Micro-Resistance method is most commonly used for the measurement of copper thickness on PCB laminate and through hole copper thickness on PCBs.
Current is passed through two probe tips and then is measured by two more tips spaced apart. The consequent drop in current is due to the resistance of the copper layer. The thicker the copper, the less drop in current (lower resistance), thus the thickness can be calculated by the instrument based on the amount of resistance.
This method can measure over a wide range of thickness and the measurement uncertainty can be as low as only ± 5-10 millionths of an inch (± 0.1-0.25 micron).