The layout of components in PCB design is crucial. A correct and reasonable layout not only makes the layout more tidy and beautiful, but also affects the length and quantity of printed wires. A good PCB component layout is extremely important for improving the performance of the whole machine. meaning.
So how can the layout be more reasonable? Today we will share with you “6 details of PCB board layout”, pure practical information! Collect first!
1. Key points of PC layout including wireless modules
Physical separation of analog circuits and digital circuits, for example, the antenna ports of MCU and wireless modules should be kept as far away as possible;
Try to avoid placing high-frequency digital traces, high-frequency analog traces, power traces and other sensitive devices under the wireless module. Copper can be laid under the module;
Wireless modules need to be kept away from transformers, high-power inductors, power supplies and other parts with high electromagnetic interference;
When placing an on-board PCB antenna or ceramic antenna, the PCB under the antenna part of the module needs to be hollowed out, no copper is allowed and the antenna part should be as close as possible to the edge of the board;
Regardless of the radio frequency signal or other signal traces, the traces should be as short as possible, and other signals should be kept away from the transmitting part of the wireless module to avoid interference;
The layout needs to consider that the wireless module needs to have a relatively complete power ground, and the RF wiring needs to leave space for ground holes;
The voltage ripple required by the wireless module is relatively high, so it is best to add a more suitable filter capacitor close to the module voltage pin, such as 10uF;
The wireless module has a fast transmission frequency and has certain requirements for the transient response of the power supply. In addition to selecting a power supply solution with excellent performance during design, attention should also be paid to the reasonable arrangement of the power supply circuit during layout to give full play to the power supply performance; for example, the DC-DC layout is It is necessary to pay attention to the distance between the freewheeling diode ground and the IC ground as close as possible to ensure the return flow, the distance between the power inductor and the capacitor needs to be as close as possible, etc.
2. Setting of line width and line spacing
The settings of line width and line spacing have a huge impact on the performance improvement of the entire board. Reasonable setting of line width and line spacing can effectively improve the electromagnetic compatibility and all aspects of performance of the entire board.
For example, the line width setting of the power line should be considered from the current size of the load of the whole machine, the size of the supply voltage, the copper thickness of the PCB, the length of the traces, etc. Generally, a trace with a width of 1.0mm and a copper thickness of 1oz (0.035mm) can be used. Passes a current of approximately 2A. Reasonable setting of line spacing can effectively reduce crosstalk and other phenomena, such as the commonly used 3W principle (that is, when the center distance between conductors is not less than 3 times the line width, 70% of the electric field can be maintained without mutual interference).
Power supply wiring: Based on the comprehensive consideration of load current, voltage and PCB copper thickness, usually the current needs to be reserved 2 times the normal operating current, and the line distance should try to meet the 3W principle.
Signal wiring: Based on comprehensive considerations such as signal transmission rate, transmission type (analog or digital), wiring length, etc., it is recommended that the spacing of ordinary signal lines meet the 3W principle, while differential lines are considered separately.
RF wiring: The line width of RF wiring needs to consider the characteristic impedance. Commonly used RF module antenna interfaces are all 50Ω characteristic impedance. According to experience, the RF line width for power ≤30dBm (1W) is 0.55mm, and the copper spacing is 0.5mm. , a more accurate characteristic impedance of about 50Ω can also be obtained through adjustment by the board manufacturer.
3. Spacing settings between devices
The spacing between devices is something we must consider during PCB Layout. If the spacing is too small, it will easily lead to soldering tin connection and affect production;
Distance recommendations are as follows:
Similar devices: ≥0.3mm
Different devices: ≥0.13*h 0.3mm (h is the maximum height difference between adjacent devices)
Recommended distance between components that can only be hand-soldered: ≥1.5mm
In-line components and SMD components should also be kept at a sufficient distance for production, and it is recommended to be between 1-3mm;
4. Spacing control between board edges, components and traces
During PCB layout and wiring, it is also very important to ensure that the distance between components and traces from the edge of the board is reasonable. For example, in the actual production process, most of the panels are assembled, so if the components are too close to the edge of the board, it will cause the PCB to split. The solder pads may fall off or even the device may be damaged. If the lines are too close, the lines may break during production and affect the circuit function.
Recommended distance and placement:
Device placement: It is recommended that the device pads be parallel to the “V cut” direction of the board. The purpose is to make the mechanical stress on the device pads even and in the same direction when splitting, and reduce the possibility of the pads falling off.
Device distance: The distance between the device and the edge of the board is ≥0.5mm
Trace distance: The distance between the trace and the edge of the board is ≥0.5mm
5. Adjacent pad connections with teardrops
If the adjacent pins of the IC need to be connected, it should be noted that it is best not to connect directly on the pad, but to connect outside the pad. This can prevent the IC pins from being short-circuited during production. In addition, the width of the lines drawn between adjacent pads also needs to be noted. It is best not to exceed the size of the IC pins, except for some special pins such as power pins.
Teardrops can effectively reduce reflections caused by sudden changes in line width, allowing for smooth connection between traces and pads;
Adding teardrops solves the problem that the connection between the trace and the pad is easily broken by impact force;
From the appearance point of view, adding teardrops can also make the PCB look more reasonable and beautiful;
6. Parameters and placement of vias
The reasonable setting of the size of the via hole has a great impact on the performance of the circuit. The reasonable setting of the via hole size needs to consider the current that the via hole bears, the frequency of the signal, the difficulty of the production process, etc., so the PCB Layout requires special attention.
In addition, the location of the vias is equally important. If the vias are placed on the pad, it will easily lead to poor welding of the device during production. Therefore, the vias are generally placed outside the pads. Of course, the vias should be placed when space is extremely tight. It is also possible to add the board manufacturer’s hole-in-the-plate process to the pad, but this will increase the production cost.
Key points of via setting:
Vias of different sizes can be placed in a PCB due to different routing needs, but it is usually not recommended to exceed 3 types to avoid causing great inconvenience to production and raising costs;
The depth-to-diameter ratio of via holes is generally ≤6, because when it exceeds 6 times, it is difficult to ensure that the hole wall can be evenly plated with copper;
The parasitic inductance and capacitance of vias also need to be paid attention to, especially in high-speed circuits, special attention needs to be paid to their distribution performance parameters;
The smaller the via hole and the smaller the distribution parameters, the more suitable it is for high-speed circuits, but its cost is also high;
The above 6 points are some precautions about PCBLayout compiled this time. I hope they can be helpful to everyone.Keywords: 4G network IO controller