075582814553
The PCB (printed circuit board) design process includes schematic design, electronic component database login, design preparation, block division, electronic component configuration, configuration confirmation, wiring, and final inspection. Wherever a problem is found in the process, it must be returned to the previous process for re-confirmation or correction.
The component layout is critical in PCB layout design, which determines the neatness and beauty of the board and the length and number of printed wires, and also has a certain impact on the reliability of the whole machine. The following are the principles of the PCB components layout.
(1)The components are evenly distributed and packed on the whole board. Components do not occupy the board surface, and the board around the edge should leave some space. The size of the space to be left depends on the size of the PCB and the way of the fixation. Components that are located on the edge of the printed circuit board, should be at least greater than 3mm away from the edge of the PCB. Generally, there is leave a space of 5~ 10mm around each side of the PCB in electronic equipment.
(2)In general, the components should be set up on one side of the PCB, and each component of the lead pin should occupy a separate pad.
Component layout
(3)The layout of the components should not be crossed up and down. It is necessary to maintain a certain distance between the two adjacent components. Spacing shall not be too small. If the potential difference between adjacent components is high, a safe distance should be maintained. The safety voltage for the gap in the general environment is 200 V/mm.
Component layout direction
(4)The installation height of the components should be as low as possible. The lead of general components should not exceed 5mm away from the board, otherwise, the stability to withstand vibration and the deterioration is affected.
(5)According to the installation position and state of the PCB in the whole machine, we can determine the axis of the component direction. Components that are regular arrangement, should make the axis of the larger components in the vertical state, which can improve the stability of the components fixed on the board.
(6)Components at both ends of the pad span should be slightly larger than the axial dimensions of the component body.
Component assembly
There two ways of installing and fixing components on the PCB: vertical and horizontal. Horizontal means that the direction of the axis of the components parallel to the PCB surface, shown in Figure. These two ways have their own characteristics. In the design of the PCB, you can use one of them, and can also use both ways. However, it is necessary to ensure that the circuit has good anti-vibration performance and evenly arranged components.
Component installation method
Vertically mounted components occupy a small area and can accommodate a large number of components per board surface. This kind of installation method is suitable for products with a dense and compact arrangement of components, such as semiconductor radios, hearing aids, etc. The components in many small portable instruments also often adopt the vertical installation method. The vertically fixed components require small volume and lightweight, and oversized and overweight components should not be mounted vertically. Otherwise, the machine's mechanical strength deteriorates, vibration resistance is weakened, and the components are easy to collapse caused by a mutual contact, reducing the reliability of the circuit.
Compared with the vertical fixation, the primary device horizontal installation has the advantages of good mechanical stability, the board surface is arranged neatly, etc... The horizontal fixation makes the span of the element enlarge, which is very advantageous for laying out the printed wire.
Components should be evenly, neatly, and compactly arranged on the printed circuit board, minimizing and shortening the leads and connections between the individual cell circuits and between each component. There are two ways of arranging components on the PCB, irregular and regular format. These two ways on the PCB can be used alone or may appear at the same time.
The irregular arrangement of components
The axis direction of the components is not consistent with each other, the arrangement order on the board also does not have certain rules. In this way, the arrangement of components looks messy and disorderly. But because the components are not restricted to the position and direction, the printed wire layout is convenient. It can shorten and reduce the connection of components, greatly reducing the total length of the printed wire on the board. This kind of layout format reduces the distribution parameter and the suppression interference of the circuit board. This is more advantageous for the high-frequency circuit pole. This arrangement is also generally adopted in the vertical installation of fixed components.
The axis of the components is aligned in the same direction and is perpendicular and parallel to the four sides of the board.
In addition to high-frequency circuits, the components in general electronic products should be arranged as parallel or perpendicularly as possible. When fixed components are installed horizontally, they should be arranged regularly. This is not only for the beautiful and neat surface of the board, but also for easy assembly, welding, debugging, and easy production and maintenance. The regular arrangement is especially suitable for low-frequency circuits with relatively loose board surface, relatively few types of components, and a large number of components. Components in electronic instruments often use this arrangement. However, because the regular arrangement of components is subject to certain restrictions on the direction or position, the layout of the wires on the PCB may be more complicated, and the total length of the wires will increase accordingly.
Each lead wire of a component must occupy a pad on the PCB, and the position of the pad varies with the size of the component and its fixing method. For vertical fixed and irregularly arranged boards, the position of the pads can not be restricted by the size and spacing of components; for regularly arranged boards, the position of each pad and the distance between each other should comply with certain standards. No matter which fixing method or arrangement rule is adopted, the center of the pad (ie the center of the lead hole) should not be too close to the edge of the PCB. Generally, the distance should be more than 2.5mm, at least greater than the thickness of the board.
The location of the pads is generally required to fall on the intersection of the standard coordinate grid.
Standard coordinate network
In the International Electrotechnical Commission (IEC) standard, the basic grid spacing of the standard coordinate grid is 2.54mm, and the auxiliary grid spacing is 1.27mm or 0.635mil. This grid spacing standard only has practical significance in computer automatic design, automatic drilling, and automatic assembly and welding of components. For general manual drilling, in addition to the pins of dual in-line integrated circuits, the positions of the pads of other components may not be strictly restricted by this grid spacing. However, in the layout design, the position of the pads should be arranged as neatly and uniformly as possible, and for components with similar dimensions, the pad spacing should be unified (the pad center distance should not be less than the thickness of the board). In this way, it is not only neat and beautiful but also convenient for component assembly and lead bending. Of course, the so-called neatness and consistency are also relatively speaking, special circumstances should be adjusted to specific conditions.
Flexibility in arrangement
The layout principle of the whole circuit is: the whole circuit is divided into a number of circuit units according to their functions, and the position of each functional circuit unit on the board is arranged one by one according to the flow direction of the electric signal so that the layout is convenient for the signal flow, and the signal flow can be kept in the same direction as far as possible. In most cases, the signal flow direction is arranged from left to right (left input, right output) or from top to bottom (top input, bottom output). Components that are directly connected to the inputs and outputs should be placed near the input and output connectors. Center the core components of each functional circuit and layout around it. For example, semiconductor devices such as transistors or integrated circuits are usually used as core components, and other components are arranged according to the location of their respective electrodes. Consider the shape, size, polarity, and number of pins of each component, and adjust their orientation and location with the goal of shortening the connection.
The problem of interference with electronic products is more complex, it may be caused by a variety of electrical, magnetic, thermal, mechanical and other factors. Therefore, when proceeding to design the layout of the PCB, to decide the layout of the machine circuit, you should analyze the principle of the circuit. First of all, determine the location of special components, and then arrange for other components, try to avoid possible interference with the factors, and take measures so that PCB interference may arise from the maximum suppression.
The so-called special components refer to those components that have an impact on the performance of the whole machine from the aspects of electricity, magnetism, heat, and mechanical strength, or are fixed in position according to operating requirements.
Electromagnetic interference is a phenomenon that often occurs in the work of the whole machine, and its reasons are also various. In addition to interference caused by external factors (such as spatial electromagnetic waves), unreasonable PCB wiring and improper component installation positions can all cause interference. These interference factors can be completely avoided if they are prioritized in the layout design. On the contrary, if the design is not well considered, interference will occur and the design will fail. Here, several electromagnetic interferences and suppression methods that may be caused by PCB design schemes are discussed.
Components that may affect or interfere with each other should be separated or shielded as much as possible. Try to shorten the wiring between the high-frequency components, reduce their distribution parameters, and mutual electromagnetic interference (if you need to use a metal shield for the high-frequency part, you should also leave the shielding cover on the board. area). Components that are susceptible to interference should not be too close. The components of the strong current part (220V) and the weak current part (DC power supply), the input stage, and the output stage should be separated as much as possible. When the DC power lead is long, filter components should be added to prevent 50Hz interference.
As some components or wires may have a higher potential difference, the distance between them should be increased to avoid accidental short circuits due to discharge and breakdown. The components of the metal shell should avoid touching each other. For example, the shell of the NPN transistor or the heat sink of the high-power tube is generally connected to the collector C of the core, which is connected to the positive or high potential of the power supply in the circuit; the shell of the electrolytic capacitor is the negative electrode, which is grounded or connected to the low potential in the circuit. If the two shells are not insulated, the distance between them must be considered when designing the circuit board. Otherwise, when the circuit is working, the collision between the two will cause a short circuit accident.
The interference caused by temperature rise should also be paid attention to in PCB design. For example, a transistor is a temperature-sensitive device, especially a semiconductor device made of germanium material. It is more susceptible to the influence of ambient temperature to cause its operating point to drift, causing changes in the electrical performance of the entire circuit. When designing the PCB layout, you should first analyze and distinguish which are heating components and which are temperature sensitive components.
The heating components installed on the board (such as resistors with high power consumption) should be placed near the shell or in a well-ventilated place, so as to use the vent holes excavated on the shell to dissipate heat. Try not to put several heating components together, and consider using devices such as radiators or small fans so that the temperature rise of the components does not exceed the allowable value. High-power devices can be directly fixed on the chassis, and use the metal shell to conduct heat; if they must be installed on the printed circuit board, pay special attention to not installing them close to the board, but to configure a large enough heat sink. It should also keep a certain distance from other components to avoid heat conduction or heat radiation from heating components to surrounding components.
Temperature-sensitive components, such as transistors, integrated circuits, and other thermal components, large-capacity electrolytic capacitors, etc., should not be placed near the heat source or on the upper part of the equipment. The temperature rise caused by the long-term operation of the circuit will affect the working status and performance of these components.
Pay attention to the balance and stability of the center of gravity of the entire circuit board. For those components that are large, heavy, and generate more heat (such as power transformers, large electrolytic capacitors, and high-power transistors with heat sinks, etc.), generally do not directly install and fix them on the printed circuit board. They should be fixed on the bottom of the chassis so that the center of gravity of the whole machine is lowered and it is easy to stabilize. Otherwise, these large components will not only occupy a large amount of effective area and space on the PCB, but when they are fixed, the PCB may be bent and deformed, causing mechanical damage to other components, and poor contact of the externally connected connectors. If large components weighing more than 15g must be mounted on a circuit board, they cannot be fixed by soldering only by soldering pads, but auxiliary fixing measures such as brackets or clips should be used.
When the size of the printed circuit board is larger than 200mm×150mm, considering the mechanical stress caused by gravity and vibration on the circuit board, it should be reinforced with a mechanical frame to avoid deformation. And you should leave the positions for fixing brackets, positioning screws, and connecting sockets on the board.
Regarding the layout of adjustment elements such as potentiometers, variable capacitors, or adjustable inductance coils, the overall structure of the machine should be considered. If it is adjusted outside the machine, its position should be adapted to the position of the adjustment knob on the chassis panel. If it is adjusted inside the machine, it should be placed on the PCB where it can be easily adjusted.
In order to ensure the safety of debugging and maintenance, pay special attention to the components with high voltage (such as the anode high voltage circuit components of the display), and try to arrange them in places that are not easy to reach during operation.
The main content of printed circuit design is layout design. The reasonable layout of electronic components on a certain board area is the first step in designing printed circuits. The layout design is not just simply connecting components through printed lines according to circuit principles. In order to make the whole machine work stably and reliably, it is necessary to carry out a reasonable layout of the components and their connections on the PCB. If the layout is unreasonable, various interferences may occur, so that a reasonable principle scheme cannot be realized, or the technical indicators of the whole machine are reduced. Although some layout designs can reach the technical parameters of the principle, the arrangement of the components is uneven and disorderly, which not only affects the appearance but also brings inconvenience to assembly and maintenance. Of course, such a design cannot be considered reasonable.
Catalogue
FPGA / CPLD
Memory
MOS 
MCU 
DSP
OCEP
Secondary 
Other 