What Copper Weight Means?
Copper weight refers to the amount of copper on each square foot of the PCB surface, measured in ounces per square foot (oz/ft²). However, we often encounter another term, copper thickness. What is the relationship between copper weight and thickness? The measure of copper thickness is when a specific weight of copper is rolled to occupy 1 square foot. So, if 1 oz. of copper is rolled out to have an area of 1 square foot; the resulting foil thickness will be 1.37 mils or 0.0348 mm.
Here is a chart representing copper weight in ounces against copper thickness.
Selecting Optimal Copper Weight
When calculating copper weight, trace width, signal integrity, and voltage drop are essential considerations. For example, if you need to carry a high current, you'll need to use a thicker copper weight to ensure that the traces don't overheat. If you need to maintain high signal integrity, you'll need to use a thinner copper weight to minimize the inductance of the traces. PCBs with copper weights exceeding 3 oz and approaching 4 oz or more are generally considered "heavy copper." Due to their thicker copper layers, heavy copper PCBs are designed to handle high current loads and provide superior thermal management.
Design considerations for heavy copper
Increased minimum track spacing: As the copper weight increases on a PCB, the required spacing between copper features increases as well. It is important to pay attention to the minimum track spacing requirements for different copper weights, thus preventing signal degradation and overheating. At PCBWay, the minimum track spacing for 3 oz copper is 10 mil, 4 oz copper is 13 mil, and 5 oz copper is 18 mil.
Increased via size: Since heavy copper designs are often used in circuits that handle higher currents, it is essential to ensure that the vias can carry the required current without causing excessive resistive losses. Larger vias offer lower impedance paths for current flow, reducing resistive heating and minimizing voltage drops. This design choice helps ensure that the vias can effectively carry the higher current loads required in heavy copper designs, maintaining reliable connections between different layers of the PCB.
Increased solder mask thickness: Heavy copper has a higher surface tension, which can make it difficult for the solder mask to adhere to the copper surface. If the solder mask does not adhere properly, it can peel away from the copper surface, leaving the copper traces exposed. This can lead to shorts, corrosion, and other problems. Increasing the solder mask thickness can help to overcome this problem by providing more surface area for the solder mask molecules to adhere to.
