Copper Weight: it defines the number of ounce copper on 1 sq. ft. area, such as 0.5, 1.0, 2.0, 3.0 oz, etc. Usually this parameter is used to specify the thickness of the copper on each layer of the board. It is very easy to work out the thickness of 1.0 oz copper on 1 sq. ft. board based on the copper density (8.9 g/cm3), which is 34.29 µm and rounded up to 35 µm. So we have the following list.

0.5 oz means 17
.5 µm copper thichness
1.0 oz means 35 µm copper thichness
2.0 oz means 70 µm copper thichness

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