PCB Copper Weight
When ordering printed circuit boards, one of the key steps faced by most of us is to specify the weight of PCB copper.
The most common measurement unit for copper thickness on the PCB is ounce (oz). A 1oz copper thickness is the result of 1oz of copper spread evenly over a one square foot area. The resulting thickness is 1.37 mils, or 0.0347 millimeters.
Most PCB manufacturers have a default PCB copper weight of 1 ounce. 2oz copper thickness is often more than is necessary, while 0.5 ounce of copper may not be enough, especially for maintaining a higher current on ground plane. If the board designer determines that the board must handle more current than 1 ounce, the PCB copper weight or trace width can be increased. It is worth mentioning that the thicker the thickness the higher the cost.
Minimum line width and PCB copper weight are interdependent. During the etching process, the copper traces are protected from the top only. This means that when the copper is etched away, the sides of the trace are also partially etched, and the finished shape of the trace will have a trapezoidal cross-section; and, thicker copper will require more traces and space.
Here are some general guidelines for minimal spacing based on PCB copper weight:
For weights greater than 4oz, it is generally recommended to double a layer rather than using thicker copper.
Keep in mind that heavy copper on the outer layers is problematic. For example, due to the trapezoidal cross section resulted from the etching process, an SMT pad may be thinner than designed and this may lead to weak solder joints or difficult component placement. Also, the solder mask will require multiple coats in order to protect heavy copper traces.