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Question:I have a question about the manufacturing capability for copper weights. Is it possible to use different copper weights on the same PCB? I have a few traces that will carry a high current, while the majority of the traces will carry low current. Because I have package constraints, I may be unable to use uniform copper weight with wide traces. Any feedback would be appreciated.
Answer:
Yes, using different copper weights on the same PCB is possible. This is called “multiple copper thicknesses” or “dual copper weight” PCBs. This option is meant for designs that need to handle varying current levels across different traces. There are generally two options to accomplish this:
- Layer-specific copper weight: Different copper weights applied to different layers
- Localized copper weight: Different copper weights are applied to different areas on the same layer.
Mechanical considerations
Thicker copper traces improve heat dissipation, which is beneficial for high-power designs. However, thicker copper traces might also impact how the board expands and contracts during temperature changes. It is important to “balance” the board so that the copper weights appear symmetrically across all layers (see Figure 1). This means that if a heavy copper weight is used for the top outer layer, it should also be mirrored on the bottom outer layer. Balancing ensures that the board does not experience excessive mechanical strains from thermal stress or during the stack-up assembly and lamination stage, where the layers are stacked per the original design and put in a press to achieve cohesion.
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Figure 1: A 4-layer stackup with thick 3-ounce copper used for high current traces on the outer layer is possible. But you must create balance for both outer layers to prevent mechanical failures such as bowing or twisting under high temperatures.
Several factors could cause mechanical issues:
- Uneven dielectric thickness
- Unbalanced board cross-section
- Use of mixed materials
As PCBs experience high temperatures, the internal materials expand and contract according to their coefficient of thermal expansion (CTE). Differential thermal strain from asymmetries can lead to warpage, bowing, or twisting of the PCB materials. For instance, the differences between the copper and foil and substrate can lead to warpage.
For localized copper weighting, consider the copper distribution of copper across the entire layer to ensure that the etching across the board, and thus the plating, is more predictable. Etching involves submerging the board in a bath to remove the exposed extra copper (the copper that is not covered in photoresist), leaving the desired resist/copper circuit pattern. The chemical process is intrinsically less precise where smaller copper features, such as traces and pads surrounded with less copper, will tend to experience more etching than larger copper features surrounded with copper. These will also tend to experience more plating than densely populated areas. An even copper weighting across the layer would ensure that the etch and stripping process will remove and add a similar amount of copper, mitigating any potential thermal issues (e.g., warping, bowing, twisting) that the board may later experience.
Tolerance considerations
Even with dual copper weights, it is essential to design the traces carefully to handle current appropriately. For high-current traces, ensure sufficient width and thickness while considering the overall design constraints, such as space and thermal management. Table 1 shows the minimum copper trace width and copper spacing for 1-ounce, 2-ounce, 3-ounce, and 4-ounce copper boards. The copper spacing is the minimum air gap between any two adjacent copper features; the trace width is the minimum width of a copper feature, usually traces.
Cost and lead-time considerations
PCBs with multiple copper weights typically require special manufacturing processes, including selective copper plating, which could increase both cost and lead time. These are important factors to consider as a design progresses. The introduction of board complexities can yield unexpected performance degradations, such as poor signal integrity and mechanical failures, and influence cost and time commitments. Consult with your PCB manufacturer early in the design to determine whether they can meet your specific requirements.
If you are working within packaging constraints and need to balance high-current and low-current traces, this approach is effective. Discuss your specific needs directly with us to ensure we can support this design strategy. Reach out to our Design Team here.
For localized copper weighting, consider the copper distribution of copper across the entire layer to ensure that the etching across the board, and thus the plating, is more predictable. Etching involves submerging the board in a bath to remove the exposed extra copper (the copper that is not covered in photoresist), leaving the desired resist/copper circuit pattern. The chemical process is intrinsically less precise where smaller copper features, such as traces and pads surrounded with less copper, will tend to experience more etching than larger copper features surrounded with copper. These will also tend to experience more plating than densely populated areas. An even copper weighting across the layer would ensure that the etch and stripping process will remove and add a similar amount of copper, mitigating any potential thermal issues (e.g., warping, bowing, twisting) that the board may later experience.
Tolerance considerations
Even with dual copper weights, it is essential to design the traces carefully to handle current appropriately. For high-current traces, ensure sufficient width and thickness while considering the overall design constraints, such as space and thermal management. Table 1 shows the minimum copper trace width and copper spacing for 1-ounce, 2-ounce, 3-ounce, and 4-ounce copper boards. The copper spacing is the minimum air gap between any two adjacent copper features; the trace width is the minimum width of a copper feature, usually traces.
| Copper weight | Minimum trace width/Copper spacing |
|---|---|
| 1 ounce | 0.003 inch, 3 mils |
| 2 ounces | 0.005 inch, 5 mils |
| 3 ounces | 0.009 inch, 9 mils |
| 4 ounces | 0.010 inch, 10 mils |
Table 1: Minimum trace width and copper spacing for boards with differently weighted copper boards.
Cost and lead-time considerations
PCBs with multiple copper weights typically require special manufacturing processes, including selective copper plating, which could increase both cost and lead time. These are important factors to consider as a design progresses. The introduction of board complexities can yield unexpected performance degradations, such as poor signal integrity and mechanical failures, and influence cost and time commitments. Consult with your PCB manufacturer early in the design to determine whether they can meet your specific requirements.
If you are working within packaging constraints and need to balance high-current and low-current traces, this approach is effective. Discuss your specific needs directly with us to ensure we can support this design strategy. Reach out to our Design Team here.
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