PCB circuit board design, which important points should be checked later

a few important points

[Weiwenxin PCBworld] When a PCB board has completed the layout and wiring, and the connectivity and spacing are checked and no problems are found, is a PCB completed? Of course, the answer is no. Many beginners, even some experienced engineers, because of time constraints or impatient or overconfidence, tend to sloppy things and ignore post-checks, resulting in some very low-level bugs, such as insufficient line width, component labeling The silk screen is pressed on the via hole, the socket is too close, the signal has a loop, etc., which leads to electrical problems or process problems. In serious cases, the board needs to be re-printed, resulting in waste. Therefore, after a PCB has been placed and routed, post-checking is a very important step.

The inspection of PCB contains many detailed elements. This article lists some of the elements that the author considers to be the most basic and most error-prone, so as to focus on later inspections.

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 screen printing. The outline screen printing of the device is preferably a little larger than the actual size to ensure that the device can be installed smoothly.

2. Layout

(1) IC should not be close to the edge of the board.

(2) Devices of the same module circuit should be placed close together. 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 the same area first, with clear layers to ensure the realization of the function.

(3) Arrange the socket position according to the actual installation. The sockets are connected to other modules through leads. According to the actual structure, for the convenience of installation, the position of the socket is generally arranged according to the principle of proximity, 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 can 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, analog circuits). Flooring can be used to separate them.

3. Wiring

(1) Line width size. The line width should be selected according to the process and current carrying capacity. The minimum line width cannot be smaller than the minimum line width of the PCB manufacturer. At the same time, it is necessary to ensure the current carrying capacity. Generally, 1mm/A is used to select the appropriate line width.

(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)高速线需注意回流路径。高速线容易产生电磁辐射，如果走线路径与回流路径之间形成的面积过大，就会构成一个单匝线圈向外辐射电磁干扰，所以走线的时候要注意旁边是否有回流路径，采用多层板设置电源层和地平面可以有效解决这个问题。

(4)注意模拟信号线。模拟信号线应该与数字信号隔开，走线尽量避免从干扰源(如时钟、DC-DC电源)旁边走过，而且走线越短越好。

4、EMC和信号完整性

(1)端接电阻。高速线或者频率较高并且走线较长的数字信号线最好在末端串联一个匹配电阻。

(2)输入信号线并接小电容。从接口输入的信号线，最好在靠近接口的地方并接一个皮法级小电容。电容大小根据信号的强度以及频率决定，不能太大，否则影响信号完整性。对于低速的输入信号，比如按键输入，可以选用330pF的小电容。

(3)驱动能力。比如驱动电流较大的开关信号可以加三极管驱动;对于扇出数较大的总线可以加缓冲器驱动。

5、丝印

(1)板名、时间、PN码。

(2)标注。对一些接口(如排阵)的管脚或者关键信号进行标注。

(3)元件标号。元件标号要摆放在合适的位置，密集的元件标号可以分组摆放。注意不要摆放在过孔的位置。

6、其他

Mark点。对于需要机器焊接的PCB，需要加入两到三个Mark点。