What are the factors affecting PCB impedance

1. When the line is less than 25 mm from the edge of the board, the line impedance value is 1~4 ohms smaller than the middle of the board, and the impedance value is less affected by the position when the line is more than 50 mm from the board edge. Next, it is recommended to preferentially select the material size to meet the impedance line to the edge of the board is greater than 25 mm;

2. The most important factor affecting the impedance consistency of PCB imposition is the uniformity of thickness at different locations, and secondly the uniformity of line width;

3. The difference in residual copper ratio between different positions of circuit board will result in a difference of impedance of 1~3 ohm. When the uniformity of pattern distribution is poor (the residual copper ratio is quite different), it is recommended to lay down the flow block without affecting the electrical performance. Point and plating shunt points to reduce the difference in dielectric thickness and copper plating thickness at different locations;

4. The lower the gel content of the prepreg, the better the uniformity of the thickness after lamination. The thickness of the edge of the circuit board leads to a small thickness and a large dielectric constant, so that the impedance of the near-plate line is smaller than the imposition. Intermediate area

5. For the inner layer, the difference in impedance consistency due to line width and copper thickness is small in different positions of the imposition; for the outer layer, the difference in copper thickness affects the impedance within 2 ohm, but the etching line caused by the difference in copper thickness Wide difference has a great influence on impedance consistency, and it is necessary to improve the uniformity of copper plating on the outer layer.