Analysis of the key points of PCB design

In today’s society, a large number of electronic products are widely used in our daily work and life, so their reliability needs to be guaranteed, and most electronic systems and equipment that use printed circuit boards must have reasonable design schematics and correct printed circuit boards to fundamentally improve their reliability. For example: assuming that the distance between two printed thin parallel lines is very close, it will cause signal waveform delay, and eventually form a large amount of reflected noise in the terminal equipment.

1.Key points of ground wire design

Most interference problems in electrical equipment can be solved by correct shielding and reasonable grounding, so we must pay enough attention to grounding design. The grounding system consists of four parts: analog ground, digital ground, chassis ground and system ground. Among them, digital ground is also called logic ground, and chassis ground is also called shielding ground. Here we introduce several aspects that need to be paid attention to in grounding design:

2.Reasonable selection of grounding method

There are usually two grounding methods: multi-point grounding and single-point grounding, so we must make a reasonable choice. When the operating frequency of the equipment exceeds 10MHz, the excessive ground resistance will have an adverse effect on the normal operation of the equipment, so we should try to choose multi-point grounding to achieve the purpose of reducing ground impedance. Similarly, when the operating frequency of the circuit does not reach 1MHz, we have to adopt a point grounding method to avoid the influence of the formed loop current on interference. Therefore, for circuits with an operating frequency of 1 to 10MHz, multi-point grounding can be used when the wavelength is within 20 times the length of its ground wire, otherwise a single-point grounding method is required.

3.Separate analog circuits from digital circuits

Since the circuit board is very complex, there are both linear circuits and high-speed logic circuits on it, so we should separate them to avoid confusion between the two, and avoid mixed connection by grounding them to the power supply end separately. At the same time, the grounding area of ​​the linear circuit should be expanded as much as possible.

4.Choose a thicker grounding wire

When a thinner grounding wire is selected, the change in current will drive the change in grounding potential, and finally the electronic equipment cannot operate stably, greatly reducing its anti-noise performance. Therefore, we should choose a thicker grounding wire to achieve the purpose of stabilizing the device signal by increasing its allowable current. If conditions permit, choose a grounding wire with a width of more than 3mm.

5.Key points of electromagnetic compatibility design

Due to the complex and changeable working environment of electronic equipment, we require it to have better adaptability to the electromagnetic environment and reduce electromagnetic interference to other electronic equipment. This requires corresponding design of electromagnetic compatibility, so the electromagnetic compatibility design of electronic equipment is also one of the focuses of our work.

6.Choose the correct wiring method

The inductance of the wire can be greatly reduced by adopting the method of parallel routing, but it will cause the distributed capacitance and mutual inductance between the wires to continue to increase. Therefore, if conditions permit, we can use a tic-tac-toe structure when wiring. The specific wiring method is to adopt different wiring methods on the two sides of the printed circuit board, one side is longitudinal and the other side is horizontal, and metalized holes are used to connect at the cross holes. Because there is crosstalk between the wires of the printed circuit board, we should control the situation of long-distance parallel routing when it is not obvious.

7.Choose the wire of the correct width.

Because of the frequent impact interference, we must control the transient current when printing the wire. The main method is to control the generation of inductance when printing the wire. The amount of inductance is inversely proportional to the width of the wire and directly proportional to the inverse length, so we should try to choose some thick and short wires, which is very effective in suppressing interference. Since the signals of bus drivers, row drivers and clock leads often have very large incident currents, short wires should be selected when selecting the above wires. For those integrated circuits, we should control the width of the wire between 1 and 0.2 mm, and for discrete component circuits, the width should be controlled at about 1.5 mm.

8.Design points of devices and sizes on circuit boards

The size of the printed circuit board should be moderate. If it is too large, the printed lines will be long and the impedance will increase, which will not only reduce the anti-noise ability but also increase the cost; if it is too small, the heat dissipation will be poor and it will be easily interfered by adjacent lines. In terms of device layout, like other logic circuits, related devices should be placed as close as possible to obtain better anti-noise effect. The clock input terminals of the clock generator, crystal oscillator and CPU are all prone to noise and should be closer to each other. Devices that are prone to noise, low current circuits, high current circuits, etc. should be kept as far away from logic circuits as possible. If possible, a separate circuit board should be made. This is very important.

9.Key points of heat dissipation design

From the perspective of heat dissipation, the printed board is best installed upright. The distance between the boards should generally not be less than 2cm, and the arrangement of devices on the printed board should follow certain rules:
For equipment using free convection air cooling, it is best to arrange the integrated circuits (or other devices) in a longitudinal manner; for equipment using forced air cooling, it is best to arrange the integrated circuits (or other devices) in a horizontal manner.

The devices on the same printed board should be arranged as far as possible according to their heat generation and heat dissipation degree.

Devices with low heat generation or poor heat resistance (such as small signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed at the uppermost stream (entrance) of the cooling airflow, and devices with high heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) should be placed at the lowest downstream of the cooling airflow. In the horizontal direction, high-power devices should be arranged as close to the edge of the printed circuit board as possible to shorten the heat transfer path; in the vertical direction, high-power devices should be arranged as close to the top of the printed circuit board as possible to reduce the impact of these devices on the temperature of other devices when they are working.

Temperature-sensitive devices are best placed in the lowest temperature area (such as the bottom of the device), and never place it directly above the heating device. Multiple devices are best arranged in a staggered manner on the horizontal plane.

The heat dissipation of the printed circuit board in the equipment mainly depends on air flow, so when designing, the air flow path should be studied and the devices or printed circuit boards should be reasonably configured. When air flows, it always tends to flow to places with low resistance, so when configuring devices on the printed circuit board, avoid leaving a large airspace in a certain area.