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32 The PCB Design Magazine • September 2016 In Part 1 of this series, I deliberated on how dangling via stubs distort signals passing through an interconnect and also decrease the usable bandwidth of the signal. This is due to the via stub acting as a transmission line an- tenna, which has a resonant frequency deter- mined by the quarter wavelength of the struc- ture. The conventional solution to this problem is to back-drill (or control depth drill) the vias to bore out the via stub barrels, so that the via stubs are reduced in length if not completely re- moved. This month I will look into all the pos- sible solutions to alleviate this issue. 1. Back-drill the Stub Back-drilling is a process to remove the stub portion of a plated through-hole (PTH) via. It is a post-fabrication drilling process where the back-drilled hole is of larger diameter than the original PTH. This technology is often used in- stead of blind via technology to remove the stubs of connector vias in very thick high-speed backplane designs. State-of-the-art board fabri- cation shops are able to back-drill to within 8 mils of the signal layer, so there will always be a small stub portion attached to the via. High-speed, SERDES, serial link-based back- planes generally have thick substrates. This is due to the system architecture and backplane to card interconnect requirements such as press-fit connectors. Back-drilling the via stub is a com- mon practice, on thick PCBs, to minimize stub length for bit-rates greater than 3Gbps (1.5GHz). However, at transmission rates >10Gbps (5GHz), back-drilling alone may not be adequate to re- duce jitter and bit error rate (BER). Figure 1 shows the effects of excessively long via stubs on a high-speed differential pair. On the left, the differential pair is simu- lated using a pseudo random bit stream (PRBS) with lossy transmission lines enabled; note the open eye pattern. However, on the right, I had included via modelling, which enables the via parasitics and highlights the effects of via resonance. The high-frequency harmonics are attenuated, rolling off the signal rise time, by Barry Olney IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA How to Handle the Dreaded Danglers, Part 2 BEYOND DESIGN Figure 1: High-speed differential signal with via resonance (simulated in HyperLynx).

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