PCB Blog - The essentials of paying attention to details in the process of drawing the PCB board

Wiring is the most delicate and limited skill in the PCB board design process. Even engineers who have laid wiring for more than ten years often feel that they cannot wire because they have seen all kinds of problems and know what will happen if the wire is laid out. Bad consequences, so I don't know how to spread it. But there are still masters. They have very rational knowledge, and at the same time, they carry some self-created emotions to wire, and the wires they lay out are quite beautiful and artistic.

Here are some good wiring tips and essentials:
First of all, let's make a basic introduction. The number of layers of the PCB board can be divided into single-layer, double-layer and multi-layer, and the single-layer is basically eliminated now. Double-layer boards are used a lot in audio systems now, and they are generally used as power amplifiers. Will suffice. From the perspective of via holes, it 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 passed from the top layer or bottom layer to the middle layer, and then does not continue to pass through. The advantage is that the position of the via hole is not blocked from the beginning to the end. Other layers can still be routed at the position of this via; a buried via means that this via is from the middle layer to the middle layer, buried, and the surface is completely invisible. Before automatic routing, route the lines with higher requirements interactively in advance, and the edges of the input end and the output end should not be adjacent and parallel to avoid reflection interference. When necessary, a ground wire can be added for isolation, and the wirings 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 routing rules can be pre-set, such as the number of wire bends, the number of vias, and the number of steps. Generally, exploratory wiring is performed first, and short lines are quickly connected, and then through labyrinth wiring, the global wiring path is optimized for the wiring to be routed. Overall wiring effect. For layout, a principle is to separate digital and analog as much as possible, and a principle is that low-speed should not be close to high-speed. The basic principle is to separate digital grounding and analog grounding. Since digital grounding is a switching device, the current is large at the moment of switching, and small when it is not moving. Therefore, digital grounding cannot be mixed with analog grounding. A recommended layout can look like the image below.

1. Precautions for wiring between power supply and ground wire
1) A decoupling capacitor should be added between the power supply and the ground wire. The power supply must be connected to the pins of the chip after passing through the decoupling capacitor. The following figure lists several wrong connection methods and a correct connection method. Please refer to it. Have you made such a mistake? Generally speaking, decoupling capacitors have two functions, one is to provide a large current of the chip instantaneously, and the other is to remove power supply noise. On the one hand, the noise of the power supply can affect the chip as little as possible. power supply.
2) Widen the power and ground wires as much as possible. The ground wire is wider than the power wire, and the relationship is: ground wire > power wire > signal wire.
3) You can use a large area of copper layer as the ground wire, connect the unused places to the ground on the printed board, use it as a ground wire, or make a multi-layer board, the power supply and the ground wire each occupy one layer .

2. Processing when digital circuits and analog circuits are mixed
Nowadays, many PCB boards are no longer a single-function circuit, but are composed of a mixture of digital circuits and analog circuits, so it is necessary to consider the problem of mutual interference between them when wiring, especially the noise interference on the ground wire . Due to the high frequency of digital circuits and strong sensitivity of analog circuits, for signal lines, high-frequency signal lines should be as far away from sensitive analog circuit devices as possible, but for the entire PCB board, the ground wire of the PCB board is not sensitive to the external There can only be one node, so the problem of the common ground of the digital circuit and the analog circuit must be dealt with inside the PCB board, and inside the circuit board, the ground of the digital circuit and the ground of the analog circuit are actually separated, only in the PCB The interface between the board and the outside world (such as a plug, 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, and some have no common ground on the PCB board, which is determined by the system design.

3. Processing of line corners
Usually there will be changes in the thickness of the corners of the wire, but when the thickness of the wire diameter changes, some reflections will occur. For the change of the thickness of the line, the right angle is bad, the 45-degree angle is better, and the rounded corner is good. However, rounded corners are more troublesome for PCB board design, so it is generally determined by the sensitivity of the signal. Generally, a 45-degree angle is sufficient for general signals. Only those very sensitive lines need to use rounded corners.

4. Check the design rules after wiring
No matter what you do, you must check it after you finish it. Just like when we take an exam, if there is time left, we must check our answers. This is an important way for us to get high scores. Similarly, we draw PCB boards. In this way, we can be more sure that the circuit board we draw is a qualified product. We generally check the following aspects:
1) Whether the distance between wire and wire, wire and component pad, wire and through hole, component pad and through hole, and through hole and through hole is reasonable, and whether it meets the production requirements.
2) Whether the width of the power line and the ground line is appropriate, whether the power supply and the ground line are tightly coupled (low wave impedance), and whether there is any place in the PCB board that can widen the ground line.
3) Whether measures have been taken for key signal lines, such as short lengths, adding protection lines, and clearly separating input lines and output lines.
4) Whether the analog circuit and the digital circuit part have their own independent ground wires.
5) Whether the graphics added to the PCB board will cause a short circuit of the signal.
6) Modify some unsatisfactory line shapes.
7) Whether there is a process line on the PCB board, 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, so as not to affect the quality of the electrical equipment.
8) Whether the edge of the outer frame of the power ground layer in the multi-layer board is reduced, such as the copper foil of the power ground layer exposed outside the board, it is easy to cause a short circuit.
In a word, the above skills and methods are based on experience, and it is very worthwhile for us to learn and draw lessons from when drawing PCB boards. In the process of drawing PCB board drawings, in addition to using drawing tools and software proficiently, we must also have solid theoretical knowledge and rich experience. Practical experience, these can help you complete your PCB board diagram quickly and efficiently. But there is also a very important point, that is, you must be careful, whether it is wiring or the overall layout, you must be very careful and serious, because your small mistake may cause your final product to become waste, and then find There is nothing wrong, so we would rather spend more time carefully checking the details during the drawing process than go back and check if there is a problem, which may take more time. In short, the process of drawing the PCB board pays attention to the details.