When drawing PCB, some very good wiring skills
Wiring is the most delicate and limited part of PCB design. Even engineers who have been wiring for more than ten years often feel that they can’t do it, because they have seen all kinds of problems and know what bad consequences will be caused by this line, so they don’t know how to do it. But there are still experts. They have very rational knowledge and at the same time they use some self-creative emotions to wire. The wires they wire are quite beautiful and artistic.
Here are some good wiring skills and essentials:
First of all, let’s make a basic introduction. The number of layers of PCB can be divided into single layer, double layer and multi-layer. Single layer is basically eliminated now. Double-layer boards are now widely used in audio systems, generally as rough boards for power amplifiers. Multi-layer boards refer to boards with 4 layers or more. Generally speaking, 4 layers are enough for components with low density requirements.
From the perspective of vias, they can be divided into through holes, blind holes, and buried holes.
A through hole is a hole that goes directly from the top layer to the bottom layer; a blind hole is a hole from the top or bottom layer to the middle layer, and then does not continue to penetrate. The advantage is that the position of this via is not blocked from beginning to end, and other layers can still route at the position of this via; a buried via is a via from the middle layer to the middle layer, which is buried and completely invisible on the surface. The specific situation is shown in the figure below.
Before automatic routing, interactive routing is used in advance for the lines with higher requirements.
The edge lines of the input and output ends should not be adjacent and parallel to avoid reflection interference. If necessary, a ground wire can be added for isolation, and the wiring of two adjacent layers should be perpendicular to each other, because parallelism is more likely to produce parasitic coupling. The routing rate of automatic routing depends on a good layout, and 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 routing is performed first to quickly connect short lines, and then the maze routing is used to optimize the global routing path of the lines to be routed. It can disconnect the already routed lines as needed and try to re-route them, thereby improving the overall routing effect.
For layout, one principle is to separate digital and analog as much as possible, and another principle is not to get low-speed devices close to high-speed devices. The most basic principle is to separate digital ground from analog ground. Since digital ground is a switching device, the current is very large at the moment of switching and very small when it is not moving, so digital ground cannot be mixed with analog ground. A recommended layout can be shown in the figure below.
1.Precautions for wiring between power supply and ground
(1) Add decoupling capacitors between power supply and ground. The power supply must be connected to the chip pins after passing through the decoupling capacitor. The figure below lists several incorrect connection methods and a correct connection method. Please refer to them and see if you have made such a mistake. 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 wire. The relationship is: ground wire>power wire>signal wire.
(3) You can use a large copper layer as a ground wire, connect all unused areas on the printed circuit board to the ground, or make a multi-layer board with the power supply and ground wire occupying one layer each.
2.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 wire.
Due to the high frequency of digital circuits and the strong sensitivity of analog circuits, 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 wire 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 connects to the outside world (such as plugs, etc.). The ground of the digital circuit and the ground of the analog circuit are a little short-circuited. Please note that there is only one connection point. There are also some that are not common ground on the PCB. This is determined by the system design.
3.Handling of line corners
Usually, the thickness of the line will change at the corners, but when the thickness of the line changes, some reflection will occur. For the change of line thickness, the right angle is the worst, the 45-degree angle is better, and the rounded corner is the best. However, rounded corners are more troublesome to handle for PCB design, so it is generally determined by the sensitivity of the signal. For general signals, a 45-degree angle is sufficient, and only those very sensitive lines need rounded corners.
4.Check the design rules after laying out the lines
No matter what you do, you must check it after completion, just like when we have time left in the 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 board we draw is a qualified product.
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 are reasonable and meet 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 where the ground line can be widened.
(3) Whether the best measures have been taken for key signal lines, such as the shortest length, adding protection lines, and clearly separating the input and output lines.
(4) Whether the analog circuit and digital circuit parts have their own independent ground lines.
(5) Whether the graphics added to the PCB later (such as diagrams and annotations) will cause signal short circuits.
(6) Modify some undesirable line shapes.
(7) Whether process lines are added to 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 are worth learning when drawing PCB boards. In addition to being proficient in using drawing tools and software, you must also have solid theoretical knowledge and rich practical experience, which can help you quickly and effectively complete your PCB drawings. 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, because a small mistake may cause your final product to become waste, and then you can’t find where the mistake is, so in the process of drawing, we would rather spend more time to carefully check the details than go back to check after a problem occurs, which may take more time. In short, pay attention to the details when drawing PCBs.
