Tom, Foiol thickness is a widely misunderstood number and based on weight. If yoiu look in IPC-MF-140, it wil give you the nominal and tolerances on foil as received. If you look in IPC-RB-276 Para 2.9.2.2 and 2.9.2.3., it will give the numbers after processing. For example in RB-276, for internal layers after processing, the minimum for 1 oz. foil is 0.0001 inch. If 1 oz. is used externally the as plated the minimum is 0.0018 inch. The common practice of say on the drawing that the "Copper thickness is 1 oz" is very misleading and subject to all sorts of interpretations and lawsuits. Best practice is to state what copper foil starting thickness is as part of the material description on the drawing. If finshed thickness is critical and does noit fall into numbers in RB-276 it is necessary to put the finished number on the drawing as a real number. For example. "Copper conductor including foil and plating shall be 0.0002 inches min. thick". I think that IPC-D-275 or its replacement to be, IPC2221, will give you some guidelines. I also agree that copper suppliers an laminate suppliers almost always provide copper that is 10% below the nominal thickness. So, if it is critical, go to RB-276 for minimum thickness after processing. Phil Hinton Hinton "PWB" Engineering [log in to unmask]