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Research on the pad and copper design of high frequency PCB

writer: G January 22, 2019

1. Pad and aperture

In the case of ensuring that the minimum spacing of the wiring does not violate the electrical spacing of the design, the pad design should be large to ensure a sufficient loop width. Generally, the inner hole of the pad is slightly larger than the lead diameter of the component, and the design is too large to form a solder joint in the soldering. The outer diameter D of the pad is generally not less than (d + 1.2) mm, where d is the inner aperture of the pad. For some PCBs with a relatively high density, the minimum value of the pad can be (d + 1.0) mm. The shape of the pad is usually set to a circular shape, but the pad of the integrated circuit for the DIP package is preferably in the shape of a racetrack, which can increase the area of the pad in a limited space and facilitate the soldering of the integrated circuit. The connection between the wiring and the pad should be smoothly transitioned. That is, when the width of the wiring entering the circular pad is smaller than the diameter of the circular pad, a teardrop design should be adopted.

It should be noted that the size of the aperture d in the pad is different and should be considered according to the diameter of the actual component lead, such as component holes, mounting holes and slots. The hole pitch of the pad should also be considered according to the installation method of the actual components. For example, resistors, diodes, tubular capacitors and other components have "vertical" and "horizontal" mounting modes. The pitch of the two methods is different. In addition, the pad pitch is designed to take into account the minimum gap requirements between components, especially the gap between special components needs to be ensured by the pitch between the pads.

In high-frequency PCBs, the number of vias should also be minimized, which reduces the distributed capacitance and increases the mechanical strength of the PCB. In short, in the design of high-frequency PCB, the design of the pad and its shape, aperture and pitch should be considered in consideration of its particularity and the requirements of the production process. Adopting a standardized design can not only reduce product costs, but also improve production efficiency while ensuring product quality.

2. Apply copper

The main purpose of copper coating is to improve the anti-interference ability of the circuit. At the same time, it has great advantages for PCB heat dissipation and PCB strength. The copper grounding can also play a shielding role. However, it is not possible to use a large-area strip of copper foil because it will generate a large amount of heat when the PCB is used for a long time. At this time, the strip-shaped copper foil is liable to expand and fall off. Therefore, it is preferable to use a grid when applying copper. The copper foil is connected to the grounding network of the circuit, so that the grid will have a better shielding effect. The size of the grid is determined by the interference frequency to be shielded.

After completing the design of the wiring, pads, and vias, DRC (Design Rule Check) should be performed. The difference between the designed map and the defined rules is detailed in the inspection results, and the network that does not meet the requirements can be found. However, the DRC should be parameterized before wiring to run the DRC, that is, execute the ToolsDesignRuleCheck command.

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