Analysis of key points of PCB design and wiring

Some friends in PCB wiring, often found a lot of problems after the board is made. Due to the lack of sufficient analysis in the early stage or no analysis at all, the post-processing is very difficult. For example, the power cord and some miscellaneous lines are well deployed, but a group of important signal lines are missing, and the result is that this group of lines can not be placed on the same group of the same layer, or even a complete reference plane, which requires significant modifications to the previous wiring work to complete, time-consuming and labor-intensive. We can compare the PCB board to a city, the components are like a forest of various buildings, and the signal line is the city’s streets, Bridges and roundabout islands. The appearance of each road is planned in detail, as is the wiring.

Priority when wiring:

a) Priority processing of key signals: key signals such as power supply, analog small signals, high-speed signals, clock signals and synchronization signals should be routed first.

b) Prioritize areas with high wiring density: Start wiring from the most complex connected devices on the board, and start from the most densely wired areas.

c) Key signal processing precautions: Provide a special wiring layer for key signals such as clock signals, high-frequency signals and sensitive signals to ensure the minimum loop area. If necessary, the method of shielding and increasing the safety distance is adopted to ensure the signal quality.

d) Impedance control: The network with impedance control requirements should be arranged on the impedance control layer to avoid the signal crossing the segmented area.

Cabling crosstalk control:

a) Interpretation of the 3W principle: The distance between lines should be maintained at least 3 times the width of the lines. This is to reduce crosstalk between the lines and ensure that the line spacing is large enough. If the center distance of the lines is not less than 3 times the width of the lines, 70% of the electric fields between the lines can not interfere with each other, which is called the 3W rule.

b) CrossTalk solution: Crosstalk refers to the interference between different networks on the PCB due to long parallel wiring, mainly due to the distributed capacitance and distributed inductance effect between parallel lines.

i. Increase the spacing of parallel wiring and follow the 3W rule to ensure that the spacing is large enough.

ii. Insert a ground isolation cable between parallel lines.

iii. Reduce the distance between the wiring layer and the ground plane.

General rule requirements for wiring

a) The plane is orthogonal to the direction of the line. The signal lines of adjacent layers should go in an orthogonal structure to avoid different signal lines going in the same direction in adjacent layers, thereby reducing unnecessary cross-talk between layers. When some board structure is limited (such as some backplanes), it is difficult to avoid this situation, especially when the signal rate is high, the ground plane should be considered to isolate different wiring layers, while the ground wire is used to isolate each signal line.

b) The wiring of small discrete devices should be symmetrical. Dense (SMT) pad leads should be connected outside the pad and are not allowed to be connected directly in the middle of the pad.

c) Minimize loop rules. Ensure that the area of the loop formed by the signal line and its loop is as small as possible. The smaller the area of the ring, the less interference radiated to the outside, thus reducing the reception of external interference.

d) Avoid STUB parts of cables. Make sure there are no unconnected protrusions.

e) Keep the same cable width of the same network. Different widths of the track will lead to uneven characteristic impedance of the line, especially at high speed transmission may cause reflection. In some cases, such as connector leads or BGA-packaged leads, the effective length of inconsistent parts is minimized.

f) Prevent the signal line from forming a self-loop. This problem is particularly easy to occur in multi-layer board design, and the self-loop will cause radiation interference.