In the process of PCB design, some engineers don’t want to lay copper on the bottom of the surface layer in order to save time. Is this the right thing to do? Is it necessary to lay copper on the surface layer of PCB?
First of all, we need to be clear: it is beneficial and necessary for PCB to lay copper on the bottom surface, but there are some conditions for laying copper on the whole board.
Benefits of copper plating on the whole surface layer
1. From EMC’s point of view, the whole floor of the surface layer is covered with copper, which provides additional shielding protection and noise suppression for the inner layer signals, as well as some shielding protection for the surface layer devices and signals.
2. From the point of view of heat dissipation, as the current PCB board is getting denser and denser, the BGA main chip also needs to consider the thermal problem more and more. The whole floor copper improves the heat dissipation capacity of PCB.
3. From the process point of view, the whole board is paved with copper, so that the PCB board is evenly distributed, and the bending and warping of the board can be avoided during PCB processing and pressing, and meanwhile, the warping deformation of the PCB can be avoided due to the different stress caused by the uneven copper foil during PCB reflow soldering.
Reminder: For two-layer boards, copper cladding is necessary.
On the one hand, because there is no complete reference plane for the two-layer board, paving the floor can provide a backflow path and can also be used as a coplanar reference to achieve the purpose of impedance control. Generally, we can lay the floor on the ground floor, and put the main devices and power lines and signal lines on the top floor. For high impedance circuits, it’s a good idea to coat copper for analog circuits (analog-to-digital conversion circuit, switching mode power conversion circuit).
Conditions of copper laying on the surface layer
Although copper plating on the surface layer is beneficial to PCB, some conditions need to be followed:
1. At the same time, shop by hand as much as possible. Do not shop all at once to avoid broken copper skin, and properly add vias to the ground plane in the copper shop area.
Reason: The copper-clad plane on the surface layer will be broken by the separation of the components and signal lines on the surface layer. If there is a copper foil with poor grounding (especially the thin and long broken copper), it will become an antenna, resulting in EMI problems.
2. Consider the heat balance of small devices, such as 0402 0603, to avoid the monument effect.
Cause: If the whole board is covered with copper and the components pins are fully connected with copper, the heat will be dissipated too quickly, which will make it difficult to disassemble and repair welding.
3. The whole floor should be paved continuously, and the distance from the floor to the signal should be controlled to avoid the discontinuity of transmission line impedance.
Cause: The copper skin that is too close to the ground will change the impedance of microstrip transmission line, and the discontinuous copper skin will also have a negative impact on the impedance discontinuity of transmission line.
4. Some special situations depend on the application scenario. PCB design should not be absolute, but should be weighed and applied in combination with various theories.
Reason: Besides sensitive signals need to be covered with ground, if there are many high-speed signal lines and components, which will produce a lot of small and long pieces of copper, and the wiring channel is tight, it is necessary to try to avoid the surface copper skin from punching holes to connect with the ground plane. At this time, the surface layer can be chosen not to be covered with copper.
