PCB layout principle

writer: G March 29, 2019

1. Layout "Aesthetics"

Avoid corners when cornering, try to use a diagonal or arc transition.

The lines should be neat and orderly, and arranged in different categories, which can not only avoid mutual interference of signals of different natures, but also facilitate 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. However, control signals like reading, writing, and clock should be used alone. It is best to use ground protection. stand up.

When laying a large area (discussed further below), the ground line (in fact, it should be the "face") should be kept at a reasonable equal distance from the signal line, and should be as close as possible to prevent short circuit and leakage.

For weak current systems, the ground and power lines should be as close as possible.

For systems that use surface mount components, the signal lines are as far as possible.

2. Ground layout

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 of the ground wire arrangement in the actual PCB. 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-scale paving".

When paving the ground, it must generally be grid-like, except for sporadic sites that are separated by other lines. The grid-like heating performance and high-frequency conductivity are much better than the entire ground. In double-panel wiring, sometimes the ground line has to be split in order to walk the signal line, which is extremely disadvantageous for maintaining a sufficiently low ground resistance. To this end, a series of "small clever" means must be used to ensure the "unobstructed" current. These tips include:

A large number of surface mount components are used, eliminating the space that the "original" occupied by the solder holes should belong to the ground.

Make full use of the front space: In the case of extensive use of surface mount components, try to make the signal line as far as possible to the top layer, and let the bottom layer "selflessly" to the ground line, which involves countless small tricks, I will make "PCB" One of the tricks: Exchange Pins has a trick, and there are many similar spells that will be written in the future.

Reasonably arrange the signal line, and let the important areas on the board, especially the "belly" (which is related to the communication of the entire board and ground), "give" to the ground line, as long as it is carefully designed, this can still be done.

The cooperation between the front side and the back side: Sometimes on one side of the board, the ground line is really "no way to go". At this time, we can try to make the wiring on both sides coordinate with each other. "There is no place to stay here, there is no place to stay." Corresponding position vacates a sufficient ground to lay the ground wire, and then pass through a sufficient number of well-located vias (considering that the via has a large resistance), through which the "bridge" will be traversed by the signal line Forced to split but reluctantly, looking forward to the unified two sides of the two sides into a conductive enough overall.

The number of dogs rushing to the wall: If you can't get out of the place and you are not willing to be huge, the ground wire is cut off by a signal line in the district, let this signal be wronged, take the jumper. Sometimes, I am not willing to pull only a bare wire. This signal happens to pass through a resistor or other "long-legged" device. I can justify the pin of this device and make it a jumper. Both pass the signal and avoid the indecent name of the jumper: - (Of course, in most cases, I can always let such a signal pass from the right place and avoid crossing with the ground. The only thing needed is 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 advance side by side. Sometimes, choosing a reasonable path, a row of ground lines is worthy of a group army that is unreasonably configured.

Finally, by the way, there is a famous saying: "You can trust your mother, but never trust your land." In the case of extremely weak signal processing (below microvolts), even if the ground potential is consistent by any means, the ground potential difference at the key points on the circuit still exceeds the amplitude of the processed signal, at least the same magnitude, even if the static potential is appropriate. The instantaneous potential difference can still be large. For such a situation, it is first necessary to make the operation of the circuit as independent as possible from the ground potential.

3. Power line layout and power supply filtering

The general literature agrees that the power cord should be as thick as possible, and I don't quite agree with it. Only in the case of high power (the average supply current may reach 1A in 1 second), it is necessary to ensure sufficient power line width (my experience, 50 mil per 1A current can meet the needs of most occasions). The width of the power cord is not critical if it is only to prevent signal interference. Even sometimes, a thinner power cord is more advantageous! The quality of the power supply is generally not mainly in its absolute value, but in the fluctuation of the power supply and the interference of the superposition. The key to solving the power supply interference is the filter capacitor! If your application has strict requirements on the power quality, don't worry about the filter capacitor! Pay attention to the following when using the filter capacitor:

The power input of the whole circuit should have "total" filtering measures, and all types of capacitors should be matched with each other. "There should be no less." At least not bad. J must have at least 100uF electrolysis + 10uF for digital systems. +0.1uF patch + 1nF patch. Higher frequency (100kHz) 100uF electrolysis + 10uF film + 0.47uF patch + 0.1uF patch. AC analog system: For DC and low frequency analog systems: 1000uF|1000uF electrolysis + 10uF chip 钽 u uu patch + 0.1uF patch.

There should be a "set" of filter capacitors around each important chip. For digital systems, a 0.1uF chip is generally sufficient. A chip with a large or large operating current should be connected to a 10uF chip or a 1uF chip. The chip with the highest operating frequency (CPU, crystal oscillator) and 10nF |470pF or a 1nF. The capacitor should be as close as possible to the power supply pins of the chip and connected as directly as possible. The smaller the closer it should be.

For the chip filter capacitor, the length of the filter capacitor to the chip power pin should be as thick as possible. It is better to use multiple thin wires side by side. With a filter capacitor to provide a low (AC) impedance voltage source and to suppress AC-coupled interference, the power line outside the capacitor pin (referring to the section from the total power supply to the filter capacitor) is not so important, the line width does not have to be too thick, at least It is not necessary to occupy a large amount of board area for this purpose. In some analog systems, the UPS filter network is required for the power input to further suppress the interference, and the thinner power supply line sometimes has the effect of the resistor in the RC filter, which is advantageous.

For systems with a wide range of operating temperature changes, it should be noted that the performance of aluminum electrolytic capacitors will be reduced or even lost at low temperatures. In this case, replace them with appropriate tantalum capacitors. For example, instead of 470 uF aluminum with 100 uF|1000 uF aluminum, or 100 uF aluminum with 22 uF.

Note that the aluminum electrolytic capacitor should not be too close to the high-power heating device.