My general advice to people about silkscreens is "Don't use
          them!"  They will cost you an average of 0.50 per side, and
          now with surface mount, people are putting them on two sides
          of the board.  Gone are the days when a field service man
          goes to a field site with a flashlight looking for C25.
          Most people use field replacement units, and bring the
          faulty unit back to the shop to fix it.  With everyone
          looking to shave pennies of the cost of a board, what a
          terrific cost savings!
          Because of the screen tolerance involved, most board shops
          will "clip" (remove) any alpha-numerics coming within .008"
          of a solderable feature.  "R"s start looking like "P"s, etc.
          With today's tightly designed boards, there is really no
          room for legends.
          If you cannot create the culture change required at your
          company to eliminate silkscreens, then here are some
          thoughts for you:
          * Soldermask is not required for silkscreens, but you should
          be building your boards as SMOBC (soldermask over bare
          copper), which will of course necessitate soldermask.  If
          you have parts without soldermask, I would suggest that you
          use a black legend ink for contrast.
          * Use an .008" draw to create your alpha-numerics.
          * Use a .050" minimum character height; otherwise, zero's
          and "O"s start to fill in.
          Good luck!
          Tom Coyle
          Field Services Engineer
          HADCO Corporation
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