There are many factors that affect the impedance of PCB boards, such as dielectric thickness, line width/line spacing, copper thickness, and dielectric constant. Let's take a look at the relationship between these factors and the impedance value of PCB boards.

1. Dielectric thickness---is the most important factor affecting the impedance value of PCB board

Increasing dielectric thickness can increase impedance, and reducing dielectric thickness can reduce impedance;

In impedance design, dielectrics are divided into prepregs and plates. Prepregs of the same thickness have different glue contents, and the same is true for plates. The thickness of the prepreg after pressing is related to the flatness of the press, the program of the press plate and the design of the circuit;

2. Line width/line spacing

Increasing line width can reduce impedance, and reducing line width can increase impedance.

In impedance design, the finished width of the line is controlled within a tolerance of +/-10% to better meet the requirements of controlling impedance. When the dielectric thickness and dielectric constant of the product are confirmed, the impedance can only be fine-tuned by adjusting the line width and line spacing. At the same time, a large copper surface must be laid at the corresponding position of the impedance line to ensure the stability of the signal.

3. Copper thickness

Reducing line thickness can increase impedance, and increasing line thickness can reduce impedance;

Copper thickness can be controlled by graphic electroplating or selecting copper foil of different thicknesses.

For copper thickness, the copper plating distribution should be uniform. For thin wires and isolated wires, shunt blocks should be added to balance the current to prevent uneven copper thickness on the wires and affect the impedance. For the situation where the copper distribution on the CS and SS surfaces is extremely uneven, the boards should be cross-plated to achieve the purpose of uniform copper thickness on both sides.

4. Dielectric constant:

Increasing the dielectric constant can reduce the impedance, and reducing the dielectric constant can increase the impedance. The dielectric constant is mainly controlled by the material.

Different boards have different dielectric constants, which are related to the resin material used: the dielectric constant of FR4 board is 3.9-4.8, which will decrease with the increase of the frequency of use, and the dielectric constant of polytetrafluoroethylene board is between 2.2-3.9.