PCB Tech - PCB design skills question and answer record summary

1. Does rigid-flex board design require special design software and specifications? Where can we undertake such circuit board processing in China?

You can use general PCB design software to design a flexible printed circuit (Flexible Printed Circuit). It is also produced by FPC manufacturers in Gerber format. Since the manufacturing process is different from that of general PCBs, various manufacturers will have their limitations on the minimum line width, minimum line spacing, and minimum vias based on their manufacturing capabilities. In addition, it can be reinforced by laying some copper skin at the turning point of the flexible circuit board. As for the manufacturer, you can find it on the Internet "FPC" as a keyword query.

2. What is the principle of properly selecting the grounding point between the PCB and the case?

The principle of selecting the PCB and shell grounding points is to use the chassis ground to provide a low-impedance path for the returning current and to control the path of the returning current. For example, usually near high-frequency devices or clock generators, fixed screws can be used to connect the ground layer of the PCB to the chassis ground to minimize the area of the entire current loop and reduce electromagnetic radiation.

3. From which aspects should the circuit board DEBUG start?

Circuit board DEBUG As far as digital circuits are concerned, first determine three things in order:

1. Confirm that all power supply values meet the design requirements. Some systems with multiple power supplies may require certain specifications for the order and speed of the power supplies.

2. Confirm that all clock signal frequencies are working properly and there are no non-monotonic problems on the signal edges.

3. Confirm whether the reset signal meets the specification requirements. If these are normal, the chip should send out the first cycle (cycle) signal. Next, debug according to the operating principle of the system and the bus protocol.

4. When the PCB size is fixed, if the design needs to accommodate more functions, it is often necessary to increase the PCB trace density, but this may increase the mutual interference of the traces, and at the same time, the impedance of the traces is too thin. Decrease, please introduce the skills in high-speed (>100MHz) high-density PCB design?

When designing high-speed and high-density PCBs, crosstalk interference (crosstalk interference) really needs special attention, because it has a great impact on timing and signal integrity. Here are a few points to note:

1. Control the continuity and matching of the characteristic impedance of the wiring.

2. The size of the trace spacing. It is generally seen that the spacing is twice the line width. It is possible to know the influence of trace spacing on timing and signal integrity through simulation, and find the minimum tolerable spacing. The result of different chip signals may be different.

3. Choose the appropriate termination method.

4. Avoid two adjacent layers with the same wiring direction, even if the wiring overlaps up and down, because this kind of crosstalk is larger than that of adjacent wiring on the same layer. 5. Use blind/buried vias to increase the trace area. However, the manufacturing cost of the PCB board will increase. It is indeed difficult to achieve complete parallelism and equal length in actual implementation, but it is still necessary to do it as much as possible. In addition, differential termination and common mode termination can be reserved to alleviate the impact on timing and signal integrity.

5. The filtering at the analog power supply often uses an LC circuit. But why sometimes LC is less effective than RC filtering?

The comparison of LC and RC filtering effects must consider whether the selection of the frequency band to be filtered and the inductance value is appropriate. Because the inductance (reactance) of the inductor is related to the inductance value and frequency. If the noise frequency of the power supply is low and the inductance value is not large enough, the filtering effect may not be as good as RC. However, the price to pay for using RC filtering is that the resistor itself consumes energy and has poor efficiency, and pay attention to the power that the selected resistor can withstand.

6. What is the method of selecting inductor and capacitor value for filtering?

In addition to the noise frequency that you want to filter out, the choice of inductance value should also consider the response capability of the instantaneous current. If the output terminal of the LC has a chance to output a large current instantaneously, too large an inductance value will hinder the speed of the large current flowing through the inductor and increase ripple noise. The capacitance value is related to the size of the ripple noise specification value that can be tolerated. The smaller the ripple noise value requirement, the larger the capacitance value. The ESR/ESL of the capacitor will also have an impact. In addition, if the LC is placed on the output terminal of a switching regulation power (switching regulation power), pay attention to the influence of the pole/zero generated by the LC on the stability of the negative feedback control loop. .

7. How to meet EMC requirements as much as possible without causing too much cost pressure?

The cost increase on the PCB due to EMC is usually due to the increase in the number of ground layers to enhance the shielding effect and the addition of ferrite bead, choke and other high-frequency harmonic suppression devices. In addition, it is usually necessary to match the shielding structure on other institutions to make the entire system pass the EMC requirements.

The following only provides several PCB design techniques to reduce the electromagnetic radiation effect generated by the circuit.

1. Try to choose devices with slower signal slew rate to reduce the high frequency components generated by the signal.

2. Pay attention to the placement of high-frequency components, not too close to the external connector.

3. Pay attention to the impedance matching of high-speed signals, the wiring layer and its return current path to reduce high-frequency reflection and radiation.

4. Place sufficient and appropriate decoupling capacitors on the power supply pins of each device to alleviate the noise on the power plane and ground plane. Pay special attention to whether the frequency response and temperature characteristics of the capacitor meet the design requirements.

5. The ground near the external connector can be properly separated from the ground, and the ground of the connector can be connected to the chassis ground nearby.

6. Ground guard/shunt traces can be appropriately used beside some special high-speed signals. But pay attention to the influence of guard/shunt traces on the characteristic impedance of the trace.

7. The power layer shrinks 20H from the ground layer, and H is the distance between the power layer and the ground layer.

8. When there are multiple digital/analog function blocks in a PCB board, the conventional method is to separate the digital/analog ground. What is the reason?

The reason for separating the digital/analog ground is because the digital circuit will generate noise in the power and ground when switching between high and low potentials. The magnitude of the noise is related to the speed of the signal and the magnitude of the current. If the ground plane is not divided and the noise generated by the digital area circuit is relatively large and the analog area circuits are very close, even if the digital-to-analog signals do not cross, the analog signal will still be disturbed by the ground noise. That is to say, the non-divided digital-to-analog method can only be used when the analog circuit area is far from the digital circuit area that generates large noise.