1, the line width is wider than the fine line: because there is a fine line limit in the manufacturing process, the width is not limited. Adjusting the line width to adjust the impedance at a later stage and reach the limit can be cumbersome, which increases the cost or relaxes the impedance control.

　　2, there is an overall trend: there may be multiple impedance control targets in the design, so the overall size is too large or too small, and there is no need to be out of sync.

　　3, consider the residual copper rate and glue flow: when one or both sides of the prepreg are etched lines, during the pressing process, the glue will fill the etching gap, so the glue thickness time between the two layers is reduced. If the residual copper rate and glue flow rate are not calculated correctly, and the dielectric coefficient of the new material is not consistent with the nominal value, signal integrity problems may occur.

　　4, specify the glass cloth and glue content: different glue content of different glass cloth, prepreg or core plate dielectric coefficient is different, even if the height is the same, it may be the difference between 3.5 and 4. The reason is that the single-line impedance changes about 3ohm.