Marcy’s Musing: From Pitch to PO—The Sales Stack
It’s Only Common Sense: Knowing When to Walk Away
Dan’s Biz Bookshelf: ‘Elevate: Push Beyond Your Limits'
Beyond Design: How to Handle the Dreaded Danglers, Part 2
October 12, 2016 | Barry Olney, In-Circuit Design Pty LtdEstimated reading time: 2 minutes
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 antenna, which has a resonant frequency determined by the quarter wavelength of the structure. 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 removed. This month I will look into all the possible 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 instead of blind via technology to remove the stubs of connector vias in very thick high-speed backplane designs. State-of-the-art board fabrication 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 backplanes 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 common 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 reduce 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 simulated 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, distorting the signal, reducing bandwidth and closing the eye.
Vias can appear as capacitive and/or inductive discontinuities. These parasitics contribute to the degradation of the signal as it passes through the via. At high frequencies and with thick backplane substrates, it is imperative that these issues are addressed.
Back-drilling typically requires specialized equipment, and further requires that the back-drill be precisely located over the vias. As such, the back-drilling process, especially two sided back-drilling, is expensive due to drill breakage and yield issues and is very time-consuming.
To read this entire article, which appeared in the September 2016 issue of The PCB Design Magazine, click here.
Share on:
Suggested Items
New Podcast Series Launches: Optimize the Interconnect
07/16/2025 | I-Connect007I-Connect007 is excited to announce the debut of Optimize the Interconnect—a new podcast series featuring guest Chris Ryder, senior director of business development at MKS’ ESI. This insightful series explores how MKS’ ESI is rethinking microvia formation for today’s most advanced HDI PCB and substrate designs.
Trouble in Your Tank: Can You Drill the Perfect Hole?
07/07/2025 | Michael Carano -- Column: Trouble in Your TankIn the movie “Friday Night Lights,” the head football coach (played by Billy Bob Thornton) addresses his high school football team on a hot day in August in West Texas. He asks his players one question: “Can you be perfect?” That is an interesting question, in football and the printed circuit board fabrication world, where being perfect is somewhat elusive. When it comes to mechanical drilling and via formation, can you drill the perfect hole time after time?
Excellon Installs COBRA Hybrid Laser at Innovative Circuits
06/23/2025 | ExcellonExcellon is pleased to announce the successful installation of a second COBRA Hybrid Laser System at Innovative Circuits, located in Alpharetta, Georgia. The Excellon COBRA Hybrid Laser System uniquely combines both UV and CO₂ (IR) laser sources on a single platform—making it ideal for high-density prototype and production printed circuit boards (PCBs).
Gorilla Circuits Elevates PCB Precision with Schmoll’s Optiflex II Alignment System
06/23/2025 | Schmoll MaschinenGorilla Circuits, a leading PCB manufacturer based in Silicon Valley, has enhanced its production capabilities with the addition of Schmoll Maschinen’s Optiflex II Post-Etch Punch system—bringing a new level of precision to multilayer board fabrication.
The Evolution of Picosecond Laser Drilling
06/19/2025 | Marcy LaRont, PCB007 MagazineIs it hard to imagine a single laser pulse reduced not only from nanoseconds to picoseconds in its pulse duration, but even to femtoseconds? Well, buckle up because it seems we are there. In this interview, Dr. Stefan Rung, technical director of laser machines at Schmoll Maschinen GmbH, traces the technology trajectory of the laser drill from the CO2 laser to cutting-edge picosecond and hybrid laser drilling systems, highlighting the benefits and limitations of each method, and demonstrating how laser innovations are shaping the future of PCB fabrication.