What happens after PCB layout
Source: InternetPublisher:风向西瓜 Keywords: pcb wiring Updated: 2021/05/05
Is it done after PCB layout and routing? This may not be the case. Many beginners, including some experienced engineers, often rush through the process due to time constraints, impatience, or overconfidence, and ignore post-inspection. As a result, some very basic bugs appeared, such as insufficient line width, component label silk screen printing on the via holes, sockets too close to each other, signal loops, etc. This can lead to electrical problems or process problems, and in serious cases, the board must be re-printed, resulting in waste. Therefore, after a PCB has been laid out and routed, a very important step is post-inspection.
There are many detailed elements in PCB inspection. I have listed some elements that I consider to be the most basic and error-prone elements as a post-inspection.
1. Component packaging
(1) Pad spacing. If it is a new device, you need to draw the component package yourself to ensure that the spacing is appropriate. The pad spacing directly affects the welding of the components.
(2) Via size (if any). For plug-in devices, the size of the via should leave sufficient margin, and it is generally appropriate to leave it no less than 0.2mm.
(3)Contour silk screen printing. It is best to make the outline screen printing of the device a little larger than the actual size to ensure that the device can be installed smoothly.
2. Layout
(1) The IC should not be close to the edge of the board.
(2) Devices in the same module circuit should be placed close to each other. For example, the decoupling capacitor should be close to the power pin of the IC, and the devices that form the same functional circuit should be placed in one area first, with clear layers to ensure the realization of the function.
(3) Arrange the location of the socket according to the actual installation. The sockets are all wired to other modules. According to the actual structure, in order to facilitate installation, the proximity principle is generally used to arrange the location of the sockets, and they are generally close to the edge of the board.
(4) Pay attention to the direction of the socket. The sockets all have directions. If the direction is reversed, the wires will have to be re-customized. For flat sockets, the socket direction should be toward the outside of the board.
(5) There must be no devices in the Keep Out area.
(6) Interference sources should be kept away from sensitive circuits. High-speed signals, high-speed clocks or high-current switching signals are sources of interference and should be kept away from sensitive circuits, such as reset circuits and analog circuits. You can use paving to separate them.
3. Wiring
(1) Line width size. The line width should be selected based on the process and current carrying capacity. The minimum line width cannot be less than the minimum line width of the PCB manufacturer. At the same time, the current carrying capacity is ensured. Generally, 1mm/A is used to select the appropriate line width.
(2) Differential signal lines. For differential lines such as USB and Ethernet, please note that the lines must be of equal length, parallel, and in the same plane. The spacing is determined by the impedance.
(3) Pay attention to the return path for high-speed lines. High-speed lines are prone to electromagnetic radiation. If the area formed by the wiring path and the return path is too large, a single-turn coil will be formed to radiate electromagnetic interference outwards, as shown in Figure 1. Therefore, when routing, pay attention to the return path next to it. Multilayer boards equipped with power layers and ground planes can effectively solve this problem.
(4) Pay attention to the analog signal lines. Analog signal lines should be separated from digital signals, and the traces should be kept away from sources of interference (such as clocks, DC-DC power supplies), and the shorter the traces, the better.
4. EMC and signal integrity
(1) Terminating resistor. For high-speed lines or digital signal lines with higher frequency and longer traces, it is best to have a matching resistor in series at the end.
(2) Connect the input signal line to a small capacitor. The signal line input from the interface is best placed close to the interface and connected to a picofarad-level small capacitor. The size of the capacitor is determined by the strength and frequency of the signal. It cannot 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 used, as shown in Figure 2.
(3) Driving ability. For example, a switching signal with a large drive current can be driven by a transistor; a bus with a large fan-out number can be driven by a buffer (such as 74LS224).
5. Silk screen printing
(1) Board name, time, PN code.
(2) Marking. Label the pins or key signals of some interfaces (such as arrays).
(3) Component labeling. Component labels should be placed in appropriate locations, and dense component labels can be placed in groups. Be careful not to place it where the via hole is.
6. Others
(1)Mark point. For PCBs that require machine welding, two or three Mark points need to be added. The above are some precautions after PCB layout and routing. I hope it can give you some help.
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