The reason I heard is that since the copper thickness has an accepted
tolerance plus or minus, the manufacturers ALWAYS make the copper thickness
on the low side to save money. So even though theoretically one ounce copper
is near 1.4 mils, if you measure it it will be more like 1.2

Jack

                -----Original Message-----
                From:   <Charles Barker> [mailto:[log in to unmask]]
                Sent:   Friday, June 25, 1999 7:58 AM
                Subject:        Specifying copper thickness

                I have a question for all you TechNetters:

                If one ounce of copper is .00135" thick (per
ANSI/IPC-MF-150F Metal Foil
                for Printed Wiring Board), why do the tables in IPC-6012,
such as Table
                3-9, say: "For each succeeding ounce of copper foil,
increase minimum
                conductor thickness by 30.0 um (0.0012 inch). My detailitis
infected brain
                gets hung up on things that don't compute, and this doesn't
seem to
                compute!