(1) Although the wiring spacing and wiring width are consistent, the spacing of Coupon Test point is fixed at 2.54mm (to meet the spacing of test probes), and the spacing of the end of the actual wiring in the board (i.e., the golden finger) is variable. With the emergence of QFP, PLCC, and BGA packages, the pin spacing of some chips is far less than 2.54mm (i.e., the spacing of Coupon Test point).

(2) Coupon wiring is an ideal straight line, while the actual wiring on the board is often designed and produced according to actual needs. PCB designers and production personnel can easily idealize Coupon's wiring, but the actual wiring on the PCB board may undergo irregular changes due to practical needs.

(3) The positions of Coupon and the actual wiring inside the board are different on the entire PCB board. Coupons are located in the middle or edge of the PCB board and are often removed by the manufacturer when the PCB board leaves the factory. The actual positions of the wiring inside the board are diverse, some near the edges of the board, some at the center of the board, and so on.

(4) The impedance wiring inside the board is generally surrounded by vias, pads, shielding layers, etc., while the environment around Coupon wiring is relatively single.

From this, it can be seen that there are differences between the impedance lines inside the board and the Coupon wiring, which also have a significant impact on the impedance testing.