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PCB design experience, wiring is an important step to complete product design. The quality of PCB wiring directly affects the performance of the entire system. Wiring is very important in high-speed PCB design. The wiring design process is highly restricted, with fine skills and heavy workload. PCB wiring includes single-sided wiring, double-sided wiring and multilayer wiring.
The PCB design process of PCB board is a complicated process. If you want to master it well, you need electronics enthusiasts to experience it by yourself to get the true meaning of it. The following will analyze the rationality of some situations that may be encountered in actual wiring, and give some more optimized routing strategies.
1. Some experience on PCB routing
1. The edges of the input end and the output end should be avoided adjacent and parallel to avoid reflection interference. Ground wire isolation should be added when necessary; the wiring of two adjacent layers should be perpendicular to each other, which is prone to parasitic coupling.
2. Ground wire>power wire>signal wire, usually the signal wire width is: 8mil～12mil; the power wire is 50mil～100mil. For the PCB of the digital circuit, a wide ground wire can be used to form a loop, that is, to form a ground net to use (the ground of the analog circuit cannot be used in this way)
3. You can use some island copper, and then connect it to the ground plane.
4. In the PCB board, the digital ground and analog ground are actually separated, they are not connected to each other, but at the interface (such as plugs, etc.) connecting the PCB to the outside world. There is a short connection between the digital ground and the analog ground. Please note that there is only one connection point. There are also non-common grounds on the PCB, which is determined by the system design.
5. There is really no place for wiring, you can consider laying it on the VCC layer, and then consider the GND layer.
6. The distance between the legs of standard components is 100mil (2.54mm), so the basis of the grid system is generally set to 100mil (2.54 mm) or an integral multiple of less than 100mil, such as: 50mil, 25mil, 20mil, etc. In general layout, choose a 50mil grid and a 5mil grid for wiring. The hole spacing and device distance are set to 25mil (to allow wiring between devices). 7. I think snake-shaped wiring is just for length matching! ! Inductance, filtering, I don’t think such a dumb method would be used. 8. The copper laying on the edge of the board should be 20mil away from the edge of the board.
PCB design
9. The delay on the PCB is 0.167ns/inch. However, if there are many vias, many device pins, and many constraints set on the network cable, the delay will increase.
10. The wider the wire diameter, the closer to the power/ground, or the higher the dielectric constant of the isolation layer, the smaller the characteristic impedance.
11. The traces on the PCB board can be equivalent to series and parallel capacitance, resistance and inductance structures. The typical value of series resistance is 0.25-0.55 ohms/ft. The resistance of the parallel resistor is usually very high.
12. If CMOS or TTL circuit is used for design, the operating frequency is less than 10MHz, and the wiring length should not be greater than 7 inches. The wiring length should not be greater than 1.5 inches at 50MHz. If the operating frequency reaches or exceeds 75MHz, the wiring length should be 1 inch.
13. Any high-speed and high-power devices should be placed together as much as possible to reduce the transient overshoot of the power supply voltage.
14. Only when using the * to switch the layer when using the network fly line, the Via will be automatically added. When executing Place Line, Via will not be added automatically when changing layers.
15. The PCB design method of modifying the line width before routing. After executing the routing command and pressing the starting point, Track Width will be displayed in the lower right corner of the screen, which is the current line width. At this time, you can press Tab to modify the line width. And this line width will remain until the next time the line is routed to modify the line width. This function is similar to the Current Track of the DOS version.
