What is Finished Copper

What is Finished Copper

Finished copper refers to the final thickness or weight of the copper layer on a printed circuit board after all manufacturing processes have been completed. It directly impacts the performance and functionality of the PCB.

The thickness of the copper layer is typically measured in microns or mils and is mainly determined by the base copper weight, which is often measured in ounces per square foot. For example, a 1-ounce copper weight corresponds to a certain thickness, while 2 ounces of copper will result in a thicker copper layer.

The perceived thickness based on ounces is not always accurate. The IPC-6012 standard, which is the qualification and performance standard for rigid PCBs, provides specific guidelines for copper thickness. According to this standard, the internal layer foil thickness for 1 ounce of copper is actually 34.3 microns (1.350 mils) as the target for the foil manufacturer. However, there is a 10% tolerance allowed, so a 1-ounce inner layer foil could be received at the PCB factory with a thickness of 30.9 microns (1.217 mils). Additionally, there is a 6-micron (0.236-mil) reduction allowed for the pretreatment of the copper foil during the process at the PCB factory before imaging and bonding. Therefore, the minimum copper foil thickness for 1 ounce of copper foil may be 24.9 microns (0.980 mils), which is different from the perceived 35 microns.

When it comes to the external or outer layers of the PCB, the calculation of finished copper becomes more complex. The IPC-6012 standard specifies base copper weights rather than finished copper. The minimum surface conductive finish of the finished copper is equal to the absolute minimum copper foil thickness, plus an average copper plating thickness, minus the maximum processing reduction allowance.

For example, if a customer calls for 1 ounce finished copper, the IPC minimum for 1-ounce copper weight is 30.9 microns (1.217 mils). Starting with ½ ounce (15.4 microns) and adding plating thickness of 20 microns (0.787 mils) for Class 2 or 25 microns (0.984 mils) for Class 3, and then accounting for a process reduction of 2 microns (0.079 mils), the finished copper thickness would be 33.4 microns (1.315 mils) for Class 2 and 38.4 microns (1.512 mils) for Class 3.