George,

     I discussed this very topic with one of my customers this morning.  I
     haven't received my copy of IPC-2222 yet.  However, this is what I
     recommend to designers:

     Pad sizing guideline formula:  P = D + FA + 2(AR), where

         P = Land size (pad size)
         D = Drill size, typically 0.004-0.006" larger than NOMINAL
             finished hole size; accounts for the reduction in hole size
             after plating; typical plated through hole tolerance is
             nominal +/-0.003"
        FA = Fabrication allowance (expressed as diameter about the true
             position); factors that the board fabricator can control or
             must account for include material movement, artwork tooling
             tolerance, drilling location accuracy and etch loss.  For most
             board fabricators, this value ranges between 0.010" and 0.014"
             depending on board complexity and layer count.
        AR = Minimum annular ring; defined by customer specification.

     Example:  Assume finished plated through hole size of 0.034+/-0.003";
               minimum annular ring requirement of 0.002"; and, FA = 0.014"

     Therefore:  P = (0.034 + 0.005) + 0.014 + 2(0.002) = 0.057"


     Using the information from the above example, I would answer your
     questions as follows:

     #1  Maximum finished hole size = 0.034 + 0.003 = 0.037

     #2  Value to add to nominal hole size = 0.005 + 0.014 + 0.004 = 0.023

     Regards, Dave Wolf
     Field Engineer, HADCO Corporation
     [log in to unmask]
     612/893-1466
______________________________ Reply Separator _________________________________
Subject: [DC] Hole size calculations
Author:  "Uptain; George" <[log in to unmask]> at SMTPLink-Hadco
Date:    4/23/98 1:21 PM

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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