Selection Principles and Design Skills of Devices for Printed Circuit Boards

Before the advent of printed circuit boards, the interconnection between electronic components relied on the direct connection of wires to form a complete circuit. In modern times, circuit panels exist only as effective experimental tools, and printed circuit boards have become an absolutely dominant position in the electronics industry. The following is to share selection principles and design skills of devices for printed circuit boards.

The size of the printed circuit board should be moderate. When it is too large, not only the anti-noise ability will be reduced, but the cost will also be high; when it is too small, the heat dissipation will not be good, and it will be easily interfered with by adjacent lines. As with other logic circuits in terms of device layout, the devices related to each other should be placed as close as possible to obtain a better anti-noise effect.

Clock generators, crystal oscillators, and CPU clock input terminals are all prone to noise, so they should be closer to each other. Noise-prone devices, low-current circuits, and high-current circuits should be kept away from logic circuits as much as possible.

In the DC power supply loop, the change of the load will cause the power supply noise. For example, in digital circuits, when the circuit changes from one state to another, a large spike current will be generated on the power line, forming a transient noise voltage. The configuration of decoupling capacitors can suppress noise caused by load changes, which is a common practice in the reliability design of printed circuit boards.

(1) A 10-100uF electrolytic capacitor is connected across the power input terminal. If the position of the printed circuit board allows, the anti-interference effect of using an electrolytic capacitor above 100uF will be good.

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

(3) For devices with weak noise capability and large current changes during turn-off, and storage devices such as ROM, RAM, etc., a decoupling capacitor should be directly connected between the power line and the ground line (GND) of the chip.

(4) The leader of the decoupling capacitor should not be too long, especially the high-frequency bypass capacitor should not have lead.

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

(1) For equipment that adopts free convection air cooling, it is best to arrange integrated circuits (or other devices) vertically; for equipment that uses forced air cooling, it is best to arrange integrated circuits (or other devices) horizontally.

(2) The devices on the same printed board should be arranged as far as possible according to their calorific value and degree of heat dissipation. Devices with low calorific value or poor heat resistance (such as small-signal transistors, small-scale integrated circuits, electrolytic capacitors, etc.) should be placed.

The uppermost flow (inlet) of the cooling airflow and the devices with large heat generation or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed at the most downstream of the cooling airflow.

(3) In the horizontal direction, high-power devices are arranged as close as possible to the edge of the printed board to shorten the heat transfer path; in the vertical direction, high-power devices are arranged as close as possible to the top of the printed board to reduce the temperature of other devices when these devices are working.

(4) The temperature-sensitive device is best placed in the lowest temperature area (such as the bottom of the device). Never place it directly above the heating device. It is best to stagger multiple devices on the horizontal plane.

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

Electromagnetic compatibility refers to the ability of electronic equipment to work in a coordinated and effective manner in various electromagnetic environments. The purpose of electromagnetic compatibility design is to enable electronic equipment to suppress all kinds of external interference so that the electronic equipment can work normally in a specific electromagnetic environment, and at the same time to reduce the electromagnetic interference of the electronic equipment itself to other electronic equipment.

Since the impact interference generated by the transient current on the printed lines is mainly caused by the inductance of the printed wires, the inductance of the printed wires should be reduced as much as possible. The inductance of the printed wire is proportional to its length and inversely proportional to its width, so short and precise wires are beneficial to suppress interference.

The signal wires of clock leads, row drivers, or bus drivers often carry large transient currents, and the printed wires should be as short as possible. For discrete component circuits, when the width of the printed wire is about 1.5mm, it can fully meet the requirements; for integrated circuits, the width of the printed wire can be selected between 0.2mm and 1.0mm.

The use of equal routing 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 grid-shaped wiring structure. The specific method is to wire one side of the printed board horizontally, and the other side of the printed board. Connect with metalized holes at the cross holes. In order to suppress the crosstalk between the wires of the printed board, long-distance equal wiring should be avoided when designing the wiring.

In electronic equipment, grounding is an important method to control interference. If the grounding and shielding can be properly combined and used, most of the interference problems can be solved. The ground structure of electronic equipment generally includes system ground, chassis ground (shielding ground), digital ground (logical ground), and analog ground. The following points should be paid attention to in the ground wire design:

In the low-frequency circuit, the working frequency of the signal is less than 1MHz, its wiring and the inductance between the devices have little influence, and the circulating current formed by the grounding circuit has a greater influence on the interference, so one point grounding should be adopted. When the signal working frequency is greater than 10MHz, the ground wire impedance becomes very large.

At this time, the ground wire impedance should be reduced as much as possible, and the nearest multiple points should be used for grounding. When the working frequency is 1~10MHz, if one-point grounding is used, the length of the ground wire should not exceed 1/20 of the wavelength, otherwise, the multi-point grounding method should be used.

There are both high-speed logic circuits and linear circuits on the circuit board. They should be separated as much as possible, and the ground wires of the two should not be mixed, and they should be connected to the ground wires of the power supply terminal. Try to increase the grounding area of the linear circuit.

If the ground wire is very thin, the ground potential will change with the change of the current, causing the timing signal level of the electronic device to be unstable and the anti-noise performance to deteriorate. Therefore, the ground wire should be as thick as possible so that it can pass the allowable current on the printed circuit board. If possible, the width of the ground wire should be greater than 3mm.

When designing the grounding system of a printed circuit board composed of only digital circuits, making the grounding wire a closed loop can significantly improve the anti-noise ability. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are components that consume a lot of power, due to the limitation of the thickness of the ground wire, a large potential difference will be generated on the ground junction, causing a decrease in noise resistance.

If the grounding structure is formed into a loop, the potential difference will be reduced and the anti-noise capability of electronic equipment will be improved.

PCB Quick, as an expert who has studied printed circuit boards for many years, can give you a guideline to a certain extent. If you want to know more about printed circuit boards after reading the above, you can get a comprehensive solution by contacting us.

PCB Quick is an experienced PCB product supplier. We have a professional production team and a comprehensive management system. We have carried out comprehensive quality control whether from PCB design, packaging or export. Our products are also sold all over the world because of their excellent performance. If you are interested in our printed circuit boards, please contact us immediately!