When the circuit board has been designed and laid out, and the connectivity and other issues did not go wrong during the test, is the PCB circuit board done? Either there is no error or no problem with the circuit board. Many engineers with little experience and overconfidence end the PCB testing process. In mass production, N BUGs have appeared. It’s all caused by details, such as insufficient line width, silk screen of component labels pressed on vias, sockets too close together, signal loops, etc. Therefore, after a PCB has been placed and routed, an important step is post-checking.
1, component package
(1) Pad pitch. If 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. Generally, it is more appropriate to keep it at least 0.2mm.
(3) Contour silk screen. The contour silk screen of the device should be larger than the actual size to ensure the device can be installed smoothly.
(1) IC should not be near the board edge.
(2) The components 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 are placed in an area with a clear hierarchy 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, in order to facilitate installation, the principle of proximity is generally used to arrange the location of the sockets, and it is generally near 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 wires will be re-customized. For flat sockets, the direction of the socket should face out of 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.
(1) Line width. The 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 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 cables must be equal length, parallel, and in 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 electromagnetic radiation. If the area formed by the routing path and the return path is too large, it will form a single-turn coil to radiate electromagnetic interference to the outside. Layers and ground planes can effectively solve this problem.
(4) Pay attention to analog signal lines. The analog signal line should be separated from the digital signal, and the trace should be avoided as far as possible from the interference source (such as clock, DC-DC power supply), and the shorter the trace, the better.
4, EMC and signal integrity
(1) Termination resistor. A 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 the input signal line in parallel with a small capacitor. 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 switching signal with a larger driving current can be driven by a transistor; a bus with a larger fan-out number can be driven by a buffer (such as 74LS224).
5, silk screen
(1) board name, time, PN code.
(2) mark. Mark the pins or key signals of some interfaces (such as arrays).
(3) Component number. The component labels should be placed in the appropriate position, and dense component labels can be placed in groups. Be careful not to place them in the vias.
Mark. For PCBs that require machine soldering, two to three Mark points need to be added.