Greetings;
 Everytime I revisit hole size to pad size calculations I find a
different road to follow.

 I just received my new copy of the IPC-2220 series that replaces
IPC-D-275.  While reviewing it, 2222 9.1.2, I am confused by the
calculation for land size.  It would seem that the maximum hole size did
not account for locational tolerance, only size.  We have always
provided .022" over nominal hole size but the calculation provides only
*.014" over max. My guess is that might be .017" over nominal.  Does the
fabrication allowance from 2221 Table 9-1 include the positional
tolerance? and/or size tol?


 The 1st question is:  How do you calculate maximum hole size? My usual
method is (nominal) + (size tol) + (1/2 DTP)
Usually that's nominal + .007"

 The 2nd question is: Do you have a "normal" value to add to nominal
hole size for pad size? Ours is .022 or .026 depending on complexity.
Our usual vendor recommends .021" minimum in their design guide.


*The same calculation in IPC-D-275 shows .020" over max hole size.
5.3.2.3

Thanks;
George W Uptain, Jr.
Austin Product Center of Schlumberger Oilfield Services
(512)331-3183

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