“PCB is the provider of electrical connection of electronic components, and the layout of components is very important in PCB design, which will affect the final performance of products. If the layout and arrangement of components are unreasonable, the electrical and mechanical properties of the products will decrease, and it will also bring inconvenience to assembly and maintenance. So how to lay out PCB components reasonably? ”
Precautions of PCBA control board component layout
1. In the PCB design layout of components, related components should be placed as close as possible. For example, clock generator, crystal oscillator and CPU clock input terminal are prone to noise. When placed, they should be placed close to each other. For those devices that are prone to noise, low-current circuits, high-current circuit switching circuits, etc. should be kept away from the logic control circuits and memory circuits (ROM, RAM) of the single-chip microcomputer as far as possible. If possible, these circuits can be made into circuit boards, which are conducive to anti-interference and improve the reliability of circuit work.
2. Try to install decoupling capacitors next to key components such as ROM and RAM chips. In fact, PCB wiring, pin wiring and wiring may contain large inductive effects. Large inductance may cause serious switching noise spikes in Vcc wiring. The only way to prevent switching noise spikes in Vcc wiring is to place a 0.1uF electronic decoupling capacitor between Vcc and the source. If a surface mount component is used on the circuit board, the patch capacitor can be placed directly against the component and connected to the Vcc pin. It is best to use ceramic tile capacitors because of their low ESL electrostatic loss, high frequency impedance and good dielectric stability in temperature and time range. Tantalum capacitors should not be used as much as possible because of their high impedance at high frequency. The following points should be noted when placing decoupling capacitors in PCB design of PCBA control board:
1. The power input terminal of the printed circuit board is connected to an electrolytic capacitor of about 100uF. If the volume allows, larger capacity will be better.
2. In principle, a 0.01uF ceramic capacitor should be placed next to each IC chip. If the gap between printed circuit boards is too small to be placed, 1-10 tantalum capacitors can be placed for every 10 chips.
3. A decoupling capacitor should be connected between the power line (Vcc) and the grounding line to deal with the components with weak anti-interference ability and large current change during shutdown, as well as storage elements such as RAM and ROM.
4. The capacitor lead should not be too long, especially for high-frequency bypass capacitors.
In the single-chip microcomputer control system, there are many kinds of ground wires, such as systematic, shielded, logical, simulated, etc. Whether the PCB design layout of the ground wires is reasonable or not will determine the anti-interference ability of the circuit board. The following considerations should be taken into account when designing grounding wires and connections for PCB of PCBA control board:
1. Logic and analog grounding should be wired separately and cannot be used together. Connect their respective grounding wires to the corresponding power grounding wires. In the design, the analog grounding wire should be as thick as possible, and the grounding area of the front end should be increased as much as possible. Generally speaking, for the input and output analog signals, MCU circuits are best separated by optocouplers.
2. When designing the printed circuit board of logic circuit, the grounding wire should form a closed loop to improve the anti-interference ability of the circuit.
3. The grounding wire should be as thick as possible. If the grounding wire is very thin, the resistance of the grounding wire will be very large, which will lead to the change of the grounding potential with the change of current, thus leading to the instability of the signal level, thus reducing the anti-interference ability of the circuit. If the wiring space allows, make sure that the width of the main grounding wire is at least 2〜3mm, and the grounding wire on the component pins should be about 1.5mm
4. Pay attention to the choice of venue. When the signal frequency on the circuit board is lower than 1MHz, the electromagnetic induction between the wiring and the components has little influence, while the circulating current formed by the grounding circuit has more influence on the interference, so the grounding point should be adopted. Avoid forming a cycle. When the signal frequency on the circuit board is higher than 10MHz, the impedance of the grounding line becomes very large due to the obvious inductance effect of the wiring, so the circulating current formed by the grounding circuit is no longer the main problem. Therefore, multi-point grounding should be adopted to minimize the ground impedance.
5. In addition to thickening the line width as much as possible according to the current, the power cord of 5.PCB design layout should also be wired in the same direction as the data line of square wiring. When the wiring is finished, the grounding line will not cover the bottom of the circuit board. These methods can help to enhance the anti-interference ability of the circuit.
6. The width of the data line should be as wide as possible to reduce the impedance.
With regard to the layout of components on PCB, it can be simply summarized as five items:
First, beautiful
The layout of PCB components should be as beautiful and attractive as possible. In the layout design, we should consider not only the orderly placement of components, but also the graceful and smooth routing. Usually, many users evaluate the advantages and disadvantages of circuit design unilaterally by whether the layout of components is neat or not. For the sake of product image, the former should be given priority when the performance requirements are not strict. However, in high-performance occasions, the requirements for the circuit board are high, and the components are also encapsulated in it, so they can’t be seen at ordinary times, so the aesthetic appearance of the wiring should be given priority. Second, the signal interference
PCB signal interference is an important factor to be considered in PCB design and layout, and the following aspects should be paid attention to:
1. The weak signal circuit is separated or even isolated from the strong signal circuit;
2. The AC part is separated from the DC part;
3. The high frequency part is separated from the low frequency part;
4. Pay attention to the direction of signal lines and the layout of ground wires;
5. Appropriate shielding and filtering measures. Third, stress
PCB should be able to withstand various external forces and vibrations during installation and work, so it is necessary to arrange the positions of various holes (screw holes, special holes) on the PCB reasonably. Generally, the distance between the hole and the edge of the plate should be at least larger than the diameter of the hole. At the same time, it should be noted that the weakest section of the plate caused by the special-shaped hole should also have sufficient bending strength. In particular, the connectors that directly “extend” out of the equipment shell on the board should be properly fixed to ensure the reliability of long-term use. Fourth, heated
In the layout of PCB components, attention should be paid to the placement of high-power and severe heating devices, and sufficient heat dissipation conditions should be ensured. Especially in the precision analog system, special attention should be paid to the adverse effects of the temperature field generated by these devices on the fragile preamplifier circuit. Generally, the part with very high power should be made into a single module, and certain thermal isolation measures should be taken between it and the signal processing circuit. V. Installation
PCB installation refers to a series of basic requirements put forward in specific application occasions, in order to smoothly install the circuit board into the chassis, shell, and slot, prevent accidents such as space interference and short circuit, and make the specified connector in the specified position on the chassis or shell, and have a fixed position for the layout of components.
