In the electronics industry there are many inexperienced engineers, the design of the PCB board is often overlooked in the design of the latter part of the inspection led to the PCB board there are a variety of problems, such as line width is not enough, component label silk pressure on the hole, the socket by Too close, the signal appears loop and so on. Resulting in electrical problems or process problems, serious to re-plate, resulting in waste. PCB board design later is the most important step is to check.

PCB board design post-check there are many details:

1, component package

(1) pad pitch

If it is a new device, to paint their own components to ensure that the appropriate spacing, pad spacing directly affect the component welding.

(2) via size (if any)

For plug-in devices, the via size should retain sufficient margin, generally less than 0.2mm is appropriate.

(3) contour silk screen

The outline of the device is better than the actual size of silk screen to ensure that the device can be successfully installed.

2, PCB board layout

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

(2) Devices of the same module circuit should be placed close to each other

Such as decoupling capacitors should be close to the IC's power pin, composed of the same function of the circuit device placed in a priority area, structured, to ensure the realization of the function.

(3) Arrange the position of the socket according to the actual installation

Jacks are lead to other modules, according to the actual structure, in order to facilitate the installation, generally using the nearest principle, the location of the socket, and generally close to the board edge.

(4) pay attention to the socket direction

The socket is a direction, the direction of anti, the wire will re-order. For flat plugs, the socket should be oriented towards the outside of the board.

(5) Keep Out area can not have the device

(6) interfere with the source away from the sensitive circuit

High-speed signal, high-speed clock or high-current switching signals are interference sources, should be away from sensitive circuits, such as reset circuit, analog circuit. You can use the floor to separate them.

3, PCB board wiring

(1) line width size

Line width to combine the process, the load to choose, the minimum line width can not be less than the PCB board manufacturers minimum line width. While ensuring the carrying capacity of the current, generally 1mm / A to select the appropriate line width.

(2) differential signal line

For USB, Ethernet and other differential lines, pay attention to the alignment to be long, parallel, with the plane, the distance determined by the impedance.

(3) high-speed line attention to the return path

High-speed line is easy to produce electromagnetic radiation, if the trail path and the return path to form an area is too large, it will form a single-turn coil outward radiation electromagnetic interference, as shown in Figure 1. So when the line should pay attention to the return path next to the multi-layer board set the power layer and ground plane can effectively solve this problem.
PCB board design _ high-speed line return path
Figure 1: PCB board design _ high-speed line return path
(4) Note the analog signal line

The analog signal lines should be separated from the digital signal, and the trace should be avoided as far as possible from the source of interference (such as clock, DC-DC power), and the trace is as short as possible.
    
4, PCB board electromagnetic compatibility (EMC) and signal integrity

(1) Termination resistance

High-speed lines or higher frequency and long alignment of the digital signal line is best in the end string into a matching resistor.

(2) input signal line and connect a small capacitor

From the interface input signal line, preferably in the vicinity of the interface and then the small class of small battery. Capacitance size depends on the strength and frequency of the signal, not too large, otherwise the impact of signal integrity. For low-speed input signal, such as key input, you can use a small 330pF capacitor, as shown in Figure 2.
PCB board design _ input signal line connected to a small capacitor
Figure 2: PCB design _ input signal line connected to a small capacitor
(3) drive capability

Such as the drive current of the larger switching signal can be coupled with the transistor drive; for the fan out of the larger bus can be added buffer driver.

5, PCB board screen printing

(1) board name, time, PN code

(2) marked

For some interfaces (such as array) pins or key signals are marked.

(3) component label

Component labels to be placed in the right place, dense component labels can be placed in groups. Be careful not to place the hole in the position.

6, PCB mark point

For PCB boards that require machine soldering, you need to add two to three mark points.