Ideas & Solutions
Test coupons for the measurement of impedance on PCB
As it is known, the measurement of impedance of traces on printed circuit boards (PCB) can be executed either directly on the printed-circuit board (if the parameters of the PCB make it possible to carry out such measurement), or on a test coupon, which must be specially prepared for such purpose. The test coupon can be located directly on the printed-circuit board (if size of PCB allows this arrangement), or it can be carried out at a separate board, if it cannot be placed at the PCB. In the second case, the test coupon is placed, as a rule, on the same PCB manufacturing panel, where main printed circuit board is located. To achieve a good quality of the measurement of impedance, the PCB designer and manufacturer shall follow the requirements for test coupon design (you can see more details in IPC-2221B Appendix A).
When you order to our company the PCB projects with impedance control requirements, our engineers check the initial stack-up and provide you the feedback: how your stack-up and/or PCB layout should be corrected to meet the needed impedance values. We can suggest the proper core and prepreg materials, the trace widths, and the differential pair spacing. If the PCB design allows we could make the changes by ourselves upon your confirmation. After your approval of finalized PCB layout and stack-up, our engineers create the design of impedance test coupon, which meets your requirements, and in accordance to IPC-2221B recommendations. During the manufacturing of PCB batch, if the impedance measurements of the coupons from certain manufacturing panels will not meet the initial impedance requirements, those manufacturing panels will be rejected. The customer will get only fully tested PCBs with correct impedance within required tolerance (±10% for regular orders, or ±5% for advanced technology orders), and upon request it's possible to ship the impedance test coupons together with shipped PCBs. 
See also this article on PCB etch factor problems.