In all the processes of PCB design, wiring is the most delicate, most restrictive, and most time-consuming. Many engineers spend their daily work “drawing wires” diligently. Next, 2PCB will share some very practical wiring tips with you, hoping to be helpful to you.
Wiring tips between power and ground wires
(1) Add a decoupling capacitor between the power supply and the ground wire. The power supply must pass through the decoupling capacitor before being connected to the chip pin. Decoupling capacitors generally have two functions: one is to provide the chip with a large instantaneous current, and the other is to remove power supply noise. On the one hand, the power supply noise should affect the chip as little as possible, and on the other hand, the noise generated by the chip should not affect the power supply.
(2) Try to widen the power supply and ground wires. It is best that the ground wire is wider than the power supply wire. The relationship is: ground wire>power supply wire>signal wire.
(3) You can use a large area of copper layer as the ground wire. Connect all unused areas on the printed circuit board to the ground and use it as a ground wire, or make a multi-layer board with the power supply and ground wire occupying one layer each.
Processing when digital circuits and analog circuits are mixed
Nowadays, many PCBs are no longer single-function circuits, but are composed of a mixture of digital circuits and analog circuits. Therefore, when wiring, it is necessary to consider the problem of mutual interference between them, especially the noise interference on the ground line.
Since digital circuits have high frequencies and analog circuits have strong sensitivity, for signal lines, high-frequency signal lines should be as far away from sensitive analog circuit devices as possible. However, for the entire PCB, the ground line of the PCB can only have one node to the outside world, so the problem of common ground of digital circuits and analog circuits must be handled inside the PCB. Inside the circuit board, the ground of the digital circuit and the ground of the analog circuit are actually separated, but only at the interface where the PCB is connected to the outside world (such as plugs, etc.). The ground of the digital circuit is a little short-circuited with the ground of the analog circuit. Please note that there is only one connection point, and there are also non-common grounds on the PCB, which is determined by the system design.
Processing of line corners
Usually, there will be changes in thickness at the corners of the line, but when the thickness of the line diameter changes, some reflections will occur. The corner method is the worst for changes in line thickness. A right angle is the worst, a 45-degree angle is better, and a rounded corner is the best. However, rounded corners are more difficult to handle for PCB design, so it is generally determined by the sensitivity of the signal. A 45-degree angle is sufficient for general signals, and only very sensitive lines need rounded corners.
After laying out the lines, design rule checks should be performed
No matter what you do, you must check it after completion, just like when we have time left in an exam, we must check our answers. This is an important way for us to get high scores. The same is true when we draw PCB boards. In this way, we can be more confident that the circuit boards we draw are qualified products. We generally check the following aspects:
(1) Whether the distance between lines, lines and component pads, lines and through holes, component pads and through holes, and through holes is reasonable and meets production requirements.
(2) Whether the width of the power line and the ground line is appropriate, whether the power and ground lines are tightly coupled (low wave impedance), and whether there is any place in the PCB that can widen the ground line.
(3) Have the best measures been taken for key signal lines, such as the shortest length, adding protection lines, and clearly separating input and output lines?
(4) Do the analog circuit and digital circuit parts have their own independent ground lines?
(5) Will the graphics added to the PCB (such as diagrams and annotations) cause signal short circuits?
(6) Modify some undesirable line shapes.
(7) Whether there are process lines on the PCB, whether the solder mask meets the requirements of the production process, whether the solder mask size is appropriate, and whether the character logo is pressed on the device pad to avoid affecting the quality of the electrical equipment.
(8) Whether the outer frame edge of the power ground layer in the multilayer board is reduced. If the copper foil of the power ground layer is exposed outside the board, it is easy to cause a short circuit.
In short, the above skills and methods are all based on experience, which is very worth learning when drawing PCB boards. In the process of drawing PCB drawings, in addition to being proficient in using drawing tool software, you must also have solid theoretical knowledge and rich practical experience, which can help you complete your PCB drawings quickly and effectively. But there is another very important point, that is, you must be careful. Whether it is wiring or overall layout, every step must be treated carefully and seriously!