There are many details of the final check of the circuit board design:
1.Circuit board design component package
(1) Pad pitchIf it is a new device, you must draw the component package yourself to ensure that the pitch is appropriate. The pad pitch directly affects the soldering of the component.
(2) Via size (if any)
For plug-in devices, a sufficient margin should be reserved for the via size, and it is generally more appropriate to keep it not less than 0.2mm.
(3) Outline silk screen
The contour silk screen of the device should be larger than the actual size to ensure the device can be installed smoothly.
2.PCB board design layout
(1) IC should not be near the board edge(2) The components of the same module circuit should be placed close to
For example, the decoupling capacitor should be close to the power supply pin of the IC, and the devices that make up the same functional circuit are preferentially placed in an area with clear levels to ensure the realization of the function.
(3) Arrange the location of the socket according to the actual installation
The sockets are all lead to other modules. According to the actual structure, for the convenience of installation, the principle of proximity is generally used to arrange the location of the socket, and it is generally close to the edge of the board.
(4) Pay attention to the direction of the socket
The sockets are all directional, the direction is reversed, and the wire will be customized again. For a flat socket, the socket direction should be facing outside the board.
(5) There must be no devices in the Keep Out area
(6) Keep the interference source away from sensitive circuits
High-speed signals, high-speed clocks, or high-current switching signals are all sources of interference and should be kept away from sensitive circuits, such as reset circuits and analog circuits. They can be separated by paving.
3.PCB board wiring
(1) Line widthThe line width should be selected in combination with the process and current carrying capacity. The minimum line width cannot be less than the minimum line width of the PCB board manufacturer. At the same time to ensure the ability to carry current, generally 1mm / A is used to select a suitable line width.
(2) Differential signal line
For differential cables such as USB and Ethernet, note that the traces must be of equal length, parallel, and in the same plane, and the spacing is determined by the impedance.
(3) High-speed circuit board design pay attention to the reflow path
High-speed lines are prone to electromagnetic radiation. If the area formed by the routing path and the return path is too large, a single-turn coil will radiate electromagnetic interference. Therefore, pay attention to the return path when wiring. Multi-layer boards with power plane and ground plane can effectively solve this problem.
(4) Pay attention to analog signal lines
The analog signal lines should be separated from the digital signals, and the traces should be avoided as far as possible from interference sources (such as clocks, DC-DC power supplies), and the shorter the better
4. Electromagnetic compatibility (EMC) and signal integrity of PCB board
(1) Termination resistanceA high-speed line or a digital signal line with a high frequency and a long trace is best connected with a matching resistor at the end.
(2) Connect a small capacitor in parallel with the input signal line
The signal line input from the interface, it is best to connect a picofarad small capacitor in parallel near the interface. The size of the capacitor is determined by the strength and frequency of the signal. It should not be too large, otherwise it will affect the signal integrity. For low-speed input signals, such as key input, a small capacitor of 330pF can be selected.
(3) Driving ability
For example, a switch signal with a larger driving current can be driven with a transistor; a bus with a larger fan-out number can be driven with a buffer.
5.Silk screen of PCB board
(1) Board name, time, PN code(2) Marking
Mark the pins or key signals of some interfaces (such as arrays).
(3) Component number
The component labels should be placed in a suitable position, and dense component labels can be placed in groups. Be careful not to place them in the vias.
6.Mark points of PCB board
For PCB boards that require machine soldering, the board design needs to add two to three Mark points.
