FROM: James D. Herard
*** Resending note of 11/04/96 16:17
**************************************************************
Subject: DES: Hole Sizes on PCBs for tight tolerance non-plated holes


THAD;  FOR EASY REFERENCE, SEE MY THOUGHTS IN CAPS.

     I've got a set of questions for the distinguished body.

     It regards to specifying the finished diameters of non-plated holes on
     PCBs.

        1.  What is the tightest finished hole size tolerance I can
     reasonably specify for volume production.  Is this a function of hole
     size.

 THIS IS A COST/PERFORMANCE QUESTION.  STICK WITH +/- 0.003 FROM
 0,0 LOCATION FOR LOCATION AND +/- 0.003 ON FINISHED SIZE
 AND YOU'LL GET A LOW COST ANSWER.  FOR NON-PLATED IN VOLUME
 YOU MAY BE ABLE TO REQUEST +/- 0.001" ON SIZE, BUT LOCATION
 WILL PROBABLY STILL BE +/- 3 UNLESS YOU'RE WILLING TO PAY FOR
 A SORT OR SPECIAL MFG PROCESS.
        2.  I have a standard table of drill bit sizes.  Does the PCB
     industry have custom drill bit sizes or are these all in the
     standards?  If there are custom sizes are these published anywhere?
 CUSTOM SIZES;  WELL, YOU NAME IT, WE'LL GO FIND SOMEONE TO BUILD
 A DRILL TO FIT IT, OR WE'LL FIND A WAY TO "MILL" THE HOLE TO THAT
 SIZE, BUT AGAIN, YOU'LL PAY MORE FOR NON-STANDARD SIZES.

        3.  For a given drill bit size, what will be the nominal finished
     hole size drilling in a typical FR-4,.062" thick PCB?
 TYPICAL NON-PLATED +/- 0.002 OR EVEN +/- 0.001 ON FINISHED HOLE.
  TYPICAL PLATED +/- 0.003.  THE PRINTED CIRCUIT BOARD MFG'S ABILITY
  TO STACK THE BOARDS DURING DRILLING, AND YOUR HOLE QUALITY
  REQUIREMENTS MAY HAVE AN AFFECT.

        4.  Is there a table of the different values of nominal FINISHED
     holes sizes for non-plated through holes.
  NOT THAT I'M AWARE OF.  WE'VE EVEN DONE "SQUARE" HOLES AND COMMONLY
  DO SLOTS.
     My problem is that often I need to specify tight tolerance holes and I
     need to know the "sweet spots" of finished hole sizes for these holes.

     For example I might desire to specify a hole .093" +/- .001", not
     knowing that there is no drill size near this size.  Or if there is a
     drill size, I might not know that if I drill at a given size the
     finished hole shrinks slightly.  I need to understand this to
     understand the "sweet spots" for tight tolerance hole sizes.

     I'd appreciate any feedback.

     Thanks,

     [log in to unmask]


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