"I. General Principle of wiring:
(1) Arrange the position of each functional circuit unit according to the circuit flow, make the layout convenient for signal flow, and keep the signal in the same direction as far as possible.
(2) Take the core components of each functional circuit as the center and layout around it. The components shall be evenly, neatly and compactly arranged on the PCB to minimize and shorten the lead and connection between components.
(3) For the circuit working at high frequency, the distribution parameters between components should be considered. For general circuits, components shall be arranged in parallel as far as possible. In this way, it is not only beautiful. Moreover, it is easy to assemble and weld and easy to mass produce.
(4) Components located at the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board. The best shape of the circuit board is rectangle. The aspect ratio is 3:2 to 4:3. The surface size of the circuit board is greater than 200 × At 150mm. The mechanical strength of the circuit board shall be considered.
(5) The power line and ground wire (or neutral line) shall be wired in a "well" shape.
2、 Selection and determination of wire width and spacing:
According to the current of the printed circuit board, the width of the power line shall be thickened as much as possible to reduce the loop resistance.
The minimum width of printed conductor is mainly determined by the adhesion strength between conductor and insulating base plate and the current flowing through them. When the thickness of copper foil is 0.05mm and the width is 1 ~ 1.5mm. Through 2A current, the temperature will not be higher than 3 ℃, and the conductor width of 1.5mm can meet the requirements. For integrated circuits, especially digital circuits, the conductor width of 0.02 ~ 0.3mm is usually selected. Of course, as long as possible, use wide wires, especially power and ground wires. The minimum spacing of wires is mainly determined by the insulation resistance and breakdown voltage between wires in the worst case. For integrated circuits, especially digital circuits, the spacing of wires can be as small as 5 ~ 8mm as long as the process allows.
If the line width is too small, the resistance of the printed wire is large, and the voltage drop on the line is large, which affects the performance of the circuit. If the line width is too wide, the wiring density is not high and the board area increases, which is not conducive to miniaturization in addition to increasing the cost
Relationship among ground wire, power wire and signal wire:
Ground wire ＞ power wire ＞ signal wire. Generally, the signal line width is 0.2 ~ 0.3mm, the thinnest width can reach 0.05 ~ 0.07mm, and the power wire is 1.2 ~ 2.5mm.
However, for high current, if the current load is calculated as 20A / mm2, when the thickness of copper clad foil is 0.5mm (generally so much), the current load of 1mm (about 40mil) linewidth is 1a. Therefore, the linewidth of 1-2.54mm (40-100mil) can meet the general application requirements. For the ground wire and power supply on the high-power equipment board, the linewidth can be appropriately increased according to the power, and on the low-power digital circuit, In order to improve the wiring density, the minimum line width is 0.254-1.27mm (10-15mil)
In the same circuit board, the power line and ground line are thicker than the signal line
According to the above calculation, it can be calculated that the current of 20a is 20mm
This is because when the current density is determined, the cross-sectional area of the line must be proportional to the passing current. When the circulating current is too large, the line will heat and shorten its service life. In serious cases, it will affect the stability of surrounding components or be burned out.
Relationship among ground wire, power wire and signal wire:
The ground wire is generally 1.5 ~ 3mm. For the PCB of digital circuit, a wide ground wire can be used to form a circuit, that is, a ground grid can be formed for use (the ground of analog circuit cannot be used in this way). A large area of copper layer is used as the ground wire, and the unused places on the printed board are connected with the ground as the ground wire.
(1) Wiring between high and low voltage power lines
There are high-voltage circuits and low-voltage circuits on the printed circuit board at the same time. The components of the high-voltage circuit and the low-voltage part shall be separated, and the isolation distance is related to the withstand voltage. Generally, the distance on the Board shall be 20mm at 2000V, which shall be increased in proportion. For example, if it is to withstand the withstand voltage test of 3000V, the distance between the high-voltage and low-voltage circuits shall be more than 35mm, In many cases, slots are also cut between high and low voltage on the printed circuit board to avoid creepage. The circuit board we made also has high and low voltage circuits, and the circuit spacing between high and low voltage is 10mm.
(2) Spacing between wires
The distance between adjacent conductors must meet the electrical safety requirements, and the distance shall be wider as far as possible in order to facilitate operation and production. The minimum spacing shall be at least suitable for the withstand voltage. This voltage generally includes working voltage, additional fluctuation voltage and peak voltage caused by other reasons. If the relevant technical conditions allow a certain degree of metal residues between conductors, the spacing will be reduced. Therefore, the designer should take this factor into account when considering the voltage. When the wiring density is low, the spacing of signal lines can be appropriately increased. The signal lines with great difference between high and low levels should be as short as possible and the spacing should be increased.
(3) Routing of ground wire
Shielding and grounding of printed conductor: the common ground wire of printed conductor shall be arranged at the edge of printed circuit board as far as possible. Copper foil should be reserved as much as possible on the printed circuit board as the ground wire. In this way, the shielding effect is better than a long ground wire, the transmission line characteristics and shielding effect will be improved, and the distributed capacitance will be reduced. It is better to form a loop or mesh for the common ground wire of the printed conductor, because when there are many integrated circuits on the same board, especially components with high power consumption, the grounding potential difference is generated due to the graphic limitations, resulting in the reduction of noise tolerance. When the circuit is made, the grounding potential difference is reduced. In addition, the patterns of grounding and power supply should be parallel to the flow direction of data as much as possible, which is the secret of enhancing the ability of noise suppression; Several layers of multilayer printed circuit board can be used as shielding layer, and the power layer and ground wire layer can be regarded as shielding layer. Generally, the ground wire layer and power layer are designed in the inner layer of multilayer printed circuit board, and the signal wire is designed in the inner layer and outer layer.
(4) . signal line
Firstly, the signal line should be as short as possible. This basic criterion will reduce the possibility of irrelevant signals coupling to the signal path. Generally, the distance between two legs of standard components is 0.1 inch (2.54 mm), so the foundation of signal line system is generally set as 0.1 inch (2.54 mm) or an integral multiple less than 0.1 inch, such as 0.05 inch, 0.025 inch, 0.02 inch, etc.
(5) . grounding mode
A) Series single point grounding refers to that the grounding wires of each unit circuit are connected in series to the grounding point. The mode of series single point grounding is shown in Figure 2 (a). The grounding points are connected in series by the working grounding wire and then grounded. Figure 2 (b) is the corresponding equivalent circuit diagram.
Figure 2
It can be seen from the above three formulas that the potentials of points a, B and C are not zero and are affected by other circuit currents. From the perspective of preventing noise and suppressing interference, this grounding method is the most unsuitable. Although this grounding method is very unreasonable, there are still many places to adopt this grounding method because it is relatively simple. It can be used when the level difference of each circuit is small. When the level of each circuit varies greatly, it cannot be used, because the high-level circuit will produce a large ground current, form a large ground potential difference and interfere with the low-level circuit. Single point grounding in series is easy to cause common ground resistance interference because each unit shares a ground wire. As shown in Figure 3 (a), since unit circuit a on the printed circuit board is a low-level analog amplifier and unit circuit B is a digital circuit integrated chip, the two grounding points are connected in series and led out to the external grounding of the printed circuit board. Z1 and Z2 are the impedance of corresponding ground wires. Here, the working ground wire is used as both the return line of the power supply and the return line of the signal. The power return IB of the digital circuit unit B contains high-frequency components. The voltage drop of IB on the ground wire impedance Z1 will be superimposed with the input analog signal and added to the low-level analog amplifier, resulting in common resistance anti-interference. In order to eliminate the common ground impedance interference of the circuit shown in Fig. 3 (a), it can be improved to the structure shown in Fig. 3 (b), that is, the analog power supply circuit is completely separated from the digital power supply circuit.
When using the single point grounding mode, the low-level circuit shall be placed at the place closest to the grounding point, that is, point a in Figure 2, because the point is close to the ground potential.
Figure III
B) . parallel grounding
Parallel single point grounding means that the grounding wires of each unit circuit are led to the grounding point respectively. The mode of parallel single point grounding of independent ground wires is shown in Figure 4. Each unit circuit in the figure is connected to a grounding point with ground wires, and Figure 4 (b) is its corresponding equivalent circuit diagram.
Figure IV
Because the parallel single point grounding mode requires many ground wires, it is troublesome and cumbersome to use in the design of printed circuit board. Due to separate grounding, it is bound to increase the length of the ground wire and then increase the ground wire impedance. In addition, this grounding method will also cause inductive coupling between the ground wires, and the distributed capacitance between the ground wires will also form capacitive coupling between the ground wires. With the increase of frequency, the impedance of ground wire, the inductance pot and capacitance pot between ground wires will increase, so this grounding method is not suitable for high frequency. When the frequency increases, especially when the length of the ground wire is odd times of 1 / 4 wavelength, the ground wire impedance becomes very high, and the ground wire becomes an antenna, radiating interference signals outward. Therefore, the length of the ground wire shall not exceed 1 time of the signal wavelength to prevent radiation and reduce the ground wire impedance.
When the parallel single point grounding mode is adopted, it must also be noted that the unit circuit with the lowest level shall be arranged at a close to the grounding point, so as to minimize the impact on the potential of point B and point C.
Generally, we combine these two grounding methods. The basic principles are as follows: first, divide the circuits prone to mutual interference into groups, such as analog circuits and digital circuits, low-power and high-power circuits, low-noise circuits and high-noise circuits; In each group, the grounding points of each circuit in the group are connected in series by single point series, and the circuit with the lowest level is selected as the group grounding point.
This method is often used when the frequency is low, the ground wire impedance is small, and the level difference of each circuit in the group is small. Because it is relatively simple and the wiring is similar to the circuit diagram, it is easier to route the circuit. After grouping, connect the grounding points of each group to an independent general grounding point in a single point parallel mode. The schematic diagram of hybrid connection method is shown in the figure below.
Single point series and parallel hybrid grounding mode
There are at least three kinds of ground wires in general electronic systems and equipment: signal ground wire, noise ground wire and metal part ground wire. Signal ground wire is generally used for circuits with low power, and can be further divided into analog circuit ground wire and digital circuit ground wire. The noise ground wire is used in high-power circuits, such as transistors, relays, motors and other circuits prone to high noise. The ground wire of metal parts refers to the equipment casing, frame and bottom plate. The protective ground wire in AC power supply shall be connected with the ground wire of metal parts. The basic knowledge of circuit board wiring is applicable to both analog and digital circuits. A basic rule of thumb is to use an uninterrupted ground plane. This common sense reduces the di / dt effect in digital circuits. This effect will change the ground potential and make noise enter analog circuits. The wiring skills of digital and analog circuits are basically the same, except for one thing. For analog circuits, another thing to note is to keep the digital signal line and the loop in the ground plane as far away from the analog circuit as possible. This can be achieved by connecting the analog ground plane separately to the connection end of the system, or placing the analog circuit at the farthest end of the circuit board, that is, the end of the line. This is done to minimize external interference to the signal path. This is not necessary for digital circuits, which can tolerate a large amount of noise on the ground plane without problems.
For a board containing high and low frequency signals, the high frequency and low frequency shall be separated, and the high frequency components shall be close to the connectors of the circuit board.
3、 Design principle of circuit board ground wire (1) principle of "separate ground" in printed circuit board wiring
The so-called "ground separation" means that when designing printed circuit boards, ground wires shall be laid respectively according to different power supply voltages, digital circuits and analog circuits, high-speed circuits and low-speed circuits, as well as high current circuits and low current circuits. The main purpose of ground separation is to prevent common ground wire impedance interference.
The general principles of "ground separation" design according to the circuit function module are as follows:
① The grounding of low-frequency circuit shall adopt single point parallel grounding as far as possible, or adopt partial series connection and then parallel grounding;
② Multi point series grounding shall be adopted for high-frequency circuit, and the ground wire shall be short and thick;
③ A large area of grid foil shall be used around the high-frequency element as far as possible.
If there are both logic circuits and linear circuits on the printed circuit board, they shall be separated as far as possible. Single point parallel grounding shall be adopted for the grounding of low-frequency circuit as far as possible. If it is difficult to connect the actual wiring, it can be partially connected in series and then connected in parallel. Multi point series grounding shall be adopted for high-frequency circuit, the ground wire shall be short and thick, and grid like large-area ground foil shall be used around high-frequency elements as far as possible.
(2) The grounding wire shall be as thick as possible
If the grounding wire adopts a very thin printed wire, the grounding potential changes with the change of current, which reduces the anti noise performance. Therefore, the grounding wire should be as thick as possible and can pass at least three times the allowable current on the printed circuit board. Of course, if possible, the grounding wire should be more than 2mm-3mm.
(3) The digital circuit ground wire forms a closed-loop circuit
If the printed circuit board is only composed of digital circuits, the grounding circuit shall be arranged in the form of circular loop, which can generally improve the anti noise ability.
(4) Grounding and signal loop
The control layer or signal return loop is one of the most important design considerations for EMI suppression on printed circuit boards. Each single point connection related to RF current generated by RF noise electronic circuit must be carefully analyzed, such as mechanical fixation between printed circuit board ground and base plate ground. In general, high-speed logic devices and oscillators are always arranged at a single point connection point as close as possible to minimize the RF loop in the form of eddy current“
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