Several Elements to Inspect after PCB Design

When the layout and wiring of a piece of PCB is completed, and no errors are reported for connectivity and spacing, is a piece of PCB completed? The answer is of course not. Many beginners, even some experienced engineers, because of time constraints or impatience or overconfidence, they tend to be hastily and neglect the late inspections. As a result, some very basic bugs appeared, such as insufficient conductor width, component silk screen pressed on via, too close sockets, signal loops, and so on. Accordingly, it will cause electrical problems or process problems, and it needs reprint the board in serious cases, resulting in waste. Therefore, after the layout and wiring of a piece of PCB is completed, a very important step is the late inspection.      

PCB inspection has many detailed elements. This article lists some of the most basic and most error-prone elements in late inspection.

1. Component packaging
(1) Pad pitch 
If it is a new component, you must draw the component package by yourself to ensure that the pitch is appropriate. The pitch of the pads directly affects the soldering of the components.

(2) Via size (if any)
For plug-in components, the size of via should be left with sufficient margin. Generally, it is more appropriate to keep not less than 0.2mm.

(3) silk screen Contour
The outline silk screen of the component should be larger than the actual size to ensure that the component can be installed smoothly.

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

(2) The components of the same functional 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 components that make up the same functional circuit should be placed in one area with a clear hierarchy to ensure the realization of the function.

(3) Arrange the position of the socket according to the actual installation Sockets are all down-lead to other modules. According to the actual structure, in order to facilitate installation, generally adopt the principle of proximity 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, and if the direction is reversed, the wire must be re-customized. For flat sockets, the direction of the socket should be toward the outside of the board.

(5) There must be no components in the Keep Out area

(6) The interference source should be far away from sensitive circuits. 
High-speed signals, high-speed clocks, or high-current switch signals are all interference sources and should be kept away from sensitive circuits, such as reset circuits and analog circuits. They can be separated by Ground plane.

3. Wiring
(1) The size of the conductor width
The conductor width should be selected in combination with the process and the current carrying capacity, and the minimum conductor width cannot be less than the minimum conductor width of the PCB manufacturer. At the same time, ensure the current carrying capacity, and generally select the appropriate conductor width with 1mm/A.

(2) Differential signal line
For differential cables such as USB and Ethernet, please note that the cables should be of equal length, parallel, and in the same plane, and the spacing is determined by the impedance.

(3) Pay attention to the reflow path of the high-speed line
High-speed lines are prone to electromagnetic radiation. If the area formed by the wiring path and the reflow path is too large, a single-turn coil will radiate electromagnetic interference outwards, as shown in Figure 1. Therefore, when wiring, pay attention to the reflow path next to it. The multi-layer board is provided with a power layer and a ground plane to 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 try to avoid passing by interference sources (such as clocks, DC-DC power supply), and the wiring should be as short as possible.

4. EMC and signal integrity
(1) Terminating resistor
High-speed lines or digital signal lines with higher frequencies and longer traces are best to have a matching resistor in series at the end.

(2) The input signal line is connected in parallel with a small capacitor
The signal line input from the interface is best to be connected to a small farad-grade capacitor near the interface. Capacitance is determined according to the strength and frequency of the signal, and 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 need a large driving current can be driven by a transistor; for a bus with a large number of fan-out, a buffer (such as 74LS224) can be used for driving.

5. Silk screen
(1) Board name, time and PN code

(2) Labeling
Mark the pins or key signals of some interfaces connectors (such as arrays).

(3) Component label
The component labels should be placed in a suitable position, and dense component labels can be placed in groups. Be careful not to place it in the position of via.

6. Other elements
Fiducial Mark
For PCBs that require SMT machine soldering, two to three Mark points need to be added.