The copper thickness of the layers of a Printed Circuit Board varies depending on the material and manufacturing process.
When we consider the layer copper thickness on a PCB we refer to:
- Start Copper or Base Copper – The copper thickness as received from our suppliers.
- End Copper or Finished Copper – This is the final thickness of copper on the finished PCB.
The IPC-4562 and IPC-A-600 define both the acceptable thickness and tolerances for both the Base and End copper.
Start or Base Copper Thickness
Originally the base copper thickness was defined on the basis weight over an area, as in oz/ft² (oz = ounce and ft = feet) however, today it is more commonly defined as an actual thickness measurement, for example 35 µm.
So how do we convert oz/ft² to an actual measured thickness?
This is done by theoretically spreading the oz value of copper evenly over 1ft² and then measuring the thickness of the copper.
For example, 1oz/ft² has a thickness of 1.37 mil (0.00137 Inch) or 34.798 µm (0.034798 mm).
As with all things that are manufactured there is Manufacturing Tolerance that we must take into consideration. The IPC-4562 standard allows for up to a maximum of 10% reduction in the base copper thickness as supplied by the base material manufacturer.
This means 34.798 µm (35 µm) is allowed to be a minimum of 31.31 µm thick.