Electronic Design - PCB circuit board wiring skills 6 questions and answers

1. How to choose a PCB board? How to avoid high-frequency interference from high-speed data transmission to the surrounding analog small signals? Are there any basic design ideas?

Answer: The choice of PCB board must strike a balance between meeting design requirements and mass production and cost. The design requirements include both electrical and mechanical parts. This material issue is usually more important when designing very high-speed PCB boards (frequency greater than GHz). For example, the commonly used FR-4 material, the dielectric loss at a frequency of several GHz will have a great influence on the signal attenuation, and may not be suitable. As far as electricity is concerned, pay attention to whether the dielectric constant and dielectric loss are suitable for the designed frequency. The basic idea of avoiding high-frequency interference is to minimize the electromagnetic field interference of high-frequency signals, which is the so-called crosstalk (Crosstalk). You can increase the distance between the high-speed signal and the analog signal, or add ground guard/shunt traces next to the analog signal. Also pay attention to the noise interference from the digital ground to the analog ground.

2. It is well known that the PCB board includes many layers, but the meaning of some of them is not very clear to me. mechanical, keepoutlayer, topoverlay, bottomoverlay, toppaste,bottompaste, topsolder, bottomsolder, drillguide, drilldrawing, multilayer do not know their exact meaning?

Answer: Many of the technical terms of EDA software do not have the same definition. The following explains the possible meaning literally.

Mechnical: General multi-finger board type machining dimensioning layer

Keepoutlayer: Define the area where wires, vias or parts cannot be placed. These limits can be defined independently. Topoverlay: The meaning cannot be known literally. Provide more information for further discussion.

Bottomoverlay: The meaning cannot be known literally. More information can be provided for further discussion.

Toppaste: The top layer needs to expose the part of the solder paste on the copper skin.

Bottompaste: The bottom layer needs to expose the part of the solder paste on the copper skin.

Topsolder: Should refer to the top solder mask, to avoid accidental short circuit during the manufacturing process or future repairs. Bottomsolder: Should refer to the bottom solder mask.

Drillguide: It may be a table of different aperture sizes, corresponding symbols, and numbers.

Drilldrawing: Refers to the hole drawing, each different hole diameter will have a corresponding symbol.

Multilayer: There should be no single layer, which can refer to multi-layer boards, for single-sided and double-sided boards.

3. A system is often divided into several PCBs, with power supplies, interfaces, motherboards, etc., and the ground wires between the boards are often interconnected, resulting in the formation of many loops, such as low-frequency loop noise. I don't know this problem. How to solve?

Answer: When the signal or power supply between each PCB board is connected to each other, for example, if board A has a power supply or a signal is sent to board B, there must be an equal amount of current flowing from the ground back to board A (this is Kirchoff current law) . The current on this ground will find the place with the least impedance to flow back. Therefore, at each interface, whether it is power or signal interconnection, the number of pins allocated to the ground layer should not be too small to reduce the impedance, which can reduce the noise on the ground layer. In addition, you can also analyze the entire current loop, especially the part with a large current, and adjust the connection of the ground layer or ground wire to control the current flow (for example, make a low impedance somewhere so that most of the current flows from this Go somewhere) to reduce the impact on other more sensitive signals.

4. (1) Can you provide some empirical data, formulas and methods to estimate the impedance of the wiring? (2) When the impedance matching requirements cannot be met, it is better to add a parallel matching resistance at the end of the signal line or add a series matching resistance to the signal line. (3) Can a ground wire be added to the middle of the differential signal line?

Answer: 1. The following provides two frequently referred characteristic impedance formulas: a. Microstrip Z={87/[sqrt(Er+1.41)]}ln[5.98H/(0.8W+T)] Among them, W is the line width, T is the copper thickness of the trace, H is the distance from the trace to the reference plane, and Er is the dielectric constant of the PCB material. This formula must be applied when 0.1<(W/H)<2.0 and 1<(Er)<15. b. Stripline Z=[60/sqrt(Er)]ln{4H/[0.67π(T+0.8W)]} Among them, H is the distance between the two reference planes, and the trace is located on the two reference planes in the middle. This formula must be applied when W/H<0.35 and T/H<0.25. It is better to use simulation software to calculate more accurately.

2. There are several factors to consider when choosing the method of termination: a. The structure and strength of the source driver. b. The size of power consumption. c. The impact on time delay, this is the most important consideration. Therefore, it is difficult to say which termination method is better.

3. Generally, the ground wire cannot be added in the middle of the differential signal. Because the most important point of the application principle of differential signals is to use the benefits of coupling between differential signals, such as flux cancellation and noise immunity. If you add a ground wire in the middle, it will destroy the coupling effect.

5. Introduce some foreign current high-speed PCB design levels, processing capabilities, processing levels, processing materials, and related technical books and materials?

Answer: Nowadays, high-speed digital circuits are used in related fields such as communication networks and computers. In terms of communication network, the working frequency of the PCB board has reached up to GHz, and the number of layers is as high as 40 layers as far as I know. Computer-related applications are also due to the advancement of chips, whether it is a general PC or a server (Server), the highest operating frequency on the board has also reached over 400MHz (such as Rambus). In response to this demand for high-speed and high-density wiring, the requirements for blind/buried vias, mircrovias, and build-up processes have gradually increased. These design requirements are available for mass production by manufacturers. Here are some good technical books: 1. Howard W. Johnson, "High-Speed Digital Design – Handbook of Black Magic"; 2. Stephen H. Hall, "High-Speed Digital System Design"; 3. Brian Yang, "Digital Signal Integrity";

6. Regarding the design and processing of flexible circuit boards, it is planned to use flexible circuit board design to solve the problem of signal transmission and circuit board interconnection in small imaging systems. Does rigid-flex board design require special design software and specifications? In addition, where can we undertake this type of circuit board processing in China?

Answer: 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.