One argument for using is 1/2 oz on internal layers is a reduction in line 
width variation.  There are very few PWB suppliers that can supply supply a 
4 mil line +/-20% on 1oz copper.  In addition, mixed copper thicknesses on a 
layer pair [i.e. lyr 2 (top)and lyr 3 (bottom)] result in reduced yields.  I 
also don't believe there is anyone who can honestly quote a 3 mil line 
+/-20% on 1oz copper and maintain the specification.

nwbluez
 ----------
From: TechNet-request
To: Technet
Subject: Half ounce internals
Date: Friday, February 16, 1996 4:40PM

A strong consideration for not using 1/2 ounce copper on internal
layers, is that you only half the width of copper to make a
connection at the barrel of plating.  Possible problems.  Be careful.
 May not be reliable.

Norm Einarson