At present, electronic equipment is used in various electronic products and systems, and printed circuit boards are still the basic installation method. Practice has proved that even if the circuit design drawing is appropriate and the printed circuit board is improperly designed, it will have a negative impact on the reliability of electronic products. For example, if the two thin lines of the printed board are very close, the delay of the signal waveform will be generated and the reflection noise will be generated at the end of the transmission line. Therefore, when planning the printed circuit board, pay attention to the correct EMC test method.
A. EMC design
EMC refers to the ability of electronic products to work harmoniously and efficiently in various electromagnetic environments. The purpose of EMC design is to enable electronic products to suppress various external influences, enable electronic products to operate normally in a specific electromagnetic environment, and reduce the electromagnetic interference of electronic products to other electronic products.
1. Select a reasonable total wire width, because the impact of instantaneous current on the printed wire frame is generally caused by the inductance of the printed wire, so the inductance of the printed wire should be minimized. The inductance of the printed wire is positively related to its length and inversely proportional to its total width, so the short and fine wire is beneficial to suppress the influence. The power line of clock line, line controller or bus driver often carries large instantaneous current, and the printing line should be as short as possible. For separated component circuits, when the total width of the printing line is about 1.5mm, the requirements can be fully met; For integrated circuits, the total width of printed wire can be selected from 0.2 to 1.0 mm.
2. Selecting the correct wiring strategy and selecting the fair wiring can reduce the inductance of the wire, but increase the mutual inductance and distributed capacitance between the wires. If the layout allows, it is better to select the well mesh cabling structure. The specific method is that one side of the printed board is horizontally wired, the other side is vertically wired, and then the metallized hole is connected at the cross hole. In order to prevent the crosstalk between the wires of the printed circuit board, we should try to avoid the long-distance fair wiring when planning the wiring.
B. In electronic products, grounding is the main way to control the impact.
If the grounding and shielding can be properly combined, most of the impact problems can be solved. The ground wire structure in electronic products may be system, enclosure (shield), digital (logic) and analog. The following points should be noted in the design of ground wire:
1. Correctly select single-point grounding and multi-point grounding in low-frequency circuits. The signal output power is less than 1MHz. The inductive hazard between lines and equipment is small. The circulating current formed by the grounding circuit has a great impact on the grounding circuit. Therefore, one-point grounding circuit should be used. When the signal output power exceeds 10 MHz and the ground wire impedance becomes very large, the ground wire impedance should be reduced as much as possible and the nearest multipoint grounding should be used. When the output power is 1~10, if the MHz is grounded at one point, the length of its ground wire shall not exceed 1/20 of the wavelength, otherwise the multi-point grounding method shall be adopted.
2. The digital circuit and analog circuit separation circuit board has both fast logic circuit and linear circuit, which should be separated as far as possible. The ground wire of the two does not need to be connected with the ground wire of the power supply terminal. Try to increase the grounding area of linear circuit.
3. Try to thicken the grounding wire. If the ground wire is very thin, the ground potential will change with the change of current, resulting in the instability of the signal level of electronic products on time and the deterioration of anti-noise characteristics. Therefore, the grounding wire should be thickened as much as possible to pass the three allowable currents on the printed circuit board. If possible, the width of the grounding wire should exceed 3mm.
4. When the grounding wire constitutes a closed loop and the grounding wire system of the printed circuit board composed only of digital circuits is designed, the anti-noise level can be significantly improved by making the grounding wire into a closed loop. The reason is that there are many integrated circuit components on the printed circuit board, especially when the components consume a lot of power, due to the limitation of the size of the grounding wire, a large potential difference will be caused on the grounding junction, resulting in a decline in anti-noise ability. If a loop is formed by grounding, the potential difference will be reduced and the anti-noise level of electronic products will be improved.
C. PCB size and equipment layout
The size of the printed circuit board is moderate, the printed wire frame is too large, and the impedance is increased, which not only reduces the anti-noise ability, but also has high cost; Too small, poor heat removal, and will be affected by the adjacent wire frame. At the equipment layout level, like other logic circuits, relevant equipment should be as close as possible to achieve better anti-noise effect. The clock generator, crystal vibration and CPU clock key entrance are prone to generate noise and need to be closer to each other. For equipment prone to noise, small current circuit, large current circuit, etc., Logic circuits should be avoided as much as possible. If possible, it is very important to have a circuit board.
D. Equipped with decoupling capacitor
In DC power supply circuit, the change of load will cause power supply noise. For example, in a digital circuit, when the circuit changes from one state to another, a large peak current will be generated on the power line, forming a transient noise voltage. Equipped with decoupling capacitor can suppress the noise caused by load change, which is the basic method of PCB reliability design. The configuration standard is as follows: ● The power input terminal is connected to 10~100uF. If the position of the printed circuit board allows, select 100 electrolytic capacitors. The above electrolytic capacitors have good anti-interference effect. ● Each integrated circuit chip is equipped with. 01uF ceramic capacitor. If the printed circuit board is too small to fit in, it can be equipped with 1~10uF tantalum electrolytic capacitor for every 4~10 chips. The high frequency impedance of this kind of equipment is particularly small, and the impedance is less than 1 Ω in the range of 500kHz~20MHz, and the leakage current is small (below 0.5uA). ● For devices with low noise capacity and large current change during shutdown, as well as device ROM, RAM and other storage devices, decoupling capacitors need to be connected immediately on the chip power line (Vcc) and ground wire (GND). ● The conductor of decoupling capacitor shall not be too long, especially the high-frequency bypass capacitor shall not have a conductor.
E. Heat dissipation design
From the point of view of facilitating heat removal, the printed board should be assembled standing, and the distance between the boards should not be less than 2cm, and the arrangement of equipment in the printed board should follow certain rules: · For machines with random convection evaporative cooling, it is better to sort the integrated circuits (or other equipment) by lengthwise method; For machines with forced evaporative cooling, it is better to arrange integrated circuits (or other equipment) according to the transverse length method· The equipment in the same printed board shall be sorted according to its calorific value size and heat removal level. Equipment with low heat generation or poor temperature resistance (such as small signal transistor, small integrated circuit, electrolytic capacitor, etc.) should be placed at the most famous person (entrance) of the refrigeration cyclone. Equipment with high heat value or good temperature resistance (such as power transistor, large integrated circuit, etc.) should be placed at the downstream of the refrigeration cyclone· In the horizontal direction, high-power equipment should be arranged as close as possible to the edge of the printed board to reduce the heat transfer mode; In the vertical direction, high-power equipment should be as close to the printed board as possible to reduce the impact of the equipment on the temperature of other equipment. The equipment that is particularly sensitive to temperature should be placed in the area with the lowest temperature (such as the bottom of the equipment), not above the hot equipment. Multiple equipment should overlap on the horizontal surface as much as possible· The heat removal of the printed circuit board in the equipment depends on the air flow, so the air flow mode should be studied and the equipment or printed circuit board should be reasonably configured. When air flows, it always tends to flow in areas with low resistance. Therefore, when equipping equipment on the printed circuit board, it is necessary to avoid leaving a large route in a certain area.