Electric current moves through a PCB along its copper layers. So, choosing the incorrect copper thickness can cripple the performance of a circuit assembly. However, by understanding the relationship of copper weight to signal transmission and PCB fabrication cost, you can confidently select the correct copper weight for your design. To do this, it is essential to know:
- What is copper weight/copper thickness?
- Why is copper weight important?
- Why use heavy copper?
- Copper weight and PCB manufacturing Cost
1. What is copper weight/copper thickness?
The standard copper thickness used in PCB production is 1 oz. Generally, the copper thickness is referenced to in ounces. 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.
2. Why is copper weight important?
The current carrying capacity of a PCB is determined by the thickness of its copper traces and, therefore, of the copper layer.
For impedance-controlled boards, the required trace width is an important determinant of how thick the copper should be. Moreover, the PCB weight, the type of connectors that will be assembled onto the board, and component compatibility requirements, are also important factors.
3. Why use heavy copper?
Most PCBs are manufactured using a copper thickness from 1 oz. to 3 oz. Generally, copper weight above 4 oz. is considered heavy copper and is used for:
- Larger current carrying capability
- Heightened capacity to withstand thermal stresses and better thermal management
- Improved heat dissipation
- Higher mechanical strength suitable for large/heavy connectors and through-hole components
- Planar transformers and power converters
Sometimes PCBs have layers designed with differing copper thicknesses. For instance, 1 oz. copper is used for the inner layers and 4 oz. copper for outer layers. When in operation, a PCB generates heat which can result in the PCB bending or twisting. To minimize this issue, the same copper weight should be used in opposing layers in the stack-up. For example, in an 8-layer board, the opposing layer groupings would be: 1 & 8, 2 & 7, 3 & 6, 4 & 5. So, layers 1 and 8 should have the same copper weight, and so should layers 2 and 7, and so on.
4. Copper weight and PCB manufacturing cost
If your PCB design requires heavy copper, you should know that manufacturing cost and time will be impacted. Naturally, thicker copper is more expensive than standard copper. Also, heavier copper requires more manufacturing time and resources, therefore, adding to the manufacturing cost.
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