In PCB design, many standard rules need to be followed to ensure the quality of the PCB.
1. 3W rule
In order to reduce crosstalk between lines, it should be ensured 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 fields can be kept from interfering with each other, called the 3W rule, when using a spacing of 10W, 98% of the electric fields can be reached without interfering with each other.
2. 20H rule
Since the electric field between the power supply layer and the ground layer is variable, electromagnetic interference radiates outward at the edge of the plate, which is called the edge effect.
The solution is to shrink the power supply layer so that the electric field is conducted only within the range of the ground plane. In a unit of H (the thickness of the medium between the power supply and ground), if the internal shrinkage of 20H can limit 70% of the electric field to the edge of the ground plane, and the internal shrinkage of 100H can limit the electric field of 98% to the limit.
3. Five-five rules
The number of layers of the printed board selection rules, that is, the clock frequency to 5MHz or the pulse rise time is less than 5ns, then the PCB board must use a multi-layer board, which is a general rule, sometimes for the sake of cost and other factors, when using a double-layer board structure, in this case, it is best to make one side of the printed board as a complete ground plane layer.
4. Ground circuit rules
The minimum rule of the loop, that is, the ring area formed by the signal line and its loop should be as small as possible, and the smaller the ring area, the less radiation to the outside world, and the smaller the interference received from the outside world;
When dividing the ground plane, it is necessary to consider the distribution of the ground plane and the important signal traces to prevent problems caused by slotting of the ground plane;
In the double-layer board design, in the case of leaving enough space for the power supply, the remaining part should be filled with reference ground, and some necessary holes should be added to effectively connect the double-sided ground signal, and some key signals should be isolated by ground as much as possible;
For some high frequency designs, special consideration needs to be given to the ground plane signal loop problem, and it is recommended to use multilayer boards.
5. Crosstalk control
Crosstalk refers to the mutual interference caused by long parallel wiring between different networks on the PCB, mainly due to the distribution capacitance and distribution inductance between the parallel lines, the main measures to overcome crosstalk are:
Increase the spacing of parallel wiring and follow the 3W rule;
Insert an isolation line of ground between parallel lines;
Reduce the distance between the wiring layer and the ground plane;
6. Shield protection
Corresponding to the rules of the ground line loop, in fact, is also to minimize the circuit area of the signal, more common in some of the more important signals, such as clock signals, synchronization signals; for some particularly important, particularly high frequency signals, should consider the use of copper shaft cable shielding structure design, that is, the line of the clothed up and down the ground line isolation, but also to consider how to effectively make the shielding ground and the actual ground plane effectively combined.
7. Trace direction control
That is, the routing direction of the adjacent layer is an orthogonal structure, avoiding the different signal lines in the same direction in the adjacent layer to reduce unnecessary inter-layer channeling. (Adjacent layer signals are not parallel, the actual situation does not necessarily have to be orthogonal, limited by the routing space)
When the space is limited, especially when the signal rate is high, the ground plane is inserted to isolate each wiring layer, and the local signal line is used to isolate each signal line.
8. Open loop inspection rules for the line
Unnecessary interference radiation is not allowed to appear in excess of floating wires, in order to avoid the “antenna effect”.
9. Impedance matching check rules
The wiring width of the same network should be consistent, and the change in line width will cause uneven impedance of the line characteristics, and when the transmission speed is higher, it will produce reflections, which should be avoided as much as possible in the design.
Under certain conditions, such as connector pinouts, BGA packages when similar structures to pinouts, changes in line width may not be avoided and the effective length of the inconsistencies in the middle should be minimized.
9. Impedance matching check rules
The wiring width of the same network should be consistent, and the change in line width will cause uneven impedance of the line characteristics, and when the transmission speed is higher, it will produce reflections, which should be avoided as much as possible in the design.
Under certain conditions, such as connector pinouts, BGA packages when similar structures to pinouts, changes in line width may not be avoided and the effective length of the inconsistencies in the middle should be minimized.
10. Trace matching rules
In the high-speed digital circuit, when the delay time of the PCB wiring is greater than 1/4 of the signal rise time (or falling time), the wiring can be regarded as a transmission line, in order to ensure that the input and output impedance of the signal and the impedance of the transmission line are correctly matched, a variety of forms of matching methods can be used, and the selected matching method is related to the connection mode of the network and the topology structure of the wiring.
11. Closed-loop inspection rules for routing
Prevent the signal line from forming a self-loop between different layers, which is prone to occur in multilayer board designs, and the self-ring will cause radiation interference.
12. Trace branch length control rules
Try to control the length of the branch, the general requirement is Tdelay< = Trise/20. 13. Trace resonance rules Mainly for the design of high-frequency signals, that is, the length of the wiring must not be an integer multiple of its wavelength, so as not to produce resonance phenomena. 14. Trace length control rules That is, short-term rules, the trace is as short as possible, especially important signal lines such as clock lines. 15. Chamfering rules Traces avoid right and sharp angles. 16. Device decoupling rules Add the necessary decoupling capacitors, the power supply is filtered by the capacitor before being used by the device, and the decoupling capacitors follow the principle of proximity. 17. Partition rules for device layout Devices of different frequencies, generally placed at high speed at the interface, regarding the ground plane, consider splitting the ground of the two, and then connecting at the interface at a single point. For hybrid circuits, some place digital and analog on both sides of the PCB, and the middle layer is isolated. 18. Control rules for isolated copper zones The emergence of isolated copper regions will bring some unpredictable problems, so connecting isolated copper regions with other signals will help improve signal quality, usually grounding or deleting isolated copper regions. 19. Integrity rules for power and ground planes For areas with dense conduction holes, attention should be paid to avoiding the holes being connected to each other in the hollow areas of the power supply and the formation layer, forming a division of the plane layer, thereby destroying the integrity of the plane layer, and then causing the loop area of the signal line in the formation layer to increase. 20. Overlapping power supply and ground plane rules Different power layers should be spatially avoided to overlap. Mainly to reduce the interference between different power sources, especially between some power sources with large voltage differences, the overlap of power planes must be avoided, and the middle spaced strata can be considered when it is difficult to avoid.
F22 RC Jet
Arrows Rc jet
GSM alarm system
FMS RC Car
security camera
Hobbystar motor
atten soldering gun
PCBA factory