Printed circuit boards (PCBs) are the support for 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 PCBs is getting higher and higher. The quality of PCB design has a great impact on the ability to interfere. Practice has proved that even if the schematic design of the circuit is correct and the printed circuit board is not properly designed, it will adversely affect the reliability of the electronic product. For example, if the two thin parallel lines of the printed board are in close proximity, a delay in the signal waveform is formed, and reflected noise is formed at the end of the transmission line. Therefore, when designing a printed circuit board, care should be taken to adopt the correct method, to comply with the general principles of PCB design, and to comply with the requirements of anti-interference design.
First, the general principles of PCB design
In order to get the best performance of the electronic circuit, the layout of the components and the layout of the wires are very important. In order to design a PCB of good quality and low cost, the following general principles should be followed:
First, consider the PCB size. When the PCB size is too large, the printed lines are long, the impedance is increased, the noise resistance is reduced, and the cost is also increased; if the size is too small, the heat dissipation is not good, and the adjacent lines are susceptible to interference. After determining the PCB size, determine the location of the particular component. Finally, according to the functional unit of the circuit, all the components of the circuit are laid out.
Observe the following principles when determining the location of a particular component:
(1) Try to shorten the wiring between high-frequency components as much as possible, and try to reduce their distribution parameters and mutual electromagnetic interference. Components that are susceptible to interference cannot be placed too close together, and input and output components should be kept as far away as possible.
(2) There may be a high potential difference between some components or wires. The distance between them should be increased to avoid accidental short circuit caused by discharge. Components with high voltage should be placed as far as possible in the hands of the hand when debugging.
(3) Components weighing more than 15g shall be fixed by brackets and then welded. Those components that are large, heavy, and have a lot of heat should not be mounted on the printed board, but should be installed on the chassis of the whole machine, and heat dissipation should be considered. The thermal element should be kept away from the heating element.
(4) For the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, microswitches, etc., the structural requirements of the whole machine should be considered. If it is adjusted inside the machine, it should be placed on the printed board for easy adjustment; if it is adjusted outside the machine, its position should be compatible with the position of the adjustment knob on the chassis panel.
(5) The position occupied by the positioning holes of the printed board and the fixed bracket should be left.
According to the functional unit of the circuit. When laying out all the components of a circuit, the following principles must be met:
(1) Arrange the position of each functional circuit unit according to the flow of the circuit, so that the layout facilitates signal circulation and keeps the signal as consistent as possible.
(2) Center around the core components of each functional circuit and arrange it around it. Components should be arranged evenly, neatly and compactly on the PCB. Minimize and shorten leads and connections between components.
(3) For circuits operating at high frequencies, the distribution parameters between components should be considered. In general, the circuit should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to weld and easy to mass produce.
(4) Components located at the edge of the board are generally not less than 2 mm from the edge of the board. The optimal shape of the board is rectangular. The length and width are 3:2 or 4:3. When the board surface size is larger than 200×150mm, the mechanical strength of the board should be considered.
The principles of wiring are as follows:
(1) The wires used at the input and output terminals should be avoided as far as possible. It is best to add the ground wire between the wires to avoid feedback.
(2) The minimum width of the printed board wires is mainly determined by the adhesion strength between the wires and the insulating substrate and the current value flowing through them. When the thickness of the copper foil is 0.5 mm and the width is 1 to 15 mm, the temperature is not higher than 3 ° C by the current of 2 A. Therefore, a wire width of 1.5 mm is sufficient. For integrated circuits, especially digital circuits, a wire width of 0.02 to 0.3 mm is usually selected. Of course, as far as possible, use wide lines, especially power and ground. The minimum spacing of the wires is primarily determined by the worst case interline insulation resistance and breakdown voltage. For integrated circuits, especially digital circuits, the pitch can be less than 5-8 mils as long as the process allows.
(3) The curved corner of the printed conductor generally takes a circular arc shape, and the right angle or angle affects the electrical performance in the high frequency circuit. In addition, try to avoid the use of large areas of copper foil, otherwise, when heated for a long time, copper foil expansion and falling off is easy. When large-area copper foil must be used, it is best to use a grid shape. This is advantageous in eliminating volatile gases generated by the heat of the adhesive between the copper foil and the substrate.
The center hole of the pad is slightly larger than the diameter of the device lead. The pad is too large to form a solder joint. The pad outer diameter D is generally not less than (d + 1.2) mm, where d is the lead aperture. For high-density digital circuits, the minimum pad diameter can be (d + 1.0) mm.
Second, PCB and circuit anti-interference measures
The anti-jamming design of printed circuit boards is closely related to the specific circuit. Here, only some common measures of PCB anti-interference design are explained.
1. Power cord 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, the direction of the power line and the ground line are consistent with the direction of data transmission, which helps to enhance the anti-noise capability.
2. Ground wire design
In electronic product design, grounding is an important method of controlling interference. If the grounding and shielding are properly combined, most of the interference problems can be solved. The ground wire structure in electronic products is roughly systematic, chassis ground (shielded ground), digital ground (logical ground), and analog ground. Pay attention to the following points in the ground line design:
(1) Correct selection of single point grounding and multi-point grounding
In the low-frequency circuit, the operating frequency of the signal is less than 1MHz, and the influence of the inductance between the wiring and the device is small, and the circulating current formed by the grounding circuit has a great influence on the interference, so a grounding method should be adopted. When the signal operating frequency is greater than 10MHz, the ground impedance becomes very large. At this time, the ground impedance should be reduced as much as possible. When the operating frequency is between 1 and 10 MHz, if a grounding is used, the grounding length should not exceed 1/20 of the wavelength. Otherwise, the multi-point grounding method should be used.
(2) The digital ground is separated from the simulated ground.
The circuit board has both high-speed logic circuits and linear circuits. They should be separated as much as possible, and the ground wires of the two should not be mixed, and they are connected to the ground of the power supply. The ground of the low-frequency circuit should be grounded in parallel with a single point. If the actual wiring is difficult, it can be partially connected and then grounded 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 foil should be used as much as possible around the high-frequency components. Try to increase the grounding area of the linear circuit.
(3) The grounding wire should be as thick as possible.
If the grounding wire uses a very thin line, the grounding potential changes with the change of the current, causing the timing signal level of the electronic product to be unstable and the anti-noise performance to be lowered. Therefore, the ground wire should be as thick as possible so that it can pass three times the allowable current of the printed circuit board. If possible, the width of the ground wire should be greater than 3mm.
(4) The grounding wire constitutes a closed loop.
When designing a grounding system for a printed circuit board consisting only of digital circuits, making the grounding wire closed can significantly improve the noise immunity. The reason is that there are many integrated circuit components on the printed circuit board, especially when there are many power-consuming components, due to the limitation of the grounding wire thickness, a large potential difference will be generated on the ground wire, causing the noise resistance to decrease. If the ground line is formed into a loop, the potential difference is reduced, and the noise resistance of the electronic device is improved.
3. Untwisting capacitor configuration
One of the usual practices in PCB design is to configure appropriate decoupling capacitors at various critical points in the printed board. The general configuration principle for the untwisting capacitor is:
(1) The power input terminal is connected to an electrolytic capacitor of 10 to 100 uf. If possible, it is better to pick up 100uF or more.
(2) In principle, each integrated circuit chip should be equipped with a 0.01pF ceramic capacitor. If there is insufficient gap in the printed board, a 1~10pF tantalum capacitor can be arranged every 4~8 chips.
(3) For devices with weak anti-noise capability and large power supply changes during shutdown, such as RAM and ROM storage devices, the decoupling capacitor should be directly connected between the power cable and the ground of the chip.
(4) The capacitor leads should not be too long, especially the high-frequency bypass capacitors must not have leads.
In addition, you should also pay attention to the following two points:
(1) When there are contactors, relays, buttons and other components in the printed board, a large spark discharge will occur when operating them, and an RC circuit must be used to absorb the discharge current. Generally, R takes 1 to 2K, and C takes 2.2 to 47 uF.
(2) The input impedance of CMOS is very high and it is susceptible to induction. Therefore, it is necessary to ground or connect the power supply to the unused terminal during use.
Third, PowerPCB introduction
PowerPCB is a software product of Innoveda Corporation of the United States.
PowerPCB enables users to complete high-quality design and vividly reflects all aspects of the electronic design industry. Its constraint-driven design approach reduces product completion time. You can define safety spacing, routing rules, and design rules for high-speed circuits for each signal, and apply these plans hierarchically to the board, to each layer, to each type of network, to each network, to each group. On the network, each pin is placed to ensure the correctness of the layout design. It includes a wide range of features including cluster layout tools, dynamic routing editing, dynamic electrical performance checks, automatic dimensioning and powerful CAM output capabilities. It also has the ability to integrate third-party software tools such as the SPECCTRA router.
Fourth, PowerPCB tips
PowerPCB has been promoted and used by us. Its basic use technology has been explained in detail in the training materials. For our electronic application engineers, the problem is that after mastering the wiring tools such as TANGO, How to get to the PowerPCB application. Therefore, this article does not mention such applications and training materials, and we have applied some technical skills.
1. Input specification problem
For most people who have used TANGO, when you first start using PowerPCB, you may feel that PowerPCB has too many restrictions. Because PowerPCB is responsible for the correct transfer of schematic input and schematic to PCB. Therefore, there is no function in the schematic diagram that can disconnect an electrical connection, and it is not allowed to stop an electrical connection at a certain position. It must ensure that each electrical connection has a starting tube. The foot and the termination pin are connected to the connector provided by the software for information transfer between different pages. This is a means of preventing errors from occurring. In fact, it is also a standardized schematic input method that we should abide by.
In the PowerPCB design, any changes that are inconsistent with the schematic netlist must be made in the ECO mode, but it provides the user with an OLE link, which can transfer the modifications in the schematic to the PCB, or modify the PCB. Return the schematic. In this way, it avoids errors caused by negligence, and provides convenience for real need to modify. However, it should be noted that when entering the ECO mode, the "Write ECO File" option should be selected, and the ECO file operation will be performed only when the ECO mode is exited.
2. Power layer and ground layer selection
There are two options for power and ground settings in PowerPCB, CAMPlane and Split/Mixed. Split/Mixed is mainly used when multiple power supplies or grounds share a single layer, but only one power supply and ground can be used. Its main advantage is the consistency of the diagram and the light drawing at the time of output, which is easy to check. CAMPlane is used for a single power supply or ground. This method is a negative output. Pay attention to the 25th layer when outputting. The 25th layer contains geoelectric information, which mainly means that the pad of the electric layer is 20 mils larger than the normal pad. After the metallized via is ensured, no signal is connected to the ground. This requires each pad to contain information on layer 25. And we often ignore this problem when building our own library, resulting in the use of the Split/Mixed option.
3. Push or not push
PowerPCB provides a very useful feature that is automatic push. When we manually route, the printed board is fully controlled under our full control and it is very convenient to turn on the automatic push function. However, if you want to auto-route after you have finished pre-wiring, it is best to fix the pre-wired line. Otherwise, the software will consider this line segment to be movable when it is automatically routed, and completely overthrow your work, making it unnecessary. Loss.
4. Adding positioning holes
Our printed boards often need to be equipped with some mounting holes, but for PowerPCB, this is a different device layout than the schematic, which needs to be done in ECO mode. But if in the final inspection, the software gives us many errors, it is not convenient. In this case, the positioning hole device can be set to be non-ECO registered.
In the Edit Device window, select the "Edit Electrical Properties" button. In this window, select the "Normal" item and leave the "ECO Registration" item unchecked. In this way, PowerPCB will not consider that this device needs to be compared with the netlist, and there will be no errors that should not be there.
5. Add a new power package
Since our international and US software companies do not have the same standards, we try to equip them with an international library for everyone to use. But the new symbols for power and ground must be added to the library that comes with the software, otherwise it won't think that the symbol you built is the power supply.
So when we want to build a power symbol that meets the national standard, we need to open the existing power symbol group, select the "Edit Electrical Connection" button, click the "Add" button, enter the name of your newly created symbol and so on. Then, select the "Edit Gate Package" button, select the symbol name you just created, draw the shape you need, exit the drawing state, and save. This new symbol can be called up in the schematic.
6. Empty foot settings
Among the devices we use, some of the pipe scripts are empty feet and are marked NC. When we build the library, we must pay attention, otherwise the pins marked with NC will be connected together. This is because you built the NC pin in "SINGAL_PINS" when building the library, and PowerPCB considers the pin in "SINGAL_PINS" to be the default pin, which is a useful pin, such as VCC and GND. So, if you have NC pins, you must remove them from "SINGAL_PINS", or you don't need to pay attention to it at all, and don't use it for any special definition.
7. Transistor pin control
The package of the triode changes a lot. When we build the library of the triode, we often find that the netlist of the schematic is transmitted to the PCB and is inconsistent with the connection we want. This problem is mainly in the construction of the library.
Since the pins of the triode are often marked with E, B, and C, when creating your own triode library, select the "Include alphanumeric pins" check box in the "Edit Electrical Connections" window. In this case, "Text The "digital pin" tag is illuminated, enter the tag, and change the corresponding pin of the triode to a letter. In this way, it is easier to identify when it is connected to the PCB package.
8. Pretreatment of surface mount devices
Nowadays, surface mount devices are getting more and more applications due to the demand for miniaturization. During the layout process, the handling of surface mount devices is important, especially when laying multi-layer boards. Because the surface mount device has electrical connections only on one layer, unlike the placement of the dual in-line device on the board, the tube is attached from the surface when other layers need to be connected to the surface device. Pull a short line on the foot, punch it, and connect it to other devices. This is called fan-in (FAN-IN) and fan-out (FAN-OUT) operation.
If necessary, we should first fan-in, fan-out, and then route the surface-mount device. This is because if we just choose to fan-in and fan-out in the auto-wiring setup file, the software will This operation is performed during the wiring process, and the drawn line is bent and folded, and is relatively long. Therefore, we can enter the autorouter after the layout is completed. In the setup file, only fan-in, fan-out operation, and other routing options are not selected, so that the line drawn from the surface mount device is relatively short and neat.
9. Add the board diagram to AUTOCAD
Sometimes we need to add a printed board diagram to the structure diagram, then the conversion tool can be used to convert the PCB file into a format that AUTOCAD can recognize. In the PCB drawing box, select the "Output" menu item in the "File" menu, and set the save type to DXF file in the pop-up file output window, and then save. You can open this picture in AUTOCAD.
Of course, there is an automatic labeling function in PADS, which can dimension the drawn printed board and automatically display the position of the board frame or positioning hole. It should be noted that in the Drill-Drawing layer, if you want to mark the other output diagrams, you need to add this layer to the output.
10. PowerPCB and ViewDraw interface
With the schematic diagram of ViewDraw, the PowerPCB table can be generated. After the PowerPCB is read into the netlist, the functions such as automatic routing can be performed. Moreover, the PowerPCB has a linking tool that can dynamically link, modify, and maintain electrical connection with the schematic of VIEWDRAW. Consistency.
However, due to the difference between the software modified version and the upgraded version, sometimes the definition of the device name is inconsistent between the two softwares, which may cause a netlist transmission error. To avoid this kind of error, it is best to build a library that stores the corresponding devices of ViewDraw and PowerPCB, of course, this is only for a part of the mismatched device. You can use the copy function in PowerPCB to easily copy the component packages of other libraries in the existing PowerPCB to this library and store the corresponding names in VIEWDRAW.
11. Generate a gerber file
In the past, when we made the printed board, we copied the printed board on the floppy disk and directly to the plate making factory. This practice is poorly confidential and cumbersome, requiring a very detailed documentation for the plate-making factory. Now, we can directly produce photo-painting files to manufacturers with PowerPCB. From the name of the light painting file, it can be seen that this is the first layer of the trace, silk screen or solder mask, very convenient and safe.
Steps to transfer files:
A. Change APERTURE to 999 in the DEVICESETUP of the PowerPCB's CAM output window.
B. When you move the line layer, select the document type as ROUTING, then select the board frame and what you need to put on this layer in LAYER. It is not important to remove LINE, TEXT when you transfer the line (unless you want to make copper on the line).
C. When turning soldering, select the document type as SOLD_MASK, and select the via in the top solder mask.
D. When transferring silk screen, select the document type as SILKSCREEN, and the rest refer to steps B and C.
E. When converting the drilling data, select the document type as NCDRILL and convert it directly.
Note that you should preview the file when you turn it into a file. The graphic in the preview is the graphic you want to output, so watch it carefully to prevent errors.
With the experience of printed board design, such as the power of PowerPCB, drawing complex printed boards is not annoying.
Fortunately, we now have the tools to convert TANGO's PCB into PowerPCB. The majority of scientific and technical personnel who are familiar with TANGO can more easily join the ranks of PowerPCB drawing, and draw a satisfactory printed board more conveniently and quickly.