Hi Charles,
All the replies you have received are factual, but do not cover all necessary
information.
1. Cu foil is sold by weight; the thinner you can make it within the spec the
more sq.ft. you can cover.
2. Most foil sold for electronic purposes has an adhesion tooth; so,
how/where are you measuring the foil thickness.
3. The thickness of the adhesion tooth does not vary proportionately with the
foil thickness; thus, you should not expect foil thickness values to be
multiples of the foil weight (nominal or otherwise) values.
4. Most Cu foil for electronic puposes is electrodeposited; unlike rolled
foil, ED foil has a density that is lower than the book value for Cu and is
somewhat dependent on the plating process.
4.