PCB design wiring 20 questions

Cadence Allegro has almost become the actual industry standard in high-speed board design, and the latest version is Allegro 16.5. Combined with its front-end product Capture, it can complete high-speed, high-density and multi-layer complex PCB design and wiring.
01
Q: What problems should be paid attention to when wiring high-frequency signals?
A: impedance matching of signal lines; Space isolation from other signal lines; For digital high-frequency signals, the differential line effect will be better.
02
Q: When laying the board, if the wires are dense, there may be more holes, which will of course affect the electrical performance of the board. How can we improve the electrical performance of the board?
A: For low-frequency signals, the via holes do not matter; for high-frequency signals, minimize the via holes. Multi-layer board can be considered if there are many lines.
03
Q: Is it possible to add more decoupling capacitors on the board?
A: Decoupling capacitors need to be added with appropriate values at appropriate positions. For example, it needs to be added at the power supply port of analog devices, and different capacitance values need to be used to filter out stray signals with different frequencies.
04
Q: What is the standard of a good board?
A: Reasonable layout, sufficient power redundancy of power cord, concise high-frequency impedance and low-frequency wiring.
05
Q: How big is the difference of signals affected by through holes and blind holes? What is the principle of application?
A: Using blind holes or buried holes is an effective method to improve 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 well realized in technology and low in cost, so through holes are generally used in design.
06
Q: When it comes to analog-digital hybrid systems, some people suggest that the electrical layer should be divided, and the ground plane should be coated with copper. Others suggest that the electrical layer should be divided, and different grounds should be connected at the power supply terminal. However, the return path of the signal will be far away. How should we choose the appropriate method in specific applications?
A: If there are signal lines with high frequency > 20MHz, and the length and quantity are relatively large, then at least two layers are needed for this analog high frequency signal. One layer of signal line, one layer of large area ground, and the signal line layer needs to make enough vias to the ground. The purpose of this is:
For analog signals, this provides a complete transmission medium and impedance matching;
The ground separates analog signals from other digital signals;
The ground is small enough, because you have made many vias, and the ground is a big plane.
07
Q: In the circuit board, the signal input plug-in is at the leftmost edge of the PCB and the MCU is on the right. So, in the layout, should the stabilized power supply chip be placed near the connector (the output 5V of the power supply IC goes through a long path to reach the MCU) or the power supply IC should be placed to the right of the middle (the output 5V line of the power supply IC goes through a relatively long section of PCB)? Or have a better layout?
A: First, is the signal input plug-in an analog device? If it is an analog device, it is suggested that the power supply layout should not affect the signal integrity of the analog part as much as possible. So there are a few points to consider:
First of all, is the regulated power supply chip a relatively clean power supply with small ripple? The analog part of the power supply has higher requirements for power supply;
Whether the analog part and MCU are a power supply, in the design of high-precision circuit, it is suggested to separate the power supply of analog part and digital part;
The power supply to the digital part needs to consider minimizing the influence on the analog circuit part.
08
Q: In the application of high-speed signal chain, there are analog ground and digital ground for multiple ASIC. Is it divided by ground or not divided by ground? What are the existing criteria? Which effect is better?
A: So far, there is no conclusion. In general, you can refer to the manual of the chip. All manuals of ADI’s hybrid chips recommend a grounding scheme, some of which are common ground and some are isolated ground, depending on the chip design.
09
Q: When should we consider the equal length of lines? If you want to consider using an equal length line, what is the maximum difference between the lengths of two signal lines? How to calculate?
A: Calculation idea of differential line: 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 will be completely cancelled out.
So the length difference at this time is the maximum. By analogy, the signal line difference must be less than this value.
10
Q: What kind of situation is the serpentine route suitable for high speed? Are there any shortcomings? For example, for differential routing, two sets of signals are required to be orthogonal.
A: Serpentine route has different functions due to different applications:
If the serpentine trace appears in the computer board, it mainly plays a role of filtering inductance and impedance matching to improve the anti-interference ability of the circuit. Serpentine traces in computer motherboards are mainly used in some clock signals, such as PCI-Clk, AGPCIK, IDE, DIMM and other signal lines.
In the ordinary PCB, besides the function of filter inductance, it can also be used as inductance coil of radio antenna and so on. For example, 2.4G walkie-talkies are used as inductors.
The wiring length of some signals must be strictly equal. The equal wiring length of high-speed digital PCB board is to keep the delay difference of each signal within a range and ensure the validity of the data read by the system in the same cycle (when the delay difference exceeds one clock cycle, the data of the next cycle will be misread).
For example, there are 13 HUBLink in the INTELHUB architecture, which use the frequency of 233MHz. They must be strictly of equal length to eliminate the hidden trouble caused by time delay, and winding is the only solution.
Generally, it is required that the delay difference should not exceed 1/4 clock cycle, and the delay difference per unit length of line is also fixed. The delay is related to the line width, line length, copper thickness and board structure, but too long line will increase the distributed capacitance and inductance, which will reduce the signal quality. Therefore, the clock IC pins are generally terminated, but the serpentine traces do not function as inductors.
On the contrary, inductance will phase shift the higher harmonics in the rising edge of the signal, resulting in the deterioration of the signal quality, so it is required that the pitch of the serpentine should be at least twice the line width. The smaller the rise time of the signal, the more susceptible it is to the influence of distributed capacitance and inductance.
In some special circuits, the serpentine line acts as an LC filter with distributed parameters.
11
Q: When designing PCB, how to consider EMC/EMI, and what specific aspects should be considered? What measures are taken?
A: EMI/EMC design must take into account the location of devices, the arrangement of PCB stack, the way of important connection, the selection of devices, etc. at the beginning of layout.
For example, the position of the clock generator should not be close to the external connector as far as possible, the high-speed signal should go through the inner layer as far as possible and pay attention to the continuity of characteristic impedance matching with the reference layer to reduce reflection, the slew rate of the signal pushed by the device should be as small as possible to reduce the high-frequency component, and when selecting decoupling/bypass capacitor, pay attention to whether its frequency response meets the requirements to reduce the noise of the power layer.
In addition, pay attention to the return path of high-frequency signal current to make its loop area as small as possible (that is, loop impedance as small as possible) to reduce radiation. The range of high-frequency noise can also be controlled by dividing the stratum.
Finally, properly select the chassis ground between PCB and shell.
12
Q: What should we pay attention to in the transmission line design of RF broadband circuit PCB? How to set the ground hole of the transmission line properly? Do you need to design the impedance matching yourself or cooperate with PCB manufacturers?
A: There are many factors to consider in this question. For example, various parameters of PCB materials, transmission line model finally established according to these parameters, device parameters, etc. Impedance matching is generally designed according to the data provided by the manufacturer.
13
Q: When analog circuits and digital circuits coexist, for example, half of them are digital circuits of FPGA or MCU, and the other half are analog circuits of DAC and related amplifiers. There are many power supplies with various voltage values. If you encounter power supplies with voltage values that both digital and analog circuits need, can you use a common power supply? What are your skills in wiring and bead arrangement?
A: Generally, it is not recommended to use it like this. It will be complicated and difficult to debug.
14
Q: When designing high-speed multi-layer PCB, what is the main basis for the selection of packages for resistors, capacitors and other devices? Which packages are commonly used? Can you give some examples?
A: 0402 is commonly used in mobile phones; 003 is commonly used for modules of general high-speed signals; It is based on the fact 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 positions.
15
Q: Generally, in the design of double panels, do you take the signal line or the ground line first?
A: This should be considered comprehensively. Considering layout first, consider routing.
16
Q: What are the most important issues to pay attention to when designing high-speed multilayer PCB? Can you explain the solution to the problem in detail?
A: The most important thing to pay attention to is the design, that is, how you divide the signal line, power line, ground and control line on each floor.
The general principle is that analog signals and analog signals should be grounded at least in a separate layer. It is also recommended to use a separate layer of power supply.
17
Q: When will you use 2-ply board, 4-ply board and 6-ply board? Are there any strict technical restrictions (except for the size)? Is it the frequency of CPU or the frequency of data interaction with external devices?
A: The multi-layer board can first provide a complete ground plane, and in addition, it can provide more signal layers to facilitate wiring.
For applications where CPU is going to control external storage devices, the frequency of interaction should be taken into consideration. If the frequency is high, the complete ground plane must be guaranteed, and in addition, it is best to keep the signal lines of equal length.
18
Q: How to analyze the influence of Q：PCB wiring on analog signal transmission, and how to distinguish whether the noise introduced during signal transmission is caused by wiring or by operational amplifier devices?
A: It’s difficult to distinguish this one. You can only try to avoid additional noise caused by wiring through PCB wiring.
19
Q: For high-speed multilayer PCB, what is the appropriate line width setting of power line, ground line and signal line? What are the common settings? Can you give an example? For example, how to set it when the working frequency is 300Mhz?
A: 300 MHz signal must be impedance simulated to calculate the line width and the distance between the line and the ground; The line width of the power line needs to be determined according to the current. When mixing signal PCB, the ground is generally not represented by “line”, but the whole plane, so as to ensure the minimum resistance of the loop and a complete plane under the signal line.
20
Q: What kind of layout can achieve the best heat dissipation effect?
A: There are three main sources of heat in A：PCB: the heat of electronic components; Heating of PCB itself; Heat from other parts.
Among these three heat sources, the heat generated by components is the largest, which is the main heat source, followed by the heat generated by PCB board. The heat from outside depends on the overall thermal design of the system, so it will not be considered for the time being.
Then the purpose of thermal design is to take appropriate measures and methods to reduce the temperature of components and PCB, so that the system can work normally at a suitable temperature. It is mainly achieved by reducing heat generation and accelerating heat dissipation.