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20 questions and 20 answers to PCB design and wiring

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Cadence allegro has almost become the de facto industry standard in high-speed board design. The latest version is Allegro 16.5. Combined with its front-end product Capture, it can complete high-speed, high-density, multi-layer complex PCB design and wiring. Allegro is easy to operate, friendly interface, powerful, such as simulation, signal integrity simulation, power integrity simulation can be done.

What problems should be paid attention to when high-frequency signal wiring?
A Impedance matching of signal lines; spatial isolation from other signal lines; for digital high-frequency signals, the effect of differential lines will be better.

Q When laying out the board, if the wires are dense, there may be more holes. Of course, it will affect the electrical performance of the board. How to improve the electrical performance of the board?

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

Q Is it better to add more decoupling capacitors on the board?

A Decoupling capacitors need to be added with appropriate values ​​at appropriate locations. For example, it is added at the power supply port of the analog device, and different capacitance values ​​need to be used to filter out spurious signals of different frequencies.

Q  What is the standard for a good board?

A Reasonable layout, sufficient power line redundancy, high-frequency impedance, and simple low-frequency wiring.

Q How much does the through-hole and blind-hole affect the signal difference? What are the principles applied?

A The use of blind vias or buried vias 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.

Q When it comes to analog-digital hybrid systems, some people suggest that the electrical layer is divided, and the ground plane should be copper-clad, and some people suggest that the electrical ground layer is divided, and different grounds are connected at the power terminal, but the return path of the signal is far away. , How to choose the appropriate method for specific application?

A If there is a high frequency>20MHz signal line, and the length and quantity are relatively large, then at least two layers are required for this analog high frequency signal. One layer of signal line, one layer of large area ground, and the signal line layer needs to punch enough vias to the ground. The purpose of this is: a. For analog signals, this provides a complete transmission medium and impedance matching; b. The ground plane isolates the analog signal from other digital signals; c. The ground loop is small enough because you have hit a lot. The hole and the ground are again a large plane.

Q In the circuit board, the signal input plug-in is on the leftmost edge of the PCB, and the MCU is on the right, so in the layout, the stabilized power supply chip is placed near the connector (the power supply IC outputs 5V after a relatively long path to reach the MCU ), or place the power IC to the right of the center (the output 5V line of the power IC reaches the MCU is relatively short, but the input power segment line passes through a relatively long PCB board)? Or is there a better layout?

A First, is the signal input plug-in an analog device? If it is an analog device, it is recommended that the power supply layout should not affect the signal integrity of the analog part as much as possible.

Therefore, there are several points to consider: a. First, is the regulated power supply chip a relatively clean power supply with low ripple? The power supply of the analog part requires relatively high power supply; b. Whether the analog part and MCU are the same power supply, in the design of high-precision circuit, it is recommended to separate the power supply of the analog part and the digital part; c. The power supply requirement of the digital part Consider minimizing the impact on the analog circuit part.

Q  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 is better?

A There is no conclusion so far. Under normal circumstances, you can consult 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 for isolated ground, depending on the chip design.

Q  When should the equal length of the line be considered? If you want to consider the use of equal-length cables, what is the maximum length difference between the two signal lines? How to calculate?

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

Q Snake-shaped routing in high-speed, which situation is suitable? Are there any disadvantages? For example, for differential wiring, the two sets of signals are required to be orthogonal.

A. Serpentine traces have different functions due to different applications: a. If serpentine traces appear in the computer board, they mainly play a role in 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. b. 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. c. 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 (delay When the difference exceeds one clock cycle, the data of the next cycle will be read incorrectly). For example, the HUBLink in the INTELHUB architecture has a total of 13 and uses a frequency of 233MHz. The length must be strictly equal to eliminate the hidden dangers caused by time lag. Winding is the only solution. Generally, the delay difference is not more than 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 too long a line will increase distributed capacitance and distributed inductance. , The signal quality has decreased. Therefore, clock IC pins are generally terminated, but the serpentine trace does not act as an inductor. Conversely, the inductance will shift the phase shift of the higher harmonics in the rising edge of the signal, causing deterioration of the signal quality, so the serpentine line spacing is required to be at least twice the line width. The smaller the rise time of the signal, the more susceptible to the influence of distributed capacitance and distributed inductance. d. The serpentine trace acts as a distributed parameter LC filter in some special circuits.

Q How to consider the electromagnetic compatibility emc/EMI when designing the PCB, and what aspects need to be considered? What measures are taken?

A EMI/EMC design must consider the location of the device, the arrangement of the PCB stack, the routing of important connections, and the selection of the device at the beginning of the layout. For example, the location of the clock generator should not be as close to the external connector as possible. High-speed signals should go to the inner layer as much as possible. Pay attention to the characteristic impedance matching and the continuity of the reference layer to reduce reflections. The slew rate of the signal pushed by the device should be as small as possible to reduce the height. Frequency components, when choosing a decoupling/bypass capacitor, pay attention to whether its frequency response meets the requirements to reduce noise on the power plane. In addition, pay attention to the return path of the high-frequency signal current to make the loop area as small as possible (that is, the loop impedance as small as possible) to reduce radiation. The ground can also be divided to control the range of high-frequency noise. Finally, properly choose the chassis ground between the PCB and the housing.

Q  What should I pay attention to when designing the transmission line of the RF broadband circuit PCB? How to set the ground hole of the transmission line is more appropriate, do you need to design the impedance matching yourself or cooperate with the PCB processing manufacturer?

A There are many factors to consider in this question. For example, the various parameters of PCB materials, the transmission line model finally established according to these parameters, the parameters of the device, etc. Impedance matching is generally designed according to the information provided by the manufacturer.

Q” When analog circuits and digital circuits coexist, for example, one half is the FPGA or the digital circuit part of the microcontroller, and the other half is the analog circuit part of the DAC and related amplifiers. There are many power supplies with various voltage values. When encountering power supplies with voltage values ​​that are used in both digital and analog circuits, can a common power supply be used? What skills are there in wiring and magnetic bead layout?

A It is generally not recommended to use this way because it will be more complicated to use and difficult to debug.

Q  When designing high-speed multilayer PCBs, what is the main basis for the selection of packages for devices such as resistors and capacitors? Which packages are commonly used, can you give me some examples?

A 0402 is commonly used in mobile phones; 0603 is commonly used in general high-speed signal modules; the basis is that the smaller the package, the smaller the parasitic parameters. Of course, the same package from different manufacturers has great differences in high-frequency performance. It is recommended that you use high-frequency special components in key locations.

Q  Generally, in the design of the double panel, should the signal line or the ground line be taken first?

A This should be considered comprehensively. When considering the layout first, consider routing.

Q  What should be the most important issue when designing high-speed multilayer PCBs? Can you make a detailed solution to the problem?

A The most important thing to pay attention to is the design, that is, how you divide the signal lines, power lines, ground, and control lines on each layer. The general principle is that the analog signal and analog signal ground must be at least a separate layer. A separate layer is also recommended for the power supply.

Q  When do I use 2-layer, 4-layer, and 6-layer boards? Are there any strict technical restrictions (excluding volume reasons)? Is the frequency of the CPU or the frequency of data interaction with external devices as the standard?

A  uses a multilayer board to provide a complete ground plane first, and more signal layers to facilitate wiring.

For the application of CPU to control external storage devices, the frequency of interaction should be considered. If the frequency is higher, a complete ground plane must be guaranteed. In addition, the signal lines should be kept the same length.

Q How to analyze the influence of PCB wiring on analog signal transmission, and how to distinguish whether the noise introduced in the signal transmission process is caused by wiring or op amp devices?

A This is difficult to distinguish, and the only way to avoid extra noise introduced by the wiring can be through PCB wiring.

Q  For high-speed multilayer PCBs, what are the appropriate line width settings for power lines, ground lines, and signal lines? What are the common settings? Can you give an example? For example, how to set the working frequency at 300Mhz?

A   300MHz signal must do impedance simulation to calculate


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