It is the most skillful and limited in PCB design. Even engineers who have laid wires for more than ten years often feel that they can’t lay wires. Because they see all kinds of problems and know what consequences this wire will lead to when it is laid out, they don’t know how to lay it out. But there are some masters. They have very rational knowledge, and at the same time, they lay the wires with some self-created feelings. The wires laid out are quite beautiful and artistic.
Here are some good wiring tips and essentials:
First of all, make a basic introduction. The layers of PCB can be divided into single-layer, double-layer and multi-layer, and single-layer is basically eliminated now. Double-layer boards are widely used in audio systems nowadays, and they are generally used as rough boards for power amplifiers. Multilayer boards refer to boards with four or more layers. Generally speaking, four layers are enough for components with low density requirements. From the perspective of via, it can be divided into via, blind hole and buried hole. A hole is a hole that passes directly from the top layer to the bottom layer; The blind hole passes through the top or bottom hole to the middle layer, and then it doesn’t continue to pass through. This advantage is that the position of this via hole is not blocked from beginning to end, and other layers can still be routed in this via hole position; Buried hole means that this via hole is buried from the middle layer to the middle layer, and the surface is completely invisible. The specific situation is shown in the following figure.
Before automatic wiring, wire the lines with high requirements interactively in advance, and the edges of the input and output ends should not be adjacent and parallel to avoid reflection interference. When necessary, ground wires can be added for isolation, and the wires of two adjacent layers should be perpendicular to each other, because it is easy to generate parasitic coupling in parallel. The routing rate of automatic routing depends on good layout, and the routing rules can be set in advance, such as the number of routing bends, the number of vias, the number of steps, etc. Generally, exploratory wiring is carried out first, short lines are quickly connected, and then the wiring to be laid is globally optimized through labyrinth wiring. It can disconnect the laid lines and try to rewire them according to the needs, thus improving the overall wiring effect.
As far as layout is concerned, one principle is that digital and analog should be separated as much as possible, and the other principle is that low speed should not approach high speed. The most basic principle is to separate digital grounding from analog grounding. Because digital grounding is a switching device, the current is very large at the moment of switching, and it is very small when it is stationary, so digital grounding cannot be mixed with analog grounding.
1. Matters needing attention in wiring between power supply and ground wire
(1) A decoupling capacitor should be added between the power supply and the ground wire. Make sure that the power supply is connected to the pins of the chip after the decoupling capacitor. The following figure lists several wrong connection methods and a correct connection method. Let’s refer to it. Have you made such a mistake? Generally speaking, decoupling capacitors have two functions, one is to provide the instantaneous high current of the chip, the other is to remove the noise of the power supply. On the one hand, the noise of the power supply will affect the chip as little as possible, and on the other hand, the noise generated by the chip will not affect the power supply.
(2) Try to widen the power supply and ground wire, preferably the ground wire is wider than the power wire, and the relationship is: ground wire > power wire > signal wire.
(3) A large area of copper layer can be used as the ground wire, and unused places can be connected to the ground wire on the printed board, or it can be made into a multi-layer board with one layer for power supply and one layer for ground wire.
2. Processing when digital circuit and analog circuit are mixed.
Nowadays, many PCBs are no longer single-function circuits, but are composed of digital circuits and analog circuits. Therefore, the mutual interference between them, especially the noise interference on the ground line, should be considered when wiring.
Because of the high frequency of digital circuits and the strong sensitivity of analog circuits, as far as signal lines are concerned, high-frequency signal lines should be as far away from sensitive analog circuit devices as possible. However, for the whole PCB, there is only one node between the ground of PCB and the outside world, so it is necessary to deal with the problem that digital circuits and analog circuits share the same ground inside the PCB. However, inside the PCB, the ground of digital circuits and the ground of analog circuits are actually separated, only at the interface between PCB and the outside world (such as plug). There is a short circuit between the ground of the digital circuit and the ground of the analog circuit. Please note that there is only one connection point, and some of them are not grounded on the PCB, which is determined by the system design. 3. Treatment of line corners
Usually, there will be thickness changes at the corners of the line, but when the diameter of the line changes, some reflection will occur. Corner For the change of line thickness, right angle is the worst, 45-degree angle is better, and rounded angle is the best. However, rounded corners are troublesome for PCB design, so it usually depends on the sensitivity of signals. Generally, a 45-degree angle is enough for signals, and only those very sensitive lines need rounded corners.
4. Check the design rules after laying the wires.
No matter what you do, you have to check it after you finish it, just as we have to check our answers if we have time left in the exam. This is an important way for us to get high marks. Similarly, it is the same when we draw PCB. Only in this way can we be more sure that the circuit board we drew is a qualified product. Our general inspection has the following aspects:
(1) Whether the distance between line and line, line and component pad, line and through hole, component pad and through hole, through hole and through hole is reasonable and meets the production requirements.
(2) Whether the width of the power line and the ground wire is appropriate, whether the power line and the ground wire are tightly coupled (low wave impedance), and whether there is any place in the PCB where the ground wire can be widened.
(3) Whether the best measures have been taken for the key signal lines, such as the shortest length, adding protection lines, and the input lines and output lines are clearly separated.
(4) Whether the analog circuit and digital circuit have their own independent ground wires.
(5) Whether the graphics added to the PCB (e.g. illustration, note mark) will cause signal short circuit.
(6) Revise some unsatisfactory alignment.
(7) Whether there is a process line on the PCB, whether the solder resist meets the requirements of the production process, whether the solder resist size is appropriate, and whether the character mark is pressed on the device pad, so as not to affect the quality of electrical equipment.
(8) Whether the edge of the outer frame of the power layer in the multilayer board is narrowed, such as the copper foil of the power layer exposed outside the board, which is easy to cause short circuit.
In a word, the above techniques and methods are all experience, which is very worthy of our learning when drawing PCB. In the process of drawing PCB, you should have solid theoretical knowledge and rich practical experience, which can help you finish your PCB quickly and effectively. However, it is also very important to be careful. Every step of wiring and overall layout should be taken seriously, because a small mistake of yours may cause your final product to become a waste product, and then you can’t find any mistakes. Therefore, in the process of drawing, we would rather spend more time carefully checking the details than returning to check if something goes wrong, which may take more time. In short, pay attention to the details in the process of drawing PCB.
