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PCB Tech

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PCB Tech

PCB Tech

What are the advantages of laying copper on the bottom of the PCB high-frequency board
2021-09-10
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Author:Belle

    Do you know the advantages of laying copper on the bottom of the PCB high-frequency board? In the whole process of PCB circuit board design, engineers want to ignore the link of laying copper on the bottom of the surface in order to save time. Is this right? Is it necessary for the PCB high-frequency board to lay copper on the bottom of the surface? What are the advantages of laying copper on the bottom of the PCB high-frequency board?


    First of all, we need to be clear: the copper on the bottom of the surface is beneficial and necessary for the PCB high-frequency board, but the copper on the whole board needs to comply with some conditions.


    1. Advantages of paving copper on the bottom of the PCB high-frequency board:

    1. From the perspective of heat dissipation, as the current PCB high-frequency boards are getting higher and higher density, the BGA main chip also needs to consider heat issues more and more. The copper floor of the whole board improves the heat dissipation capacity of the PCB high-frequency board.


    2. From the emc point of view, the whole board is covered with copper on the bottom of the surface, which provides additional shielding protection and noise suppression for the inner signal to the inner signal, and also has a certain shielding protection for the devices and signals on the bottom of the surface.


    3. From the perspective of process analysis, the whole board is covered with copper, which makes the PCB high-frequency board evenly distributed, avoids the bending and warping of the board during PCB processing and pressing, and avoids the PCB high-frequency board from being reflowed due to the unbalanced copper foil. The different stresses cause the PCB high-frequency board to warp and deform.


    Reminder: For double-sided circuit boards, copper coating is necessary

    On the one hand, since the double-sided circuit board does not have a complete reference plane, the ground can provide a return path and can also be used as a coplanar reference to achieve the purpose of impedance control. We can generally lay the ground plane on the bottom layer, and put the main components on the top layer and use power lines and signal lines. For high-impedance loops, analog circuits (analog-to-digital conversion circuits, switch-mode power conversion circuits), copper plating is a good practice.

PCB high-frequency board

    2. Conditions for copper paving on the surface and bottom layers:

    Although the copper on the bottom of the surface is good for the PCB high-frequency board, but it also needs to follow some conditions:

    1. Consider the thermal balance of small devices, such as 04020603, to avoid tombstone effect.


    Reason: If the whole board is covered with copper, if the component pins are fully connected with copper, the heat will be lost too quickly, and it will be difficult to desolder and rework.


    2. At the same time, try to shop by hand at the same time, do not cover it all at once, avoid broken copper skin, and properly add via holes in the copper paving area to the ground plane.


    Reason: The surface 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.


    3. It is best to pave the whole board continuously. The distance from the paving to the signal needs to be controlled to avoid discontinuity in the impedance of the transmission line.

    Reason: The copper skin that is too close when laying the ground will change the impedance of the microstrip transmission line, and the discontinuous copper skin will also cause the negative impact of the impedance discontinuity of the transmission line.


    4. Some special situations depend on the application scenario. PCB circuit board design should not be an absolute design, it should be weighed and used in conjunction with the theories of all parties.


    Reason: In addition to 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 are generated, and the wiring channels are tight, it is necessary to avoid perforating the surface copper to connect to the ground plane. You can choose not to lay copper on the surface. The above are the advantages of laying copper on the bottom of the PCB high-frequency board. I hope to help everyone.