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6 elements of PCB design post-examination

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In the PCB design process, a PCB board is completed the layout and wiring, and check connectivity and spacing are not wrong, now have a PCB board is completed? The answer, of course not. Many beginners also include experienced engineers who have missed the post-inspection due to tight deadlines or other factors.The result shows some very basic bugs, such as the line width is not enough, the symbol of the element label is pressed on the hole, the socket is too close, the signal appears ring road and so on.This can lead to electrical problems or technical problems, serious replays and waste. So one of the most important steps is the later check when completing layout and wiring of a PCB Board.


1.Component encapsulation

(1) weld spacing.If it is a new device, it is necessary to seal the components of the drawing itself, ensuring that the spacing is appropriate, and the weld spacing directly affects the welding of the components.

(2) via size. For plug-in devices, the size of the hole should be sufficient, and the general retention is not less than 0.2mm.

(3) contour screen printing.The outline of the device is the most similar to the actual size, ensuring that the device can be installed smoothly.


2.PCB layout

(1) IC should not be close to the edge of the board.

(2) the components of the same circuit should be placed close to each other. For example, the decoupling capacitor should be close to the IC power foot, and the devices that consist of the same function circuit should be placed in an area with clear levels and guaranteed functions.

(3) arrange the position of the socket according to the actual installation. Sockets are lead to other modules, according to the actual structure, in order to facilitate the installation, generally use the nearest principle, arrange the location of the socket, and generally near the edge of the board.

(4) pay attention to the direction of the socket. Sockets are in the direction of the reverse direction, the wire will be re customized. For flat plug sockets, the direction of the spigot should be outside the plate.

(5) there is no device in the Keep Out region.

(6) the interference source should be far away from the sensitive circuit. High speed signal, high speed clock or high current switch signal are all interference sources, and should be far away from sensitive circuit, such as reset circuit, analog circuit. Flooring can be used to separate them.


3.PCB wiring

(1) linewidth. The width of line should be selected according to the process and ampacity, and the minimum linewidth should not be less than the minimum linewidth of PCB manufacturer. At the same time, the load current capacity is guaranteed, and the appropriate line width is selected by 1mm/A.

(2) differential signal line. For USB, the Ethernet arithmetic line, line length, pay attention to the same plane, parallel, spacing by impedance.

(3) the return route of the high speed line. High speed line to produce electromagnetic radiation, if the wire path and the return path forming area is too large, it will form a single turn coil radiate electromagnetic interference, pay attention next to the return path to walk the line so, multilayer board is provided with a power supply layer and ground plane can effectively solve this problem.

(4) pay attention to analog signal line. Analog signal line should be separated from digital signal, the line as far as possible from interference sources (such as clock, DC-DC power) next to walk, and the shorter the line, the better.


4.EMC and signal integrity

(1) termination resistance. A high speed line or a higher frequency and longer line digital signal line is best fitted with a matching resistor at the end.

(2) input signal line and connect small capacitance. The input signal line from the interface, preferably near the interface, and the skin level method of small capacitance. The size of the capacitor depends on the intensity and frequency of the signal, not too large, otherwise it will affect the integrity of the signal. For low-speed input signals, such as button input, you can choose a small capacitor 330pF

(3) drive capability. For example, the switch signal with large driving current can be driven by a triode; for a larger fanout, the bus can be driven by a buffer (such as 74LS224).


5.Screen printing

(1) name, time, PN code.

(2) tagging. Some of the interface (such as array) pin or key signal mark.

(3) component labeling. The component labels should be placed to the right position, and the dense component labels can be grouped and placed. Be careful not to place the hole in the hole.



Mark point. For PCB that requires welding machines, two to three Mark points are required.

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