Real-world copper traces have resistance. This means that a trace has a voltage drop, power dissipation, and a temperature rise when current flows through it. The formula of resistance here is: R= (resistivity×length)/(thickness×width). It is obviously that length, thickness, and width can be used to control the resistance of a PCB trace. As resistance is a physical property of the metal, PCB designers can't really change the physical properties of copper, so the trace size plays an important role.
PCB trace thickness is measured in ounces of copper. One ounce of copper is the thickness we would measure if we evenly spread 1 oz of copper over a 1 square foot area. This thickness is 1.4 thousandths of an inch. Many PCB designers use 1 oz or 2 oz copper, also most PCB manufacturers can provide 6 oz thickness. Note that fine features like pins that are close together are hard to make in thick copper.
Use a PCB trace width calculator to determine how thick and wide your traces should be for your application. If you have extra space on the board, use bigger traces, as they don’t cost anything.
When doing a multi-layer board, remember that traces on external layers have better cooling than traces on internal layers because the heat from inner layers has to travel through layers of copper and PCB material before being conducted, radiated, or connected away.