Issue link: https://iconnect007.uberflip.com/i/1522641
84 PCB007 MAGAZINE I JUNE 2024 ory? As Einstein said, "A theory is something nobody believes in except the person who made it. An experiment is something every- body believes, except the person who made it." To investigate, RoBAT has analysed imped- ance from a production run of 4,640 small form factor PCBs: 58 panels with 80 PCBs per panel (5 columns by 16 rows); each PCB with 32 differential pairs (88Ω ±8%); 16 on top and 16 on the bottom; 0.65 mm pitch, 10 mm long. Coupon testing has been the de facto test for PCB impedance verifica- tion, and all 58 panels pres- ent with passing coupon impedance—but does the coupon impedance really represent every single sig- nal on every PCB across the entire panel? Can it with- stand the scrutiny of mod- ern SI impedance require- ments? We tested this by endeav- oring to measure 100% of these signals (all 148,480 differential pairs) and ana- lysing the impedance dis- tribution across the panel. Utilising the SCARA- TDR-M, the task isn't as daunting as it might sound. It is capable of measuring up to 12 pairs simul- taneously (4.2s/PCB, 5m41s/panel) and is fast enough to keep pace with PCB fab production. Custom modules interface directly onto the TDR measurement unit to provide the fastest rise time TDR signal onto the DUT <15ps. Fast rise time is essential for seeing imped- ance discontinuities, but that is especially true for the short 10 mm traces. Typical TDR prob- ing measurements with rise times less than 15 ps wouldn't even see the tracks, let alone be able to measure a stable impedance. Figure 1: Example of impedance discontinuities. (Source: SiGuys 2023 DesignCon) Figure 2: RoBAT TDR signal path.