Issue link: https://iconnect007.uberflip.com/i/1233021
24 DESIGN007 MAGAZINE I APRIL 2020 The TDR measurements of the 100-ohm dif- ferential impedance test coupons passed for both top and bottom microstrip signal layers (Figure 7). Comparing the modeled impedance to the actual measured impedance, in this case, we get a 1.6-ohm variation for microstrip trac- es and a 2.5-ohm variation for stripline trac- es. The Zmetrix TDR's accuracy is within 1%, across the entire impedance range from 10 to 200 ohms. Keep in mind that there is also a multitude of fabrication variables to account for—hence a tolerance of ±10%, which is the IPC standard for controlled impedance mea- surement. And fabrication variables will be different for every process line. This is where an experienced fabricator can improve yields. Some fabrication shops offer a tolerance of Figure 6: 100-ohm microstrip differential impedance calculation (iCD Stackup Planner). Figure 7: Pass/fail results for the 100-ohm differential traces (Zmetrix TDR and Zscan software).