At present various types of electronic equipment used in electronic devices and systems are still in the main printed circuit board assembly method. Practice has proved that, even if the circuit schematic design proper, improper printed circuit board design, can also adversely affect the reliability of electronic equipment. For example, if the PCB two thin parallel lines are very close, it will delay the waveform signal is formed, is formed at the terminal reflection noise transmission line. Therefore, in designing the printed circuit board when the correct approach attention.
A, ground design in an electronic device, the ground is an important method of controlling interference. The proper grounding and shielding can be used in combination, solve most of the interference. An electronic device ground structure substantially systematic and chassis ground (shield ground), digitally (logically) and analog ground and the like. In the design of the ground should note the following: 1. Select the correct single-point ground and the multi-point grounding in the low frequency circuit, the signal frequency is less than 1MHz, low inductance between the impact device and its wiring, and a ground circuit is formed circulation greater impact on interference, and thus should be a little ground. When the signal is greater than the operating frequency of 10MHz, ground impedance becomes large, the impedance of the ground at this time should be minimized, should the nearest multi-point grounding. When the operating frequency at 1 ~ 10MHz, if the ground point, its length should not exceed 1/20 of the wavelength of the ground, or be multi-point ground method. 2. The digital and analog circuits on a circuit board separate from both the high-speed logic circuit, there are linear circuits, such that they are separated as far as possible, both the ground and not to be confused, are connected to ground power terminal. To try to increase the ground contact area of linear circuits. 3. The timing of the signal level, if possible bold line ground line ground very fine, the ground potential varies with current, resulting in instability of the electronic device, the anti-noise performance deteriorates. It should therefore be bold ground line, so that it is located by three printed circuit boards allow current. When, if possible, should be greater than the width of the grounding wire 3mm. 4. The printed circuit board constituting a ground line ground line system of a closed loop design is only composed of digital circuits, the closed loop grounding wire can be made significantly improved noise immunity ability. The reason is that: there are many printed circuit board integrated circuit components, especially in case of power when the multi-component, due to limited ground line thickness, will have a greater potential difference on the junction, causing decreased noise immunity , when the ground structure in a loop, the potential difference will be reduced to improve the noise immunity of the electronic device.
B, EMC design [EMC] refers to the electronic device is still able to coordinate the various electromagnetic environments, the ability to work effectively. EMC design object is to make the electronic device can inhibit a variety of outside interference, electronic devices to work properly in a particular electromagnetic environment, while reducing electromagnetic interference to the electronic device itself to other electronic devices. 1. Select reasonable since transient current conductor width on the printed lines produced by the impact of interference is mainly caused by the inductance component of the conductor, and therefore should minimize the inductance of the conductor. Proportional to the length of the inductance of the conductor, inversely proportional to its width, which is short and precise wire is advantageous for suppressing interference. Clock wiring, or the row driver signal line bus driver often contain a large transient current conductor as short as possible. For the discrete components of the circuit, when the conductor width of about 1.5mm, can fully meet the requirements; for an integrated circuit, a printed conductor width may be selected between 0.2 ~ 1.0mm. 2. The routing strategy employs the correct alignment of equal line inductance can be reduced, but the mutual inductance and distributed capacitance between the wires, the layout allows, preferably using well-shaped mesh wiring structure, which would be the lateral side of the printed board wiring, the other side of the longitudinal wires, connected by a metallized hole and the cross hole. In order to suppress crosstalk between the conductors on printed boards, wiring design should be equal to avoid traces long distance.
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C, decoupling capacitor configurationIn the DC power supply circuit, the change in the load will cause the power supply noise. For example, in a digital circuit, when the circuit switches from one state to another state, it will produce a large current spike in the power supply line, forming a noise voltage transients. Configuring decoupling capacitors to suppress noise due to the load variation can be generated, one conventional approach is the reliability of the printed circuit board design, the configuration principle is as follows: power input connected across the electrolytic capacitor of a 10 ~ 100uF, if the printed circuit position of the plate allow, electrolytic capacitor 100uF or more anti-interference effect will be better. Configuring a 0.01uF ceramic capacitors for each integrated circuit chip. When the printed circuit board in case of a small space fit, can be 4 to 10 per a chip configuration 1 ~ 10uF tantalum electrolytic capacitors, the high frequency impedance of such a device is extremely small, in the range of 500kHz ~ 20MHz impedance is less than 1 [Omega], and low leakage current (less 0.5uA). For the weak noise, large off current change device and ROM, RAM and other storage-type devices, the chip power line should be (Vcc) and direct access between the ground (GND) decoupling capacitor. Decoupling capacitor leads can not be too long, especially not with high frequency bypass capacitor leads.
D, print size and arrangement of the components of the circuit board printed wiring board to medium size, print line length, to increase the impedance is too large, not only the noise immunity decreased, the cost is high; too small, the heat is not good, while near the line susceptible to interference. As with the other logic circuits in the arrangement of the device, the device should be related to each other as close as possible to put something, which can obtain better anti-noise effect. A clock input a clock generator, the crystal and the CPU are easy to generate noise to be close to each other some. The device is easy to produce noise, low current circuit, a large current circuit should be as far away from the logic circuit, if possible, should make another circuit board, it is important.
E, thermal design from the viewpoint of heat dissipation in favor of, preferably printed version installed upright, the distance between the board and the board is generally not less than 2cm, and the arrangement of components on the printed version should follow certain rules: • for devices using free convection air cooling, preferably to an integrated circuit (or other device) are arranged in longitudinal manner; forced air cooling apparatus for use, and is preferably an integrated circuit (or other device) by the horizontally long manner row. · Partitioning arrangement should be as their size and heat dissipation the same extent on a printed circuit board device, heat is small or of poor heat resistance device (e.g., small signal transistors and small scale integrated circuits, electrolytic capacitors, etc.) on the cooling most gas stream upstream (inlet), heat, or heat-resistant device (such as a power transistor, LSI, etc.) on the most downstream cooling air flow. · In the horizontal direction, the edge of the PCB power device disposed as close as possible in order to reduce the heat transfer path; when in the vertical direction, as close to the power device is arranged above the printed board, these devices operate in order to reduce the temperature of the other devices influences. · Temperature sensitive device is preferably arranged in the lowest temperature range (e.g., the bottom of the device), it must not be placed directly above the heating device, preferably a plurality of devices are staggered layout in the horizontal plane. PCB cooling device mainly depends on the air-flow, so the design to study the air flow path, a reasonable configuration device or the printed circuit board. Always tend to flow where resistance is small when the air flow, so that when the printed circuit board device configuration, to avoid leaving an area larger airspace.