The Later Checking After PCB Layout Design
When we completed the layout of a circuit board, and checked there were no errors that the connectivity and spacing, then a PCB design really be finished? The answer of course no,many beginners also includes a number of experienced engineers often ignore the later check and get throught a thing carelessly because time is tight or impatient or too confident.As a result, there are some basic BUG, such as insufficient line width, component labels, screen printing on the through-hole, sockets too close, signal loop, and so on.Which leads to the electrical problems or process problems, to a serious waste. So, when a PCB completes layout and wiring, one of the most important steps is the later check.
The PCB check has a lot of detail elements. Here are some of the most basic and most error prone elements for later inspection.
1. Component package
- Pad spacing. If it is a new device, it is necessary to draw the component package to ensure the proper spacing. The spacing between the pads directly affects the welding of the components.
- Through-hole size (if available). For plug-in devices, the size of the hole should be retained enough margin, generally reserved not less than 0.2mm, more appropriate.
- Silkscreen printing. The contour of screen printing is better larger than the actual size to ensure that the device can be installed smoothly.
2. Components allocation
- IC should not be near the edge of the board.
- The components of the same circuit should be placed close to. For example, decoupling capacitor should be close to the IC power pin, the components of the same functional circuit should be placed in an area, with distinct layers, and ensure the realization of the function.
- Arrange the location of the socket according to the actual installation. Sockets are lead to other modules, according to the actual structure, in order to facilitate installation, generally use the nearest principle, arrange the location of the socket, and generally near the edge of the board.
- Pay attention to the direction of the socket. If sockets are reverse direction, the wire will be rework. For flat plug sockets, the direction of the jack shall be toward the outside of the board.
- There is no components in the Keep Out region.
- The interference source is away from the sensitive circuit. High speed signals, high-speed clocks, or high current switching signals are all sources of interference and should be away from sensitive circuits, such as reset circuits and analog circuits,separate them through ground copper.
- The line width should be selected by combining process and ampacity, and the minimum linewidth should not be less than the minimum line width of the PCB manufacturer. At the same time to ensure the carrying capacity of the current, 1mm/A is generally used to select the appropriate line width.
- Differential signal line. For USB, the Ethernet arithmetic line, line length, pay attention to the same plane, parallel, spacing by impedance.
- Pay attention the high-speed line return path. High speed lines are easy to generate electromagnetic radiation, and if the line path and the return path form too large area, a single turn coil will be formed to radiate electromagnetic interference.Therefore, the line should pay attention to the next side of the return path, the multilayer board with power and ground plane can effectively solve this problem.
- Pay attention to analog signal line. Analog signal lines should be separated from digital signals, and the line should be taken as far as possible from interference sources (such as clock, DC-DC power), and the shorter the line, the better.
4. EMC and signal integrity
- Termination resistance. A high speed line, or a higher frequency, and longer line digital signal line, it is best to string a matching resistor at the end.
- Input the signal line and connect the small capacitance. The signal line input from the interface is preferably near the interface and connected to a small capacitor at the PI level. The size of the capacitor depends on the intensity and frequency of the signal. It can not be too large, otherwise it will affect the integrity of the signal.For low speed input signals, such as key inputs, a small capacitor of 330pF can be used.
- Drive capability. For example, the driving current larger switch signal can add triode drive; for fan out a larger number of bus can add buffer (such as 74LS224) drive.
5 . Silk screen
- Board name, time, PN code.
- Some of the interface (such as array) pin or key signal mark.
- Component marking. Component labels should be placed to a suitable location, and dense component labels can be placed in groups. Be careful not to place them in the through hole.
Fiducial mark. For PCB that requires machine soldering, you need to add two to three fiducial marks.
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