Wiring is the most detailed and limited technique in the PCB design process. Even engineers who have been working on the PCB for more than ten years tend to think that they will not be able to wire the PCB, because they see all kinds of problems and know what will happen if the wire cloth is put out. However, there are some masters. They have very rational knowledge, and at the same time, they take some self-created emotions to make the wiring, which is quite beautiful and artistic.
Here are some good wiring tips and tricks:
First of all, a basic introduction is made first, the number of layers of PCB can be divided into single layer, double layer and multi-layer, single layer is now basically eliminated. Double layer board is used in sound system now very much, it is to regard power to put in commonly bold type board, multilayer board is the board that points to 4 layers and 4 above, to the density of component demand not high generally speaking 4 layers is enough. It can be divided into through-hole, blind hole, and buried hole from the Angle of through-hole. A through-hole is a hole that passes directly from the top to the bottom; The blind hole goes from the top hole or the bottom hole to the middle hole and then does not continue to go through. The advantage is that the position of the hole is not from top to bottom. The buried hole is the hole that passes through the middle layer to the middle layer, buried, completely invisible to the surface. The specific situation is shown in the figure below.
Before the automatic wiring, the high-demand line should be arranged interactively in advance. The side lines of the input end and the output end should not be adjacent and parallel, so as to avoid reflection interference. When necessary, the ground can be added for isolation, and two adjacent layers of wiring should be perpendicular to each other, because parallel is more likely to generate parasitic coupling. The routing rate of automatic wiring depends on good layout, and the wiring rules can be set up in advance, such as the number of routing bending, number of conducting holes, number of steps, etc. Generally, exploratory wiring should be carried out first, and quick connection of short lines should be made, and then the global wiring path optimization should be carried out for the connection of cloth through maze wiring. It can disconnect the wiring of cloth and try to rearrange it as needed, so as to improve the overall wiring effect.
For layout, one principle is to separate the digital and the analog as much as possible, so that one principle is low speed and not close to high speed. The basic principle is to separate the digital grounding from the analog grounding. Since digital grounding is a switch device, the current is very large at the moment of switching and very small when not moving, so digital grounding cannot be mixed with analog grounding. A recommended layout can look like the following.
1. Matters needing attention for wiring between power supply and ground wire
(1) the decoupling capacitance shall be added between power supply and ground wire. It is necessary to connect the power supply to the chip pin after the decoupling capacitance. Several wrong connection methods and a correct connection method are listed in the following figure. In general, decoupling capacitors have two functions: one is to provide the high current of the chip in an instant, and the other is to remove the noise from the power supply. On the one hand, the noise from the power supply should be as low as possible to affect the chip; on the other hand, the noise generated by the chip should not affect the power supply.
(2) make the power and ground wire as wide as possible, preferably with the ground wire wider than the power cord. The relationship is: ground wire > power cord > signal wire.
(3) a large area of copper layer can be used as the ground line, connecting all unused places to the ground on the printed board to be used as the ground line, or making multi-layer board, the power supply and the ground line each occupy one layer.
2. Processing when digital circuit and analog circuit are mixed
Nowadays, many PCBS are no longer single-function circuits, but composed of a mixture of digital circuits and analog circuits. Therefore, the problem of mutual interference, especially noise interference on the ground, needs to be taken into account when wiring.
Due to the high frequency digital circuits, analog circuit sensitivity is strong, for signal lines, high frequency signal as far as possible away from the sensitive analog device, but for the whole PCB, PCB ground wire to the outside world nodes can have only one, so must be within the PCB processing, the problem of digital circuit and analog circuit, the circuit board inside, the field of digital circuit and analog circuit is actually a separate, is connected with the outside world in PCB interface (e.g., plugs, etc.). There is a short connection between the digital circuit and the analog circuit, please note that there is only one connection point, and there is non-common ground on the PCB, which is determined by the system design.
3. Processing of line corners
Usually there is a change in thickness around the corner of the line, but some reflection occurs when the diameter changes. The corner method is the worst right Angle for the change in thickness of the line, the 45 degree Angle is better, and the rounded Angle is the best. However, round corners are troublesome to PCB design, so it is usually determined by the sensitivity of the signal. The general signal can be used at a 45 degree Angle, and only those very sensitive lines need to use round corners.
4. Check the design rules after the wire is laid
No matter what we do, we should check it after finishing, just like we should check our answers if there is time left in the examination. This is an important way for us to get high scores, and the same is true for us to draw PCB boards. In this way, we can be more sure that the circuit board we drew is qualified. We generally check the following aspects:
(1) whether the distance between wire and wire, wire and component pad, wire and connector hole, component pad and connector hole is reasonable and meets the production requirements.
(2) whether the width of the power cord and ground wire is appropriate, whether the power supply and ground wire are tightly coupled (low wave impedance), and whether there is still room in the PCB for widening the ground wire.
(3) whether the best measures are taken for the key signal lines, such as the shortest length, protection line, input line and output line are clearly separated.
(4) whether the analog circuit and digital circuit have their own ground wires.
(5) whether the figure (as shown and marked) in the PCB with J after that will cause the signal short circuit.
(6) modify some unsatisfactory lines.
(7) whether there is a process line on the PCB, whether the resistance welding meets the requirements of the production process, whether the resistance welding dimension is appropriate, and whether the character mark is pressed on the device pad, so as not to affect the installation quality.
(8) whether the outer frame edge of the power layer in the multi-layer board is shrunk, for example, the copper foil exposure plate in the power layer can easily cause short circuit.
All in all, the above skills and methods are all experiences, which are worth learning when drawing PCB boards. In addition to skilled application of drawing tools and software, solid theoretical knowledge and rich practical experience are also needed in the process of drawing PCB boards, which can help you to complete your PCB drawings quickly and effectively. But there is very important, that is, must be careful, wiring and overall layout each step is very careful seriously, because you are a small mistake can lead to the final product you become a waste, and also can not find something wrong, so we would rather spend more time in drawing process carefully check the details are not willing to back to check out, it may take more time. In short, pay attention to the details of the process of drawing a PCB.