PCB design experience 1

PCB design gods 105 experience summary In electronic product design, PCB layout is the most important step, PCB layout and quality will directly affect the performance of the circuit.

Now, although there are a lot of software that can realize automatic layout and layout of PCBs, as the signal frequency continues to increase, many times, engineers need to understand the most basic principles and techniques related to PCB layout and wiring, so that their designs can be perfect. PCB (Printed Circuit Board Layout and Routing 100) covers the basic principles and design techniques of PCB layout and routing. It answers questions about PCB layout and wiring in a question and answer format. It is very difficult for PCB designers to read. Welcome everyone to supplement and improve on this basis.

1[Q] What should you pay attention to when wiring high-frequency signals? Answer 1. Impedance matching of signal lines; 2. Spatial isolation from other signal lines; 3. For digital high-frequency signals, differential lines will be better;

2[Q] In the layout, if the line is dense, there may be more vias, 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, the vias do not matter, and the high frequency signals minimize the vias. If the line is more, consider the multi-layer board;

3[Q] Is the decoupling capacitor added to the board as good as possible? A decoupling capacitor needs to add the appropriate value at the appropriate position. For example, add the power supply port of your analog device and use different capacitor values to filter out spurious signals at different frequencies.

4[Q] What is the standard for a good board? The layout is reasonable, the power line power redundancy is sufficient, the high-frequency impedance impedance, and the low-frequency trace are simple.

5 [Q] How much influence does the through hole and blind hole have on the signal difference? What is the principle of application? Answer The use of blind holes or buried holes is an effective way to increase the density of the multilayer board, reduce the number of layers and the size of the board, and greatly reduce the number of plated through holes. However, in comparison, the through hole is well realized in the process and the cost is low, so the through hole is used in the general design.

6[Q] When it comes to analog-digital hybrid systems, it is suggested that the electrical layer be divided, and the ground plane should be covered with copper. It is also suggested that the electric ground layer be divided, and different grounds are connected at the power source end, but the signal is reflowed. The path is far away. How to choose the appropriate method for specific applications? A If you have a high frequency >20MHz signal line, and the length and number are more, then you need at least two layers to give this analog high frequency signal. One layer of signal lines, one layer of large area, and the signal line layer needs to make enough vias to the ground.

The purpose of this is:

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

2. The ground plane isolates the analog signal from other digital signals;

3, the ground circuit is small enough, because you have a lot of holes, the ground is a large plane.

7[Q] In the board, the signal input plug-in is on the leftmost edge of the PCB, and the MCU is on the right side. Then, in the layout, the regulated power supply chip is placed close to the connector (the power IC outputs 5V after a relatively long path). Arriving at the MCU), or placing the power IC to the center to the right (the 5V output of the power IC reaches the MCU is shorter, but the input power line goes through a longer PCB)? Or is there a better layout? A. 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.

So there are a few things to consider

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

(2) Whether the analog part and your MCU are a power supply. In the design of high-precision circuits, it is recommended to separate the power supply between the analog part and the digital part.

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

8 [Q] In the application of high-speed signal chain, there are analog ground and digital ground for multiple ASICs. Is it ground split or undivided? What are the existing guidelines? Which effect is better? A. So far, there is no conclusion. In general, you can check the manual of the chip. All of ADI's mixed-chip manuals recommend a grounding solution, some recommended for public ground and some for recommended isolation. It depends on the chip design.

9[Q] When should I consider the length of the line? If you want to consider using the isometric line, what is the difference between the lengths of the two signal lines? How to calculate? Answer the difference line calculation idea: If you pass a sinusoidal signal, your length difference is equal to half of its transmission wavelength, and the phase difference is 180 degrees, then the two signals are completely offset. So the length difference at this time is the maximum value. By analogy, the signal line difference must be less than this value.

10 [Q] The serpentine line in the high speed, is it suitable for that situation? There are no disadvantages. For example, for differential traces, the two sets of signals are required to be orthogonal.

A serpentine trace has different functions because of different applications: (1) If a serpentine trace appears in a computer board, it mainly acts as a filter inductor 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. (2) In the general ordinary PCB board, in addition to the role of the filter inductor, it can also be used as an inductive coil of the radio antenna. For example, a 2.4G walkie-talkie is used as an inductor. (3) The length of some signal wiring lengths must be strictly equal. The length of the high-speed digital PCB board is to keep the delay difference of each signal within a range, and to ensure the validity of the data read by the system in the same period ( When the delay difference exceeds one clock cycle, the data of the next cycle is misread). For example, HUBLink in the INTELHUB architecture has a total of 13 frequencies, and the frequency of 233MHz is required to be strictly equal in length to eliminate the hidden trouble caused by time lag. Winding is the only solution. Generally, 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 line width, line length, copper thickness, and plate layer structure, but the line length increases the distributed capacitance and distributed inductance. To make the signal quality drop. Therefore, the clock IC pins are generally connected; "termination, but the serpentine trace does not act as an inductor. Conversely, the inductor will phase shift the higher harmonics in the rising edge of the signal, causing the signal quality to deteriorate, so It is required that the spacing of the serpentine line is at least twice the line width. The smaller the rise time of the signal, the more susceptible it is to the distributed capacitance and the distributed inductance. (4) The serpentine traces play a role in some special circuits. The role of the LC filter of the parameters.

11[Q] When designing a PCB, how to consider electromagnetic compatibility EMC/EMI, what needs to be considered? What measures are taken? A must consider good EMI / EMC design layout must be started when the position of the device, PCB stack arrangement, important moves online, the selection device. For example, the position of the clock generator should not be close to the external connector. The high-speed signal should go as far as possible to the inner layer and pay attention to the characteristic impedance matching and the continuity of the reference layer to reduce the reflection. The slope of the signal pushed by the device is as small as possible to reduce the height. Frequency component, choose decoupling/bypass capacitors to pay attention to whether the frequency response meets the requirements to reduce power layer noise. In addition, pay attention to the return path of the high-frequency signal current so that the loop area is as small as possible (that is, the loop impedance is as small as possible) to reduce the radiation. It is also possible to divide the formation to control the range of high-frequency noise. Finally, the appropriate choice The grounding point of the PCB and the casing. 12[Q] Please ask the RF broadband circuit PCB transmission line design what needs attention? How to set the ground hole of the transmission line is appropriate, does impedance matching need to be designed by itself or with PCB processing manufacturers? Answering this question requires consideration of many factors. For example, various parameters of the PCB material, the transmission line model finally established based on these parameters, the parameters of the device, and the like. Impedance matching is generally designed according to the information provided by the manufacturer.

13 [Q] When the analog circuit and the digital circuit coexist, half of them are the digital circuit part of the FPGA or the MCU, and the other half is the analog circuit part of the DAC and the related amplifier. There are many power sources of various voltage values. Do you have a power supply with voltage values that are used by both digital and analog circuits. Is it possible to use a common power supply? What are the techniques for wiring and magnetic bead placement? A general use is not recommended. This can be complicated to use and difficult to debug.

14[Q] Hello, what is the main basis for the selection of packages for resistors and capacitors when designing high-speed multilayer PCBs? Commonly used for those packages, can give a few examples. Answer 0402 is commonly used in mobile phones; 0603 is commonly used for high-speed signals; the basis is that the smaller the package, the smaller the parasitic parameters. Of course, the same package of different manufacturers has great differences in high-frequency performance. It is recommended that you use high frequency components in critical locations.

15 [Q] Generally, in the design, is the double panel going to the signal line first or the ground line first? Answer this to be considered comprehensively. Consider the trace when considering the layout first.

16[Q] What are the most important issues to be aware of when designing high-speed multilayer PCBs? Can you elaborate on the solution to the problem? The most important thing to note is that the design of your layer is the signal line, power line, ground, and control line. How are you divided in each layer? The general principle is to ensure at least a single layer of analog and analog signals. A separate layer is also recommended for power supplies.

17[Q] Please ask when to use 2 layers, 4 layers, 6 layers, is there a strict technical limit? (Remove the volume) Is the frequency of the CPU or the frequency of its interaction with the external device data? Answer The use of multi-layer boards can first provide a complete ground plane, and can provide more signal layers for easy routing. For the application of the CPU to control the external memory device, the frequency of interaction should be considered. If the frequency is high, the complete ground plane must be guaranteed. In addition, the signal line should preferably be of equal length.

18 [Q] How to analyze the influence of PCB layout on analog signal transmission, how to distinguish whether the noise introduced during signal transmission is caused by wiring or op amp device. A. This is difficult to distinguish, only through the PCB wiring to minimize the wiring to introduce additional noise.

19[Q] I recently studied the design of PCB. For high-speed multi-layer PCB, it is appropriate to set the line width of power, ground 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 be impedance simulation to calculate the line width and the distance between the line and the ground; the power line needs to determine the line width according to the current line. When mixing the signal PCB, generally do not need "line", but use the entire plane, so To ensure that the loop resistance is minimal and that there is a complete plane below the signal line

20[Q] What kind of layout can I achieve the best heat dissipation? There are three main sources of heat in the PCB: (1) heat generation of electronic components; (2) heat generation of P c B itself; and (3) heat from other parts. Among the three heat sources, the component generates the largest amount of heat, which is the main heat source, followed by the heat generated by the PCB board. The external heat input depends on the overall thermal design of the system, and is not considered for the time being. The purpose of the thermal design is to take appropriate measures and methods to reduce the temperature of the components and the temperature of the PCB, so that the system works properly at the right temperature. Mainly by reducing heat and speeding up heat dissipation.

21[Q] Can you explain the relationship between the line width and the size of the matching via? Answering this question is very good. It is hard to say that there is a simple proportional relationship because his two simulations are different. One is a polygon transmission and the other is a circular transmission. You can find a perforation impedance calculation software on the Internet, and then keep the impedance of the via and the impedance of the transmission line consistent.

22[Q] In a common PCB circuit board with MCU control, but the requirements of high current and high speed signals are not very high, then whether the outermost edge of the PCB is covered with a ground wire will wrap the whole circuit board. Would it be better? In general, it is enough to lay a complete land.

23[Q] 1, I know that the AD conversion chip should be connected to the single point of analog ground and digital ground, but what if there are multiple AD conversion chips on the board? 2. In a multi-layer circuit board, when a multiplexer switches analog sampling, do you need to separate the analog part from the digital part like an AD conversion chip? Answer 1. Several ADCs should be put together as much as possible. The analog ground digitally connects at a single point under the ADC. 2. Depending on the switching speed of the MUX and the ADC, the speed of the general ADC will be higher than the MUX, so it is recommended to place it below the ADC. Of course, for the sake of safety, you can also put a magnetic bead package under the MUX. When debugging, choose where to make a single point connection depending on the specific situation.

24 [Q] In the conventional network circuit design, some use several places together, and such usage? why? Thank you! A is not very clear about your problem. There are certainly several types of ground for a hybrid system, and eventually they will be connected together at one point, the purpose of which is to equal potential. Everyone needs a common ground level for reference.

25 [Q] How to effectively deal with the analog part and the digital part of the PCB, analog ground and digital ground, thank you! Answer The analog circuit and the digital circuit should be placed in separate areas so that the analog circuit is reflowed in the analog circuit area and the digital is in the digital area, so that the number does not affect the simulation. The starting point for analog ground and digital ground processing is similar, and the return of the digital signal cannot be allowed to flow to the analog ground.

26[Q] What are the differences in the design of the ground line when designing the analog circuit and the digital circuit in the PCB board? What issues should be noted? The main requirements of the analog circuit to the ground are: complete, small loop, and impedance matching. Digital signals are not particularly required if low frequency; if the speed is high, impedance matching and ground integrity are also considered.

27 [Q] Decoupling capacitors generally have two, 0.1 and 10. If the area is tight, how to place two capacitors, which one is better? A should be designed according to the specific application and what chip

28[Q] Please ask the teacher, in the RF circuit, there will always be two signals of IQ. Do you need the same length of the two lines? Answer Try to use the same in the RF circuit.

29 [Q] Is the design of the high-frequency signal circuit different from the ordinary circuit design? Can you briefly explain the layout design as an example? A high-frequency circuit design should consider the influence of many parameters. Under high-frequency signals, many parameters that can be ignored by ordinary circuits cannot be ignored, so the transmission line effect may be considered.

30 [Q] High-speed PCB, how to deal with the avoidance of vias during the wiring process, is there any good advice? A high-speed PCB, it is best to play less holes, increase the signal layer to solve the need to increase the need for vias.

31 [Q] How to choose the thickness of the power supply line in the PCB design? Is there any rules? Answer can refer to: 0.15 × line width (mm) = A, also need to consider copper thickness

32 [Q] When the digital circuit and the analog circuit are on the same multi-layer board, should the analog ground and the digital ground be arranged on different layers? A does not need to do this, but the analog circuit and the digital circuit are placed separately.

33[Q] Is it suitable to use up to several vias for general digital signal transmission? (Signal below 120Mhz) Answer It is best not to exceed two vias.

34[Q] In the circuit with analog circuit and digital circuit, how to avoid mutual interference when designing PCB board? A. Analog circuits are generally interfered if they match a reasonable amount of radiation. Sources of interference come from devices, power supplies, space, and PCBs; digital circuits are certainly sources of interference due to their large frequency components. The solution is generally reasonable device layout, power supply decoupling, PCB delamination, and if the interference characteristics are large or the analog part is very sensitive, a shield can be considered.

35[Q] For high-speed circuit boards, parasitic parameters may exist everywhere. In the face of these parasitic parameters, are we precise parameters and then eliminated, or empirical methods? How should this efficiency and performance be balanced? A general analysis of the impact of parasitic parameters on circuit performance. If the impact cannot be ignored, it must be considered and eliminated.

36 [Q] What should you pay attention to when laying out the multi-layer board? When answering the layout of the multi-layer board, because the power supply and the ground layer are in the inner layer, be careful not to have a suspended ground plane or power plane. Also, make sure that the vias hitting the ground are connected to the ground plane. Finally, it is important for some. The signal is added with some test points to facilitate measurement when debugging.

37 [Q] How to avoid crosstalk of high-speed signals? Answer can make the signal line farther away, avoid parallel lines, shield by paving or adding protection, and so on.

38[Q] I would like to ask the power plane in the multi-layer board design, but need to design the power plane in the double-layer board? Answer is difficult, because your various signal lines are almost in the double layer layout.

39 [Q] What is the effect of the thickness of the PCB on the circuit? How is it generally selected? The thickness of the answer is important when making impedance matching. The PCB manufacturer will ask if the impedance matching is calculated when the thickness is too high. The PCB manufacturer will make it according to your requirements.

40 [Q] The ground plane can make the signal minimum loop, but it will also generate parasitic capacitance with the signal line. How should this be chosen? A depends on whether the parasitic capacitance has a non-negligible effect on the signal. If you can't ignore it, then rethink it.

41 [Q] LDO output as a digital power supply or analog power supply means digital and analog which first power supply output? A If you want to use an LDO to provide power for digital and analog, it is recommended to connect the analog power supply first. After the analog power supply is filtered by the LC, it is a digital power supply.

42[Q] Please use magnetic beads between analog Vcc and digital Vcc, or should we use magnetic beads between analog ground and digital ground? A. The analog VCC is LC filtered to obtain the digital VCC, and the magnetic beads are used between the analog ground and the digital ground.

43[Q] How to route differential signal lines such as LVDS? A general need to pay attention: all wiring including the surrounding device placement, the ground plane needs to be symmetrical.

44 [Q] A good PCB design, you need to do as little as possible to emit electromagnetic radiation, but also to prevent external electromagnetic radiation to interfere with yourself, what measures should the circuit take to prevent external electromagnetic interference? The best way to answer is to block and prevent external interference from entering. On the circuit, such as INA, you need to add RFI filter in front of INA to filter out RF interference.

45 [Q] How to solve the problem of transmission line effect when designing a PCB with a fast integrated circuit chip circuit with high clock frequency? What chip is this fast integrated circuit chip? If it is a digital chip, generally do not have to consider. If it is an analog chip, it depends on whether the transmission line effect is large enough to affect the performance of the chip. 46[Q] Is there still a need for copper in a multi-layer PCB design? If copper is covered, which layer should it be connected to? A If there is a complete ground plane and power plane inside, the top and bottom layers may not be copper.

47[Q] In the high-speed multi-layer PCB design, how to carry out impedance simulation generally, what software is used? Is there any problem that requires special attention? A You can use Multisim software to simulate the resistor-capacitor effect.

48[Q] Some devices have thinner pins, but the traces on the PCB are thicker. Will the impedance mismatch be caused after the connection? If so how to solve it? Answer to see what device. And the impedance of the device is generally given in the data sheet, generally has little to do with the thickness of the pin.

49 [Q] Differential lines generally need to be the same length. If it is difficult to achieve in LAYOUT, is there any other remedy? Answers can solve the problem of equal length by taking the serpentine line. Now most PCB software can automatically take the same length line, which is very convenient.