6 details to quickly improve your PCB layout quality

The layout of components in the PCB board is crucial. Correct and reasonable layout not only makes the layout more neat and beautiful, but also affects the length and number of printed wires. Good PCB device layout is extremely important to improve the performance of the whole machine.

So how to make the layout more reasonable? Today we will share with you "6 details of PCB board layout"

01. Key points of PCB layout with wireless module

Physically separate analog circuits from digital circuits, for example, keep the antenna ports of MCU and wireless module as far away as possible;

Try to avoid arranging high-frequency digital wiring, high-frequency analog wiring, power wiring and other sensitive devices under the wireless module, and copper can be laid under the module;

The wireless module should be kept as far away from transformers and high-power power supplies as possible. Inductor, power supply and other parts with large electromagnetic interference;

When placing an onboard PCB antenna or ceramic antenna, the PCB under the antenna part of the module needs to be hollowed out, copper should not be laid, and the antenna part should be as close to the board as possible;

Whether the RF signal or other signal routing should be as short as possible, 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 routing needs to leave space for the ground hole;

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 an excellent power supply solution during design, you should also pay attention to the reasonable layout of the power supply circuit during layout to give full play to the power supply. Source performance; for example, in DC-DC layout, it is necessary to pay attention to the distance between the freewheeling diode ground and the IC ground to ensure return flow, and the distance between the power inductor and the capacitor to ensure return flow.

02. Line width and line spacing settings

The setting of line width and line spacing has a huge impact on the performance improvement of the entire board. Reasonable setting of trace width and line spacing can effectively improve the electromagnetic compatibility and various aspects of the entire board.

For example, the line width setting of the power line should be considered from the current size of the whole machine load, the power supply voltage size, the copper thickness of the PCB, the trace length, etc. Usually, a trace with a width of 1.0mm and a copper thickness of 1oz (0.035mm) can pass about 2A of current. Reasonable setting of line spacing can effectively reduce crosstalk and other phenomena, such as the commonly used 3W principle (that is, the center spacing between the wires is not less than 3 times the line width, 70% of the electric field can be kept from interfering with each other).

Power routing: According to the current, voltage and PCB copper thickness of the load, the current usually needs to be reserved twice the normal working current, and the line spacing should meet the 3W principle as much as possible.

Signal routing: According to the signal transmission rate, transmission type (analog or digital), routing length and other comprehensive considerations, the spacing of ordinary signal lines is recommended to meet the 3W principle, and differential lines are considered separately.

RF routing: The line width of RF routing needs to consider the characteristic impedance. The commonly used RF module antenna interface is 50Ω characteristic impedance. According to experience, the RF line width of ≤30dBm (1W) is 0.55mm, and the copper spacing is 0.5mm. More accurate characteristic impedance of about 50Ω can also be obtained through the assistance of the board factory.

03. Spacing between devices

During PCB layout, the spacing between devices is something we must consider. If the spacing is too small, it is easy to cause soldering and affect production;

The distance recommendations are as follows:

Similar devices: ≥0.3mm

Different devices: ≥0.13*h+0.3mm (h is the maximum height difference of the surrounding adjacent devices)

The distance between devices that can only be manually soldered is recommended: ≥1.5mm

DIP devices and SMD devices should also maintain a sufficient distance in production, and it is recommended to be between 1-3mm;

04. Spacing control between board edge and devices and traces

During PCB layout and routing, it is also very important whether the distance design between devices and traces from the board edge is reasonable. For example, in the actual production process, most of the panels are assembled. Therefore, if the device is too close to the board edge, it will cause the pad to fall off when the PCB is divided, or even damage the device. If the line is too close, it is easy to cause the line to 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 panel, so that the mechanical stress on the device pads during panel separation is uniform and the force direction is the same, reducing the possibility of pads falling off.

Device distance: The placement distance of the device from the edge of the board is ≥0.5mm

Trace distance: The distance between the trace and the edge of the board is ≥0.5mm

05. Connection of adjacent pads and 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 pads, but to lead them out to connect outside the pads, so as to prevent the IC pins from being short-circuited during production. In addition, the line width between adjacent pads should also be noted, and 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 the reflection caused by sudden changes in line width, and can allow the traces to connect to the pads smoothly.

Adding teardrops solves the problem that the connection between the trace and the pad is easily broken by impact.

From the appearance point of view, adding teardrops can also make the PCB look more reasonable and beautiful.

06. Parameters and placement of vias

The reasonableness of via size setting has a great impact on the performance of the circuit. Reasonable via size setting needs to consider the current that the via bears, the frequency of the signal, the difficulty of the manufacturing process, etc., so PCB Layout needs special attention.

In addition, the placement of the via is also important. If the via is placed on the pad, it is easy to cause poor device welding during production. Therefore, the via is generally placed outside the pad. Of course, in the case of extremely tight space, the via is placed on the pad and the via in the plate process of the board manufacturer is also possible, but this will increase the production cost.

Key points of via setting:

Different sizes of vias can be placed in a PCB due to the needs of different routings, but it is usually not recommended to exceed 3 types to avoid great inconvenience in production and increase costs.

The depth to diameter ratio of the via is generally ≤6, because when it exceeds 6 times, it is difficult to ensure that the hole wall can be evenly copper-plated.

The parasitic inductance and parasitic capacitance of the via also need to be paid attention to, especially in high-speed circuits, special attention should be paid to its distributed performance parameters.

The smaller the vias and the smaller the distribution parameters, the more suitable they are for high-speed circuits, but their costs are also high.

The above 6 points are some of the precautions for PCB Layout sorted out this time, I hope they can be helpful to everyone.