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The Printed Circuit Designer’s Guide to... High Performance Materials
by Michael Gay, Isola
Chapter 4: Copper Foil
Copper foil is the standard conductive layer used for metal-clad laminates, although other options are available. There are two main types of copper foil used for PCB boards today: electrodeposited (ED) foil and rolled annealed (RA) foil. ED copper foil is produced by a continuous process which yields a well-controlled product in mass volume and low cost as compared to RA copper foil. ED copper foil has a wide range of thicknesses, from 5–400 μm, for PCB applications. IC substrate application requires an ultra-thin foil which is supplied on an 18–72 μm copper carrier and range in thicknesses from 1.5–5 μm.
Rolled annealed foil yields a very smooth surface where the process deforms the copper crystalline structure to achieve thickness. Unfortunately, the foil is only available in a 25" wide format. Most processes are designed around a 50" wide machine direction of the glass using large hydraulic presses with platen sizes to accommodate the 50" glass width. Use of the RA foil reduces productivity and results in higher cost. With the development of newer smooth ED foils that are as smooth as RA foil, the need for RA foil and the associated cost has been largely mitigated.
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