PCB printed circuit board design is one of the five major technical design skills introduced

the size of the PCB and the arrangement of devices

The printed circuit board size should be moderate. When the printed lines are long, the impedance increases, and not only does noise immunity decrease but the cost is also high; too small, it is not good for heat dissipation and is vulnerable to interference by the adjacent lines. In the device arrangement with other logic circuits, they should be placed as close as possible to the devices related to each other so that you can get a better anti-noise effect. The clock generator, crystal, and CPU clock input are prone to noise, so be closer to each other. If possible, it should be done on a separate circuit, should be done on a separate circuit board, which is very important.

The decoupling capacitor configuration

Changes in the load can cause power supply noise in the DC power circuit. For example, in digital circuits, when the circuit is converted from one state to another, it will generate a large spike current in the power line, forming a transient noise voltage. Configuring decoupling capacitors can suppress noise due to load changes and is a regular practice in the reliability design of printed circuit boards.

Configuration principles are as follows:

The power supply input across a 10 to 100uF electrolytic capacitor. If the location of the printed circuit board allows, the anti-interference effect of using more than a 100uF electrolytic capacitor will be good.

Configure a 0.01uF ceramic capacitor for each IC chip. If you encounter the printed circuit board space is small and cannot fit, every 4 to 10 chips can be configured with a 1/10uF tantalum electrolytic capacitor. The high-frequency impedance of this device is particularly small; in the range of 500kHz to 20MHz, impedance is less than 1Ω, and the leakage current is very small (0.5uA or less).

For devices with weak noise capability, large current changes during the shutdown, and storage-type devices such as ROM and RAM, decoupling capacitors should be directly connected between the power line (Vcc) and ground line (GND) of the chip.

The lead of the decoupling capacitor should not be too long. Especially the high-frequency bypass capacitor should not be lead.

Heat dissipation design

From the perspective of facilitating heat dissipation, the printing plate is best installed upright, the distance between the plate and the board should generally not be less than 2 cm, and the arrangement of devices on the printing plate should follow certain rules:

① For the use of free convection air-cooled equipment, it is best to have integrated circuits (or other devices) arranged longitudinally; for the use of forced air-cooled equipment, it is best to be integrated circuits (or other devices) arranged in a long horizontal manner.

② The same printed board devices should be arranged as far as possible according to their heat generation and heat dissipation degree: heat generation of small or heat-resistant devices (such as small-signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) placed in the most upstream cooling airflow (entrance), heat generation or heat-resistant devices (such as power transistors, large-scale integrated circuits, etc.) placed in the most downstream cooling airflow.

③In the horizontal direction, high-power devices are as close as possible to the edge of the printing board layout, to shorten the heat transfer path; in the vertical direction, high-power devices are as close as possible to the top of the printing board layout, to reduce the impact of these devices on the temperature of other devices.

The more sensitive device is best placed in the lowest temperature area (such as the bottom of the device). Do not put it directly above the heat-generating devices. Multiple devices are best staggered in layout on the horizontal plane.

④The device within the printed circuit board heat dissipation mainly relies on airflow, so in the design to study the airflow path, reasonable configuration of the device or printed circuit board. Airflow always tends to flow to a place of less resistance, so when configuring devices on the printed circuit board, avoid leaving a large space in a certain area.

Electromagnetic compatibility design

Electromagnetic compatibility refers to the ability of electronic equipment in a variety of electromagnetic environments to still be coordinated and effective. The purpose of electromagnetic compatibility design is to make electronic equipment can suppress all kinds of external interference so that electronic equipment in a specific electromagnetic environment can work normally, but also reduce the electromagnetic interference of electronic equipment itself to other electronic equipment.

Choose the reasonable width of the wire.

As the transient current in the printed lines generated by the impact interference is mainly caused by the inductive components of the printed wire, so the inductance of the printed wire should be reduced as much as possible. The inductance of the printed wire is proportional to its length and inversely proportional to its width, thus a short and precise wire is beneficial for suppressing interference. Clock leads, line drivers, or bus driver signal lines often contain large transient currents, the printed wire should be as short as possible. For discrete component circuits, the printed wire width of about 1.5mm can fully meet the requirements; for integrated circuits, the printed wire width can be selected between 0.2 and 1.0mm.

using the correct wiring strategy

The use of equal alignment can reduce the wire inductance, but the mutual inductance and distributed capacitance between the wires increase, if the layout allows, it is best to use a tic-tac-toe mesh wiring structure, the specific practice is to print the board on one side of the horizontal wiring, the other side of the longitudinal wiring, and then connected at the cross-hole with a metalized hole. To suppress the crosstalk between the printed board wires, the long-distance of equal alignment should be avoided as much as possible when designing the wiring.

Grounding design

In electronic equipment, grounding is an important method of controlling interference. If grounding and shielding can be used in conjunction with the correct, most interference problems can be solved. Electronic equipment in the ground structure is roughly system ground, chassis ground (shield ground), digital ground (logic ground), analog ground, etc. The following points should be noted in the ground design.

① The correct choice of single-point ground and multi-point ground

In the low-frequency circuit, the signal operating frequency is less than 1MHz, and its wiring and inductance between the devices have less impact, while the grounding circuit to form a loop current on the interference impact is greater, and thus should be used as a point of grounding. When the signal operating frequency is greater than 10MHz, the ground impedance becomes very large, at this time should try to reduce the ground impedance, which should be used near the multi-point grounding. When the operating frequency is 1 ~ 10MHz, if a point of grounding, the ground length should not exceed 1/20 of the wavelength, otherwise, a multi-point grounding method should be used.

② Separate the digital circuit from the analog circuit

Circuit boards both high-speed logic circuits, and linear circuits, should be separated as far as possible, and the ground of the two do not mix, respectively, with the power end of the ground connection. To try to increase the grounding area of the linear circuit.

③ Try to thicken the ground wire

If the grounding line is very thin, the ground potential changes with the current, resulting in the timing of electronic equipment signal level instability, and anti-noise performance becomes bad. Therefore, the grounding wire should be as thick as possible, so that it can be located in the printed circuit board through the three allowable currents. If possible, the width of the grounding wire should be greater than 3mm.

④ The ground wire to form a closed-loop

When designing the ground system of the printed circuit board consisting only of digital circuits, the ground wire will be made into a closed-loop can significantly improve the noise immunity. The reason is that: the printed circuit board has many integrated circuit components, especially in the case of more power-consuming components, due to the limitations of the thickness of the ground wire, will produce a large potential difference in the ground junction, causing a decrease in noise immunity, if the ground structure into a loop, it will reduce the potential difference value, improve the noise immunity of electronic equipment.