Printed circuit board

Reveal! Why do others draw the PCB board so perfect?

Electronic product design, PCB layout wiring is an important step, the PCB layout wiring will directly affect the performance of the circuit.

Impedance matching of signal lines.
– Spatial isolation from other signal lines.
– For digital high frequency signals, differential lines will work better.

2, [Q] In the layout of the board, if the line is dense, the over-hole may have to be more, 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, it does not matter if there are too many holes, high-frequency signals try to reduce the number of holes. If more lines can consider multi-layer board.

3、[Q] Is it better to add more decoupling capacitors on the board?

[A] Decoupling capacitors need to be added at the right location with the right value. For example, in the power supply port of your analog devices, and need to use different capacitance values to filter out spurious signals of different frequencies.

4、[Q] What is the standard of a good board?

[A] A reasonable layout, power line power redundancy enough, high frequency impedance impedance, low frequency alignment simple.

5、[Q] How big is the impact of through-hole and blind hole on the difference of signal? What is the principle of application?

[A] The use of blind or buried holes is an effective way to improve the density of multilayer boards, reduce the number of layers and board size, and greatly reduce the number of plated through-hole. However, in comparison, through-hole is good to achieve in process and lower cost, so generally through-hole is used in design.

6、[Q] In the case of analog-digital hybrid systems, some people suggest that the electrical layer split, the ground plane to take the entire copper, others suggest that the electrical ground layer are split, different ground in the power source end point connection, but this is far from the return path of the signal, the specific application should be how to choose the appropriate method?

[A] If you have high frequency > 20MHz signal line, and the length and number are more, then you need at least two layers to this analog high frequency signal. One layer of signal lines, one layer of large area ground, and the signal line layer needs to be punched with enough overholes to ground. The purpose of this is.

– For analog signals, this provides a complete transmission medium and impedance matching.
– the ground plane isolates the analog signal from other digital signals.
– The ground loop is small enough because you hit a lot of vias and the ground is a large plane.

7, [Q] in the circuit board, the signal input plug-in in the left edge of the PCB, mcu in the right, then in the layout is the regulated power supply chip placed close to the plug-in (power IC output 5V after a relatively long path to reach the MCU), or the power IC placed to the right of center (power IC output 5V line to reach the MCU is relatively short, but the input power line after a relatively long (a section of the 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 try not to affect the signal integrity of the analog part. Therefore, there are several points to consider.

– First of all, whether your voltage regulator chip is relatively clean, small ripple power supply. The power supply to the analog part of the power supply requirements are relatively high.
– Whether the analog part and your MCU is a power supply. In the design of high circuits, it is recommended to separate the power supply for 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, [Q] In the application of high-speed signal chain, for multi-ASIC are present analog ground and digital ground, whether to use ground split, or not split ground? What are the existing guidelines? Which one works better? [A] So far, there is no definite answer. In general you can check the manual of the chip. The manuals of all ADI hybrid chips are recommending you a grounding scheme, some recommending a common ground, some recommending an isolated ground. It depends on the chip design.

9, [Q] when to consider the line of equal length? If you want to consider using equal length lines, the difference between the length of the two signal lines should not exceed how much? How to calculate? [A] Differential line calculation ideas: If you pass a sine signal, your length difference is equal to half of its transmission wavelength is, the phase difference is 180 degrees, at which point the two signals are completely canceled. So the length difference at this point is the value. And so on, the signal line difference must be less than this value.

10, [Q] high speed in the snake alignment, suitable for that situation? What are the disadvantages, such as for differential alignment, and requires two groups of signals are orthogonal? [A] snake alignment, because the application of different occasions and with different roles.

– If the snake alignment in the computer board, the main role of a filtering inductor and impedance matching, to improve the circuit’s anti-interference ability. Computer motherboards in the snake alignment, mainly used in some clock signals, such as PCI-Clk, AGPCIK, IDE, DIMM and other signal lines.

– If in the general ordinary PCB board, in addition to the role of filter inductor, but also as a radio antenna inductor coil and so on. Such as 2.4G intercom is used as an inductor.

– For some signal wiring length requirements must be strictly equal to the length of the high-speed digital PCB board is to keep the delay difference between the signals in a range to ensure the effectiveness of the data read by the system in the same cycle (delay difference of more than one clock cycle will read the data of the next cycle by mistake). For example, the HUBLink in the INTELHUB architecture, with a total of 13 rods, uses a frequency of 233MHz and requires strict equal length to eliminate the hidden problem caused by time lag, and wire winding is the only solution. The delay difference is generally required to be no more than 1/4 clock cycle, and the delay difference per unit length of the line is also fixed. The delay is related to the line width, line length, copper thickness, and board layer structure, but too long a line will increase the distributed capacitance and distributed inductance, which will degrade the signal quality. So the clock IC pins are generally connected; ” end connection, but the snake alignment is not the role of inductance. On the contrary, inductance will make the signal in the rising edge of the higher harmonics phase shift, resulting in deterioration of signal quality, so the snake line spacing is required to be less than twice the width of the line. The smaller the rise time of the signal, the more susceptible it is to the effects of distributed capacitance and distributed inductance.

– The snake alignment acts as an LC filter for the distribution parameters in some special circuits.

11, [Q] in the design of the PCB, how to consider EMC/EMI, what specific aspects need to be considered? What measures to take? [A] good EMI / EMC design must be considered at the beginning of the layout of the device location, PCB stack arrangement, the important linkage method, the choice of devices.

For example, the location of the clock generator as close as possible to the external connector, high-speed signals as far as possible to go inside the layer and pay attention to the characteristic impedance matching and reference layer to reduce reflections, the device pushed by the slope of the signal (slew rate) as small as possible to reduce the high-frequency components, the choice of decoupling (decoupling/bypass) capacitors to pay attention to its frequency response to meet the needs to reduce Power layer noise. In addition, pay attention to the return path of high-frequency signal current to make its loop area as small as possible (that is, the loop impedance loop impedance as small as possible) to reduce radiation, but also the way to control the range of high-frequency noise by dividing the ground layer, the appropriate choice of PCB and shell grounding point.

12, [Q] What do you need to pay attention to the design of the transmission line of the RF broadband circuit PCB? How to set the ground hole of the transmission line is more appropriate, impedance matching is required to design their own or to cooperate with the PCB fabricator?

[A] This issue requires consideration of many factors. For example, the various parameters of the PCB material, the transmission line model based on these parameters, the parameters of the device, etc.. Impedance matching is generally based on the information provided by the manufacturer to design.

13. [Q] When analog and digital circuits coexist, such as half of the FPGA or microcontroller digital circuit part, and the other half is the analog circuit part of the DAC and related amplifier. There are more power supplies of various voltage values, and when it comes to power supplies of voltage values that are used in both digital and analog circuits, is it possible to use a common power supply and what are the techniques in wiring and bead arrangement?
[A] Generally, it is not recommended to use this. This will be more complicated to use, it is also difficult to debug.

14, [Q] Hello, please ask in the high-speed multilayer PCB design €€, the resistors and capacitors and other devices on the basis of the choice of the package, what is the main basis? Those packages are commonly used, can you give a few examples.
[A] 0402 is commonly used in cell 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, different manufacturers of the same package in the high-frequency performance varies greatly. It is recommended that you use high-frequency special components in key locations.

15, [Q] in general in the design of the dual panel is the first signal line or ground line?

[A] This should be a comprehensive consideration. In the first consideration of the layout of the case, consider the alignment.

16, [Q] In the design of high-speed multilayer PCB, what should be noted that the problem? Can you do a detailed description of the solution to the problem.

[A] should pay attention to the design of your layer, that is, the signal lines, power lines, ground, control lines, how you are divided in each layer. The general principle is to ensure that the analog signal and analog signal ground at least a separate layer. Power supply is also recommended to use a separate layer.

17、[Q] Is there a strict technical limitation on when to use a 2-layer board, 4-layer board, or 6-layer board? (excluding volume reasons) is the frequency of the CPU or the frequency of its data interaction with external devices prevail?

[A] The use of multilayer boards can first provide a complete ground plane, in addition to providing more signal layers for easy routing. For applications where the CPU is going to control external memory devices, the frequency of interaction should be considered. If the frequency is higher, a complete ground plane must be ensured, and in addition, the signal lines should be kept equal in length.

18、[Q] How to analyze the impact of PCB wiring on analog signal transmission, and how to distinguish whether the noise introduced during signal transmission is caused by wiring or op amp devices.

[A] This is difficult to distinguish, only through PCB cabling to minimize the introduction of additional noise cabling.

19, [Q] I recently learned the design of PCB, for high-speed multilayer PCB, the power, ground and signal lines are set to the appropriate line width, what is the common setting, can you give an example? For example, how to set the operating frequency at 300Mhz?

[A] 300MHz signal must do impedance simulation to calculate the line width and the distance between the line and ground; power lines need to determine the line width according to the size of the current, ground in the mixed signal PCB generally do not use the “line”, but the entire plane, so as to ensure that the circuit resistance, and the signal line below a complete plane.

20, [Q] How can the layout to achieve the heat dissipation effect?

[A] There are three main aspects of heat in the PCB.

– The heat generation of electronic components.
– The heat generated by P c B itself.
– Other parts of the heat transfer.
Among these three sources of heat, the heat generated by the components, is the main source of heat, followed by the heat generated by the PCB board, and the heat coming in from outside depends on the overall thermal design of the system and is not 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 board temperature, so that the system works properly at the right temperature. This is mainly achieved by reducing heat generation, and accelerating heat dissipation.


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