Jim..

  1) We do not allow "Negative Etchback" on our designs. That said, we
     have allowed it, as a wavier to the drawing, in some hardware such              as test sets or extendard test cards on a "need" basis. To rebuild 
     the product would have a schedule impact, so we would accept these 
     "as is", with the supplier notified that he/she would eat any other
     non-conforming product.

  2) Like you, we are designing/building product to Mil-Std-275 and 
     we have used the standard Mil formula for end product requirements.
     This is what we have been using, for a long time, on our 12-14 layer
     polyimide designs.

     SMT designs: Min Land Area = A + 2B + C

     INTERNAL Worst Case Conditions : 
                                  
            .018 Plated Finished Hole to .040 Land Area
                A = .024 max drilled hole
                B = .002 Min Annular Ring Requirement
                C = .008 Std Fab Allowance 

                    .024 + .004 + .008 = .036 adjusted to .040 pad
                                          (really have .004  more 
                                           to play with)

      Of course, the key to a real tight max drill size, or as we say--
      squeezing the banana, is supplier dependent, SO TALK TO YOUR SUPPLIER!!


  3)  I am not a board fabricator, but I don't think drill size is related to
      etchback... Isn't that process related??  
      
      Hope that helps somewhat.
                                                 regards,
                                                         Bob Vanech
                                                         Norden Systems

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