The main reason is, as is so often the case, money!

The copper foil used in circuit boards is almost never truly 1 oz thick
(.00135 or .0014"), especially on a finished board.  Look at Table 3-8
in IPC 6012.  One ounce copper foil on an IL needs to be 0.001" thick on
a finished board.  As a fabricator, my goal is to use the thinnest foil
possible and still meet that finished thickness.  By the way, my
supplier has the same goal.  Anything thicker than the minimum is extra
cost.  Thinner foil also makes conductor formation, especially on fine
lines and spaces, easier.

I need about 0.0002" of foil thickness to account for the tooth
structure and thinning due to processes such as preclean and oxide.
Therefore, I'm going to try to get the foil from my supplier at about
0.0013".  This is within the tolerance of IPC-MF-150F, which is the spec
covering the purchase of the foil by the fabricator, not the foil on the
finished PWB.

Table 1 in IPC-MF-150F shows that the nominal thickness goes up in
increments of 0.00135" per ounce.  The minimum allowed is the nominal
less 10%.  That is 0.001215" per ounce.  Consistent (sort of) with the
0.0012" increment in 6012.

The point is the foil supplier and board fabricator need to be able to
use foil at the minimum in IPC-MF-150F and meet the minimum in 6012.

Now if someone could explain why the difference between 2 oz and 3 oz
copper in Table 3 is 0.0014" instead of 0.0012".....




Mike McMaster
RF Product Engineer
Merix Corporation
503-992-4263

> ----------
> From:         <[log in to unmask]>[SMTP:[log in to unmask]]
> Reply To:     TechNet E-Mail Forum.;[log in to unmask]
> Sent:         Friday, June 25, 1999 7:58 AM
> To:   [log in to unmask]
> Subject:      [TN] 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!
>