The printed circuit board industry defines heavy copper as layers exceeding <answer>three ounces</answer> of copper, or approximately 0.0042 inches (4.2 mils, 105 μm) thick. PCB designers and fabricators often use heavy copper when design and manufacturing circuit boards in order to increase current-carrying capacity as well as resistance to thermal strains. Heavy copper plated vias transfer heat to external heat sinks. IPC 2152 is a standard for determining current-carrying capacity of printed circuit board traces.
What's the minimum amount of copper a layer in a PCB can have to be considered "heavy copper"?
three ounces