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PCB Tech
What are the five common PCB design problems?
PCB Tech
What are the five common PCB design problems?

What are the five common PCB design problems?

2021-10-24
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Author:Downs

To suppress the edge radiation effect. Electromagnetic interference is radiated outward at the edge of the board. The power layer is retracted so that the electric field is only conducted within the grounding layer, which effectively improves the EMC. If you shrink by 20H, you can limit 70% of the electric field to the ground edge; if you shrink by 100H, you can limit 98% of the electric field.

We require the ground plane to be larger than the power or signal layer, which is beneficial to prevent external radiation interference and shield the external interference to itself. Generally, in the PCB design, shrinking the power layer by 1mm from the ground layer can basically meet the 20H principle.

2. How to reflect the 3W principle and 20H principle in PCB design?

First, the 3W principle is easily reflected in PCB design. Ensure that the center distance between the trace and the trace is 3 times the line width, for example, the line width of the trace is 6 mils.

So in order to meet the 3W principle, set the line-to-line rule in Allegro to 12mil. The spacing in the software is to calculate the edge-to-edge spacing

Second, the 20H principle. In the PCB design, in order to reflect the 20H principle, we generally need to shrink the power layer 1mm from the ground layer when the plane layer is divided.

pcb board

Then punch a shielding ground via on the 1mm shrink tape, one 150mil

3. What are the types of PCB signal lines and what are the differences?

PCB signal lines are divided into two types, one is microstrip line and the other is strip line.

The microstrip line is a strip line that runs on the surface layer (microstrip) and is attached to the surface of the PCB. As shown in the figure below, the blue part is the conductor, the green part is the insulating dielectric of the PCB, and the blue block on it is Microstrip line. Because one side of the microstrip line is exposed in the air, it can form radiation or be interfered by surrounding radiation, and the other side is attached to the insulating dielectric of the PCB, so part of the electric field formed by it is distributed in the air, and the other part Distributed in the insulating medium of the PCB. However, the signal transmission speed in the microstrip line is faster than the signal transmission speed in the stripline, which is its outstanding advantage.

Stripline: The stripline/double stripline that goes on the inner layer (stripline/double stripline) and is embedded in the PCB. As shown in the figure below, the blue part is the conductor, the green part is the insulating dielectric of the PCB, and the stripline is embedded in two layers. Ribbon wire between conductors. Because the stripline is embedded between two layers of conductors, its electric field is distributed between the two conductors (planes) enclosing it, and it will not radiate energy, nor will it be interfered by external radiation. But because it is surrounded by dielectric materials (the dielectric constant is greater than 1), the signal transmission speed in the stripline is slower than in the microstrip line.

4. What is EMC?

EMC is the abbreviation of Electro Magnetic Compatibility, which translates as electromagnetic compatibility, which refers to the ability of a device or system to work normally in its electromagnetic environment and does not constitute an inability to withstand electromagnetic disturbance to anything in the environment.

The electromagnetic compatibility of the sensor refers to the adaptability of the sensor in the electromagnetic environment, the ability to maintain its inherent performance and complete the specified functions. It contains two requirements: On the one hand, the electromagnetic interference generated by the sensor in the environment in the normal operation process cannot exceed a certain limit; on the other hand, the sensor is required to have a certain degree of immunity to electromagnetic interference in the environment.

5. What are the design methods to distinguish between analog ground and digital ground in PCB design?

Generally, there are several ways to deal with analog ground and digital ground:

Separate directly, connect the ground of the digital area as DGND in the schematic diagram, and connect the ground of the analog area as AGND, and then divide the ground plane in the PCB board into digital ground and analog ground, and increase the spacing;

Use magnetic beads to connect between digital ground and analog ground;

Connect the digital ground and the analog ground with a capacitor, and use the principle of blocking the direct current through the capacitor;

The digital ground and the analog ground are connected by inductance, and the inductance varies from uH to tens of uH;

A zero-ohm resistor is connected between the digital ground and the analog ground.

To sum up, the capacitor separates the direct current and causes the floating ground. If the capacitor is not connected to DC, it will cause pressure difference and static electricity accumulation, which will make your hands numb when touching the case. If the capacitor and the magnetic beads are connected in parallel, it is superfluous, because the magnetic beads will pass through and the capacitor will be invalid. If they are connected in series, they are nondescript.

The inductor has a large volume, many stray parameters, unstable characteristics, poor control of discrete distribution parameters, and large volume. Inductance is also notch, LC resonance (distributed capacitance), which has special effects on noise.

The equivalent circuit of the magnetic bead is equivalent to a band-rejection trap, which only suppresses the noise at a certain frequency. If the noise cannot be predicted, how to choose the model. Moreover, the frequency of the noise is not necessarily fixed, so the magnetic bead is not a good choice. s Choice.

A resistance of 0 ohm is equivalent to a very narrow current path, which can effectively limit the loop current and suppress noise. Resistance has attenuation in all frequency bands (0 ohm resistance also has impedance), which is stronger than magnetic beads.

In short, the key is that the analog ground and digital ground should be grounded at one point. It is recommended to connect different types of grounds with 0 ohm resistors; use magnetic beads when introducing high-frequency devices into the power supply; use small capacitors for coupling high-frequency signal lines; use inductors for high-power and low-frequency applications.