What is the copper layer thickness for power & ground layer? What is the formula for current calculations?

What is the copper layer thickness for power & ground layer? What is the formula for current calculations?

We suggest using two ounce copper layer internally. The external layers (the top and bottom) are usually one ounce. Our chip-scale devices have solder bumps or bars. Two ounce copper inner layers can carry more current, will heat less and the heat can be removed easier.