We measure by cross section for 1 oz copper foil a thickness of about 32 µm only. Why?
The value of 35 µm is a nominal thickness value only. The nominal average area weight of 305 g/m² corresponds to a value of 34.15 µm. IPC-4562 stipulates for all copper foils a tolerance of ± 10 %. This means for 1 oz (35 µm) foil the area weight might vary between 274.5 g/m² and 335.5 g/m². By simple calculation using the copper density (8.93 g/cm³), the resulting theoretical thickness can vary between 30.74 µm and 37.57 µm. For a typical average area weight for a 1 oz copper foil of 285 g/m² the calculated copper thickness is 31.9 µm. Additionally, for all thickness measurements of copper foils in cross section the roughness of the treatment side has to be taken into account. The pyramidal and dendritic shaped matte side doesn’t allow a simple determination of the thickness. The graph below should explain this effect.