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Some Basic Rules of PCB Layout
2020-09-10
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Author:Holia      Share

Layout is an important part of PCB design, and it is also the most time-consuming part of the whole PCB design. Engineers need to follow some basic rules, such as chamfering rules, 3W rules, etc.


Ground circuit rules

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The minimum loop rule is that the ring area formed by the signal line and its loop should be as small as possible. The smaller the ring area is, the less the external radiation is and the smaller the interference received from the outside.


According to this rule, the distribution of ground plane and important signal lines should be considered in ground plane segmentation to prevent the problems caused by ground plane slotting;


In the design of double-layer board, in the case of leaving enough space for power supply, the left part should be filled with reference ground, and some necessary ground crossing holes should be added to effectively connect the two-sided signals. Some key signals should be isolated by ground wire as far as possible. For some high-frequency designs, special consideration should be given to the ground plane signal circuit. It is recommended to use multi-layer boards

Shielding protection rules


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The corresponding ground circuit rule is actually to minimize the loop area of the signal, which is often seen in some important signals, such as clock signal and synchronous signal;


For some particularly important and high frequency signals, we should consider the design of copper shaft cable shielding structure, that is, the ground wire is used to isolate the line, the left and the right, and how to effectively combine the shielding ground with the actual ground plane should be considered.

Crosstalk control rules

Crosstalk refers to the mutual interference caused by long parallel wiring between different networks on PCB, mainly due to the distributed capacitance and inductance between parallel lines. The main measures to overcome crosstalk are as follows:


Increase the spacing of parallel wiring and follow the 3W rule;
Insert grounding isolation wire between parallel lines;
Reduce the distance between the wiring layer and the ground plane.

3W Rules

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In order to reduce the crosstalk between lines, it is necessary to ensure that the line spacing is large enough. When the line center spacing is not less than 3 times the line width, 70% of the electric field can not interfere with each other, which is called 3W rule. To achieve 98% of the electric field without mutual interference, a 10 W spacing can be used.

Direction control rules of routing

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The direction control rule of routing, that is, the routing direction of adjacent layers is orthogonal structure. Different signal lines should be avoided to go in the same direction in the adjacent layers to reduce unnecessary inter layer interference; when this situation is difficult to avoid due to board structure constraints (such as some back boards), especially when the signal rate is high, it should be considered to isolate the wiring layers with the ground plane and isolate the signal lines with the land signal lines.



Open loop inspection rules for wiring

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Generally, floating wiring at one end is not allowed(Dangling Line),It is mainly to avoid "antenna effect" and reduce unnecessary interference radiation and acceptance, otherwise it may bring unpredictable results.


Inspection rules for closed loop wiring

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Prevent signal line from forming self loop between different layers. This kind of problem is easy to occur in the design of multi-layer plate, and the self-loop will cause radiation interference.


Chamfering rules


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In PCB design, it is necessary to avoid sharp angle and right angle to produce unnecessary radiation, and at the same time, the process performance is not good.

Device decoupling rules


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The necessary decoupling capacitance is added to the printed board to filter out the interference signal on the power supply to stabilize the power signal. It is recommended that the power supply be connected to the power pin after passing through the filter capacitor.

Power supply ground plane integrity rules

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For the area with dense through holes, attention should be paid to avoid connecting the holes in the hollowed out area between the power supply and the stratum, forming the segmentation of the plane layer, thus damaging the integrity of the plane layer and increasing the loop area of the signal line in the formation. In order to avoid damaging the plane layer, the distance between vias should be at least one signal line.


Overlap rules of power ground plane

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Different power layers should avoid overlapping in space. In order to reduce the interference between different power sources, especially between some power sources with large voltage difference, the overlapping problem of power supply plane must be avoided. If it is difficult to avoid, the interlayer can be considered.


20H Rules

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Because the electric field between the power layer and the stratum is variable, electromagnetic interference will radiate from the edge of the plate. It's called edge effect.


The solution is to shrink the power layer, so that the electric field is only conducted in the range of the ground plane. Taking an H (dielectric thickness between power supply and ground) as a unit, if the electric field is indented by 20h, 70% of the electric field can be limited to the edge of the ground plane, and the electric field of 98% can be limited to within 100h.