What should I pay attention to when wiring on PCB?

In order to obtain the best performance of electronic circuits, the arrangement of components and wires is very important. For design quality

High-quality, low-cost PCB .. should follow the following general principles:

1. layout

First consider the size of the PCB. When the PCB size is too large, the printed circuit is longer, the impedance is increased and the noise resistance is improved.

Decline, the cost also increases; Too small, heat dissipation is not good, and adjacent lines are easily disturbed. Confirm the PCB size again.

Determine the position of special parts. Finally, according to the functional units of the circuit, all components of the circuit are laid out.

When determining the position of special parts, please follow the following principles:

(1) Shorten the connection between high-frequency components as much as possible, and minimize their distribution parameters and electromagnetic interference between them.

Excuse me. Do not get too close to the easily disturbed components, and keep the input and output components as far away as possible.

(2) There may be a high potential difference between some elements or wires, so the distance between them should be increased to avoid discharge.

Unexpected short circuit. High-voltage components should be arranged as far as possible in places that are hard to reach when debugging.

(3) Parts with a weight exceeding 15g shall be fixed by brackets and then welded. Those are big, heavy and high in heat.

Many components should not be installed on printed boards, but should be installed on the floor of the chassis of the whole machine, and heat dissipation should be considered. Temperature-sensitivity

Components should be kept away from heating components.

(4) The layout of adjustable elements such as potentiometer, adjustable inductance coil, variable capacitor and microswitch should be considered.

Structural requirements of. If it is adjusted in the machine, it should be placed above the printed board where it is easy to adjust; In case of off-board adjustment, its position

So as to adapt to the position of the adjusting knob on the chassis panel.

(5) Reserve the position occupied by the positioning hole of the printing wrench and the fixed bracket.

According to the functional units of the circuit, the layout of all components of the circuit shall conform to the following principles:

(1) Arrange the position of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal circulation and signal discharge.

May keep the same direction.

(2) Layout around the core components of each functional circuit. Components should be uniform, neat and tight.

Arranged together on the PCB. Minimize and shorten leads and connections between components.

(3) For circuits operating at high frequency, the distribution parameters between components should be considered. General circuits should be made as components as possible.

Parallel arrangement. This is not only beautiful, but also easy to assemble, weld and mass production.

(4) Components located at the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board. The best shape of the circuit board is rectangular.

Form. The aspect ratio is 3: 2 to 4: 3. When the surface size of the circuit board is greater than 200x 150mm, the mechanical stress on the circuit board should be considered.

Strength.

Step 2 connect wires

The wiring principle is as follows:

(1) The wires used at the input and output terminals should be as parallel as possible. It is best to add grounding wire between lines to avoid feedback coupling.

(2) The minimum width of printed conductor is mainly determined by the adhesive strength between conductor and insulating substrate and the current flowing through them.

Settings. When the thickness of copper foil is 0.05mm and the width is 1~ 15mm, the current is 2 years and the temperature will not be higher than 3℃, because

The width of the conductor is 1.5mm, which can meet the requirements. For integrated circuits, especially digital circuits, 0.02~0.3mm is usually selected.

Wire width. Of course, as long as it is allowed, try to use wide wires, especially power and ground wires. Minimum spacing of wires

It is mainly determined by the worst insulation resistance and breakdown voltage between lines. Only applicable to integrated circuits, especially digital circuits.

If the process allows, the spacing can be as small as 5 ~ 8 mm

(3) The bending of printed wires is generally circular, and the right angle or included angle will affect the electrical performance in high-frequency circuits. this

Outside, try to avoid using a large area of copper foil, otherwise the copper foil will easily swell and fall off if heated for a long time. must

When using a large area copper foil, it is best to use a grid shape, which is beneficial to eliminate the volatile gas generated by the heating of the glue between the copper foil and the substrate.

Body.

Step 3: Pad

The central hole of the pad is slightly larger than the diameter of the device lead. If the pad is too large, it is easy to form virtual welding. The outer diameter d of the pad is usually not small.

Inch (d+ 1.2) mm, where d is the lead aperture. For high-density digital circuits, the minimum diameter of the pad is (d+1.0) mm. ..

Anti-interference measures of PCB and circuit

The anti-interference design of printed circuit board is closely related to the specific circuit. This paper only introduces the anti-interference design of several PCBs.

Some common measures are explained.

1. Power line design

According to the current of the printed circuit board, increase the width of the power line as much as possible to reduce the loop resistance. At the same time, make the power supply

The direction of electric wire and ground wire is consistent with the direction of data transmission, which helps to enhance the anti-noise ability.

2. Batch design

The principle of grounding wire design is:

(1) is digitally isolated from analog ground. If there are both logic circuits and linear circuits on the circuit board, it should be divided as much as possible.

Open. The grounding of low-frequency circuits should be connected in parallel at a single point as far as possible. When the actual wiring is difficult, it can be partially connected in series and then connected in parallel.

Land. Multi-point series grounding should be adopted for high-frequency circuits, the grounding wire should be short and rented, and grid grounding should be adopted around high-frequency components as far as possible.

Tin foil.

(2) The grounding wire should be as thick as possible. If the grounding wire is made of very soft wire, the grounding potential changes with the change of current, so

Anti-noise performance is reduced. Therefore, the grounding wire should be thick so that it can pass three times the allowable current on the printed board. if possible

Yes, the grounding wire should be more than 2 ~ 3 mm

(3) The grounding wire forms a closed loop. For printed boards composed only of digital circuits, grounding circuits are mostly divided into loops.

High noise resistance.

3. Decoupling capacitor configuration

A common practice in PCB design is to configure appropriate decoupling capacitors in each key part of PCB. Decoupling capacitor

The general configuration principle of is:

The input of (1) power supply is connected across the electrolytic capacitor of 10~ 100uf. If possible, it is better to connect it above 100uF.

(2) In principle, each IC chip should be equipped with a ceramic capacitor of 0.0 1pF. In the case that the gap on the printed board is insufficient,

A capacitor of 1~ 10pF can be arranged every 4~8 chips.

(3) Devices with weak anti-noise ability and large power supply changes when turned off, such as RAM and ROM storage devices, should be on the chip.

The decoupling capacitor is directly connected between the power supply line and the ground line.

(4) Capacitor leads should not be too long, especially for high-frequency bypass capacitors. In addition, we should also pay attention to the following two points:

(1) When there are contactors, relays, buttons and other components in the printed board, there will be a big fire during operation.

For flower discharge, the RC circuit shown in the attached figure must be used to absorb the discharge current. Generally, r is 1~2K, and c is .2 ~ 47uf.

(CMOS has a high input impedance and is easy to be induced. When in use, the unused terminals should be grounded or directly connected.

Power supply.