PCB Blog - PCB board layout analog circuit and digital circuit part

1. The placement direction of the PCB board components determines the direction of the wiring

2. The wiring directions of the adjacent layers are different, the surface layer of the two panels and the wiring body of the solder layer are at 90 degrees

3. The wiring direction of the rectangular circuit board is vertical, and the horizontal wiring can easily cause congestion or even impossible wiring.

4. Try to ensure the wiring space. When this is not possible, use the wiring under specific components to avoid setting connection holes under the components.

Because when the circuit board fails, it is impossible to visually see the state of the connection hole under the component, and whether it is short-circuited with other wiring or component pins.

Analog circuit part and the digital circuit part

Including wiring, the analog circuit part and the digital circuit part should be kept more than 5mm to ensure that there is no signal interference with each other. When a symbol is used to represent the ground wire in the circuit diagram, the circuit board designer needs to analyze the circuit diagram and set a set area. Power line and ground line The power line and ground line were originally designed. For two-panel and four-layer boards, the wiring composition is completely different, because the power and ground lines are set on the inner layer, and the main attention is paid. Just focus on the layout of the signal lines. For beginners, it is recommended to learn from the design of the four-layer board. The wiring of power and ground wires has a very large impact on electrical and clutter, so design carefully.

Take two panels as an example:

1) The power line and the ground line are designed on the same layer, and the effect is very poor

2) The ground wire is on the surface layer, and the power wire is on the welding layer. General design

3) The ground wire is on the surface layer, the power wire is on the welding layer, and the copper foil is used for wiring. The anti-clutter effect is better. Because of the uncontrollability of CAD design, the design time is longer than that of simple wiring. Pay attention to ensuring a small wiring width to ensure that there will be no disconnection and blocking.

To put it simply: the power and ground wires are equivalent to the aorta and veins of the human body. It can also be considered simply a water pipe. The wider the line width, the greater the current flow that can be passed, and the faster the heat dissipation. The narrower the line width, the greater the resistance under the same voltage, the smaller the current flow that can be passed, and the slower the heat dissipation.

welding surface

For power lines and ground lines, a large area of copper foil wiring is used. Precautions for two-layer circuit board power and ground wiring. Usually, the power line is wired on the welding surface, the ground line is wired on the surface, and the copper foil is used for wide-area wiring, and then some more capacitors are added between the power line and the ground line, and there is basically no problem. But when it comes to electromagnetic interference, the problem is different. When it exceeds 8MHz, there may be such and such problems. When it exceeds 25MHz, it will be quite unstable. At this time, it is necessary to surround the ground wire copper foil around the important components, and also design the ground wire copper foil on the welding surface.

The wiring of the crystal oscillator

For anti-interference, the surrounding components is surrounded by ground wire copper foil as much as possible. What is not shown in the figure is that the ground wire copper foil can also be placed under the crystal oscillator of the solder layer, and then the surface and the solder surface are connected by connecting holes. Strengthen anti-interference ability.

Use of thermal pads

When using bulk copper foil for power and ground wiring, design heat-resistant pads as much as possible. This is because if the component pads are directly connected to a large piece of copper foil, the heat will dissipate quickly during soldering, and the temperature of the melting solder is not enough, resulting in poor soldering or virtual soldering.

Thermal pad

The power supply of the analog circuit: the output part should be close to the power supply, and the high-sensitivity input part should be separated from the output part by a certain distance in order not to be affected by the output part. DC power supply: When supplying power from the outside, it must first pass through the electrolytic capacitor and then supply it to the internal circuit. The wiring method is generally as follows. The two-layer panel, not through point A, but through point B, provides power to the internal circuits. The multi-layer PCB board also imports power to the inner layer after passing through point B.