As an electronic engineer, the miniaturization of electronic products and the complexity of system design make PCB design more and more complex. Today's highly integrated circuit boards are getting smaller and smaller, and the pins of packaged devices are getting denser and denser, all of which have brought enormous pressure to wiring.
As the top priority of the PCB design process, wiring will directly affect the performance of the PCB board. The design process is also the most complicated and requires higher requirements. Although many advanced EDA tools now provide automatic routing functions and are quite intelligent, automatic routing cannot guarantee a 100% routing rate. Therefore, many engineers are not satisfied with the results of automatic routing, and manual routing is still the choice of most engineers, and the requirements of signal integrity are achieved by constraining the routing of electrical rules.
In the PCB design process, wiring can be roughly divided into three realms: " The first is routing, which is also the most basic requirement for PCB design. If the circuit is blocked, then the basic function of the board is useless, it is a waste board, not to mention other. The second is performance, which is a measure of how good a printed circuit board is. After the wiring is completed, it is necessary to consider how to achieve the best performance and avoid all kinds of interference. The third is beautiful, smooth lines and excellent performance, but if the picture is beautiful but you dare not look at it, you should consider how to beautify the messy lines. The wiring is neat and uniform, which can also bring great convenience to future testing and maintenance, which is also the basic quality that senior engineers should have.Wiring is far from being as simple as imagined, and it is not just a simple job, but if wiring is done as an art, there will be different harvests. There are pictures and the truth, let us feel the wiring difference between the novice and the master together.
So how to route elegantly? The wiring method can be very different. In order to avoid the reflection interference caused by the adjacent parallel lines of the input end and the output end during wiring, and the interference such as parasitic coupling between the two adjacent wiring layers being parallel to each other, the stability of the line will be affected, and even when the interference is serious. The circuit board cannot work at all, and many factors need to be considered when wiring. Now I will give you the rules of PCB wiring.
3W rule
Here 3W is the distance between lines to keep 3 times the line width. You said 3H is also ok. But here H refers to the line width. Not the media thickness. In order to reduce the crosstalk between lines, the line spacing should be large enough. If the line center distance is not less than 3 times the line width, 70% of the electric field between lines can be kept from interfering with each other, which is called the 3W rule. To achieve 98% of the electric fields do not interfere with each other, the 10W rule can be used.
The adoption of the "20H rule" refers to ensuring that the edge of the power plane is at least 20 times smaller than the edge of the 0V plane equivalent to the distance between the two planes. It refers to the distance that the power supply layer is retracted by 20H relative to the ground layer, of course, it is also to suppress the edge radiation effect. Electromagnetic interference is radiated outward at the edge of the board. Retract the power plane so that the electric field is only conducted within the confines of the ground plane. Taking one H (the thickness of the medium between the power supply and the ground) as the unit, if it is retracted by 20H, 70% of the electric field can be confined within the edge of the ground layer; if it is retracted by 100H, 98% of the electric field can be confined within.
The selection rule for the number of layers of the printed board, that is, if the clock frequency reaches 5MHz or the pulse rise time is less than 5ns, the PCB board must use a multi-layer board. This is a general rule. Sometimes, for cost and other factors, a double-layer board is used. In this case, it is best to use one side of the printed board as a complete ground plane layer.
Power and ground rules
The interference between the power supply and the bottom line directly affects the performance of the product, so the wiring of the power supply and the ground wire should try to minimize the noise interference to ensure the quality of the product. Every engineer who is engaged in the design of electronic products understands the reason for the noise between the ground wire and the power wire, and now only the reduced noise suppression is expressed: it is well known that adding a noise between the power supply and the ground wire coupling capacitor. Try to widen the width of the power supply and the ground wire, preferably the ground wire is wider than the power supply wire. 0.05~0.07mm, the power line is 1.2~2.5mm. For the PCB of the digital circuit, a wide ground wire can be used to form a loop, that is, a ground net can be used (the ground of the analog circuit cannot be used in this way) Use a large area copper layer as the ground wire For use, connect the unused places on the printed board to the ground as a ground wire. Or make a multi-layer board, power supply, ground wire each occupy one layer.
Signal line layout rules on the electrical (ground) layer
In the wiring of multi-layer printed boards, since there are not many lines left in the signal line layer, adding more layers will cause waste and increase the production workload, and the cost will increase accordingly. To solve this contradiction, we can consider wiring on the electrical (ground) layer. The power plane should be considered first, followed by the ground plane. Because it is best to preserve the integrity of the formation.
Common ground rules for digital circuits and analog circuits
Now PCBs are mostly composed of digital circuits and analog circuits, and the interference between each other should be considered when wiring, especially the noise interference on the ground. The frequency of the digital circuit is high, and the sensitivity of the analog circuit is strong. For the signal line, the high-frequency signal line should be kept away from the sensitive analog circuit devices as much as possible. For the ground wire, the whole PCB has only one node to the outside world, so The problem of digital and analog common ground must be dealt with inside the PCB, and the digital ground and the analog ground are actually separated inside the board, and they are not connected to each other, but at the interface (such as a plug, etc.) where the PCB is connected to the outside world. The digital ground is a little shorted to the analog ground, note that there is only one connection point. There are also different grounds on the PCB, which are determined by the system design.
Handling Rules for Connecting Legs in Large Area Conductors
In a large area of grounding (electricity), the legs of commonly used components are connected to it, and the handling of the connecting legs needs to be comprehensively considered. There are some bad hidden dangers in the welding assembly of components, such as: ① Welding requires high-power heaters. ②It is easy to cause virtual solder joints. Therefore, taking into account the electrical performance and process needs, a cross-shaped pad is made, which is called heat shield, commonly known as thermal pad. Sex is greatly reduced. The electrical (ground) leg of a multilayer board is treated the same way.
network system rules
A grid ground system is an effective way to reduce trace inductance and provide a return path for RF currents. If the grid is too dense, although the number of channels is increased, the step is too small, and the amount of data in the image field is too large, which must have higher requirements on the storage space of the equipment, and also affect the computing speed of computer electronic products. great influence. And some vias are invalid, such as those occupied by pads of component legs or occupied by mounting holes and fixed holes. Too sparse grids and too few channels have a great impact on the distribution rate. Therefore, there must be a grid system with reasonable density to support the wiring. The distance between the legs of standard components is 0.1 inches (2.54mm), so the basis of the grid system is generally set to 0.1 inches (2.54 mm) or less than an integral multiple of 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 inches etc.
Design Rule Check (DRC) Rules
After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules made by the designer, and it is also necessary to confirm whether the rules made meet the requirements of the printed board production process. Generally, the following aspects are checked:
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. Is the width of the power line and the ground line appropriate, and is the power and ground line tightly coupled (low wave impedance)? Is there any place in the PCB that can widen the ground line?
3. Whether the best measures have been taken for the key signal lines, such as the shortest length, the addition of protection lines, and the obvious separation of input lines and output lines.
4. Whether the analog circuit and the digital circuit have their own independent ground wires.
5. Whether the graphics (such as icons, labels) added to the PCB will cause short circuit of the signal.
6. Modify some unsatisfactory line shapes. Is there a process line 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, so as not to affect the electrical quality.
7. Whether the edge of the outer frame of the power ground layer in the multi-layer board is reduced, for example, the copper foil of the power ground layer is exposed outside the board, which may easily cause a short circuit.
There are many points to pay attention to in PCB layout, the above rules are only part of them, and there will be more problems encountered in actual work. Moreover, the routing rate of automatic routing depends on a good layout. The routing rules, including the number of bending times of the traces, the number of vias, and the number of steps, need to be predetermined in advance. Therefore, if you want to truly master PCB wiring, you still need to practice diligently, accumulate rich experience, and master more rules and skills, so that you can perform PCB wiring gracefully and design exquisite and beautiful PCB boards.
Reprinted from: https://mp.weixin.qq.com/s?__biz=MjM5NDQ0NjM5Mg==&mid=2650411296&idx=1&sn=252c94ddf8edb287a040677deed6f485&scene=21#wechat_redirect