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    • Copper Layer Thickness

      The copper in a PCB is rated in ounces, and represents the thickness of 1 ounce of copper rolled out to an area of 1 square foot. For example a PCB that uses 1 oz. copper has a thickness of 1.4mils.

      The vast majority of PCBs are manufactured with "1 ounce copper" on the outer layers. If there are inner layers, they are almost always manufactured with "1/2 ounce copper".


      In general terms the most commonly used values are

      1/2 oz. = 0.7 mils
      1 oz. = 1.4 mils
      2 oz. = 2.8 mils
      1 mil = 0.0254 millimetres

      The thickness of a thin slab of metal with a given top surface area is always exactly


      The area is "1 square foot" (144 square inches)
      The density of copper is 8.96 mg/mm3 = 5.18 ounce/(inch3)
      and for 1/2 oz copper clad FR4 the mass is 0.5 "1/2 ounce of copper"
      for 1oz copper clad FR4 the mass is 1.0 "1 ounce of copper"


      Electrodeposited (ED) copper foil is the standard copper used in the laminate industry. ED foil is deposited from solution onto a moving titanium or steel drum, from which it is subsequently stripped. The grain structure formed by this process forms the dendritic "tooth" of the copper foil on the bath side of the copper. The drum side takes the smooth texture of the polished drum surface onto which it is plated.

      Rolled copper is made by running a copper strip through successively smaller and smaller gaps in a rolling mill until it reaches the desired thickness. Rolled copper is smoother and can be made very flexible by annealing (The acronym "RA" means "Rolled Annealed"). Much of the RA foil in the laminate industry is used in flexible circuitry, typically bonded to a polyimide film with an acrylic adhesive, used in applications where ductility is essential.

      The primary attribute of the copper in a board will be its thickness. Copper thickness is often specified in ounces; where one ounce copper would be the thickness resulting from taking one ounce of copper and spreading it evenly over a one square foot surface. Referring to copper thickness in ounces is a tradition dating back to when many roofs were covered with copper, and the tradesmen specified copper in this way.

      Using thinner copper makes it easier to control trace widths, but thicker copper can carry more current without as much heat.

      Though copper is specified with a theoretically exact thickness, the actual thickness in any particular board construction can vary with accepted tolerances and as the result of certain processing steps in bare board fabrication.

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