The vendors I use suppy the following;

Drill size (.005 over Nominal finished size) + .014 (Mfg. Addr) + 2X annular
ring requirement

Example:
.032 finished hole
.037 + .014 + .004 = .050

AND

Drill Size (.005 over Nominal finished size) + .012 + 2X ann. ring + .0028

Example:
.032 finished hole
.037 + .012 + .004 + .0028 = .0558

IPC 2221 specifies; (Sect. 9.0, pg 70,71 and Sect. 1.6.3, pg 2)
---------------------------------------------------------------
Max. dia finished hole + 2X annular ring            + Std. Fab. Allowance
                         Internal Supported   .00118  Level A   .0157
                         External Supported   .00196  Level B   .0098
                         External UNsupported .00590  Level C   .0078

For copper weights greated than 1oz/sq. ft. add .05mm (.0196) min. of each
additional oz/sq. ft of copper used.
For more than 8 layers add .05mm (.0196
Level A = General Design Complexity (Preferred)
Level B = Moderate Design Complexity (Standard)
Level C = High Design Complexity (Reduced)

Example
.032 finished hole
.032 + (2 x .00196= .00392) + .0098 = .04572
------------------------------------------------
Hope this helps,

Michael Kuczynski       Librarian/Sr. Designer
Allied Signal           201-393-2122 (Phone)
699 Rt46E E/K4          201-393-2357 (Fax)
Teterboro NJ 07608
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