Skin depth is a measure of how far electrical conduction takes place in a conductor, and is a function of frequency. At DC (0 Hz) the entire conductor is used, no matter how thick it is. At microwave frequencies, the conduction takes place at the top surface only. As we go down the surface, the current density decrease.
Below image depicts the conduction of RF signals, red color depicts the largest current density and blue color depicts absence of RF signal.
That is why the top metal is most important to conductivity in a microstrip multi-metal stack-up.
At RF frequencies, top conductor thickness is choosen as per skin depth of the material. There is a limitation on the conductance that you can achieve, and increasing the thickness of precious metals to reduce losses RF can be a waste of money if you don’t know what you are doing. Because in DC (0IF) increasing the thickness increase the conductance but this is not the case in RF.
Skin Depth Formula:
Rule of thumb:
The skin depth for copper is about 2 microns at 1 GHz and scales inversely with the sqrt(f).