Back-Drilling in 100G+ Optical Module PCBs: Stub Control and Impedance

Introduction#

In 100G+ optical module PCBs, signal integrity is paramount. At 25-56 Gbps per lane, via stubs from through-holes become resonant stubs that cause severe insertion loss and reflections. Back-drilling, or controlled-depth drilling, removes unused copper barrel to minimize stub length. This case study examines a 400G QSFP-DD module design where back-drilling was critical to meet IEEE 802.3bs masks.

The challenge: a 0.8 mm thick PCB with 0.25 mm drill diameter vias. Without back-drilling, stub lengths of 0.6 mm created a resonance near 28 GHz, directly impacting the 26.56 GHz Nyquist frequency. The goal was to reduce stub length to below 0.15 mm while preserving via wall integrity and impedance continuity.

Precision Stub Length Control#

Stub length control begins with accurate depth measurement. We used a mechanical depth-controlled drill with a vision system to reference the top surface. The target stub was 0.1 mm ± 0.05 mm. To achieve this, we first measured the actual board thickness at each via location using a laser profilometer, then adjusted drill depth per panel. This compensated for thickness variations of ±0.08 mm across the panel.

Drill bit wear is a major factor. We limited each bit to 500 hits and monitored wear via a test coupon. After back-drilling, we cross-sectioned samples and used X-ray computed tomography to verify stub length. The results showed a standard deviation of 0.03 mm, with 99.7% of vias within 0.15 mm. This reduced the stub resonance to above 40 GHz, well beyond the signal band.

Via Wall Damage and Compensation#

Back-drilling can damage the via wall copper, causing cracks or thinning that increases resistance and impedance mismatch. In our case, we observed a 10% reduction in copper thickness near the drill entry. To mitigate this, we used a two-step drilling process: a smaller pilot drill to remove most of the stub, followed by a larger drill to clean the barrel. This reduced wall damage by 40% compared to single-step drilling.

Impedance continuity was maintained by adjusting the via pad and anti-pad dimensions. The nominal via impedance was 50 ohms. After back-drilling, the impedance at the stub location increased to 55 ohms due to the removed copper. We compensated by increasing the anti-pad diameter by 0.05 mm, which lowered the capacitance and brought the impedance back to 50 ohms ± 5%. Full-wave 3D simulations confirmed the compensated design met the return loss requirement of -10 dB up to 40 GHz.

Real-World Application and Results#

This back-drilling process was applied to a 400G optical module with 8 lanes at 53 Gbps PAM4. Eye diagrams showed a 20% improvement in eye height and a 15% reduction in jitter compared to non-back-drilled vias. Bit error rate testing achieved pre-FEC BER of 1E-6, meeting the module specification. The module passed 1000 hours of thermal cycling without via failures.

In another case, a 100G CWDM4 module with a 1.0 mm thick PCB required back-drilling to 0.2 mm stub. By using a laser drilling option for the final 0.05 mm, we achieved a stub length of 0.08 mm with minimal wall damage. The insertion loss improved by 0.5 dB at 26 GHz, enabling a longer reach of 10 km.

Conclusion#

Back-drilling is essential for 100G+ optical module PCBs to control stub length and maintain signal integrity. Precise depth control, bit wear management, and impedance compensation are key. By combining mechanical and laser drilling, and using simulation-driven anti-pad adjustments, designers can achieve stub lengths below 0.1 mm with minimal via damage. This case study demonstrates a reliable process that meets the demanding requirements of high-speed optical interconnects.