High quality PCB design should pay attention to inventory
Reasonable component layout is the basIC prEMIse for designing high-quality products
1 Layout of components
The required layout of components mainly includes installation, force, heat, signal, and aesthetics
Refer to a series of basic requirements for the smooth installation of the circuit board into the chassis requirement. I won't go into details here
The circuit board should be able to withstand various external forces and vibrations during installation and work. Therefore, the positions of variable holes (screw holes, special shaped holes) on the board should be reasonably arranged In general, 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 be noted that the weak part of the plate caused by the special-shaped hole should also have sufficient bending strength In particular, connectors that "extend" directly from the equipment housing on the board shall be properly fixed to ensure long-term reliability
For high power devices with serial heat generation, in addition to ensuring heat dissipation conditions, it should also be placed in an appropriate position Especially in complex analog systems, special attention should be paid to the adverse effects of temperature fields generated by these devices on fragile preamplifier circuits Generally, components with very high power shall be made into separate modules, and certain thermal isolation measures shall be taken between them and signal processing circuits
Signal interference is an important factor to be considered in PCB layout design Several basic aspects are: weak signal circuit and strong signal circuit are separated or even isolated; 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 grounding wire; Measurement
It is not only necessary to consider the near and orderly placement of components Because ordinary laymen sometimes emphasize the former, in order to one-sided evaluate the advantages and disadvantages of circuit design, when the performance requirements are not strict, the former should be given priority to the product image However, in high-performance occasions, if double-sided boards must be used, the circuit boards are also packaged in them, which are usually invisible. First, the aesthetics of wiring should be emphasized The next section will discuss the "aesthetics" of cabling in detail
2. Wiring principles
Some anti-jamming measures not commonly found in the literature are detaiLED below. Considering that a large number of double-sided panels are still used in practical applications, especially in the trial production of products, the following contents mainly focus on double-sided panels
2.1. Wiring "Aesthetics"
When turning, avoid right angles and try to use diagonal lines or arcs for transition The wiring should be orderly and centrally arranged, which not only avoids mutual interference of signals of different properties, 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 the control signals, such as read and write, should be isolated and protected by a ground wire When playing the ground on a large area (discussed further below), try to keep a reasonable and equal distance between the ground wire (in fact, it should be the ground "surface") and the signal wire as close as possible under the premise of preventing short circuit and leakage For weak current system, ground wire and power wire shall be as close as possible For systems using surface mount components, the signal line should go all the way to the front
2.2. Ground wire arrangement
There are many discussions on the importance and the ground wire principle in the layout literature, but there is still a lack of detailed and accurate introduction to the ground wire layout My experience is that in order to improve the reliability of the system (her than just making an experimental prototype) To this end, we must spare no effort to implement the principle of "paving roads in large areas"
2.3. Power cord layout and power filter
The general literature says that the power core should be as thick as possible, I disagree 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). If only to prevent signal interference, the width of the power cord is not important Even though, sometimes thinner power cords are more beneficial The quality of power supply usually does not depend mainly on it, but in the fluctuation and superimposed interference of power supply The key to solve power interference is filter capacitor! If your application has strict requirements on power quality, do not be stingy in purchasing filter capacitors! When using filter capacitors, pay attention to the following points: the power input terminal of the whole circuit should have "total" filtering measures, and various types of capacitors should match each other, "the same cannot be less", at least J is not a bad thing For digital systems, 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 SIMulation system: DC and low-frequency simulation system: 1000uF | 1000uF electromagnetism+10uF tantalum+1uF patch+0.1uF patch Each important chip should have a set of filter capacitors around it For digital systems, a 0.1uF patch is usually sufficient, and the chip with operating frequency (CPU, crystal) should also be connected with 10nF | 470pF or 1nF The capacitor should be as close to the power supply pin of the chip as possible and connected directly as possible. The SMAller the capacitor, the closer it is the inner section (filter capacitor to chip power pins) should be as thick as possible It would be better if multiple thin lines could be used side by side With the filter capacitor to provide a low (AC) impact 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 In some analog systems, the power input also requires an RC filter network to further suppress interference, while the thinner power line sometimes only acts as a resistor in the RC filter, which is beneficial For systems with a wide range of operating temperatures, it should be noted that at low temperatures, the efficiency of aluminum electrolytic capacitors will decrease, or even lose their filtering effect In this case, suitable tantalum capacitors should be used For example, replace 470uF aluminum | 1000uF aluminum with 100uF tantalum, or replace 100uF aluminum with 22uF tantalum sheet Be careful not to place the aluminum electrolytic capacitor too close to the high-power heating device
3. Experience in reducing noise and electromagnetic interference
(1) High speed chips are not needed if low speed chips can be used
(2) A resistor can be connected in series to reduce the transition rate of the upper and lower edges of the control circuit.
(3) Try to provide some form of damping for relays etc.
(4) Use a frequency clock that meets the system requirements.
(5) The clock generator is as close as possible to the device using the clock. The case of quartz crystal oscillator shall be grounded
(6) Circle the clock area with a ground wire
(7) Use large-capacity tantalum capacitors or polycooled capacitors instead of electrolytic capacitors as circuit charge and discharge energy storage capacitors. When using tubular capacitors, the case should be grounded
The above is the explanation given by the editor of PCB circuit board company.
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