Factors affecting PCB impedance

Through the above tests and analysis, the main factors affecting the consistency of PCB impedance and the degree of influence of each factor are recognized. The main conclusions and suggestions for improvement are as follows:

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 at different positions of the imposition 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 the choke point reasonably without affecting the electrical performance. Electroplating split 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 sheet is small, and the dielectric constant is too large, so that the impedance value of the near-plate line is smaller than the intermediate area of the imposition. ;

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.