Layout principle of PCB

A. In accordance with the "first big after small, first difficult after easy" layout principle, that is important unit circuit, core components should be prioritized layout.

B. The layout should refer to the schematic diagram, according to the main signal flow rules of the single Board to arrange the main components

C. The layout should meet the following requirements: The total wiring is as short as possible, the key signal line is shortest; high voltage, large current signal
The weak signal is completely separated from the small current, the low voltage, the analog signal is separated from the digital signal, the high frequency signal and the low frequency letter
The high-frequency components must be spaced sufficiently.

In order to reduce the electromagnetic interference of inter-layer signal, the direction of signal line of adjacent wiring layer should be perpendicular.

1. Cabling Prioritization

Key signal Line priority: power supply, touch small signal, high-speed signal, clock signal and synchronous signal, such as key signal priority
Wiring

Density priority principle: the wiring is initiated from the most complex devices connected on a single board. The most densely connected area on a single board
Start cabling.

Loop minimum rule, that is, the signal line and its return loop area to be as small as possible, the smaller the ring area, external radiation
The less it gets, the smaller the interference from the receiving world. For this rule, in the ground plane division, we should consider the ground plane and the heavy
To signal the distribution of the line, to prevent the problem caused by the ground plane slotting, etc. in the double board design, the power supply is left
Sufficient space, the left part should be filled with reference and add some necessary holes to the double-sided
Ground signal is effectively connected, to some of the key signals as far as possible to use ground insulation, some high frequency design, need to
Especially considering the problem of ground plane signal circuit, it is advisable to adopt multilayer board.

2) harassed control
Crosstalk (CrossTalk) refers to the different networks on the PCB due to the longer parallel wiring caused by mutual interference, mainly
Due to the effect of distributed capacitance and distributed inductance between parallel lines. The main measures for overcoming crosstalk are:
Increase the spacing of parallel cabling and follow the 3W rules.
Insert a grounded isolation line between parallel lines.
Reduce the distance between the routing layer and the ground plane.

That is, the direction of the adjacent layer is orthogonal structure. Avoid moving different signal lines in the same direction on adjacent layers to reduce
Unnecessary inter-interlayer harassed, which is difficult to avoid due to board structure limitations, such as some backplanes, especially the letter
When the number rate is high, the ground plane should be considered to isolate each wiring layer, and the signal lines are isolated by the ground signal line.

5) Open-loop inspection rules for the route

It is generally not permissible to have one end of a floating cable (dangling line),
Mainly to avoid the "antenna effect", reduce unnecessary interference radiation and acceptance, otherwise it may bring non-
The results of the knowledge.

The wiring width of the same network should be consistent, the change of line width will cause the line characteristic impedance uneven, when the transmission
When the speed is high, reflections are generated and should be avoided in the design. Under certain conditions, such as the plug-in
In the case of a similar structure, the wire-width variation may not be avoided when the BGA package is
The effective length of the inconsistent part between the

11) Route length control rules:

Short-term rules, in the design should try to make the length of the wiring as short as possible, in order to reduce the line too long caused by the interference asked
Important signal lines, such as clock lines, be sure to place their oscillators close to the device. To drive
Depending on the circumstances of the device, the network topology should be determined.

12) Chamfer rule:

PCB design should avoid the sharp angle and right angle, produce unnecessary radiation, while the process performance is not good.

Overlapping power and ground layer rules:

The different power layers have to avoid overlapping in space. The main purpose is to reduce the interference between different power supplies, especially some electrical
Voltage difference between the power supply, the power plane overlap problem must try to avoid, difficult to avoid when considering the intermediate
Isolated strata.

3W Rules:

In order to reduce crosstalk between lines, it is necessary to ensure that the line spacing is large enough that when the centerline spacing is not less than 3 times times the line width, it can be maintained
70% of the electric field does not interfere with each other, called the 3W rule. To achieve 98% of the electric field does not interfere with each other, you can use 10W
Spacing.

20H Rules:

Because the electric field between the power layer and the stratum is changed, the electromagnetic interference is radiated outward at the edge of the plate. Called Edge effect
Should.
The solution is to shrink the power supply layer so that the electric field is conducted only within the range of the ground plane. With an H (power and ground
The medium thickness) is the unit, if the 20H can be reduced to 70% of the electric field within the edge of the ground plane;
100H can limit 98% of the electric field.