PCB Tech - What are the principles of pcb design

1: During PCB layout, ensure that the input and output lines of the filter circuit (filter), isolation and protection circuit do not couple with each other.

Reason: When the input and output traces of the above circuit are coupled with each other, the filtering, isolation or protection effect will be weakened.

2: If a "clean ground" interface is designed on the board, the filtering and isolation components should be placed on the isolation band between the "clean ground" and the working ground.

Reason: Avoid coupling between filtering or isolation devices through the plane layer, which weakens the effect.

3: On the "clean ground", apart from filtering and protection devices, no other devices can be placed. Reason: the purpose of the "clean ground" design is to ensure the minimum interface radiation, and the "clean ground" is easily coupled by external interference, so Do not have other irrelevant circuits and devices on the "clean ground".

4: Crystals, crystal oscillators, relays, switching power supplies and other strong radiation devices should be at least 1000 mil away from the board interface connector.

Reason: The interference will radiate directly or the current will be coupled to the outgoing cable to radiate outward.

5: Sensitive circuits or devices (such as reset circuits, WATCHDOG circuits, etc.) should be at least 1000 mils away from each edge of the board, especially the edge of the board interface.

Reason: Places similar to the board interface are the places most easily coupled by external interference (such as static electricity), and sensitive circuits such as reset circuits and watchdog circuits can easily cause system misoperation.

6: The filter capacitors for IC filtering should be placed as close as possible to the power supply pins of the chip.

Reason: The closer the capacitor is to the pin, the smaller the area of the high-frequency loop and the smaller the radiation.

7: For the start-end series matching resistor, it should be placed close to its signal output end.

Reason: The design purpose of the starting-end series matching resistor is to add the output impedance of the chip output end and the impedance of the series resistance to the characteristic impedance of the trace. The matching resistance is placed at the end, which cannot satisfy the above equation.

8: PCB traces cannot have right-angle or acute-angle traces.

Reason: Right-angle wiring leads to discontinuity in impedance, leading to signal transmission, resulting in ringing or overshoot, and strong EMI radiation.

9: Avoid layer settings of adjacent wiring layers as much as possible. When it is unavoidable, try to make the traces in the two wiring layers perpendicular to each other or the length of parallel traces is less than 1000mil.

Reason: to reduce the crosstalk between parallel traces.

10: If the board has an internal signal wiring layer, key signal lines such as clocks should be laid on the inner layer (the preferred wiring layer is preferred).

Reason: Deploying key signals on the internal wiring layer can play a shielding role.

11: It is recommended to wrap the ground wire on both sides of the clock line, and the ground wire shall be grounded every 3000mil.

Reason: Ensure that the potentials of all points on the ground line of the package are equal.

12: Key signal traces such as clocks, buses, and radio frequency lines and other parallel traces on the same layer should meet the 3W principle.

Reason: Avoid crosstalk between signals.

13: The pads of surface mount fuses, magnetic beads, inductors, and tantalum capacitors used for power supplies with current ≥ 1A should not be less than two vias connected to the plane layer.

Reason: Reduce the equivalent impedance of the via.

14: Differential signal lines should be on the same layer, of equal length, and run in parallel to maintain the same impedance, and there should be no other wiring between the differential lines.

Reason: to ensure that the common mode impedance of the differential line pair is equal to improve its anti-interference ability.

15: The key signal traces must not cross the partitioned area (including the reference plane gap caused by vias and pads).

Reason: The wiring across the partition will increase the area of the signal loop.

16: When it is inevitable to divide the signal line across its return plane, it is recommended to use a bridge capacitor approach near the signal span division. The value of the capacitor is 1nF.

Reason: When the signal span is divided, the loop area is often increased. The bridge grounding method is artificially set up a signal loop for it.

17: Do not have other irrelevant signal traces under the filter (filter circuit) on the board.

Reason: Distributed capacitance will weaken the filtering effect of the filter.

18: The input and output signal lines of the filter (filter circuit) cannot be parallel or crossed.

Reason: Avoid direct noise coupling between the traces before and after filtering.

19: The distance between the key signal line and the edge of the reference plane ≥ 3H (H is the height of the line from the reference plane).

Reason: suppress the edge radiation effect.

20: For metal shell grounding components, ground copper should be laid on the top layer of the projection area.

Reason: The distributed capacitance between the metal shell and the grounded copper is used to suppress the external radiation and improve the immunity.

21: In single-layer boards or double-layer boards, pay attention to the design of "minimizing loop area" when wiring.

Reason: The smaller the loop area, the smaller the loop's external radiation, and the stronger the anti-interference ability.

22: When the signal line (especially the key signal line) is changed layer, the ground via should be designed near the layer change via hole.

Reason: The area of the signal loop can be reduced.

23: Clock line, bus, radio frequency line, etc.: The strong radiation signal line should be far away from the interface signal line.

Reason: Avoid the interference of strong radiation signal lines from coupling to outgoing signal lines and radiating outwards.

24: Sensitive signal lines such as reset signal lines, chip select signal lines, system control signals, etc. are far away from the interface and outgoing signal lines.

Reason: The signal line going out of the interface often brings in external interference, and when it is coupled to the sensitive signal line, it will cause the system to malfunction.

25: In the PCB single-sided and double-sided PCB, the routing of the filter capacitor should be filtered by the filter capacitor first, and then to the device pins.

Reason: The power supply voltage is filtered before supplying power to the IC, and the noise fed back by the IC to the power supply will also be filtered out by the capacitor.

26: In a single or double panel, if the power line is very long, decoupling capacitors should be added to the ground every 3000mil, and the value of the capacitor is 10uF+1000pF.

Reason: filter out high frequency noise on the power line.

27: The ground wire and power wire of the filter capacitor should be as thick and short as possible.

Reason: The equivalent series inductance will reduce the resonant frequency of the capacitor and weaken its high-frequency filtering effect