PCB layout skills, SMArt engineers understand
PCB board, also known as printed circuit board (printed circuit board), can realize circuit connection and function realization between electronIC components, and is also an important part of power circuit design Today, this artICle will introduce PCB board layout and wiring
PCB board
1. Basic rules of component layout
1) According to the layout of circuit modules, the relevant circuits that achieve the same function are calLED modules. The components in the circuit modules shall adopt the principle of nearest concentration, and the digital circuits and analog circuits shall be separated at the same time;

2) Do not install components within 1.27mm around non mounting holes such as positioning holes and standard holes, and do not install components within 3.5mm (M2.5) and 4mm (M3) around mounting holes such as screws;
3) Avoid placing vias under horizontally installed resistors, inductors (挿 parts), electrolytic capacitors and other components to avoid short circuit between vias and component shells after wave soldering;
4) The distance between the exterior of the component and the edge of the plate is 5mm;
5) The distance between the outside of the installation component pad and the outside of the adjacent installation components is greater than 2mm;
6) The metal shell components and metal parts (mask box, etc.) shall not contact other components, and shall not be close to the printed line and bonding pad. The spacing shall be greater than 2mm. The size of square holes such as positioning holes, fastener mounting holes and elliptical holes on the plate is greater than 3mm from the edge of the plate;
7) Heating elements shall not be close to wires and thermal elements; High heating elements shall be evenly distributed;
8) The power socket shall be arranged around the printed board as much as possible, and the bus terminal connected with the power socket shall be arranged on the same side. Special attention shall be paid not to arrange power sockets and other welding connectors between connectors, so as to facilitate the welding of these sockets and connectors and the design and binding of power cables. The spacing between power sockets and welding joints shall be considered to facilitate the insertion and removal of power plugs;
9) Arrangement of other components: all IC components are aligned on one side, with clear polarity MARKs on the polarity components, and the polarity marks on the same printed board shall not exceed two directions. When there are two directions, they are perpendicular to each other;
10) The wiring on the board shall be properly dense. When the density difference is too large, the mesh copper foil shall be filled, and the mesh size shall be greater than 8mil (or 0.2mm);
11) There should be no through-hole on the terminal block to avoid loss of solder paste and lead to parts being welded. Important signal cables are not allowed to pass through the socket pins;
12) The patches are aligned on one side, with the same character direction and packaging direction;
13) For equipment with polarity, the direction of polarity marks on the same circuit board shall be as consistent as possible.
2. Component wiring rules
1) No wiring is allowed in the area where the wiring area is less than or equal to 1mm from the edge of PCB board and within 1mm around the mounting hole;
2) The power cord shall be as wide as possible and not less than 18mil; The signal line width shall not be less than 12 mils; CPU input and output lines shall not be less than 10mil (or 8mil); The line spacing shall not be less than 10mil;
3) Normal through-hole is not less than 30mil;
4) Double in-line type: gasket 60ml, aperture 40ml; 1/4W resistance: 51 * 55mil (0805 surface mounted); The embedded gasket is 62mil, and the hole diameter is 42mil; Stepless capacitor: 51 * 55mil (0805 surface mounted); When coaxial, the liner is 50mil, and the hole diameter is 28mil;
5) Please note that the power line and ground wire should be as radial as possible, and the signal line 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 shall pay special attention to anti electromagnetic interference:
1) The microcontroller has a very high clock frequency and a very fast bus cycle.
2) The system includes high power and high current driving circuits, such as spark generator relay, high current switch, etc.
3) System with weak analog signal circuit and high A/D conversion circuit.
3.2 Take the following measures to improve the anti electromagnetic interference capability of the system:
1) Select low frequency microcontroller: Select a microcontroller with a lower external clock frequency to effectively reduce noise and improve the anti-interference capability of the system. For square wave and sine wave with the same frequency, the high-frequency component of square wave is much greater than that of 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 high frequency noise produced by the microcontroller is about 3 times of the clock frequency.
2) Reduce distortion in signal transmission
Microcontrollers are mainly manufactured with high-speed CMOS technology. The static input current of the signal input terminal is about 1mA, the input capacitance is about 10PF, and the input impedance is quite high. The output terminal of the high-speed CMOS circuit has a considerable load capacity, that is, a considerable output value. When a long line is introduced into an input with a high input impedance, the reflection problem is very serious, which will lead to signal distortion and new system noise. When Tpd>Tr, it becomes a transmission line problem, and signal reflection and impedance matching must be considered. The delay time of the signal on the PCB is related to the characteristic impedance of the wire, that is, the dielectric constant of the PCB data. It can be roughly considered that the signal transmission rate on the PCB leads is about 1/3 to 1/2 of the speed of light. In a system composed of microcontrollers, Tr (standard delay time) of common logic telephone elements is between 3 and 18 ns. On the printed circuit board, the signal passes through a 7W resistor and a 25cm long wire, and the wire delay time is about 4~20ns. In other words, the shorter the signal line on the printed circuit, the longer it should not exceed 25cm. And the number of through holes shall be as small 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. In this case, the impedance matching of the transmission line should be considered. For signal transmission between integrated blocks on PCB, Td>Trd should be avoided. The larger the PCB, the faster the system.
3) Reduce cross interference between signal lines: the step signal with rise time of Tr at point A is transmitted to terminal B through wire AB. The delay time of signal on AB line is Td. At point D, due to the forward transmission of the signal at point A, the reflection of the signal after arriving at point B and the delay of line AB, a Tr width page pulse signal will be sensed after Td time. At point C, due to the transmission and reflection of the signal on AB, a positive pulse signal with a width twice the delay time of the signal on AB line is generated, that is, 2Td. This is cross interference between signals. The intensity of the interference 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. The micro controller made by CMOS technology has high input impedance, high noise and high noise tolerance. The digital circuit superimposes 100~200mv noise, which does not affect its operation. If the AB line in the figure is an analog signal, such interference will become intolerable. For example, when the printed circuit board is a four layer board, one of which is a large-area grounded or double-sided board, and the reverse side of the signal line is large-area grounded, the cross interference between signals will be reduced. The reason is that the large area of the ground reduces the characteristic impedance of the signal line, and the reflection of the signal at the D terminal is greatly reduced. The characteristic impedance is inversely proportional to the square of the dielectric constant between the signal line and the ground, and is proportional to the natural logarithm of the dielectric thickness. If the AB line is an analog signal, in order to avoid the interference of digital circuit signal line CD to AB, there should be a large area of ground below the AB line, and the distance between AB line and CD line should be 2-3 times greater than the distance between AB line and ground. The local mask can be used, and the grounding wire is arranged on the left and right sides of the wire at the side of the wire connector.
4) Reduce power noise
While supplying power to the system, the power supply will also add noise to the power supply. The reset line, interrupt line and other control lines of the microcontroller in the circuit are easily interfered by external noise. The strong interference on the power grid enters the circuit through the power supply. Even in the battery power supply system, the battery itself also has high-frequency noise. The analog signal in the analog circuit is more resistant to the interference of power supply.
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 leads, vias, resistors, capacitors and connectors on the printed circuit board cannot be ignored. The distributed inductance of capacitor and the distributed capacitance of inductance cannot be ignored. Resistance generates reflection of high-frequency signal, and the distributed capacitance of wire will play a role. When the length is greater than 1/20 of the wavelength corresponding to the noise frequency, the antenna effect will be generated, and the noise will be transmitted through the wire. The through-hole of the printed circuit board generates a capacitance of about 0.6 pf. 2~6pf capacitor is introduced into the package data of integrated circuit. The connector on the circuit board with 520nH distributed inductance. A dual in-line 24 pin IC socket with 4~18nH distributed inductance. For this series of low frequency microcontroller systems, these small distributed parameters can be ignored; Special attention must be paid to high-speed systems.
6) The Layout of components shall be reasonably divided
Anti electromagnetic interference shall be fully considered for the position of components arranged on the printed circuit board. One of the principles is that the leads between components should be as short as possible. In the layout, analog signal part, high-speed digital circuit part and noise source part (such as relay, high current switch, etc.) shall be reasonably separated to facilitate signal coupling between them.
G Processing ground wire
On a printed circuit board, power and ground wires are important. In order to overcome electromagnetic interference, the main method is grounding. For double-sided panels, the ground wire layout is very special. By using the single point grounding method, the power supply and grounding are connected to the printed circuit board from both ends of the power supply. One contact is used for the power supply and one contact is used for grounding. On the printed circuit board, there must be multiple loop ground wires, which will gather on the contact of the loop power supply, which is called single point grounding. The separation of analog grounding, digital grounding and high-power equipment grounding means that the wiring is separated, and all these are connected to this grounding point. When connecting to signals outside the printed circuit board, mask cables are usually used. For high-frequency and digital signals, both ends of the shield cable are grounded. The shield cable of low-frequency analog signal shall be grounded at one end. The circuit which is very sensitive to noise and interference or the circuit with high frequency noise shall be shielded with metal cover.