During the PCB design process, some engineers do not want to lay copper on the surface and bottom of the entire board in order to save time. Is this right? Is it necessary for PCB to lay copper on the bottom layer of the surface?
First of all, we need to be clear: laying copper on the bottom layer of the surface is beneficial and necessary for PCB, but laying copper on the entire board must comply with some conditions.
The benefits of laying copper on the entire surface and bottom layer
1. From an EMC perspective, the entire bottom layer of the surface is paved with ground copper, which provides additional shielding protection and noise suppression for the inner layer signals. At the same time, it also provides certain shielding protection for the devices and signals on the bottom layer of the surface.
2. From the perspective of heat dissipation, as current PCB boards are becoming more and more dense, BGA main chips increasingly need to consider thermal issues. The entire board is paved with copper to improve the heat dissipation capacity of the PCB board.
3. From a process perspective, the entire board is paved with copper to make the PCB board evenly distributed. This avoids board bending and warping during PCB processing and lamination. At the same time, it avoids the difference in stress caused by the uneven copper foil during reflow soldering of the PCB. The PCB is warped and deformed.
Reminder: For two-layer boards, copper pouring is necessary
On the one hand, since the two-layer 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 place the main components and power and signal lines on the top layer. For high-impedance loops, analog circuits (analog-to-digital conversion circuits, switch-mode power conversion circuits), pouring copper is a good practice.
Conditions for laying copper on the bottom layer of the surface
Although it is good for PCB to lay copper on the bottom layer of the surface, some conditions need to be followed:
1. Try to lay it by hand at the same time. Do not lay it all at once to avoid broken copper sheets. Appropriately add via holes to the ground plane in the copper laying area.
Reason: The surface copper-clad plane will definitely be separated and fragmented by the surface components and signal lines. If there is poorly grounded copper foil (especially the thin and long broken copper), it will become an antenna and cause EMI problems. .
2. Consider the thermal balance of small devices, such as 0402 0603 and other small packages, to avoid the tombstone effect.
Reason: If the entire board is covered with copper and the component pins are fully connected with copper, it will cause heat to dissipate too quickly, making it difficult to desolder and rework the soldering.
3. It is best to use continuous flooring for the entire board. The distance from the flooring to the signal needs to be controlled to avoid discontinuous transmission line impedance.
Reason: Copper sheets that are too close to each other when laying the ground will change the impedance of the microstrip transmission line, and discontinuous copper sheets will also cause the negative impact of discontinuous impedance on the transmission line.
4. Some special situations depend on the application scenario. PCB design should not be an absolute design, but should be weighed and applied based on 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 broken copper will be produced, and the wiring channels are tight, it is necessary to try to avoid drilling holes in the surface copper to connect to the ground plane. You can choose not to lay copper on the surface.