trace thickness
The thickness of copper clad on a circuit board is usually given in ounces (oz). It is the weight of copper on one square foot of board. One ounce copper is 1.378mils (~35um) thick.
Normal PCB copper thicknesses are 1/4oz, 1/2oz, 1oz, and 2oz. Other thicknesses are possible, depending on the capabilities of the fabricator that makes your board. However, copper that is too thick creates lamination problems.
A typical board would use 1/2oz (.7mil or 18um) copper on inner layers, and 1oz (1.4mil or 35um) copper on planes and outside layers. If the board is thin and high density, thinner copper clad would be used to keep the weight and finished board thickness as small as possible.