PCB Tech - PCB layout and routing skills quiz 1

PCB layout and routing skills quiz 1

In the design of electronic products, PCB layout and routing is an important step, and the quality of PCB layout and routing will directly affect the performance of the circuit.

Now, although there are a lot of software that can realize PCB automatic placement and routing. However, as the signal frequency continues to increase, in many cases, engineers need to understand the basic principles and techniques of PCB layout and routing in order to make their designs perfect.

The following covers the relevant basic principles and design skills of PCB layout, and answers difficult questions about PCB layout in the form of question and answer.

1. Question: What problems should be paid attention to when wiring high-frequency signals?

Answer: The impedance of the signal line is matched; the space is isolated from other signal lines; for digital high-frequency signals, the effect of the differential line will be better.

2. Question: In the layout of the board, if the wires are dense, there may be more vias, which of course will affect the electrical performance of the board. How can I improve the electrical performance of the board?

Answer: For low-frequency signals, vias do not matter. For high-frequency signals, minimize vias. If there are many lines, consider multi-layer boards.

3. Question: Is it better to add more decoupling capacitors on the board?

Answer: The decoupling capacitor needs to be added with the appropriate value at the appropriate location. For example, add it to the power supply port of your analog device, and you need to use different capacitance values to filter out spurious signals of different frequencies.

4. Question: What are the standards for a good board?

Answer: The layout is reasonable, the power redundancy of the power line is sufficient, the high-frequency impedance impedance, and the low-frequency wiring are simple.

5. Question: How much influence does the through hole and blind hole have on the signal difference? What are the principles applied?

Answer: The use of blind holes or buried holes is an effective way to increase the density of multilayer boards, reduce the number of layers and board size, and greatly reduce the number of plated through holes. However, in comparison, through holes are easy to implement in process and low cost, so through holes are generally used in the design.

6. Question: When it comes to analog-digital hybrid systems, some people suggest that the electrical layer should be divided, and the ground plane should be copper-clad, and some people suggest that the electrical ground layer should be divided, and different grounds should be connected at the power source terminal, but this will return the signal The path is far, how to choose the appropriate method for specific applications?

Answer: If you have a high-frequency signal line> 20MHz, and the length and quantity are relatively large, then you need at least two layers for this analog high-frequency signal. A layer of signal lines, a layer of large area ground, and the signal line layer needs to punch enough vias to the ground. The purpose of this is:

For analog signals, this provides a complete transmission medium and impedance matching;

The ground plane isolates analog signals from other digital signals;

The ground loop is small enough, because you have made a lot of vias, and the ground is a large plane.

7. Question: In the circuit board, the signal input plug-in is on the left edge of the PCB, and the MCU is on the right. Then the stabilized power supply chip is placed close to the plug-in during the layout (the power IC output 5V is reached after a relatively long path MCU), or place the power IC to the right of the center (the output 5V line of the power IC is shorter when it reaches the MCU, but the input power line passes through a longer PCB board)? Or is there a better layout?

Answer: First of all, is your so-called signal input plug-in an analog device? If it is an analog device, it is recommended that your power supply layout should not affect the signal integrity of the analog part as much as possible. Therefore, there are several considerations:

First of all, whether your regulated power supply chip is a relatively clean power supply with low ripple. For the power supply of the analog part, the requirements for the power supply are relatively high;

Whether the analog part and your MCU are the same power supply, in the design of high circuit, it is recommended to separate the power supply of the analog part and the digital part;

The power supply to the digital part needs to be considered to minimize the impact on the analog circuit part.

8. Question: In the application of high-speed signal chain, there are analog ground and digital ground for multiple ASICs. Is the ground divided or not divided? What are the existing guidelines? Which effect is better?

Answer: So far, there is no conclusion. Under normal circumstances, you can refer to the manual of the chip. The manuals of all ADI hybrid chips recommend you a grounding scheme, some are recommended for common ground, and some are recommended for isolation. It depends on the chip design.

9. Question: When should the equal length of the line be considered? If you want to consider using equal length wires, how much can the difference between the lengths of the two signal wires not exceed? How to calculate?

Answer: Differential line calculation idea: If you transmit a sinusoidal signal, your length difference is equal to half of its transmission wavelength, and the phase difference is 180 degrees. At this time, the two signals are completely canceled. So the difference in length at this time is the value. By analogy, the signal line difference must be less than this value.

10. Q: Which kind of situation is suitable for serpentine routing in high speed? Are there any disadvantages, for example, for differential wiring, the two sets of signals are required to be orthogonal?

Answer: Serpentine routing has different functions due to different applications:

If the serpentine trace appears in the computer board, it mainly plays a role of filter inductance and impedance matching to improve the anti-interference ability of the circuit. The serpentine traces in the computer motherboard are mainly used in some clock signals, such as PCI-Clk, AGPCIK, IDE, DIMM and other signal lines.

If it is used in a general PCB board, in addition to the filter inductance, it can also be used as the inductance coil of the radio antenna and so on. For example, it is used as an inductor in 2.4G walkie-talkies.

The wiring length requirements for some signals must be strictly equal. The equal line length of high-speed digital PCB boards is to keep the delay difference of each signal within a range to ensure the validity of the data read by the system in the same cycle (the delay difference exceeds In one clock cycle, the data of the next cycle will be read incorrectly).

For example, there are 13 HUBLinks in the INTELHUB architecture, which use a frequency of 233MHz. They must be strictly equal in length to eliminate hidden dangers caused by time lag. Winding is the only solution. Generally, it is required that the delay difference does not exceed 1/4 clock cycle, and the line delay difference per unit length is also fixed. The delay is related to the line width, line length, copper thickness, and layer structure, but excessively long lines will increase distributed capacitance and distributed inductance., The signal quality has decreased.

Therefore, the clock IC pins are generally connected; "terminated, but the serpentine trace does not play the role of inductance. On the contrary, the inductance will cause the phase shift of the higher harmonics in the rising edge of the signal, causing the signal quality to deteriorate, so The serpentine line spacing is required to be less than twice the line width.

The smaller the rise time of the signal, the more susceptible to the influence of distributed capacitance and distributed inductance.

The serpentine trace acts as a distributed parameter LC filter in some special circuits.