In the PCB design process, some engineers do not want to lay copper on the entire surface of the bottom layer in order to save time. Is this correct? Does the PCB have to be copper plated?

First of all, we need to be clear: the bottom copper plating is beneficial and necessary for the PCB, but the copper plating on the entire board must meet certain conditions.

The benefits of bottom copper plating
1. From the perspective of EMC, the entire surface of the bottom layer is covered with copper, which provides additional shielding protection and noise suppression for the inner signal and inner signal. At the same time, it also has a certain shielding protection for the underlying equipment and signals.

2. From the perspective of heat dissipation, due to the current increase in PCB board density, the BGA main chip also needs to consider heat dissipation issues more and more. The entire circuit board is grounded with copper to improve the heat dissipation capacity of the PCB.

3. From a process point of view, the whole board is grounded with copper to make the PCB board evenly distributed. PCB bending and warping should be avoided during PCB processing and pressing. At the same time, the stress caused by PCB reflow soldering will not be caused by the uneven copper foil. PCB warpage.

Reminder: For two-layer boards, copper coating is required

On the one hand, because the two-layer board does not have a complete reference plane, the paved ground can provide a return path, and can also be used as a coplanar reference to achieve the purpose of controlling impedance. We can usually put the ground plane on the bottom layer, and then put the main components and power lines and signal lines on the top layer. For high impedance circuits, analog circuits (analog-to-digital conversion circuits, switch-mode power conversion circuits), copper plating is a good habit.

Conditions for copper plating on the bottom
Although the bottom layer of copper is very suitable for PCB, it still needs to meet some conditions:

1. Lay as much as possible at the same time, do not cover all at once, avoid the copper skin from cracking, and add through holes on the ground layer of the copper area.

Reason: The copper layer on the surface layer must be broken and destroyed by the components and signal lines on the surface layer. If the copper foil is poorly grounded (especially the thin and long copper foil is broken), it will become an antenna and cause EMI problems.

2. Consider the thermal balance of small packages, especially small packages, such as 0402 0603, to avoid monumental effects.

Reason: If the entire circuit board is copper-plated, the copper of the component pins will be completely connected to the copper, which will cause the heat to dissipate too quickly, which will cause difficulties in desoldering and rework.

3. The grounding of the entire PCB circuit board is preferably continuous grounding. The distance from ground to signal needs to be controlled to avoid discontinuities in the impedance of the transmission line.

Reason: The copper sheet is too close to the ground will change the impedance of the microstrip transmission line, and the discontinuous copper sheet will also have a negative impact on the impedance discontinuity of the transmission line.

4. Some special cases depend on the application scenario. PCB design should not be an absolute design, but should be weighed and combined with various theories.

Reason: In addition to sensitive signals that need to be grounded, if there are many high-speed signal lines and components, a large number of small and long copper breaks will be generated, and the wiring channels are tight. It is necessary to avoid as many copper holes on the surface as possible to connect to the ground layer. The surface layer can optionally be other than copper.