A good circuit board should not only realize the function of circuit principle, but also consider the electrical characteristics such as EMI, EMC, ESD (electrostatic discharge), signal integrity, as well as the heat dissipation of mechanical structure and high power consumption chips. On this basis, consider the beauty of the circuit board, just like artistic carving, and consider every detail.
1. Common PCB layout constraint principles
The following aspects are often considered in the layout of PCB components.
(1) Is the shape of 1)PCB matched with the whole machine?
(2) Is the spacing between components reasonable? Is there any conflict in level or height?
(3) Does 3)PCB need imposition? Is the process edge reserved? Are mounting holes reserved? How to arrange positioning holes?
(4) How to place the power module and dissipate heat?
(5) Is it convenient to replace the components that need to be replaced frequently? Is the adjustable element convenient to adjust?
(6) Is the distance between the heat-sensitive element and the heating element considered?
(7) What is the EMC performance of the whole board? How can layout effectively enhance anti-interference ability?
As for the distance between components, based on the different distance requirements of different packages and the characteristics of Altium Designer itself, the setting is too complicated and difficult to realize if it is constrained by rule setting. Generally, lines are drawn on the mechanical layer to mark the peripheral dimensions of the components, as shown in Figure 9-1, so that when other components approach, the distance between them can be roughly known. This is very practical for beginners, and it can also make beginners develop good PCB design habits.
Figure 9-1 Mechanical auxiliary line
Through the above consideration and analysis, common PCB layout constraint principles can be classified as follows.
2. Principle of element arrangement
(1) Under normal conditions, all components should be arranged on the same side of PCB. Only when the top components are too dense, can some components with limited height and low calorific value (such as SMD resistor, SMD capacitor, SMD IC, etc.) be placed on the bottom.
(2) On the premise of ensuring the electrical performance, the components should be placed on a grid and arranged parallel or vertically to each other, so as to be neat and beautiful. Under normal circumstances, the components are not allowed to overlap, and the components should be arranged compactly. The input components and the output components should be separated as far as possible, so as not to cross.
(3) There may be high voltage between some components or wires, so the distance between them should be increased to avoid accidental short circuit caused by discharge and breakdown. Pay attention to the layout space of these signals as much as possible during layout.
(4) Components with high voltage should be arranged as far as possible in places where hands are not easy to touch during debugging.
(5) The components located at the edge of the board should be as far as possible at a distance of two board thicknesses from the edge of the board.
(6) The components should be evenly distributed on the whole panel, and one area should not be dense while the other area is loose, so as to improve the reliability of products.
3. According to the principle of signal layout
(1) After placing the fixed elements, arrange the positions of each functional circuit unit one by one according to the signal flow direction, and make a local layout around the core element of each functional circuit.
(2) The layout of components should be convenient for signal circulation, so as to keep the signals in the same direction as possible. In most cases, the signal flow direction is arranged from left to right or from top to bottom, and the components directly connected with the input and output terminals should be placed close to the input and output connectors or connectors.
4. Prevent electromagnetic interference.
Figure 9-2 The inductor is laid out 90 perpendicular to the inductor.
(1) For components with strong radiation electromagnetic field and sensitive to electromagnetic induction, the distance between them should be increased, or consider adding shielding cover to shield them.
(2) Try to avoid the high and low voltage components being mixed with each other and the strong and weak signal components being staggered together.
(3) For components that can generate magnetic fields, such as transformers, speakers, inductors, etc., attention should be paid to reducing the cutting of printed wires by magnetic lines of force during layout. The magnetic fields of adjacent components should be perpendicular to each other to reduce the coupling between them. Figure 9-2 shows the layout of the inductor 90 perpendicular to the inductor.
(4) Shielding the interference source or the module susceptible to interference, and the shielding cover should be well grounded.
5. Restrain thermal interference.
(1) For the heating elements, priority should be given to the location conducive to heat dissipation. If necessary, a radiator or a small fan can be set separately to lower the temperature and reduce the impact on adjacent elements, as shown in Figure 9-4.
(2) Some integrated blocks, high-power tubes, resistors, etc. with high power consumption should be arranged in places where heat dissipation is easy, and separated from other components by a certain distance.
Figure 9-4 Heat dissipation considerations of layout
(3) The heat-sensitive element should be close to the measured element and far away from the high temperature area, so as to avoid being influenced by other heat-generating equivalent elements and cause misoperation.
(4) When placing components on both sides, heating components are generally not placed on the bottom layer.
6. Layout principle of adjustable components
For the layout of adjustable components such as potentiometer, variable capacitor, adjustable inductance coil, microswitch, etc., the structural requirements of the whole machine should be considered: if it is adjusted outside the machine, its position should be adapted to the position of the adjusting knob on the chassis panel; If it is adjusted in the machine, it should be placed on the PCB where it is easy to adjust.
