PCB Blog - Part of the requirements for PCB layout in high-quality PCB design

In order to ensure the quality of the printed circuit boards design, when designing the PCB board, it is necessary to pay attention to whether the wiring part of the PCB board diagram meets the requirements.

1. Component layout
Reasonable arrangement of components is the basic premise of designing a high-quality PCB layout. The requirements for component layout mainly include installation, force, heat, signal, and aesthetics.
1.1. Installation
Refers to a series of basic requirements for the smooth installation of the PCB board into the chassis, casing, and slot in a specific application, so as to avoid accidents such as space interference and short circuit, and to keep the designated connector in the designated position on the chassis or casing. Require. I won't go into details here.

1.2. Force
The circuit board should be able to withstand various external forces and vibrations during installation and work. For this reason, the PCB board should have a reasonable shape, and the positions of various holes (screw holes, special-shaped holes) on the board should be reasonably arranged. Generally, the distance between the hole and the edge of the plate should be at least greater than the diameter of the hole. At the same time, it should also be noted that the weak section of the plate caused by the special-shaped hole should also have sufficient bending strength. In particular, the connectors that "extend" directly out of the device shell on the board should be reasonably fixed to ensure long-term reliability.
1.3. Heat
For high-power devices with serious heat generation, in addition to ensuring heat dissipation conditions, attention should also be paid to placing them in appropriate positions. Especially in sophisticated analog systems, special attention should be paid to the detrimental effects of temperature fields generated by these devices on fragile preamp circuits. Generally, the part with very large power should be made into a separate module, and certain thermal isolation measures should be taken between it and the signal processing circuit.
1.4. Signals
Signal interference is an important factor to be considered in PCB layout design. Several basic aspects are: the weak signal circuit is separated or even isolated from the strong signal circuit; the AC part is separated from the DC part; the high frequency part is separated from the low frequency part; pay attention to the direction of the signal line; the layout of the ground wire; measure. These have been repeatedly emphasized in a large number of treatises and will not be repeated here.
1.5. Beautiful
It is not only necessary to consider the neat and orderly placement of components, but also to consider the graceful and smooth wiring. Because the general layman sometimes emphasizes the former, in order to one-sidedly evaluate the pros and cons of the circuit design, for the image of the product, the former should be given priority when the performance requirements are not harsh. However, in high-performance occasions, if a double-sided board has to be used, and the circuit board is also encapsulated in it, it is usually invisible, and the aesthetics of the traces should be given priority. The next section will discuss the "aesthetics" of the wiring in detail.

2. Wiring principles
Some anti-jamming measures not commonly found in the literature are detailed below. Considering that in practical applications, especially in product trial production, a large number of double-sided panels are still used, and the following contents are mainly aimed at double-sided panels.

2.1. Wiring "Aesthetics"
When turning, avoid right angles and try to use slashes or arcs for transitions. The wiring should be neat and orderly, and arranged in a centralized manner, which not only avoids the mutual interference of signals of different natures, but also facilitates inspection and modification. For digital systems, there is no need to worry about interference between signal lines (such as data lines and address lines) of the same camp, but control signals such as read, write, and clock should be isolated and protected with ground wires. When laying the ground in a large area (discussed further below), try to maintain a reasonable and equal distance between the ground wire (in fact, it should be the ground "surface") and the signal wire, and try to be as close as possible under the premise of preventing short circuits and leakage. For weak current systems, the ground wire and the power wire should be as close as possible. For systems using surface mount components, the signal lines should go all the way to the front.

2.2. Ground wire arrangement
There are many discussions on the importance and layout principles of the ground wire in the literature, but there is still a lack of detailed and accurate introduction to the ground wire layout in the actual PCB board. My experience is that in order to improve the reliability of the system (rather than just making an experimental prototype), the ground wire cannot be overemphasized, especially in weak signal processing. To this end, we must spare no effort to implement the principle of "large area paving". When laying the ground, it must generally be grid-like, unless those scattered sites are divided by other lines. The thermal performance and high-frequency conductivity of the grid ground are much better than that of the whole ground wire. In double-sided wiring, sometimes the ground wires have to be separated in order to route the signal wires, which is extremely unfavorable for keeping the ground resistance low enough. To this end, a series of "smart" means must be used to ensure the "smooth" ground current. These tricks include: making heavy use of surface mount components, eliminating the space that "should" belong to ground. Make full use of the frontal space: In the case of using a large number of surface mount components, try to make the signal line go to the top layer as much as possible, and give the bottom layer "selfless" to the ground line, which involves countless small tips, my own writing "PCB" One of the board skills: Swap pins” has a trick, and there are many similar spells, which will be written one after another in the future. Reasonably arrange the signal lines, and "give" important areas on the board, especially the "hintland" (this is related to the communication of the entire board ground line) to the ground line, as long as it is carefully designed, this can still be done. The coordination of the front and the back: Sometimes on one side of the board, the ground wire is really "no way", then you can try to make the wiring on both sides coordinate with each other. There is enough space in the corresponding position to lay the ground wire, and then pass through enough and reasonably located vias (considering that the vias have a large resistance), through this "bridge" will be crossed by the signal line. Forcibly divided, but reluctantly, and looking forward to reunification, the two sides of the strait are connected into a whole with sufficient electrical conductivity. The point of the dog jumping over the wall: When the huge ground wire that can't find a place and is not willing to be cut off by a mere signal line, let this Signal grievances, go with jumper wires. Sometimes, I am not willing to just pull a bare wire, this signal happens to pass through a resistor or other "long-legged" device, I can justifiably extend the pins of this device, making it double duty as a jumper, both passing the signal and avoiding the unseemly name of a jumper. Of course, in most cases, I can always let such a signal go through the right place and avoid it with The intersection of ground wires requires observation and imagination. The minimum principle: the path of the ground current should be reasonable, and the large current and the weak signal current must not move side by side. Sometimes, choose a reasonable path, and a row of ground wires An army with an unreasonable configuration. By the way, there is a famous saying: "You can trust your mother, but never trust your ground". In the case of extremely weak signal processing (below microvolts), even if the means are guaranteed In order to keep the ground potential consistent, the ground potential difference of the key points on the circuit still exceeds the amplitude of the signal being processed, at least by the same order of magnitude. Even if the static potential is appropriate, the instantaneous potential difference may still be very large. For such occasions, first of all In principle, the operation of the circuit should be as independent of the ground potential as possible.

2.3. Power line layout and power filter
The general literature says that the power cord should be as thick as possible, which I don't quite agree with. Only in the case of high power (the average power supply current may reach 1A in 1 second), it is necessary to ensure sufficient power line width (in my experience, 50mil per 1A current can meet the needs of most occasions). The width of the power line is not critical if it is only to prevent signal interference. Even, sometimes thinner power cords are more beneficial! The quality of the power supply generally lies not mainly in it, but in the fluctuation of the power supply and the superimposed interference. The key to solving power interference is the filter capacitor! If your application does have strict requirements on power quality, don't be stingy with the money for filter capacitors! When using filter capacitors, pay attention to the following: the power input of the entire circuit should have "total" filtering measures, and various types of capacitors should be matched with each other, "the same can not be less", at least not bad for the digital J The system must have at least 100uF electrolysis + 10uF tantalum + 0.1uF patch + 1nF patch. Higher frequency (100kHz) 100uF electrolysis + 10uF tantalum + 0.47uF patch + 0.1uF patch. AC analog system: For DC and low frequency analog systems: 1000uF | 1000uF electrolysis + 10uF tantalum + 1uF patch + 0.1uF patch. There should be a "set" of filter capacitors around every important chip. For digital systems, a 0.1uF patch is generally enough, an important chip or a chip with a large working current should also be connected with a 10uF chip tantalum or 1uF chip, and the chip with operating frequency (CPU, crystal oscillator) should also be connected with 10nF| 470pF or a 1nF. The capacitor should be as close as possible to the power pins of the chip and connected as directly as possible, the smaller the closer. For the chip filter capacitor, the inner section (from the filter capacitor to the chip power pin) should be as thick as possible. It is better if multiple thin wires can be used side by side. With the filter capacitor to provide a low (AC) impedance voltage source and suppress AC coupling interference, the power line outside the capacitor pin (referring to the section from the main power supply to the filter capacitor) is not so important, the line width does not need to be too thick, at least It is not necessary to take up a lot of printed circuit boards area for this.