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PCB Tech

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PCB Tech

PCB Tech

What are the basic rules of PCB layout?
2021-10-08
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Author:Downs

Basic rules of component PCB layout

1. Layout according to circuit modules, and related circuits that realize the same function are called a module. The components in the circuit module should adopt the principle of nearby concentration, and the digital circuit and the analog circuit should be separated;

2. No components or devices shall be mounted within 1.27mm around non-mounting holes such as positioning holes, standard holes, and 3.5mm (for M2.5) and 4mm (for M3) components shall not be mounted around the mounting holes such as screws;

3. Avoid placing via holes under the components such as horizontal resistors, inductors (plug-ins), electrolytic capacitors, etc., so as to avoid short circuit between the via holes and the component housing 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 mounting component pad and the outside of the adjacent interposing component is greater than 2mm;

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

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7. The heating element should not be close to the wire and the heat-sensitive element; the high-heating device should be evenly distributed;

8. The power socket should be arranged around the printed board as far as possible, and the power socket and the bus bar terminal connected to it should be arranged on the same side. Particular care should be taken not to arrange power sockets and other welding connectors between the connectors to facilitate the welding of these sockets and connectors, as well as the design and tie-up of power cables. The arrangement spacing of power sockets and welding connectors should be considered to facilitate the plugging and unplugging of power plugs;

9. Arrangement of other components: All IC components are aligned on one side, and the polarity of polar components is clearly marked. The polarity of the same printed board cannot be marked in more than two directions. When two directions appear, the two directions are perpendicular to each other. ;

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

11. There should be no through holes on the SMD pads to avoid the loss of solder paste and cause false soldering of the components. 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. As far as possible, the polarized devices should be consistent with the polarity marking direction on the same board.

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Component wiring rules

1. Draw the wiring area within 1mm from the edge of the PCB board, and within 1mm around the mounting hole, wiring is forbidden;

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 is not less than 30mil;

 4. Dual in-line: 60mil pad, 40mil aperture;

1/4W resistance: 51*55mil (0805 surface mount); when in-line, the pad is 62mil, and the aperture is 42mil;

Infinite capacitance: 51*55mil (0805 surface mount); when in-line, the pad is 50mil, and the aperture is 28mil;

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

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How to improve anti-interference ability and electromagnetic compatibility?

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

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

(1) A system where the microcontroller clock frequency is extremely high and the bus cycle is extremely fast.

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

(3) A system containing a weak analog signal circuit and a high-precision A/D conversion circuit.

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

(1) Choose a microcontroller with low frequency:

Choosing a microcontroller with a low external clock frequency can effectively reduce noise and improve the system's anti-interference ability. For square waves and sine waves of the same frequency, the high frequency components in the square wave are much more than that in the sine wave. Although the amplitude of the high-frequency component of the square wave is smaller than the fundamental wave, the higher the frequency, the easier it is to emit as a noise source. The most 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, and the input impedance is quite high. The output terminal of the high-speed CMOS circuit has a considerable load capacity, that is, a relatively large output value. The long wire leads to the input terminal with quite high input impedance, the reflection problem is very serious, it will cause signal distortion and increase system noise. When Tpd>Tr, it becomes a transmission line problem, and problems such as signal reflection and impedance matching must be considered.

(3) Reduce the cross * interference between signal lines:

A step signal with a rise time of Tr at point A is transmitted to terminal 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 from point A, the signal reflection after reaching point B and the delay of the AB line, 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 the cross*interference between signals.

 (4) Reduce noise from power supply

While the power supply provides energy to the system, it also adds its noise to the power supply. The reset line, interrupt line, and other control lines of the microcontroller in the circuit are most susceptible to interference from external noise. Strong interference on the power grid enters the circuit through the power supply. Even in a battery-powered system, the battery itself has high-frequency noise. The analog signal in the analog circuit is even less able to withstand the interference from the power supply.

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

In the case of high frequency, the leads, vias, resistors, capacitors, and the distributed inductance and capacitance of the 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 is produced, and the noise is emitted through the lead.

(6) The layout of components should be reasonably partitioned

The position of the components 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 to minimize the signal coupling between each other.

(7) Handle the grounding wire

On the printed circuit board, the power line and the ground line are the most important. The most important means to overcome electromagnetic interference is to ground.