PCB Blog - PCB board layout skills, smart engineers understand

PCB board, also known as printed circuit board (Printed Circuit Board), can realize the circuit connection and function realization between electronic components, and is also an important part of power circuit design. Today, this article will introduce the basic rules of PCB board layout and wiring.

1. Basic Rules of Component Layout

1) Layout according to the circuit module, the related circuit that realizes the same function is called a module, the components in the circuit module should adopt the principle of nearest concentration, and the digital circuit and the analog circuit should be separated at the same time;

2) Do not mount components and devices within 1.27mm around non-mounting holes such as positioning holes and standard holes, and do not mount components within 3.5mm (for M2.5) and 4mm (for M3) around mounting holes such as screws;

3) Avoid placing vias below components such as horizontally mounted resistors, inductors (plug-ins), and electrolytic capacitors to avoid short circuits between the vias and the component shell after wave soldering;

4) The distance between the outside of the component and the edge of the board is 5mm;

5) The distance between the outer side of the mounted component pad and the outer side of the adjacent mounted component is greater than 2mm;

6) Metal shell components and metal parts (shielding boxes, etc.) cannot touch other components, and cannot be close to printed lines and pads, and the spacing should be greater than 2mm. The size of the positioning holes, fastener installation holes, elliptical holes and other square holes in the plate is greater than 3mm from the edge of the plate;

7) The heating element cannot be close to the wire and the thermal element; the high-heating element should be evenly distributed;

8) The power socket should be arranged around the printed board as much as possible, and the bus bar terminals connected to the power socket should be arranged on the same side. Special care should be taken not to arrange power sockets and other soldered connectors between the connectors, in order to facilitate the soldering of these sockets and connectors, and the design and tying of power cables. The arrangement spacing of power sockets and welding connectors should be considered to facilitate the insertion and removal of power plugs;

9) Arrangement of other components: All IC components are unilaterally aligned, polar components are clearly marked with polarities, and the polarity markings on the same printed board should not be more than two directions. When two directions appear, the two directions are perpendicular to each other. ;

10) The wiring on the board should be properly dense, and when the difference in density is too large, it should be filled with mesh copper foil, and the mesh size should be greater than 8mil (or 0.2mm);

11) There should be no through-holes on the patch pads, so as to avoid the loss of solder paste and cause the components to be soldered. Important signal lines are not allowed to pass between the socket pins;

12) The patch is aligned on one side, the character direction is the same, and the packaging direction is the same;

13) For devices with polarity, the direction of polarity marking on the same board should be as consistent as possible.

2. Component wiring rules

1) In the area where the wiring area is less than or equal to 1mm from the edge of the PCB board, and within 1mm around the mounting hole, wiring is prohibited;

2) The power line should be as wide as possible and should not be less than 18mil; the signal line width should not be less than 12mil; the cpu input and output lines should not be less than 10mil (or 8mil); the line spacing should not be less than 10mil;

3) The normal via hole is not less than 30mil;

4) Dual in-line: pad 60mil, aperture 40mil; 1/4W resistance: 51*55mil (0805 surface mount); in-line pad 62mil, aperture 42mil; electrodeless capacitor: 51*55mil (0805 surface mount); When in-line, the pad is 50mil, and the aperture is 28mil;

5) Note that the power wire and the ground wire should be as radial as possible, and the signal wire should not be looped.

3. How to improve anti-interference ability and electromagnetic compatibility

How to improve anti-interference ability and electromagnetic compatibility when developing electronic products with processors?

3.1 The following systems should pay special attention to anti-electromagnetic interference:

1) A system in which the microcontroller clock frequency is particularly high and the bus cycle is particularly fast.

2) The system contains high-power, high-current drive circuits, such as spark-generating relays, high-current switches, etc.

3) Systems with weak analog signal circuits and high A/D conversion circuits.

3.2 Take the following measures to increase the anti-electromagnetic interference capability of the system:

1) Select a microcontroller with low frequency: Selecting a microcontroller with a low external clock frequency can effectively reduce noise and improve the anti-interference ability of the system. Square waves and sine waves of the same frequency, the high frequency components in the square wave are much more than the sine wave. Although the amplitude of the high-frequency component of the square wave is smaller than that of the fundamental wave, the higher the frequency, the easier it is to emit and become a noise source. The influential high-frequency noise generated by the microcontroller is about 3 times the clock frequency.

2) Reduce distortion in signal transmission

Microcontrollers are mainly manufactured using high-speed CMOS technology. The static input current of the signal input terminal is about 1mA, the input capacitance is about 10PF, the input impedance is quite high, and the output terminal of the high-speed CMOS circuit has a considerable load capacity, that is, a considerable output value. The reflection problem is very serious when the long line is led to the input end with a relatively high input impedance, which will cause signal distortion and increase the system noise. When Tpd>Tr, it becomes a transmission line problem, and problems such as signal reflection and impedance matching must be considered. The delay time of the signal on the printed circuit board is related to the characteristic impedance of the lead, that is, related to the dielectric constant of the printed circuit board material. It can be roughly considered that the transmission speed of the signal on the printed board leads is about 1/3 to 1/2 of the speed of light. The Tr (standard delay time) of commonly used logic telephone components in a system composed of microcontrollers is between 3 and 18ns. On the printed circuit board, the signal passes through a 7W resistor and a 25cm long lead, and the wire delay time is roughly between 4~20ns. That is to say, the shorter the signal leads on the printed circuit, the longer it should not exceed 25cm. And the number of vias should be as few as possible, no more than 2. When the rise time of the signal is faster than the delay time of the signal, it is processed according to fast electronics. At this time, the impedance matching of the transmission line should be considered. For the signal transmission between the integrated blocks on a printed circuit board, the situation of Td>Trd should be avoided. The larger the printed circuit board, the faster the system speed.

3) Reduce cross-interference between signal lines: A step signal with rise time Tr at point A is transmitted to end B through lead AB. The delay time of the signal on the AB line is Td. At point D, due to the forward transmission of the signal at point A, the reflection of the signal after reaching point B and the delay of line AB, a page pulse signal with a width of Tr will be induced after Td time. At point C, due to the transmission and reflection of the signal on AB, a positive pulse signal with a width of twice the delay time of the signal on the AB line, that is, 2Td, is induced. This is cross-interference between signals. The strength of the interfering signal is related to the di/at of the signal at point C, which is related to the distance between lines. When the two signal lines are not very long, what is actually seen on AB is the superposition of two pulses. Microcontrollers manufactured by CMOS process have high input impedance, high noise, and high noise tolerance. Digital circuits are superimposed 100~200mv noise and do not affect their work. If the AB line in the figure is an analog signal, this kind of interference becomes intolerable. For example, when the printed circuit board is a four-layer board, one of which is a large-area ground, or a double-sided board, when the reverse side of the signal line is a large-area ground, the cross-interference between the signals will be reduced. The reason is that the large area of ground reduces the characteristic impedance of the signal line, and the reflection of the signal at the D end is greatly reduced. The characteristic impedance is inversely proportional to the square of the dielectric constant of the medium between the signal line and the ground, and is proportional to the natural logarithm of the thickness of the medium. If the AB line is an analog signal, to avoid the interference of the digital circuit signal line CD to AB, there should be a large area of ground below the AB line, and the distance from the AB line to the CD line should be greater than 2~3 times the distance between the AB line and the ground. Partial shielding can be used, and ground wires are arranged on the left and right sides of the lead on the side with the lead junction.

4) Reduce the noise from the power supply

While supplying energy to the system, the power supply also adds its noise to the power supply being supplied. The reset line, interrupt line, and other control lines of the microcontroller in the circuit are easily disturbed by external noise. Strong disturbances on the grid enter the circuit through the power supply, and even in battery-powered systems, the battery itself has high-frequency noise. Analog signals in analog circuits are more resistant to interference from power sources.

5) Pay attention to the high frequency characteristics of printed circuit boards and components

In the case of high frequency, the distributed inductance and capacitance of the leads, vias, resistors, capacitors, and connectors on the printed circuit board cannot be ignored. The distributed inductance of the capacitor cannot be ignored, and the distributed capacitance of the inductor cannot be ignored. The resistance produces the reflection of the high frequency signal, and the distributed capacitance of the lead will play a role. When the length is greater than 1/20 of the corresponding wavelength of the noise frequency, an antenna effect will be generated, and the noise will be emitted through the lead. The vias of the printed circuit board cause about 0.6pf of capacitance. The packaging material of an integrated circuit itself introduces 2~6pf capacitance. A connector on a circuit board with a distributed inductance of 520nH. A dual in-line 24-pin IC socket with 4~18nH distributed inductance. These small distribution parameters are negligible for this line of microcontroller systems at lower frequencies; special attention must be paid to high-speed systems.

6) Component layout should be reasonably partitioned

The position of the components arranged on the printed circuit board should fully consider the problem of anti-electromagnetic interference. One of the principles is that the leads between the components should be as short as possible. In the layout, the analog signal part, the high-speed digital circuit part, and the noise source part (such as relays, high-current switches, etc.) should be reasonably separated, so that the signal coupling between them is .

G Handle the ground wire

On printed circuit boards, power and ground wires are important. To overcome electromagnetic interference, the main means is grounding. For the double-sided panel, the ground wire layout is very particular. By adopting the single-point grounding method, the power supply and the ground are connected to the printed circuit board from both ends of the power supply, with one contact for the power supply and one contact for the ground. On the printed circuit board, there must be multiple return ground wires, and these will be gathered on the contact of the return power supply, which is the so-called single-point grounding. The so-called separation of analog ground, digital ground, and high-power device ground means that the wiring is separated, and all of them are brought together to this ground point. When connecting to signals other than the printed circuit board, shielded cables are usually used. For high frequency and digital signals, the shielded cable is grounded at both ends. Shielded cables for low-frequency analog signals should be grounded at one end. Circuits that are very sensitive to noise and interference or circuits that are particularly high-frequency noise should be shielded with a metal cover.

7) Make good use of decoupling capacitors

A good high frequency decoupling capacitor can remove high frequency components up to 1GHZ. Ceramic chip capacitors or multilayer ceramic capacitors have better high-frequency characteristics. When designing a printed circuit board, a decoupling capacitor should be added between the power supply of each integrated circuit and the ground. The decoupling capacitor has two functions: on the one hand, it is the energy storage capacitor of the integrated circuit, which provides and absorbs the charging and discharging energy of the integrated circuit at the moment of opening and closing the door; on the other hand, it bypasses the high-frequency noise of the device. The typical decoupling capacitor in the digital circuit is 0.1uf. The decoupling capacitor has a distributed inductance of 5nH, and its parallel resonance frequency is about 7MHz, which means that it has a good decoupling effect for noise below 10MHz. Noise hardly works. 1uf, 10uf capacitors, the parallel resonance frequency is above 20MHz, the effect of removing high frequency noise is better. Where the power goes to the board and a 1uf or 10uf de-high frequency capacitor is often beneficial, even battery powered systems require this. Every 10 pieces of integrated circuits should add a charge and discharge capacitor, or a storage and discharge capacitor, and the size of the capacitor can be 10uf. No electrolytic capacitors are used. The electrolytic capacitors are rolled up with two layers of Pu film. This rolled up structure behaves as an inductance at high frequencies. Use bile capacitors or polycarbonate capacitors. The selection of decoupling capacitor value is not strict, it can be calculated according to C=1/f; that is, 10MHz takes 0.1uf, and for the system composed of microcontroller, it can be between 0.1~0.01uf.

3. Experience in reducing noise and electromagnetic interference.

1) If you can use low-speed chips, you don't need high-speed chips. High-speed chips are used in key places.

2) A resistor can be connected in series to reduce the transition rate of the upper and lower edges of the control circuit.

3) Try to provide some form of damping for relays etc.

4) Use a frequency clock that meets the system requirements.

5) The clock generator should be as close as possible to the device using the clock, and the case of the quartz crystal oscillator should be grounded.

6) Circle the clock area with a ground wire, and keep the clock wire as short as possible.

7) The I/O drive circuit should be as close to the edge of the printed board as possible, and let it leave the printed board as soon as possible. The signal entering the printed board should be filtered, and the signal from the high-noise area should also be filtered. At the same time, the method of serial terminal resistance should be used to reduce the signal reflection.

8) The useless end of MCD should be connected to high, or grounded, or defined as an output end. The end of the integrated circuit that should be connected to the power supply ground should be connected, and should not be left floating.

9) Do not float the input terminal of the gate circuit that is not in use, connect the positive input terminal of the operational amplifier that is not in use to the ground, and connect the negative input terminal to the output terminal.

10) The printed board should try to use 45 fold lines instead of 90 fold lines to reduce the external emission and coupling of high-frequency signals.

11) The printed board is divided according to frequency and current switching characteristics, and the distance between noise components and non-noise components should be farther.

12) Single-point connection to power supply and single-point grounding for single-panel and double-panel, the power line and ground line should be as thick as possible. If the economy can afford it, use a multi-layer board to reduce the capacitive inductance of the power supply and the ground.

13) Keep clocks, buses, and chip select signals away from I/O lines and connectors.

14) The analog voltage input line and the reference voltage terminal should be as far away as possible from the digital circuit signal line, especially the clock.

15) For A/D devices, the digital part and the analog part should be unified rather than crossed.

16) The clock line perpendicular to the I/O line has less interference than the parallel I/O line, and the clock component pins are far away from the I/O cable.

17) The component pins should be as short as possible, and the decoupling capacitor pins should be as short as possible.

18) The key lines should be as thick as possible, and protective ground should be added on both sides. High-speed lines should be short and straight.

19) Do not run lines sensitive to noise in parallel with high current, high-speed switching lines.

20) Do not run traces under quartz crystals and under noise-sensitive devices.

21) Weak signal circuits, do not form current loops around low frequency circuits.

22) Do not form a loop for any signal, if it is unavoidable, make the loop area as small as possible.

23) One decoupling capacitor per IC. A small high frequency bypass capacitor should be added beside each electrolytic capacitor.

24) Use large-capacity tantalum capacitors or polycooled capacitors instead of electrolytic capacitors as circuit charging and discharging energy storage capacitors. When using tubular capacitors, the case should be grounded on PCB board.