pad size needed to sustain annular ring:
               INNER LAYER: D + .014" + (2 X ANN. RING REQ.)
               OUTER LAYER: D + .012" + (2 X ANN. RING REQ.)
               D = drill size used
               These formulae are used to assure that 100% of product
               run meets annular ring requirements.  Most reputable
               board fabricators are working diligently to bring these
               numbers down, and we are also developing/licensing
               processes for photo-defined vias and laser drilling,
               which will make blind via design more cost-effective.
              Finally, yes, board size does affect registration.  The
               larger the board, the more misregistration will come
               into play.  The formulae above are meant to cover worst
               case scenarios.

               If you would like a copy of HADCO's DFM manual, please
          e:mail me at:  [log in to unmask]  and I will send you one.  I
          hope this answers your question.
          Regards,
          Tom Coyle
          Field Services Engineer
          HADCO Corporation

***************************************************************************
* TechNet mail list is provided as a service by IPC using SmartList v3.05 *
***************************************************************************
* To unsubscribe from this list at any time, send a message to:           *
* [log in to unmask] with <subject: unsubscribe> and no text.        *
***************************************************************************