Should copper be applied to the PCB design surface?

Should copper be applied to the PCB design surface? Cheersonic

In pcb design, we often wonder whether the surface of pcb should be coated with copper? This actually depends on the situation. First of all, we need to understand the advantages and disadvantages of surface copper coating.

First of all, let’s look at the benefits of copper coating on the surface

1. The surface copper layer can provide additional shielding protection and noise suppression for the inner layer signal;
2. It can improve a heat dissipation capacity of pcb;
3. In the PCB production process, save the amount of corrosive agent;
4. Avoid warping and deformation of the PCB due to the unbalanced copper foil caused by the different stress generated when the PCB is reflowed.

The corresponding surface copper also has corresponding disadvantages:

1. The copper-clad plane on the outer layer will be fragmented by the components and signal lines on the surface. If there is a poorly grounded copper foil (especially the slender and long broken copper), it will become an antenna and generate EMI. Problem; (for this copper skin, we can also dig it out through the function in the software)

2. If the component pins are fully connected with copper, it will cause the heat to dissipate too quickly, making it difficult to desolder and rework. Therefore, we usually use cross-connected copper for SMD components.

Therefore, there are the following conclusions for the analysis of whether the surface is coated with copper:

1. PCB design For two-layer boards, copper cladding is necessary. Generally, the ground plane is laid on the bottom layer, and the main components and power lines and signal lines are placed on the top layer;

2. For high-impedance circuits, analog circuits (analog-to-digital conversion circuits, switch-mode power conversion circuits), copper cladding is a good practice;

3. For multi-layer high-speed digital circuits with complete power supply and ground plane, note that this refers to high-speed digital circuits, and copper cladding on the outer layer will not bring great benefits;

4. For digital circuits using multi-layer boards, the inner layer has a complete power supply and ground plane. Copper coating on the surface cannot significantly reduce crosstalk. On the contrary, too close copper will change the impedance of the microstrip transmission line. Copper skin will also negatively affect the impedance discontinuity of the transmission line;

5. For multi-layer boards, the distance between the microstrip line and the reference plane is less than 10mil, and the return path of the signal will directly select the reference plane under the signal line instead of the surrounding copper, because its impedance is lower. For a double-layer board with a distance of 60 mil between the signal line and the reference plane, wrapping a complete copper layer along the entire signal line path can significantly reduce noise;

6. For multi-layer boards, if there are many surface devices and traces, do not apply copper to avoid excessive broken copper. If there are few surface components and high-speed signals, and the board is relatively empty, you can choose to lay copper on the surface layer for the requirements of PCB processing technology, but it should be noted that the distance between the copper skin and the high-speed signal line during PCB design is at least 4W or more to avoid changes The characteristic impedance of the signal line.

The articles are from Fanyi PCB, author Chen Hu

Cheersonic is the leading developer and manufacturer of ultrasonic coating systems for applying precise, thin film coatings to protect, strengthen or smooth surfaces on parts and components for the microelectronics/electronics, alternative energy, medical and industrial markets, including specialized glass applications in construction and automotive.