PCB board anti-interference

Electronic systems with microprocessors, anti-jamming and electromagnetic compatibility are issues that must be considered in the design process, especially for systems with high clock rates and fast bus cycles; systems with high power, high current drive circuits; Signal and high-precision A / D conversion circuit system. In order to increase the system anti-electromagnetic interference ability should consider the following measures:

1) Select the microprocessor with low clock frequency

As long as the controller performance to meet the requirements, the lower the clock frequency the better, low clock can effectively reduce noise and improve the system's anti-jamming capability. As the square wave contains a variety of frequency components, the high-frequency components can easily become a noise source, under normal circumstances, the clock frequency of 3 times the high-frequency noise is the most dangerous.

2) to reduce the distortion in signal transmission

When the high-speed signal (high signal frequency = rising edge and falling edge of the fast signal) in the copper film transmission, due to the copper film line inductance and capacitance, the signal will be distorted, when the distortion is too large, it will make System work is unreliable. General requirements, the signal on the PCB board to transfer the copper film as short as possible, the less the better the number of vias. Typical values: no more than 25cm in length, no more than 2 holes.

3) to reduce the cross-interference between signals

When a signal line has a pulse signal, it will interfere with another weak signal line with a high input impedance. In this case, it is necessary to isolate the weak signal line by adding a grounded contour to surround the weak signal, or Increase the distance between lines, for different levels of interference can be used to increase the power and ground level solution.

4) Reduce the noise from the power supply

Power supply to the system while providing energy, but also its noise added to the power supply system, the system reset, interrupt and some other control signals are most vulnerable to external noise interference, so should be appropriate to increase the capacitance to filter out these Noise from the power supply.

5) pay attention to PCB board and components of the high-frequency characteristics

In the case of high frequency, PCB board on the copper film, pads, vias, resistors, capacitors, connectors, distributed inductors and capacitors can not be ignored. Due to the influence of these distributed inductors and capacitors, when the length of the copper film is 1/20 of the wavelength of the signal or noise, the antenna effect is generated, and electromagnetic interference is generated internally. Under normal circumstances, the vias and pads will produce 0.6pF capacitor, an integrated circuit package will produce 2 ~ 6pF capacitor, a PCB board connector will produce 520mH inductance, and a DIP-24 socket with 18nH Inductors, these capacitors and inductors have no effect on circuits with low clock frequencies, and attention must be paid to circuits with high clock frequencies.

6) components to be a reasonable partition

The location of the components on the circuit board should take into account the anti-electromagnetic interference problem. One of the principles is that the copper film lines between the various components should be as short as possible. In the layout, the analog circuits, digital circuits and circuits that generate large noise (relays, high current switches, etc.) are reasonably separated so that they are The signal coupling is minimal.

7) handle the ground

Ground the ground according to the previously mentioned single point grounding or multi-point grounding. Will be analog ground, digital, high-power devices to connect separately, and then converged to the power of the grounding point. PCB board outside the lead to use shielded wire, for high-frequency and digital signals, shielded cable at both ends of the ground, low-frequency analog signal with the shielded wire, the general use of single-ended ground. Circuites that are very sensitive to noise and interference or circuits with severe high frequency noise should be shielded with metal shields.

8) Decoupling capacitors

Decoupling capacitors to ceramic capacitors or multilayer ceramic capacitors have high frequency characteristics. When designing the PCB board, add a decoupling capacitor between each power supply of the integrated circuit and ground. Decoupling capacitors have two effects, on the one hand is the IC's energy storage capacitor, to provide and absorb the integrated circuit to open the door and close the moment of charge and discharge power, on the other hand, bypass the device generated high-frequency noise. Digital circuit in the typical decoupling capacitor is 0.1μF, this capacitor has 5nH distributed inductance, can be less than 10MHz noise decoupling better. Under normal circumstances, select 0.01 ~ 0.1μF capacitor can be.

Generally require no 10 or so integrated circuit to add a 10μF charge and discharge capacitors. In addition, the power supply side, the four corners of the circuit board and other locations should be connected to a 10 ~ 100μF capacitor.

PCB board pad

Pad size: The hole size of the pad must be given in terms of the diameter of the lead and the tolerance dimension, the thickness of the tin layer, the pore size tolerance, and the thickness of the hole metallized plating. The inner diameter of the pad. For example, the resistance of the metal pin diameter of 0.5mm, the pad hole diameter of 0.7mm, and the pad diameter should be pad diameter plus 1.2mm, the minimum should be pad diameter plus 1.0mm. When the pad diameter is 1.5 mm, a square pad may be used in order to increase the peel strength of the pad. For holes with a hole diameter less than 0.4 mm, the outer diameter of the pad / pad hole diameter is 0.5 to 3. For holes with a hole diameter greater than 2 mm, the pad diameter / pad hole diameter is 1.5 to 2.

Common pad size:

Pad hole diameter / mm

0.4; 0.5; 0.6; 0.8; 1.0; 1.2; 1.6; 2.0

Pad outer diameter / mm

1.5; 1.5; 2.0; 2.0; 2.5; 3.0; 3.5;

Consider the following when designing the pad:

1) The edge of the pad hole to the edge of the PCB board is greater than 1mm, which can avoid the processing lead to pad defects.

2) When the copper film is connected with the pad, the connection between the pad and the copper film is designed as a teardrop-like, so that the pad can not be easily peeled off. The connection between the copper film and the pad is not easy to disconnect.

3) adjacent pads to avoid an acute angle.

PCB board filled with large area

PCB board on the purpose of a large area filled with two, one is the heat, the other is to use shield to reduce interference, in order to avoid the heat generated when welding the circuit board to produce the gas nowhere to let the copper film off, The area is filled with a window, which fills the grid. The use of copper can also achieve the purpose of anti-interference, and copper can automatically bypass the pad and can be connected to the ground.

PCB board jumper wire

In the design of single-sided PCB board, when some copper film can not be connected, the usual practice is to use jumper, the length of the jumper should choose the following: 6mm, 8mm and 10mm.

PCB board high-frequency wiring

In order to make high-frequency PCB board design more reasonable, better anti-jamming performance, in the PCB design should be considered from the following aspects:

1) Reasonable selection of layers

The use of intermediate inner plane as the power and ground layer, can play a shielding effect, effectively reduce the parasitic inductance, shorten the length of the signal line and reduce the cross-interference between the signal, under normal circumstances, 20dB.

2) way of alignment

The traces must be rotated at an angle of 45 ° so that the emission of the high frequency signal and the coupling between each other can be reduced.

3) Length of alignment

The shorter the trace length, the better the parallel distance between the two lines.

4) Number of vias

The smaller the number of vias, the better.

5) Layered wiring direction

Inter-layer routing direction should take the vertical direction, that is, the top layer for the horizontal direction, the bottom of the vertical direction, which can reduce the interference between the signals.

6) copper

Increasing the ground copper can reduce the interference between the signals.

7) enclave

The important signal line for packet processing, can significantly improve the anti-jamming capability of the signal, of course, can also be carried out interference source interference, so that it can not interfere with other signals.

8) signal line

Signal traces can not loop, need to follow the daisy chain way wiring.

9) Decoupling capacitors

Connect the decoupling capacitors across the power supply side of the integrated circuit.

10) High frequency choke

Digital ground, analog ground, etc. to connect the public ground to connect high-frequency choke device, usually the center hole through the wire of high-frequency ferrite beads.