Gregg:
               The simple answer to your question is that the holes
          will be within .0085" DTP, or +/-.003" linearly from the
          intended location.  This is on finished product.  Bringing
          in the hole-to-pad issue is another animal.  The
          relationship of the hole to its pad involves multiple
          disciplines:
               
   drilling (hole location)
               
   imaging (artwork location)
               
   etching (pad size reduction)
               
   material movement
               
   tooling system
                    Since we can't assume that everything wrong will
          happen to the same hole and pad, we use something akin to
          the "root mean square" to calculate worst case scenarios.
          In order to make 100% of your product fit our manufacturing
          window in volume, the following formula is used at HADCO
          to calculate the minimum pad size needed to sustain a
          customer's annular ring requirements:
              Drill Size* + Mfg. Allowance# + (2 X annular ring req'd)
          * Drill size is nominal finished hole size + .005"
          # Allowance for inner layers = .014"; outer layers = .012"
          The mfg. allowance captures the multiple disciplines listed
          above.  For example:
          .039" finished hole diameter; .001" annular ring
          INNER LAYER: .044" + .014" + .002" = .060" min. pad size
          OUTER LAYER: .044" + .012" + .002" = .058" min. pad size
          Using these formulae, 100% of your boards will meet your
          requirements in our volume environment.  Not every hole will
          be located in exactly the same spot on every pad.  That is
          why we need a manufacturing allowance.
          Regards,
          Tom Coyle
          Field Services Engineer
          HADCO Corporation

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