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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