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PCB design is never an arbitrary thing, from drawing the schematic to layout lines, designers need to follow certain rules. Here we share the PCB design need to know the 16 principles.
1.PCB layout design time, should fully comply with the design principles of straight line placed along the signal flow, as far as possible to avoid back and forth loop.
Reason: to avoid direct signal coupling, affecting signal quality.
2.On the PCB board, the interface circuit filtering, protection and isolation devices should be placed close to the interface.
Reason: can effectively realize the effect of protection, filtering and isolation.
3.If there are both filtering and protection circuits at the interface, the principle of protection before filtering should be followed.
Reason: The protection circuit is used for external over-voltage and over-current suppression, if the protection circuit is placed after the filter circuit, the filter circuit will be damaged by over-voltage and over-current.
4.PCB clock frequency exceeds 5MHZ or signal rise time is less than 5ns, generally need to use multi-layer PCB board design.
Reason: the use of multi-layer PCB board design signal return area can be well controlled.
5.For multi-layer PCB boards, the key wiring layer (clock line, bus, interface signal lines,RF lines, reset signal lines,chip selection signal lines, and various control signal lines, etc.) should be adjacent to the complete ground plane, preferably between the two ground planes.
Reason: the key signal lines are generally strong radiation or extremely sensitive signal lines, close to the ground plane layout can make its signal return area to reduce the intensity of radiation or improve interference resistance.
6.Multi-layer PCB boards, the power plane should be relative to its neighboring ground plane inward shrinkage of 5H-20H (H is the distance between the power supply and ground plane).
Reason: the power plane relative to its return to the ground plane inward shrinkage can effectively inhibit edge radiation problems.
7.Avoid setting wiring layers next to each other in the layering design as much as possible.If it is impossible to avoid adjacent wiring layers, the distance between two wiring layers should be appropriately enlarged, and the distance between the wiring layer and its signal loop should be reduced.
Reason: Parallel signal alignment on neighboring wiring layers will cause signal crosstalk.
8.Keep crystals, transistors,relays,switching power supplies,and other high-radiation devices at least 1000 mils away from the single board interface connector.
Reason: The interference will be directly radiated outward or in the outgoing cable coupled to the current to be radiated outward.
9.Sensitive circuits or devices (e.g., reset circuits,WATCHDOG circuits, etc.) should be located at least 1000 mils away from all edges of the pcb board, especially from the side edges of the board interface.
Reason: similar to the single-board interface and other places are most likely to be external interference (such as static electricity) coupled to the place, and like the reset circuit, watchdog circuit and other sensitive circuits are very likely to cause the system to operate incorrectly.
10.PCB alignment can not have a right angle or sharp angle alignment.
Reason: Right-angle alignment leads to impedance discontinuity, resulting in signal emission, thus generating ringing or overshooting, the formation of strong EMI radiation.
11.Avoid the layer setup of neighboring wiring layers as much as possible, and when it can't be avoided, try to make the alignments in the two wiring layers perpendicular to each other or parallel to each other with a length of less than 1,000mil.
Reason: To minimize crosstalk between parallel wires.
12.Critical signal alignments such as clocks, buses, RF lines, and other parallel alignments on the same layer should meet the 3W principle.
Reason: to avoid crosstalk between signals.
13.The critical signal alignment must not be routed across the partition (including the hole, the soldering pad resulting in the gap of the reference plane).
Reason: Cross-segmentation will result in an increase in signal return area.
14.When a signal line (especially a critical signal line) is being relaminated, it should be designed with a ground via near the relaminated aperture.
Reason: It can reduce the signal return area.
15.The distance of the key signal line from the edge of the reference plane is ≥3H (H is the height of the line from the reference plane).
Reason: To inhibit the edge radiation effect.
16.For the metal shell grounding element, the top layer of the projected area should be covered with grounding copper.
Reason: To inhibit external radiation and improve interference resistance by distributing capacitance between the metal casing and the grounding copper skin.
