What Are the Anti-Interference Measures for PCB and Circuits

The Printed Circuit Board (PCB) is the support for circuit components and devices in electronic products, which 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 significant impact on anti-interference capability. Practice has proved that even if the circuit schematic is designed correctly, improper design of the printed circuit board will adversely affect the reliability of electronic products. For example, if two thin parallel lines on the printed board are very close to each other, it will cause a delay of the signal waveform and form reflected noise at the terminal of the transmission line. Therefore, when designing a printed circuit board, attention should be paid to adopting correct methods, complying with the general principles of PCB design, and meeting the requirements of anti-interference design.

Analysis of the Application of PowerPCB in PCB Design

To achieve optimal performance of electronic circuits, the layout of components and the routing of conductors are critical. To design a PCB with high quality and low cost, the following general principles shall be followed:

First, the size of the PCB must be considered. If the PCB size is too large, the printed traces will be long, the impedance will increase, the anti-noise capability will decrease, and the cost will also rise. If the size is too small, the heat dissipation will be poor, and adjacent traces will be susceptible to interference. After determining the PCB size, the position of special components shall be defined. Finally, the layout of all components of the circuit shall be carried out according to the functional units of the circuit.

The following principles shall be complied with when determining the position of special components: (1) Shorten the wiring between high-frequency components as much as possible, and strive to reduce their distributed parameters and mutual electromagnetic interference. Components susceptible to interference shall not be placed too close to each other, and input and output components shall be kept as far away as possible. (2) Some components or conductors may have a high potential difference, so the distance between them shall be increased to avoid accidental short circuit caused by discharge. Components with high voltage shall be arranged in areas that are not easily touched by hands during debugging as much as possible. (3) Components weighing more than 15g shall be fixed with brackets before soldering. Large, heavy components with high heat generation should not be mounted on the printed board, but on the chassis bottom plate of the complete machine, and heat dissipation issues shall be considered. Thermosensitive components shall be kept away from heating components. (4) For the layout of adjustable components such as potentiometers, adjustable inductance coils, variable capacitors and micro switches, the structural requirements of the complete machine shall be considered. For internal adjustment, they shall be placed in a position convenient for adjustment on the printed board; for external adjustment, their position shall be adapted to the position of the adjustment knob on the chassis panel. (5) The position occupied by the printed board positioning holes and the fixed bracket shall be reserved.

When carrying out the layout of all components of the circuit according to the functional units of the circuit, the following principles shall be met: (1) Arrange the position of each functional circuit unit according to the circuit flow, so that the layout facilitates signal circulation and keeps the signal in the same direction as much as possible. (2) Take the core component of each functional circuit as the center, and carry out the layout around it. Components shall be arranged evenly, neatly and compactly on the PCB. Minimize and shorten the leads and connections between components as much as possible. (3) For circuits operating at high frequencies, the distributed parameters between components must be considered. For general circuits, components shall be arranged in parallel as much as possible. This is not only aesthetically pleasing, but also easy to mount and solder, and convenient for mass production. (4) Components located at the edge of the circuit board are generally no less than 2mm away from the edge of the circuit board. The shape of the circuit board is rectangular, with a length-width ratio of 3:2 or 4:3. When the size of the circuit board surface is greater than 200×150mm, the mechanical strength of the circuit board shall be considered.

The routing principles are as follows: (1) Conductors used at the input and output ends shall avoid adjacent parallelism as much as possible. A ground wire shall be added between the lines to avoid feedback coupling. (2) The width of the printed board conductor is mainly determined by the adhesion strength between the conductor and the insulating substrate, as well as the current value flowing through them. When the copper foil thickness is 0.5mm and the width is 1~15mm, the temperature will not be higher than 3℃ when a current of 2A passes through. Therefore, a conductor width of 1.5mm can meet the requirements. For integrated circuits, especially digital circuits, a conductor width of 0.02~0.3mm is usually selected. Of course, as long as permitted, wider lines should be used as much as possible, especially for power lines and ground lines. The spacing between conductors is mainly determined by the insulation resistance and breakdown voltage between lines under worst-case conditions. For integrated circuits, especially digital circuits, the spacing can be less than 5~8mil as long as the process permits. (3) The corners of printed conductors generally adopt a circular arc shape, while right angles or included angles will affect the electrical performance in high-frequency circuits. In addition, the use of large-area copper foil shall be avoided as much as possible; otherwise, the copper foil is prone to expansion and falling off when heated for a long time. When large-area copper foil must be used, it should be in a grid shape. This is conducive to exhausting the volatile gas generated by the heating of the adhesive between the copper foil and the substrate.

The center hole of the pad shall be slightly larger than the diameter of the device lead. An excessively large pad is prone to form cold solder joints. The outer diameter D of the pad is generally not less than (d+1.2) mm, where d is the lead hole diameter. For high-density digital circuits, the pad diameter can be (d+1.0) mm.

The anti-interference design of printed circuit boards is closely related to the specific circuit. Here, only several commonly used measures for PCB anti-interference design are explained.

According to the current of the printed circuit board, widen the power line width as much as possible to reduce the loop resistance. At the same time, make the direction of the power line and ground line consistent with the direction of data transmission, which helps to enhance the anti-noise capability.

In the design of electronic products, grounding is an important method to control interference. If grounding and shielding are correctly combined and used, most interference problems can be solved. The ground wire structure in electronic products generally includes system ground, chassis ground (shielding ground), digital ground (logic ground) and analog ground. The following points shall be noted in the ground line design:

(1) Correct selection between single-point grounding and multi-point grounding In low-frequency circuits, the operating frequency of the signal is less than 1MHz, the inductance effect of its wiring and devices is small, and the circulation formed by the grounding circuit has a great impact on interference, so the single-point grounding method should be adopted. When the signal operating frequency is greater than 10MHz, the ground wire impedance becomes very large. At this time, the ground wire impedance should be reduced as much as possible, and the nearby multi-point grounding method should be adopted. When the operating frequency is between 1~10MHz, if single-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) Separation of digital ground and analog ground The circuit board has both high-speed logic circuits and linear circuits, which should be separated as much as possible, and their ground wires should not be mixed, but connected to the ground wire at the power supply end respectively. The ground of low-frequency circuits should adopt single-point parallel grounding as much as possible; when the actual wiring is difficult, they can be partially connected in series and then grounded in parallel. High-frequency circuits should adopt multi-point series grounding, the ground wire should be short and thick, and grid-shaped large-area ground foil should be used around high-frequency components as much as possible. The grounding area of linear circuits should be increased as much as possible.

(3) The ground wire should be thickened as much as possible If a very thin line is used for the ground wire, the ground potential will change with the change of current, resulting in unstable timing signal level and reduced anti-noise performance of electronic products. Therefore, the ground 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 ground wire should be greater than 3mm.

(4) The ground wire forms a closed loop When designing the ground wire system of a printed circuit board composed only of digital circuits, making the ground wire into a closed loop can significantly improve the anti-noise capability. The reason is: there are many integrated circuit components on the printed circuit board, especially for components with high power consumption, a large potential difference will be generated on the ground wire due to the limitation of the ground wire thickness, resulting in reduced anti-noise capability. If the ground wire forms a loop, the potential difference will be reduced, and the anti-noise capability of electronic equipment will be improved.

One of the conventional practices in PCB design is to configure appropriate decoupling capacitors at various key parts of the printed board. The general configuration principles for decoupling capacitors are as follows: (1) Connect a 10~100μF electrolytic capacitor across the power input end. If possible, it is better to connect one above 100μF. (2) In principle, a 0.01μF ceramic chip capacitor should be arranged for each integrated circuit chip. If the space on the printed board is insufficient, a 1~10pF tantalum capacitor can be arranged for every 4~8 chips. (3) For devices with weak anti-noise capability and large power supply changes during turn-off, such as RAM and ROM memory devices, a decoupling capacitor should be directly connected between the power line and ground line of the chip. (4) Capacitor leads must not be too long, especially high-frequency bypass capacitors must not have leads.

In addition, the following two points should be noted: (1) When there are components such as contactors, relays and buttons on the printed board, a large spark discharge will be generated when they are operated. An RC circuit must be used to absorb the discharge current. Generally, R is 1~2KΩ, and C is 2.2~47μF. (2) CMOS has a very high input impedance and is susceptible to induction, so unused terminals must be grounded or connected to the positive power supply during use.

PowerPCB is a software product of Innoveda Corporation in the United States.

PowerPCB enables users to complete high-quality designs, vividly reflecting all aspects of the electronic design industry. Its constraint-driven design method can reduce product completion time. You can define safety spacing, routing rules and high-speed circuit design rules for each signal, and apply these rules 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 the layout and routing design. It includes a rich variety of functions, including cluster layout tools, dynamic routing editing, dynamic electrical performance inspection, automatic dimensioning and powerful CAM output capability. It also has the ability to integrate third-party software tools, such as the SPECCTRA router.

PowerPCB has been widely promoted and used in our institute at present, and its basic usage technology has been explained in detail in the training materials. However, for the majority of electronic application engineers in our institute, the problem lies in how to switch to the application of PowerPCB after being proficient in routing tools such as TANGO. Therefore, this article discusses such applications and some technical tips that are not covered in the training materials but are widely used by us.

For most people who have used TANGO, when they first start using PowerPCB, they may feel that PowerPCB has too many restrictions. Because PowerPCB is based on ensuring the correctness of the schematic input and the rule transmission from the schematic to the PCB. Therefore, its schematic does not have the function of disconnecting an electrical connection, nor can it arbitrarily stop an electrical connection at a certain position. It must ensure that each electrical connection has a start pin and an end pin, or is connected to a connector provided by the software for information transmission between different pages. This is a means for it to prevent errors, and in fact, it is also a standardized schematic input method that we should comply with.

In PowerPCB design, any changes inconsistent with the schematic netlist must be carried out in ECO mode. However, it provides users with an OLE link, which can transfer the modifications in the schematic to the PCB, and also transfer the modifications in the PCB back to the schematic. This not only prevents errors caused by negligence, but also facilitates the modifications that are really needed. However, it should be noted that when entering ECO mode, the "Write ECO File" option must be selected, and the ECO file writing operation will only be performed when exiting ECO mode.

There are two options for setting the power layer and ground plane in PowerPCB: CAM Plane and Split/Mixed. Split/Mixed is mainly used for the situation where multiple power supplies or grounds share one layer, but it can also be used when there is only one power supply and ground. Its main advantage is that the output drawing is consistent with the photoplot, which is convenient for inspection. CAM Plane is used for a single power supply or ground, and this mode outputs negative film. Attention should be paid to adding Layer 25 when outputting. Layer 25 contains the ground electrical information, mainly referring to the safety distance that the pad of the electrical layer is about 20mil larger than the normal pad, to ensure that after the via is metalized, no signal is connected to the ground. This requires each pad to contain the information of Layer 25. However, we often ignore this problem when building the library ourselves, resulting in the use of the Split/Mixed option.

PowerPCB provides a very useful function called automatic push. When we perform manual routing, the printed board is under our full control, and turning on the automatic push function will be very convenient. However, after you have completed the pre-routing and want to perform automatic routing, fix the pre-routed lines; otherwise, during automatic routing, the software will consider this line segment movable and completely overturn your work, causing unnecessary losses.

Our printed boards often need to add some mounting and positioning holes, but for PowerPCB, this belongs to component placement that is different from the schematic, which needs to be carried out in ECO mode. However, it is not convenient if the software gives us many errors because of this in the final inspection. In this case, the positioning hole device can be set as non-ECO registered.

In the edit device window, select the "Edit Electrical Properties" button, in this window, select the "Normal" item, and do not select the "ECO Registered" item. In this way, during inspection, PowerPCB will not consider this device to be compared with the netlist, and no unnecessary errors will occur.

Since our national standards are not quite consistent with those of American software companies, we have equipped international libraries for everyone to use as much as possible. However, new symbols for power supply and ground must be added to the library built into the software; otherwise, it will not recognize the symbols you created as power supplies.

So when you want to build a power supply symbol that meets the national standards, you need to first open the existing power supply symbol group, select the "Edit Electrical Connections" button, click the "Add" button, and enter the name of the new symbol you created and other information. 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 then be called up in the schematic.

Some pins of the devices we use are unused pins, marked as NC. We must pay attention to this when building the library; otherwise, the pins marked as NC will be connected together. This is because you built the NC pins in "SIGNAL_PINS" when building the library, and PowerPCB considers the pins in "SIGNAL_PINS" as implicit default pins and useful pins, such as VCC and GND. Therefore, for NC pins, they must be deleted from "SIGNAL_PINS", or you don't need to pay any attention to them at all and don't make any special definitions.

There are many variations in the package of triodes. When building the library of triodes by yourself, you will often find that after the schematic netlist is transferred to the PCB, the connection is inconsistent with what you expect. This problem is mainly caused by the library building.

Since the pins of triodes are often marked with E, B, and C, when creating your own triode library, you need to select the "Include Alphanumeric Pins" checkbox in the "Edit Electrical Connections" window. At this time, the "Alphanumeric Pins" tab is highlighted. Enter this tab and change the corresponding pins of the triode to letters. In this way, it will be easier to identify when connecting to the PCB package.

Now, due to the demand for miniaturization, surface mount devices are more and more widely used. The handling of surface mount devices is very important in the layout process, especially when routing multilayer boards. Because surface mount devices only have electrical connections on one layer, unlike dual in-line devices which have through holes on the board, when other layers need to be connected to surface devices, a short line needs to be pulled out from the pins of the surface mount device, a hole is drilled, and then connected to other devices. This is the so-called fan-in and fan-out operation.

If necessary, we should first perform fan-in and fan-out operations on the surface mount device before routing. This is because if we only select the fan-in and fan-out operations to be performed in the setting file of automatic routing, the software will perform this operation during the routing process, and the lines pulled out will be tortuous and relatively long. Therefore, after the layout is completed, we can first enter the automatic router, select only the fan-in and fan-out operations in the setting file, and do not select other routing options. In this way, the lines pulled out from the surface mount device are relatively short and neat.

Sometimes we need to add the printed board drawing to the structural drawing. At this time, we can convert the PCB file into a format recognizable by AUTOCAD through the conversion tool. In the PCB drawing frame, select the "Export" menu item in the "File" menu, set the save type as DXF file in the pop-up file export window, and then save it. You can open this drawing in AUTOCAD.

Of course, PADS has an automatic dimensioning function, which can dimension the printed board drawn and automatically display the position of the board frame or positioning holes. It should be noted that the dimensioning result is on the Drill-Drawing layer. If you want to add dimensioning to other output drawings, you need to specially add this layer when outputting.

The schematic drawn with ViewDraw can generate the netlist for PowerPCB. After PowerPCB reads the netlist, it can also perform functions such as automatic routing. Moreover, PowerPCB has a link tool, which can dynamically link and modify the schematic of VIEWDRAW to maintain the consistency of electrical connections.

However, due to the version difference of software modification and upgrade, sometimes the definition of device names between the two software is inconsistent, which will cause netlist transmission errors. To avoid this error, a special library can be built to store the corresponding devices between ViewDraw and PowerPCB. Of course, this is only for a part of mismatched devices. You can use the copy function in PowerPCB to easily copy the component packages from other existing libraries in PowerPCB to this library, and save them with the corresponding names in VIEWDRAW.

In the past, when we made printed boards, we copied the printed board drawings to a floppy disk and gave them directly to the board making factory. This method has poor confidentiality and is very cumbersome, requiring a very detailed description document to be written to the board making factory. Now, we can directly generate photoplot files with PowerPCB and send them to the manufacturer. The name of the photoplot file can indicate which layer of the routing it is, whether it is silkscreen or solder mask, which is very convenient and safe.

Steps to convert photoplot files: A. Change APERTURE to 999 in DEVICE SETUP of the CAM output window in PowerPCB. B. When converting the routing layer, set the document type to ROUTING, and then select the board frame and the things you need to put on this layer in LAYER. Note that LINE and TEXT should be removed when converting routing (unless you want to make copper characters on the lines). C. When converting the solder mask, set the document type to SOLD_MASK, and select the vias in the top solder mask. D. When converting the silkscreen, set the document type to SILKSCREEN, and refer to steps B and C for the rest. E. When converting the drilling data, set the document type to NC DRILL and convert directly.

Note that you must preview the photoplot file before converting it. The graphics in the preview are the graphics of the photoplot output you want, so check carefully to prevent errors.

With the experience in printed board design and the powerful functions of PowerPCB, drawing complex printed boards is no longer a troublesome thing. It is worth noting that we now have a tool to convert TANGO PCB to PowerPCB, so that the majority of technical personnel familiar with TANGO can more easily join the ranks of PowerPCB drawing, and draw satisfactory printed boards more conveniently and quickly.

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