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Focus! Don't pull PCB track carelessly


Blindly pull thread, even pulling is useless!

When some small partners do PCB layout, board is ready to go. Due to insufficient or lack of analysis at an early stage, post-processing is difficult.

For example, after extruding a power line and different lines, but without a group of important signal lines, this group of lines cannot be in same group and on same layer, and there is not even a complete reference plane. It is necessary to make serious changes to previous electrical work. Complete, time-consuming and labor-intensive.

If a printed circuit board is compared to our city, then components are like various buildings row by row, and signal lines are streets and alleys in city, overpasses and islands, and the appearance of each road has its own detailed layout. The same goes for wiring.


Routing Priority Requirements

a) Main signal line priority: Key signals such as power supply, analog weak signal, high speed signal, clock signal and clock signal are given priority.

b) Wiring Density Priority Principle: Start wiring from device with most complex connection on board. Start wiring from most densely connected area on board.

c) Precautions when handling key signals: try to provide dedicated wiring layers for key signals such as clock signals, high frequency signals and sensitive signals, and ensure smallest loop area. If necessary, methods such as shielding and increasing safety distance should be applied. Ensure signal quality.

d) Networks with impedance control requirements should be located at impedance control layer, and cross signal separation should be avoided.


Waste disposal

a) Interpretation of 3W principles

Space between lines remains 3 times line width. To reduce crosstalk between lines, distance between lines should be large enough. If distance between centers of lines is at least 3 times width of line, 70% of electric field between lines can be kept without interfering with each other, which is called 3W rule.

b) Crosstalk control: Crosstalk (CrossTalk) refers to mutual interference between different circuits on a PCB caused by long parallel wires, mainly due to role of distributed capacitance and distributed inductance between parallel lines. Basic measures to overcome crosstalk:

i. Increase spacing between parallel wires and follow 3W rule;

ii. Insert a grounded insulated wire between parallel wires

iii. Reduce distance between wiring layer and ground plane.


General Routing Rules

a) The routing directions of neighboring planes form an orthogonal structure. Avoid running different signal lines in same direction on adjacent layers to reduce unnecessary crosstalk between layers; when it is difficult to avoid this situation due to limitations of board structure (such as some backplanes), especially when signal speed is high, each layer of wiring should be considered to be isolated by a ground plate, and each signal line by a ground signal line.

b) Traces of small discrete devices should be symmetrical, and SMT pad pins with relatively tight pitch should be connected outside pads, and direct connections in middle of pads are not allowed.

c) The loop minimum rule, that is, area of ​​the loop formed by signal line and its loop, should be as small as possible. The smaller loop area, less external radiation, and less interference from outside world.

d) STUB is not allowed in wiring.

e) The wiring width of same network must be same. Changing line width will cause uneven line characteristic impedance, and reflection will occur at high transmission speed. Under certain conditions, such as connector lead wires and similar BGA package lead wire designs, line width variation cannot be avoided due to distance being too small, and effective length of mismatched portion in middle must be kept to a minimum.

f) Avoid loops between signal lines between different layers. Such problems can occur in a multi-layer board design, and self-looping will cause radiation interference.

g) PCB design should avoid sharp and right angles, resulting in unnecessary radiation, and performance of PCB manufacturing process is degraded.