Small details that are easily overlooked in PCB design can help you summarize in one minute

PCB design is a rigorous and careful work. There are many small details in the PCB design process. If some small details are not paid attention to, they may greatly affect the performance of the entire PCB, and even determine the success or failure of the entire product.

PCB Layout Specification Details

1. In the high-voltage board of the switching power supply, the high-voltage and high-current signals are completely separated from the low-voltage and low-current weak current signals.

2. In high frequency digital circuits, crystals and crystal oscillators need to be placed close to the chip. The output capacity of the crystal oscillator also has a certain limit. If the crystal oscillator is too far away from the chip layout, the square wave signal output after receiving the crystal oscillator signal inside the chip will be disturbed to a certain extent, and it will not be a fixed frequency to a certain extent. , which will cause the digital circuits to fail to work synchronously.

3. For circuits with the same modules and the same structure, the "symmetrical" layout is adopted, and the quick layout method can use the module reuse function in the PCB design software.

4. The arrangement of components needs to be considered for later debugging and maintenance, that is, do not place large components around small components; there should be enough space around components that need to be debugged.

5. Place the decoupling capacitors close to the IC power pins and make the shortest loop between them and the power supply and ground.

PCB layout specification details

1. Cross-segmentation: It means that the signal reference plane is discontinuous, and the signal line spans two different reference planes, which produces a series of EMI and crosstalk to the signal. Cross-segmentation in the PCB may not have a great impact on low-frequency signals, but for signals in some high-frequency digital circuits, it is very important to avoid cross-segmentation.

2. Pad wiring problem: In PCB design, pad wiring is also a detail that needs attention. If the two pads of the 0402 resistor package are routed separately at the opposite corners, plus the solder mask deviation caused by the PCB production accuracy (one side of the solder mask window is 0.1mm larger than the pad), it will form as shown on the left in the following figure. pad. In such a case, due to the effect of the surface tension of the solder during resistance welding, there will be a bad rotation as shown on the right in the figure below.

Using a reasonable wiring method, the pad connection adopts a fan-out method that is symmetrical about the long axis, which can effectively reduce the bad rotation after the CHIP component is mounted. If the lines of the pad fan-out are also symmetrical about the short axis, the drift after the placement of the CHIP components can also be reduced.

And the adjacent network cannot be directly connected to the network. It is necessary to connect the pads first and then connect. As shown in the figure, the straight chain is easy to cause continuous welding during manual welding.

3. Equal length details of differential trace pairs

Compared with ordinary single-ended signal traces, differential signals have the most obvious advantages in strong anti-interference ability, effective EMI suppression, and accurate timing positioning. Many designers believe that maintaining equal spacing is more important than matching line lengths. The most important rule in the design of PCB differential traces is to match the line length. Other rules can be based on it, and the design can be flexibly handled according to the actual application. The following figure lists some detailed methods of inner equal length.

4. Clock signals and high-frequency signals should be shielded as much as possible. If there is no space to cover the ground, try to separate the distance of 3W.

5. When making holes for wiring, attention should be paid to the phenomenon of reference plane splitting on the adjacent layer. Such splitting will cause the signal transmission path to be too long. It is better to keep a distance that can pass a line between the holes, so as to prevent the plane layer from being cut. Affects the integrity of the reference plane.

Production-related details in PCB design

1. Window opening details of golden fingers: I believe everyone knows about golden fingers. During the design process, we must pay attention to the small details of golden fingers. All windows must be opened in the golden finger area. When making the package, the window area will be added to prevent forgetting to add it later in the PCB. Of course, some careless engineers will forget this little detail, which will greatly reduce the performance and service life of the product after the PCB is produced.
Among them, the processing method of opening the window in the PCB: drawing the window area in the solder mask area corresponding to the gold finger, which can be drawn with copper or 2D lines.
The role of gold finger window opening: Gold finger window opening means that the green oil is not applied between the device pads and the pads, so as to avoid the green oil falling off due to long-term unplugging and plugging, thus affecting the performance and quality of the product.

Device pad tombstone phenomenon: This detail involves the design of the PCB package, and is also a little bit related to the way the pad outlet was introduced to you before. PCB packaging is also very important for PCB design. The packaging on the PCB board is a mapping of the actual device. Therefore, when designing the packaging, it must be noted that the pad design should be strictly symmetrical, and the size of the design should not be too different from the actual size. .
For example, in a resistor-capacitor package, the pads on both sides need to be symmetrical and have the same size to ensure that when the solder is melted, the surface tension of all the solder joints on the component can be balanced and an ideal solder joint has been formed. The pads are asymmetrical. During reflow soldering, the solder on the larger end of the pad cannot achieve the desired melting and wetting effect, resulting in the phenomenon of device offset and tombstone.

• Mr. Bai Jilong has been engaged in the electronics industry for 15 years. He has developed more than 100 products so far, and most of them have been mass-produced.

• It took 5 years since 2018 to record thousands of practical-level electronic engineer series courses, from components to core modules to complete products

• Lao Bai's original intention is "May the world's engineers not take detours" Among them, there are courses explaining MOS tubes and IGBTs in detail