PCB Blog - High frequency PCB board wiring rules

1. Component arrangement rules
1) Under normal conditions, all components should be arranged on the same side of the printed circuit boards. Only when the top components are too dense, some devices with limited height and low heat generation, such as chip resistors and chip capacitors, can be placed., paste IC, etc. on the bottom layer.
2) On the premise of ensuring the electrical performance, the components should be placed on the grid and arranged in parallel or perpendicular to each other, so as to be neat and beautiful. Generally, overlapping of components is not allowed; the arrangement of components should be compact, and the input and output components should be kept as far away as possible.
3) There may be a high potential difference between certain components or wires, and the distance between them should be increased to avoid accidental short circuit due to discharge and breakdown.
4) Components with high voltage should be arranged in places that are not easily accessible by hand during debugging.
5) Components located at the edge of the board, at least 2 board thicknesses away from the edge of the board
6) The components should be evenly distributed and uniform in density on the entire board surface.

2. The principle of layout according to the direction of the signal
1) Usually, the positions of each functional circuit unit are arranged one by one according to the signal flow, and the components of each functional circuit are taken as the center, and the layout is carried out around it.
2) The layout of the components should facilitate the flow of signals and keep the signals in the same direction as possible. In most cases, the signal flow is arranged from left to right or from top to bottom, and the components directly connected to the input and output terminals should be placed close to the input and output connectors or connectors.

3. Prevent electromagnetic interference
1) For components with strong radiated electromagnetic fields and components sensitive to electromagnetic induction, the distance between them should be increased or shielded, and the direction of component placement should cross the adjacent printed wires.
2) Try to avoid high and low voltage devices being mixed with each other, and devices with strong and weak signals being staggered together.
3) For components that generate magnetic fields, such as transformers, speakers, inductors, etc., attention should be paid to reducing the cutting of magnetic lines of force to printed wires during layout, and the magnetic field directions of adjacent components should be perpendicular to each other to reduce the coupling between each other.
4) Shield the interference source, and the shield should be well grounded.
5) For circuits operating at high frequencies, the influence of the distribution parameters between components should be considered.

4. Suppress thermal interference
1) For the heating element, it should be arranged in a position that is conducive to heat dissipation. If necessary, a radiator or a small fan can be set up separately to reduce the temperature and reduce the impact on adjacent elements.
2) Some integrated blocks, large or medium power tubes, resistors and other components with high power consumption should be arranged in places where heat is easy to dissipate, and should be separated from other components.
3) The thermal element should be close to the measured element and away from the high temperature area, so as not to be affected by other heating power equivalent elements and cause malfunction.
4) When placing components on both sides, the bottom layer generally does not place heating components.

5. The layout of adjustable components
For the layout of adjustable components such as potentiometers, variable capacitors, adjustable inductance coils or micro switches, the structural requirements of the whole machine should be considered. If it is adjusted in the machine, it should be placed in the place where the printed circuit board is adjusted.

Design of printed circuit boards
SMT circuit board is one of the indispensable components in surface mount design. SMT circuit board is the support of circuit components and devices in electronic products, which realizes the electrical connection between circuit components and devices. With the development of electronic technology, the volume of the PCB board is getting smaller and smaller, the density is getting higher and higher, and the PCB board layer is continuously increasing. Therefore, the PCB board is required to be in the overall layout, anti-interference ability, technology and manufacturability. Increasingly demanding.

The main steps of printed circuit board design
1) Draw a schematic diagram.
2) Creation of component library.
3) Establish the network connection relationship between the schematic diagram and the components on the printed board.
4) Wiring and layout.
5) Create printed board production usage data and placement production usage data.

The following issues should be considered during the design of printed circuit boards:
1) It is necessary to ensure that the component graphics of the circuit schematic diagram are consistent with the real object and that the network connection in the circuit schematic diagram is correct.
2) The design of the printed circuit board not only considers the network connection relationship of the schematic diagram, but also considers some requirements of circuit engineering. The requirements of circuit engineering are mainly the width of power lines, ground wires and other wires, the connection of lines, High-frequency characteristics of some components, impedance of components, anti-interference, etc.
3) The installation requirements of the whole system of the printed circuit board mainly consider that the installation holes, plugs, positioning holes, reference points, etc. must meet the requirements, and the placement positions of various components must be accurately installed in the specified positions, and at the same time, it is necessary to facilitate Installation, system debugging, and ventilation and heat dissipation.
4) The manufacturability of the printed circuit board and its technological requirements, it is necessary to be familiar with the design specifications and meet the production process requirements, so that the designed printed circuit board can be produced smoothly.
5) Considering that the components are easy to install, debug, and repair in production, and at the same time, the graphics, pads, vias, etc. on the printed circuit board must be standardized to ensure that there will be no collision between components and easy installation.
6) The purpose of designing a printed circuit board is mainly for application, so we have to consider its practicability and reliability, and at the same time reduce the layer and area of the printed circuit board, thereby reducing costs, and appropriately larger pads, Through holes, wiring, etc. are conducive to the improvement of reliability, reducing the number of vias, optimizing the wiring, making it uniform in density and consistency, and making the overall layout of the board more beautiful. To make the designed circuit board achieve the expected purpose, the overall layout of the printed circuit board and the placement of components play a key role, which directly affects the installation, reliability, ventilation and heat dissipation of the entire printed circuit board, and the straight-through of wiring. Rate.

The outer size of the printed circuit board is given priority. When the size of the PCB board is too large, the printed lines will be long, the impedance will increase, the anti-noise ability will decrease, and the cost will also increase. If it is too small, the heat dissipation will be poor, and the adjacent lines will be easily interfered., First of all, give a reasonable positioning for the size and shape of the PCB board. Then determine the location of special components and unit circuits, etc., according to the circuit process, divide the whole circuit into several unit circuits or modules, and center the components of each unit circuit (such as integrated circuits), and other components must be The order of the components is evenly, neatly and compactly arranged on the PCB board, but do not get too close to these large components, there must be a certain distance, especially some relatively large and relatively high components to maintain a certain distance around, which is helpful for welding and Rework. For integrated circuits with high power, the color heat sink should be considered, and enough space should be reserved for it, and it should be placed in a place where the ventilation and heat dissipation of the printed board are good. At the same time, don't be too concentrated. Several large components are on the same board, and there must be a certain distance, and they should be in the direction of 45 degrees. Smaller integrated circuits such as (SOP) should be arranged in the axial direction. The components are aligned vertically and axially, all of which are relative to the direction of conveyance in the production process of the PCB board. In this way, the components are arranged regularly, thereby reducing defects in welding. Light-emitting diodes used for display, etc., should be considered to be placed at the edge of the printed board because they are used for observation during the application process. Some switches, trim elements, etc. should be placed where they are easily accessible. In the same frequency circuit, the distribution parameters between components should be considered. Generally, the distribution parameters between components should be considered in high-frequency circuits. In general, the components should be arranged in parallel as much as possible, which is not only beautiful, but also easy to install and weld. Easy to mass-produce, components located on the edge of the circuit board must be 3-5 cm away from the edge. When considering the position of the components, the thermal expansion coefficient, thermal conductivity, heat resistance and bending strength of the PCB board should be fully considered to avoid adverse effects on the components or the PCB board during production.After the position and shape of the components on the PCB are determined, the wiring of the PCB is considered.

With the location of the components, it is a principle to route the wiring according to the location of the components, and the traces on the printed board are as short as possible. The traces are short, occupying a small channel and area, so the pass-through rate will be higher. The wires of the input end and the output end on the PCB board should avoid the adjacent parallel as far as possible, and place a ground wire between the two wires. To avoid circuit feedback coupling. If the printed board is a multi-layer board, the routing direction of the signal lines of each layer is different from the routing direction of the adjacent board layers. For some important signal lines, you should reach an agreement with the circuit designer. In particular, differential signal lines should be routed in pairs, try to make them parallel, close to each other, and have little difference in length. All components on the PCB board minimize and shorten the leads and connections between the components. The width of the wires in the PCB board is mainly determined by the adhesion strength between the wires and the insulating layer substrate and the current value flowing through them. When the thickness of the copper foil is 0.05mm and the width is 1-1.5mm, the temperature will not be higher than 3 degrees through the current of 2A. When the wire width is 1.5mm, it can meet the requirements. For integrated circuits, especially digital circuits, 0.02-0.03mm is usually selected. Of course, as long as it is allowed, we use wide wires as much as possible, especially the power wires and ground wires on the PCB board. The spacing of the wires is mainly determined by the insulation resistance and breakdown voltage between wires in a good condition. For some integrated circuits (ICs), the pitch can be less than 5-8mm from a process point of view. The curved part of the printed wire is generally arced, and the use of traces with a bend of less than 90 degrees is avoided. The right angle and the included angle will affect the electrical performance in high-frequency circuits. In short, the wiring of the printed board should be uniform, the density should be appropriate, and the consistency should be good. Avoid using large-area copper foil in the circuit as much as possible. Otherwise, when heat is generated during use for a long time, the copper foil will easily expand and fall off. If a large-area copper foil must be used, grid-shaped conductors can be used. The terminals of the wires are the pads. The pad center hole is larger than the device lead diameter. If the pad is too large, it is easy to form virtual welding during welding. The outer diameter D of the pad is generally not less than (d+1.2) mm, where d is the aperture. For some components with relatively high density, the diameter of the pad is preferably (d+1.0) mm, After the pad design is completed, the outline frame of the device should be drawn around the pad of the printed board, and the text and characters should be marked at the same time. Generally, the height of the text or frame should be about 0.9mm, and the line width should be about 0.2mm. And the lines such as marked text and characters should not be pressed on the pads. In the case of a two-layer board, the underlying characters should be mirrored.

In order to make the designed product work better and effectively, the PCB board has to consider its anti-interference ability in the design, and has a close relationship with the specific circuit.
The design of power lines and ground lines in the circuit board is particularly important. According to the magnitude of the current flowing through different circuit boards, try to increase the width of the power line to reduce the loop resistance. At the same time, the direction of the power line and the ground line and the data The transmission direction remains the same. Contribute to the enhancement of the noise immunity of the circuit. There are logic circuits and linear circuits on the PCB, so that they are separated as much as possible. The low-frequency circuit can be grounded in parallel at a single point. The actual wiring can be connected in series and then grounded in parallel. The high-frequency circuit can be connected in series at multiple points. The ground wire should be short and thick. For high-frequency components, a large-area ground foil can be used. The ground wire should be as thick as possible. If the ground wire is very thin, the ground potential will change with the current, which will reduce the anti-noise performance. Therefore, the ground wire should be thickened so that it can reach three allowable currents on the circuit board. If the design allows the ground wire to have a diameter width of more than 2-3mm, in digital circuits, most of the ground wires are arranged in a loop to improve the anti-noise capability. In the design of PCB board, it is generally routine to configure appropriate decoupling capacitors in key parts of the printed board. Connect a 10-100uF electrolytic capacitor across the line at the power input. Generally, a 0.01PF ceramic capacitor should be arranged near the 20-30 pin. Generally, the power supply pin of the integrated circuit chip with 20-30 pins should be arranged. Nearby, a 0.01PF magnetic chip capacitor should be arranged. For larger chips, there will be several power pins, and a decoupling capacitor should be added near them. For chips with more than 200 pins, add them on all four sides. At least two decoupling capacitors are installed. If the space is insufficient, a 1-10PF tantalum capacitor can also be arranged on 4-8 chips. For components with weak anti-interference ability and large changes in power off, the decoupling capacitor should be directly connected between the power line and the ground line of the component., the lead wire connected to the capacitor is not easy to be too long. After the component and circuit design of the circuit board is completed, it is necessary to consider its process design. The purpose is to eliminate various adverse factors before the production starts, and at the same time, the manufacturability of the circuit board must be taken into account in order to produce high-quality products and batches. in production.

The process of the circuit board has already been involved in the positioning and wiring of components. The process design of the circuit board is mainly to organically assemble the circuit board and components we designed through the SMT production line, so as to achieve good electrical connection and achieve the location layout of our designed products. Pad design, wiring and anti-interference, etc., also need to consider whether the board we designed is easy to produce, can it be assembled with modern assembly technology - SMT technology, and at the same time, it is necessary to achieve the conditions that do not allow defective products to be produced in production. Design high. Specifically, there are the following aspects:
1) Different SMT production lines have their own different production conditions, but in terms of the size of the PCB board, the single board size of the PCB board is not less than 200*150mm. If the long side is too small, imposition can be used. At the same time, the ratio of length to width is 3:2 or 4:3. When the size of the circuit board is larger than 200*150mm, the mechanical strength of the circuit board should be considered.
2) When the size of the circuit board is too small, it is difficult for the SMT whole line production process, and it is not easy to mass-produce. At the same time, a whole board suitable for mass production is formed, and the size of the whole board should be suitable for the size of the pasteable range.
3) In order to adapt to the placement of the production line, the veneer should leave a range of 3-5mm without placing any components, and the panel should have a process edge of 3-8mm. There are three forms of connection between the process edge and the PCB board: A no overlapping edge, with a separation groove, B has a lap edge and a separation groove, and C has a lap edge and no separation groove. Equipped with blanking technology. According to the shape of the PCB board, there are different types of jigsaw puzzles. The process edge of the PCB board has different positioning methods according to different models. Some have positioning holes on the process edge. The diameter of the hole is 4-5 cm. In comparison, the positioning accuracy is higher than that of the edge. The positioning hole positioning model should be equipped with positioning holes when processing PCB boards, and the hole design should be standard to avoid inconvenience to production.
4) In order to better locate and achieve higher placement accuracy, it is necessary to set a reference point for the PCB board. Whether there is a reference point and whether the setting is good or bad directly affects the mass production of the SMT production line. The shape of the reference point can be square, circle, triangle, etc. And the diameter should be within the range of 1-2mm, and the surrounding of the reference point should be within the range of 3-5mm, without placing any components and leads. At the same time, the reference point should be smooth and flat, without any pollution. The design of the reference point should not be too close to the edge of the board, but a distance of 3-5mm.
5) From the overall production process, the shape of the board is pitch-shaped, especially for wave soldering. The use of a rectangle is convenient for conveying. If there is a gap in the PCB board, the gap should be filled in the form of a process edge. For a single SMT board, a gap is allowed. However, the groove is not easy to be too large and should be less than 1/3 of the length of the side.
In short, the generation of defective products is possible in every link, but as for the PCB board design, we should consider all aspects, so that it can not only achieve the purpose of designing the product, but also be suitable for the SMT production line in production. Mass production, try our best to design high-quality PCB board, and reduce the chance of defective products.