The PCB layout, routing principle [PCB design]
Update date：2012-7-31
A good performance monitoring system, in addition to the rational design of circuits and selection of high-quality components, the correct structural design of the PCB component layout and electrical connection direction is to determine the reliable operation of electronic equipment and measurement and control systems as a key problem. The same parameters of the circuit or a component, it will produce different results due to the different component layout and electrical connection, the difference can be significant. Therefore, aspects of the process of monitoring and control system structure and how to properly design PCB component layout and the right choice for wiring comprehensive consideration, not only can eliminate the noise generated due to improper wiring design, technology reasonable structure, but also facilitate the production installation, commissioning and maintenance.
PCB as monitoring and control systems, electronic devices in the circuit components of the text support components provide the electrical connection between the circuit components. PCB design, the adequacy of anti-interference ability a great impact, not to mention getting higher and higher density of the PCB as the rapid development of electronic technology, electromagnetic compatibility (EMC) concept design must follow the general principles of the design to comply with anti- interference design requirements, the layout and wire layout of the components is a key factor in the decision circuit can get the best performance. The quality of the PCB design and not a strict requirement to respond to each specific circuit, including the installation of the whole structure, panel layout, electrical performance requirements, take the appropriate design can also be integrated several design, multi- times repeatedly modified.
Layout principles
First, determine the number of layers of printed circuit board design. In general, the complexity of the circuit to complete to determine the number of layers of the PCB board, single panel, double-sided adhesive for low and medium density circuits and less integrated circuits, multilayer high-density wiring highly integrated chip digital circuits.
Secondly, the reasonable selection of the PCB feet small size. The size of the printed circuit board should be moderate, is too large will increase the printed lines of copper length, increase the impedance, increase costs and reduce the ability to resist noise. If the size is too small, the component mounting density is too large, the various devices as well as between adjacent lines interfere with each other, ventilation and heat dissipation effect is not good. The best shape of the circuit board is rectangular, the aspect ratio select 4:3 or 3:2. If the circuit board size is too large, you should consider its mechanical strength. PCB size is determined according to the specific requirements of the circuit functional unit, the components of the circuit die to rationalize the layout. Layout of components and other circuit is interconnected components as far as possible be placed close to, such as a variety of generator, crystal oscillator and the CPU clock input are easy to produce noise, these should be close to each other, away from the logic circuit . Near the outlet terminals of the logic circuit should be placed high-speed devices, a little distance to place low-speed circuits, in order to reduce the total impedance, crosstalk and other noise. Should increase the distance between the two, for some there may be a higher potential difference components or wires to prevent short circuit accident. The arrangement of the components should be considered away from heat generating components are key integrated circuits, the best parts of the ROM, RAM, clock generator, such as fever higher place better ventilation, and magnetic components to be shielded, sensitive devices to be far away from the CPU clock generator. Some heavy, heat a large quantity of components should not be installed on the PC circuit board, such as a stent support member fixed to the machine on the chassis backplane, and better ventilation. Adjustable components, such as potentiometer, adjustable inductance, capacitance, if adjusted the machine should be placed in easy-to-regulation, external regulation should be installed in the panel.
Block layout, circuit board components should be classified as a group about to complete certain functions of the circuit components, the same group together, in order to ensure that each group components will not interfere with each other in space. Generally the press to use the grouping of the supply voltage, and then digital and analog, high-speed and low-speed and current size, further grouping. Each group circuit with its core components. External components are uniform, neat and compact arrangement, to minimize the arrangement distance, to reduce the connection length.
Circuit unit group and the group should be arranged in accordance with the circuit processes the position of each functional unit circuit layout to facilitate signal flow and signal direction as consistent as possible. If the line connector, should determine the components on the PCB position. All of the connectors is best placed on the printed circuit board side, as far as possible to avoid leads from both sides of the cable, in order to reduce electromagnetic radiation elbow. High-speed devices should be as far away as possible from the connector; the I / O drive should be close to the connector, to avoid I / O signals on the PCB traces long distance, by the gospel of the interference signal. In addition, the deleted circuit board components distance lines near the edge of the edge of the government to maintain a certain length, usually not less than 2MM.
PCB printed circuit board components Bu distribution should be uniform and reasonable, and strive to neat, attractive, compact. There are some components, such as resistors, diodes, etc., can be horizontal, vertical placement. Small number of circuit elements, and board size allowed under the commonly used flat; if the components and printed circuit board size be - given limit, then enhance the effect of the antenna, easy to receive the radiated interference noise . The same token, to minimize the printing plate with IC sockets, but considering the commissioning, maintenance convenience, the use of IC sockets, sometimes there is a huge benefit, be sure to use pin orientation and integrated circuits, which is the beginning of scholars easy to overlook, should attract attention. It should be pointed out, the components layout should strive for is reasonable, try to avoid using the jumper, jumper will increase inductance and reduce anti-jamming performance.
Wiring principle
In the component layout of the basic considerations, you can route the design. The printed board wiring, electromagnetic compatibility analysis, in any time domain signal from the source to the load transmission line must form a complete loop. If not arrive at the destination load through the design loop, you must reach through the loop of an objective existence, the majority of the objective existence of the circuit is connected by the distribution of the stove components constitute some of this non-normal loop The device will be the impact of electromagnetic interference. This distributed coupler components are often ignored. According to the law of electromagnetic induction, magnetic flux change generates an electromotive force in a closed loop, any AC current will generate electromagnetic fields in space, which is the mechanism of PC printing plate interference noise. Easy to overlook is to delete the printed circuit board design process, components, wires, trace and connector parasitic inductance, capacitance and admittance, such as capacitance equivalent series circuit consisting of capacitors, inductors and resistors .
PCB printed circuit board layout design process, you should first determine the location of the components on the board, the first layout of the ground, and then arrange for high-speed signal lines, and finally consider the low-speed signal lines. Should normally follow the following principles:
(1) power lines, ground handling. Even if the wiring in the entire PCB are better, but the interference caused by power lines, ground mishandling also cause lower performance, and even affect the success rate of the product. Need to be taken seriously, so the power cord, ground handling, power cord, ground to produce interference to a minimum, to ensure that the concept of the quality of the printing plate.
(2) signal lines, clock lines. In the circuit, the signal line, clock line and the ground distance close to the formation of the loop area is relatively small, common-mode loop voltage is small, the interference noise is weak; the contrary, strong interference noise. Input, output wire parallel to each other should be avoided. If the two line current in the opposite direction should be parallel lines, the mutual offset the magnetic field generated by currents, thus eliminating interference. Clock lines, signal lines most likely to produce electromagnetic radiation interference, the alignment should be ground loop near; drive close to the connector. Cabling to the data bus, the drive should be close to their desire to drive the bus. High impedance traces easily absorb the noise signal, causing the circuit unstable, and thus their traces as short as possible; low impedance traces can be relatively long. In the same group of circuits, the ground should be as close as possible, and the level circuit with capacitance should also be in the vicinity of the ground point, especially some of the signal ground point can not be too far away, otherwise they will be caused by the copper foil is too long interfere with self-excited. Actually printed board conductor inductance and its width is inversely proportional to its length is proportional to, so short and thick wire interference suppression is beneficial. Clock leads, the signal line of the bus driver often contains a relatively large transient current, the wire is short and wide, two-panel design in a high-speed signal lines on both sides, if necessary, and isolated ground; multilayer high-speed The clock line should be based on the length of the clock to take the appropriate shielding measures. Impedance matching of the signal lines should also pay attention to the so-called impedance matching is the signal line load effect and the signal line characteristic impedance equal. The characteristic impedance of the signal line width, the distance of the ground layer and the lining of the dielectric constant and other physical factors related to the inherent characteristics of the signal line. If the impedance does not match, will cause the oscillation of the digital transfer of John signal waveform, resulting in confusion logic. Under normal circumstances the load signal line chip is basically stable. The main reasons that do not match the signal line characteristic impedance in the alignment process itself has changed. If the width of the alignment change, alignment right angle bend, and through vias, thus wiring should take some measures to make the signal line characteristic impedance unchanged in the full alignment. General digital signal lines should be avoided through the two - death of the through-hole, high-speed signals as far as possible arranged in the same layer in order to avoid the through hole; signal line copper foil should be avoided right angle bend; signal line copper foil is not from the printed circuit board too close to the edge, to leave the distance of more than three coffee, or the characteristic impedance will change, but also prone to the edge of the field, external radiation; clock signal line load is not a tree structure alignment, and should be used spider net-shaped structure alignment, all of the clock load is directly connected with the clock power.
(3) to avoid the discontinuity of the conductor. Deleted pull wiring design to minimize the discontinuity of the printed conductors, the conductors do not suddenly widened * Requires a gradual process; corner should be greater than 90 degrees, to prohibit annular alignment to avoid the branch or tangles to inhibition of high-frequency signals generated by reflection interference and harmonic interference. In the printed board is not allowed to have any electrical connection and vacant metal, the Ministry of empty Wen-pin, heat sink, metal shield, bracket and the board does not use the metal surface of integrated circuits should be connected to the ground line .
(4) high-frequency devices. High-frequency circuit device pin lead is the shorter the better, the lead layer between the pin cross Hong smaller the better, that is used in the hole as possible in the component connection process. A hole would be able to produce the 0.5pF distributed capacitance and 1nH parasitic inductance, the parasitic inductance of vias harm is often greater than the parasitic capacitance. Reduce the number of vias can greatly improve the speed, reduce in the disturbance. High-frequency circuit wiring to the signal line wiring close the introduction of cross-interference, if you can not avoid parallel distribution, a large area of ground in the opposite arrangement of parallel signal lines to reduce the interference noise.
中文版