How awesome is high-speed PCB design?

High-speed PCB design is a relatively complicated process. Because of the need to fully consider many technical factors such as signal, impedance, transmission line, etc. in high-speed PCB design, it often becomes a major difficulty for beginners in PCB design. The basic concepts and technical points of high-speed PCB design provided in this paper will provide some technical references for beginners.
Reasonable use of multilayer board for PCB wiring
In the actual design process of PCB board, most engineers will choose multilayer board to complete high-speed signal wiring, and multilayer board is an effective means to reduce circuit interference. When using multilayer boards to complete the design of high-speed signal circuit of PCB, engineers need to reasonably select the number of layers to reduce the size of PCB, make full use of the middle layer to set the shield, realize the grounding nearby, effectively reduce parasitic inductance, shorten the signal transmission length, reduce the cross interference between signals, etc. All these methods are very beneficial to the reliability of high-speed circuits.
Figure 1: Common low-cost 6-layer laminate scheme: there are two inner wiring layers.
When stacking, two signal layers should be avoided as far as possible. If it is inevitable, as shown in Figure 1, the routing directions of the two adjacent layers must be perpendicular to each other.
Figure 2: Common 8-layer laminate scheme: better laminate.
In addition to the above mentioned methods of using multilayer boards to improve the reliability of PCB signal transmission, some authoritative data show that the noise of four-layer boards with the same material is 20dB lower than that of double-layer boards. The less the lead is bent, the better. It is best to use a straight line, which needs turning. A 45-degree folding line or arc turning can be used, which can reduce the external transmission and mutual coupling of high-speed signals and reduce the radiation and reflection of signals.
In the process of PCB high-speed signal circuit design and wiring, engineers need to shorten the leads between the pins of high-speed circuit devices as much as possible, thinking that the longer the leads, the greater the distributed inductance and distributed capacitance, and the PCB factory | Shenzhen PCB factory | Medical PCB factory | Shenzhen PCB factory | Shenzhen deep connection circuit-specializing in PCB production, which will lead to reflection and oscillation of high-speed circuit system.
In addition to shortening the leads between the pins of high-speed circuit components as much as possible, in the process of PCB wiring, the less alternation between the leads between pins of each high-speed circuit component, the better, that is, the fewer vias used in the process of component connection, the better. Generally speaking, a via can bring about 0.5pF distributed capacitance, which will lead to a significant increase in the delay of the circuit. At the same time, high-speed circuit wiring should pay attention to the “cross-interference” caused by short-distance parallel wiring of signal lines. If parallel distribution cannot be avoided, a large area of “ground” can be arranged on the opposite side of parallel signal lines to reduce the interference. In the two adjacent floors, the directions of wiring must be perpendicular to each other.
Surround particularly important signal lines or local units with ground wires.
In the process of PCB wiring design, engineers can surround some very important signal lines with ground wires, and add protective ground wires on the periphery at the same time when these signals which are not easily disturbed, such as clock signals and high-speed analog signals, and sandwich the signal lines to be protected.
Because in the design process, all kinds of signal traces can’t form a loop, and the ground wire can’t form a current loop. However, if the loop wiring circuit is generated, it will cause great interference in the system. The wiring method that the ground wire surrounds the signal wire can effectively avoid the formation of a loop during wiring.
One or several high-frequency decoupling capacitors should be set near each integrated circuit block.
When analog ground wire and digital ground wire are connected to public ground wire, high frequency choke link should be used. Some high-speed signal lines should be specially treated: differential signals should be on the same layer and as close as possible to parallel traces, and no signals should be inserted between differential signal lines, and they should be of equal length.
In addition to the design methods mentioned above, when designing PCB signal lines, engineers should try their best to avoid branching of high-speed signal lines or forming tree stumps. High-frequency signal lines are easy to generate large electromagnetic radiation when they walk on the surface layer. If the high-frequency signal lines are wired between the power supply and the ground wire, the electromagnetic wave will be absorbed by the power supply and the bottom layer, and the generated radiation will be greatly reduced.