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PCB Tech
Should the outer layer of the pcb design be coated with copper?
PCB Tech
Should the outer layer of the pcb design be coated with copper?

Should the outer layer of the pcb design be coated with copper?

2021-11-08
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Author:Jack

We often see in textbooks or in the PCB design guides of the original IC manufacturers that at the end of the layout, we should pour copper on the outer layer of the PCB, that is, cover the blank area of the PCB with a well-grounded copper foil.

PCB design guides

The advantages of copper coating on the outer layer of the PCB are as follows:
         Provide additional shielding protection and noise suppression for the inner signal to improve the heat dissipation capacity of the PCB. During the PCB production process, the amount of corrosives is saved. (Can this reduce the cost?) Avoid the PCB warping and deformation caused by the different stress caused by the PCB over reflow due to the unbalanced copper foil
But doing so will also bring some disadvantages:
       The outer copper-clad plane must be separated by the surface components and signal lines. If there is a poorly grounded copper foil (especially the thin and long copper), it will become an antenna and cause EMI problems. Full copper-clad connections for component pins will cause excessive heat loss and difficulty in desoldering and rework soldering. As mentioned earlier, the outer copper-clad plane must be well grounded, and more vias and main ground must be drilled. For planar connections, too many vias are punched, which will inevitably affect the wiring channels, unless buried blind vias are used.
        PCB design is very necessary for two-layer boards. Copper is usually laid on the bottom layer, and the top layer is used to put the main components and power lines and signal lines. For high-impedance circuits, analog circuits (analog-to-digital conversion circuits, switch-mode power conversion circuits), copper plating is a good practice.
         For high-speed digital circuits on multi-layer boards with complete power and ground planes, note that this refers to high-speed digital circuits, and copper-cladding on the outer layer will not bring great benefits. For digital circuits using multi-layer boards, the inner layer has a complete power supply and ground plane. Copper coating on the surface layer does not significantly reduce crosstalk. On the contrary, the copper skin that is too close will change the impedance of the microstrip transmission line. Discontinuous copper skin It will also cause the negative impact of discontinuous impedance on the transmission line.
        For multi-layer boards, the distance between the microstrip line and the reference plane is less than 10 mils, and the return path of the signal will directly choose the reference plane under the signal line instead of the surrounding copper because its impedance is lower. For a double-layer board with a 60 mil distance between the signal line and the reference plane, a complete copper sheet along the entire signal line path can significantly reduce noise.
        Therefore, whether copper should be laid on the surface layer depends on the application scenario. Except for sensitive signals that need to be grounded, if there are many high-speed signal lines and components, a lot of small and long copper fragments will be generated, and the wiring channels are tight, so you need to avoid it as much as possible. The surface layer of copper is connected to the ground plane through holes. At this time, the surface layer can choose not to be covered with copper. If there are few surface components and high-speed signals, the board is relatively open. For PCB processing requirements, you can choose to lay copper on the surface, but pay attention to the distance between the copper skin and the high-speed signal line at least 4W during the PCB design to avoid changes. The characteristic impedance of the signal line, and the copper on the surface should be well connected to the main ground plane with holes at tenths of the wavelength of the highest signal frequency.