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Analysis of common rule of thumb in PCB
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Analysis of common rule of thumb in PCB

There are some common rules of thumb in PCB design and wiring, these rules are not universal matching, whether applicable to the specific application scenarios, then PCB design company electronics for everyone to analyze the application of common rules of thumb in PCB design and wiring. First, vertical wiring.

The rule of vertical wiring is that the leads in adjacent signal layers need to be perpendicular to each other to reduce crosstalk caused by mutual inductance. In high frequency signals, crosstalk through capacitive coupling is the main component, creating current spikes between vertical leads. When the signal changes along time, or the frequency is low (less than a few GHz), the coupling capacitance of the adjacent signal layer vertical wiring components has little interference. In the radio frequency (RF) band (dozens of GHz), the interweaving between the leads produces hole resonance, and the conductor structure that is not surrounded by the ground wire will produce electromagnetic resonance at some special frequency points. Even if the leads are vertical, there will be strong crosstalk between them.

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In order to eliminate interference at all frequency points, a simple and effective method is to use a multi-layer board and use an isolation layer between the signal layers. It is especially important in the application of high-speed signal change. When you are unsure about the strength of the coupling between orthogonal lines, you need to use basic crosstalk simulation software to check the vertical leads to see if the crosstalk between them is within the noise tolerance range. At this time, you need to plan the signal return path, which is a major problem in vertical wiring.Two, heat dissipation through the hole.

This is a classic "follow/avoid" rule, and taste the argument. Some PCB designers say that they never use heat dissipation holes and never experience welding and assembly problems. And another group of people insist on preventing heat dissipation through the hole needs to be used when each plane is connected. Which of them is right?Their views apply to different trombones. If you weld the circuit board manually, you need to increase the temperature of the soldering iron tip to compensate for the welding problems caused by the heat dissipation of the copper layer through the welding hole. However, if you use wave crest welding, you need to use the heat dissipation hole to prevent the device loose, cold welding, monument and other phenomena, so I suggest that you better bite the bullet and stick to the design of the heat dissipation hole.

3. Right Angle wiring

This PCB routing rule is perhaps the most loved and hated. Today, I still see many PCB designers insist that the wiring can not be turned right angles at any time, for a variety of reasons. For example, they say that it is difficult for electrons to turn right angles when moving through the lead, but they also don't want to think that all the holes in the circuit board are perpendicular to the lead. There are also some reasons that are more reliable, such as the lead length can be reduced by 45° corners, and all right-angle corner wiring needs chamfering. It is also said that the right Angle corner will produce an acid corrosion trap in the circuit board acid corrosion solution, and there is no such problem in the widely used alkaline circuit board corrosion solution.

Unless your board is working on a high frequency (millimeter wave radar /5G communication) circuit above 50GHz, you don't need to worry about the lead turn right Angle. In fact, when wiring the board you can use any Angle you like to lay the lead. If the PCB design software you use has a built-in electromagnetic field solving function, this will make your wiring easier.

4. The "3W" rule

Three rules of thumb for wiring. The first version of the "3W rule "states that the spacing between two adjacent leads should be at least three times the width of the lead, in order to reduce the magnetic flux coupling between the leads and thus reduce the electromagnetic interference of the leads.

This rule may forget that the electromagnetic coupling between the leads is proportional to the overlap area of the lead loops, not the distance between the leads; Therefore, the overlap area of the lead loop is reduced, and the lead spacing is not limited by the 3W rule. As with the previous vertical wiring, the effects of different wiring spacing can be checked through basic electromagnetic interference simulation.

Another version of the "3W" rule states that when using sawtooth wiring for lead length matching, the sawtooth width needs to be at least three times the width of the lead, which minimizes the discontinuity of the lead impedance.

The "20H" ruleThis rule defines the overlap distance between the ground layer and the power layer in the PCB, and the power supply needs to be laid near the ground line in modern PCB design, so as to ensure that there is sufficient interlayer capacitance between them, and thus reduce power supply fluctuations on the high-speed circuit board.But the actual measurements are complicated. Some time results standard names follow the 20H rule at 300MHz to reduce electromagnetic radiation. However, there will be high-frequency resonance between the ground line and the power supply layer, and their structure is similar to the waveguide, which will aggravate the high-frequency interference between the lines.

So in practical applications, if your circuit frequency is within GHz, you can follow the 20H rule, otherwise, the 20H rule may bring worse results.

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