Describe the basic rules of PCB layout and wiring in detail
Printed circuit board, also known as 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 the basic rules of PCB layout and routing
1. Basic Rules of Component Layout
1. Layout according to circuit modules. Relevant circuits that achieve the same function are called modules. The components in circuit modules should be centralized nearby, and the digital circuit and the analog circuit should be separated;
2. Do not install components and equipment in 1 27mm around non installation holes such as positioning holes and standard holes, and do not install components within 3 5mm (for M2.5) and 4mm (for M3) around mounting holes such as screws;
3. Avoid placing vias under 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 outside of the pad of the mounted component and the outside of the adjacent 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. The spacing should be greater than 2mm Dimension of locating hole, fastener mounting hole, 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 shall be arranged around the printed board as far as possible, and the bus terminals connected to 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. In order to facilitate welding these sockets and connectors, power cables shall be designed and bound 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 shall be aligned on one side, and the polarity component marks shall be clear. 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 circuit board shall be appropriately dense When the density difference is too large, it should be filled with mesh copper foil, and the mesh should be greater than 8mil (or 0.2mm);
11. There should be no through-hole on the patch panel to avoid loss of solder paste and lead to welding of components Important signal lines are not allowed to pass between the socket pins;
12. The patch is unilaterally aligned, with the same character direction, and the packaging direction is the same;
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
In the area where the wiring area is â¤ 1mm from the edge of the PCB, wiring is prohibited within 1mm around the mounting hole; 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; cpu The incoming and outgoing lines should not be less than 10mil (or 8mil); the line spacing should not be less than 10mil; 3. The normal through-hole shall not be less than 30mil; 4. Double line: pad 60 mil, aperture 40 mil; 55mil (0805 surface mount); 62mil pad, 42mil aperture when plugged directly; electrodeless capacitor: 51*55mil (0805 surface mount); 50mil pad, the aperture is 28mil when directly inserted; 5 Note that the power line and ground wire should be as radial as possible, and the signal line should not have loopback
2.1 The following systems should pay special attention to anti-electromagnetic interference:
(1) The microcontroller clock frequency is particularly high
(2) The system contains high power, large current drive circuit, such as spark generator relay, large current switch, etc
(3) A system with a weak analog signal circuit and a high precision A/D conversion circuit
2.2 take the following measures:
(1) Select a microcontroller with low frequency: Selecting a microcontroller with low external clock frequency can effectively reduce noise and improve the anti interaction ability 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 high-frequency square wave component is smaller than the fundamental wave component, the higher the frequency, the easier it is to transmit and become a noise source The noise generated by an influential high-frequency microcontroller is about three times the clock frequency
(2) The microcontroller to reduce the distortion in signal transmission is mainly made of 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 relatively 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 nanoseconds. On the printed circuit board, the signal passes through a 7W resistor and a 25cm long lead, and the online delay time is about 4 to 20ns. In other words, the shorter the signal lead on the printed circuit, the better, and the length should not exceed 25cm. The number of vias 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, it is necessary to avoid Td>Trd. The larger the printed circuit board, the slower the system may be. The rule of thumb for PCB design is summarized as follows: The delay time of signal transmission on PCB should not be greater than the nominal delay time of the equipment used.
(3) Reduce the interference between signal lines:
A step signal with a rise time of Tr at point A is transmitted to terminal B through lead AB 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, after Td time, a page pulse signal with a width of Tr will be generated 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, that is, 2Td, will be induced This is cross interference between signals The intensity of interference signal is related to di/signal at point C and 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 manufactured by CMOS process has high input impedance, squeal and high noise tolerance The digital circuit adds 100~200mv noise, which does not affect its operation If the AB line in the figure is an analog signal, the interference becomes intolerable For example, if the printed circuit board is a four layer board, one of which is a large area of ground, or a double-sided board, when the back of the signal line is a large area of ground, the cross interference between these signals will become smaller The reason is that the large area grounding reduces the characteristic impedance of the signal line and greatly reduces the reflection of the signal at the D terminal 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 AB line is an analog signal to avoid the interference of digital circuit signal line CD to AB, there should be a large area of ground below 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 A partial mask can be used, and the ground wire is arranged on the left and right sides of the wire at the wire connector side
The above is the explanation given by the editor of pcb circuit board company.
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