There are many inexperienced engineers in the electronics industry. The designed PCB boards often have various problems due to ignoring certain checks in the later stage of the design, such as insufficient line width, component label silk screen printing on the via hole, socket Too close, the signal loops, etc. As a result, electrical problems or process problems are caused, and in serious cases, the board needs to be re-printed, resulting in waste. One of the more important steps in the later stage of PCB design is inspection.
There are many details in the post-check of PCB board design:
1. Component packaging
(1) Pad spacing
If it is a new device, you must draw the component package yourself to ensure proper spacing. The pad spacing directly affects the soldering of components.
(2) Via size (if any)
For plug-in devices, the size of the via hole should have enough margin, and it is generally appropriate to reserve not less than 0.2mm.
(3) Outline silk screen printing
The outline screen printing of the device is better than the actual size to ensure that the device can be installed smoothly.
2. PCB board layout
(1) IC should not be close to the edge of the board.
(2) Devices of the same module circuit should be placed close to each other
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 should be placed in one area first, with clear layers to ensure the realization of the function.
(3) Arrange the position of the socket according to the actual installation
The sockets are all led to other modules. According to the actual structure, for the convenience of installation, the principle of proximity is generally used to arrange the position 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, if the direction is reversed, the wire will have to be customized. For flat plug sockets, the direction of the socket should be towards the outside of the board.
(5) There should be no devices in the Keep Out area
(6) The source of interference should be kept 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. Flooring can be used to separate them.
3. PCB board wiring
(1) Line width size
The line width should be selected according to the process and current carrying capacity. The smaller line width cannot be smaller than the smaller line width of the PCB board manufacturer. At the same time, the current carrying capacity is guaranteed, and the appropriate line width is generally selected at 1mm/A.
(2) Differential signal line
For differential lines such as USB and Ethernet, note that the traces should be of equal length, parallel, and on the same plane, and the spacing is determined by the impedance.
(3) Pay attention to the return path of high-speed lines
High-speed lines are prone to generate electromagnetic radiation. If the area formed by the routing path and the return path is too large, a single-turn coil will be formed to radiate electromagnetic interference, as shown in Figure 1. Therefore, when routing, pay attention to the return path next to it. The multi-layer board is provided with a power layer and a ground plane, which can effectively solve this problem.
(4) Pay attention to the analog signal line
The analog signal line should be separated from the digital signal, and the wiring should be avoided as far as possible from the interference source (such as clock, DC-DC power supply), and the wiring should be as short as possible.
4. Electromagnetic compatibility (EMC) and signal integrity of PCB boards
(1) Termination resistance
For high-speed lines or digital signal lines with high frequency and long traces, it is better to put a matching resistor in series at the end.
(2) The input signal line is connected in parallel with a small capacitor
It is better to connect the signal line input from the interface near the interface and connect a small picofarad capacitor. The size of the capacitor is determined according to the strength and frequency of the signal, and should not be too large, otherwise the signal integrity will be affected. For low-speed input signals, such as key input, a small capacitor of 330pF can be used, as shown in Figure 2.
Figure 2: PCB board design_input signal line connected to small capacitor
Figure 2: PCB board design_input signal line connected to small capacitor
(3) Driving ability
For example, a switch signal with a large driving current can be driven by a triode; for a bus with a large number of fan-outs, a buffer can be added.
5. Screen printing of PCB board
(1) Board name, time, PN code
(2) Labeling
Mark the pins or key signals of some interfaces (such as arrays).
(3) Component label
Component labels should be placed in appropriate positions, and dense component labels can be placed in groups. Be careful not to place it in the position of the via.
6. Mark point of PCB board
For PCB boards that require machine soldering, two to three Mark points need to be added.
Post time: Aug-11-2022