Application of the hottest powerpcb in PCB design

2022-07-24
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Application of powerpcb in printed circuit board design

printed circuit board (PCB) is the support of circuit components and devices in electronic products. It provides electrical connections between circuit components and devices. With the rapid development of electronic technology, the density of PCB is getting higher and higher. The quality of PCB design has a great impact on the anti-interference ability. Practice has proved that even if the circuit schematic design is correct and the printed circuit board design is improper, it will have an adverse impact on the reliability of electronic products. For example, if two thin parallel lines of the printed board are close together, the delay of the signal waveform will be formed, and the reflected noise will be formed at the terminal of the transmission line. Therefore, when designing printed circuit boards, attention should be paid to adopting correct methods, following the general principles of PCB design, and meeting the requirements of anti-interference design

I. General principles of PCB design

in order to obtain the best performance of electronic circuits, the layout of components and wires is very important. In order to design PCB with good quality and low cost, the following general principles should be followed:

1 Layout

first, the PCB size should be considered. When the PCB size is too large, the printed line is long, the impedance increases, the anti noise ability decreases, and the cost also increases; If it is too small, the heat dissipation is poor, and the adjacent lines are easy to be disturbed. After determining the PCB size, determine the position of special components. Finally, according to the functional units of the circuit, all the components of the circuit are laid out

the following principles shall be observed when determining the position of special components:

(1) shorten the connection between high-frequency components as far as possible, and try to reduce their distribution parameters and mutual electromagnetic interference. The easily disturbed components shall not be too close to each other, and the input and output components shall be as far away as possible

(2) there may be high potential difference between some components or wires, so the distance between them should be increased to avoid accidental short circuit caused by discharge. Components and parts with high voltage shall be arranged at places that are not easy to reach during commissioning

(3) components weighing more than 15g shall be fixed with brackets and then welded. Those large, heavy and heat generating components should not be installed on the printed board, but on the chassis bottom plate of the whole machine, and the heat dissipation should be considered. Thermal components shall be kept away from heating components

(4) for the layout of potentiometer, adjustable inductance coil, variable capacitor, microswitch and other adjustable components, the structural requirements of the whole machine shall be considered. If it is adjusted in the machine, it should be placed on the printed board where it is convenient to adjust; If it is adjusted outside the machine, its position shall be consistent with the position of the adjustment knob on the chassis panel

(5) the position occupied by the positioning hole of the printed board and the fixed support shall be reserved

according to the functional unit of the circuit. The layout of all components and parts of the circuit shall comply with the following principles:

(1) arrange the position of each functional circuit unit according to the circuit process, so that the layout is convenient for signal flow and the signal is kept in the same direction as far as possible

(2) take the core components of each functional circuit as the center and carry out layout around it. Components and parts shall be evenly, neatly and compactly arranged on PCB. Try to reduce and shorten the leads and connections between components

(3) for the circuit working at high frequency, the distribution parameters between components shall be considered. For general circuits, components shall be arranged in parallel as far as possible. In this way, it is not only beautiful, but also easy to assemble and weld, and easy to mass produce

(4) components and parts 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. Length and width are 3:2 or 4:3. The size of circuit board is greater than 200 × At 150mm, the mechanical strength of the circuit board shall be considered

2. Wiring

the principle of wiring is as follows:

(1) the wires used at the input and output terminals should avoid adjacent parallel as much as possible. It is better to add ground wires between lines to avoid feedback coupling in reporting the development trends of the discipline

(2) the minimum width of the printed board conductor is mainly determined by the adhesion strength between the conductor and the insulating substrate and the current flowing through them. When the thickness of copper foil is 0.5mm and the width is 1 ~ 15mm, the temperature will not be higher than 3 ℃ through 2A current. Therefore, the wire width of 1.5mm can meet the requirements. For integrated circuits, especially digital circuits, the wire width of 0.02 ~ 0.3mm is usually selected. Of course, as long as it is allowed, use as wide a line as possible, especially the power line and ground line. 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, as long as the technology allows, the spacing can be less than 5 ~ 8mil

(3) the curve of printed wire is generally circular arc, and the right angle or included angle will affect the electrical performance in high-frequency circuit. In addition, try to avoid the use of large-area copper foil, otherwise, copper foil will easily expand and fall off when heated for a long time. When large area copper foil must be used, it is better to use grid shape. This is beneficial to eliminate the volatile gas generated by the heating of the adhesive between the copper foil and the substrate

3. The central hole of the pad

is slightly larger than the diameter of the device lead. If the pad is too large, it is easy to form false soldering. The outer diameter D of the pad is generally not less than (d+1.2) mm, where D is the diameter of the lead. For high-density digital circuits, the minimum diameter of the pad can be (d+1.0) mm

II. PCB and circuit anti-interference measures

the anti-interference design of printed circuit board is closely related to the specific circuit. Here, only a few common measures of PCB anti-interference design are described

1. Power line design

according to the current of the printed circuit board, try to increase the width of the power line and reduce the loop resistance. At the same time, make the direction of power line and ground line consistent with the direction of data transmission, which helps to enhance the anti noise ability

2. Ground wire design

in the design of electronic products, grounding is an important method to control interference. Most interference problems can be solved if grounding and shielding can be properly combined. The ground wire structures in electronic products generally include system ground, chassis ground (shielding ground), digital ground (logic ground) and simulation ground. The following points should be paid attention to in the design of ground wire:

(1) correctly select single point grounding and multi-point grounding

in the low-frequency circuit, the working frequency of the signal is less than 1MHz, its wiring and the inductance between devices have little impact, while the circulating current formed by the grounding circuit has a great impact on the interference, so the one point grounding method should be adopted. When the signal operating frequency is greater than 10MHz, the ground wire impedance becomes large. At this time, the ground wire impedance should be reduced as much as possible, and the nearest multi-point grounding should be adopted. When the operating frequency is 1 ~ 10MHz, if one point grounding is adopted, the length of the ground wire shall not exceed 1/20 of the wavelength, otherwise the multi-point grounding method shall be adopted

(2) digital and simulation are separated

there are both high-speed logic circuits and linear circuits on the circuit board, so they should be separated as far as possible, and the ground wires of the two should not be mixed, and they should be connected with the ground wires of the power terminal respectively. The low-frequency circuit shall be grounded at a single point in parallel as far as possible. If the actual wiring is difficult, it can be partially connected in series and then connected in parallel. The high-frequency circuit should adopt multi-point series grounding, the ground wire should be short and thick, and the grid like large-area ground foil should be used around the high-frequency components as much as possible. The grounding area of linear circuit shall be increased as much as possible

(3) the grounding wire shall be thickened as much as possible

if the grounding wire uses a very thin line, the grounding potential will change with the change of current, resulting in unstable timing signal level and reduced anti noise performance of electronic products. Therefore, the grounding wire should be thickened as much as possible so that it can pass three times the allowable current of the printed circuit board. If possible, the width of the grounding wire shall be greater than 3mm

(4) the grounding wire forms a dead cycle loop

when designing the ground wire system of a printed circuit board composed only of digital circuits, making the ground wire into a closed circuit can significantly improve the anti noise ability. The reason is that there are many integrated circuit components on the printed circuit board, especially in the case of components with high power consumption, due to the limitation of the thickness of the grounding wire, a large potential difference will be generated on the grounding wire, resulting in the decline of the anti noise ability. If the grounding wire is formed into a loop, the potential difference will be reduced and the anti noise ability of electronic equipment will be improved

3. Configuration of decoupling capacitor one of the conventional methods of PCB design is to configure appropriate decoupling capacitors at each key part of the PCB. The general configuration principle of decoupling capacitor is:

(1) the power input terminal is connected with 10 ~ 100uF electrolytic capacitor. If possible, it is better to connect to the one above 100uF

(2) in principle, each integrated circuit chip shall be equipped with a 0.01pf ceramic chip capacitor. In case of insufficient gap in the printed board, a 1 ~ 10PF tantalum capacitor can be arranged every 4 ~ 8 chips

(3) for devices with weak noise resistance and large power change during shutdown, such as RAM and ROM memory devices, decoupling capacitors shall be directly connected between the power line and ground line of the chip

(4) the capacitor lead shall not be too long, especially the high-frequency bypass capacitor shall not have a lead

in addition, the following two points should be noted:

(1) when there are contactors, relays, buttons and other components in the printed board, large spark discharge will occur when operating them, and RC circuit must be used to absorb the discharge current. Generally, R is 1 ~ 2K, and C is 2.2 ~ 47uF

(2) the input impedance of CMOS is very high, and it is easy to be induced. Therefore, it is necessary to ground or correct the power supply for the unused terminal during use

III. introduction to powerpcb

powerpcb is a software product of innoveda company in the United States, which enables users to complete high-quality design and vividly reflects the contents of all aspects of the electronic design industry. The constraint driven design method can reduce the product completion time. You can define safe spacing, wiring rules and design rules for high-speed circuits for each signal, and apply these plans hierarchically to the board, each layer, each type of network, each network, each group of networks, and each pin pair to ensure the correctness of layout and wiring design. It includes a variety of functions, including cluster layout tools, dynamic routing, dynamic electrical performance inspection, automatic dimensioning and powerful cam output capabilities. It also has the ability to integrate third-party software tools, such as specctra router

IV. powerpcb usage skills

powerpcb has been popularized and used in our institute. Its basic usage technology has been explained in detail in the training materials. For the majority of electronic application engineers in our institute, the problem is how to switch to the application of powerpcb after they have mastered the wiring tools such as tango. Therefore, this paper discusses some technical skills that are not mentioned in this kind of application and training materials, but we use more

In 2010, it accounted for about 40% of the global market

1 Input specification problem

for most people who have used tango, when they first use powerpcb, they may feel that there are too many restrictions on powerpcb. This is because the correctness of the schematic input and the rule transmission from the schematic to the PCB is the premise of powerpcb. Therefore, in its schematic diagram, there is no function to disconnect one electrical connection, nor can one electrical connection be stopped at a certain position at will. It should ensure that each electrical connection has a starting pin and a terminating pin, or is connected to the connector provided by the software for information transmission between different pages. This is a means to prevent errors. In fact, it is also a standardized schematic input method that we should follow