From my experience:

1)      If a designer can get away with using ½ ounce copper he should,
because it is easier to etch and control trace widths. The only reason to
use thicker copper on internal layers would be so that the same trace width
can handle more current. Internal traces need way more copper to handle the
same thermal rise than external traces, but if current isn't an issue I
don't think there is a reason for one ounce.
2)      A small dielectric thickness between power and ground will increase
capacitance, so in effect the planes coupled together act as one big cap.
Maybe some decoupling caps can be removed?
3)      I only specify material thicknesses for impedance controlled boards.
For example, we do lots of 100 ohm differential pairs that need 5mil or 6mil
cores, but the rest we don't exactly care about. Don't constrain the vendor
without a reason, in my opinion.

Jack

                -----Original Message-----
                From:   Eileen Ong [R&D]
[mailto:[log in to unmask]]
                Sent:   Wednesday, July 21, 1999 1:57 AM
                Subject:        Stackup assignment for pcb boards.

                I have seen different specification given to pcb house for
pcb fabrication.
                What is the guidelines regarding
                the thickness of copper for different layers for best
performance and cost
                effective.

                1) For example for a 2/4 layer pcb,examples of some of the
spec. are :
                   2 layer pcb : a) FR4 material with 1/2 oz copper
                                  b) FR4 material with 1oz copper

                   4 layer pcb : c) FR4 material with 1oz copper in all
layers
                                  d) 1/2oz copper clad FR4 plate up 1oz,
inner layer use 1oz

                Any differences between a) & b)  or c) & d) ?

                2) I have read from "somewhere" that it is better to use
smallest distance
                spacing between power & ground plane
                in the inner layer for lowest power impedance. What is this
smallest
                distance ?

                3) How about core and prepreg, do you'll usually specify
their thickness
                also ?

                Thanks in advance for all contributions given.

                Best Regards,
                EILEEN ONG (R&D Dept.)
                Paradise Innovations (Asia) Pte Ltd
                Email : mailto:[log in to unmask]
                Web: http://www.paradisemmp.com