Signal Integrity in Immersion

Overview

Signal Integrity (SI) in immersion cooling addresses how submersion of IT equipment in dielectric fluid affects high-speed electrical signal transmission. Dielectric immersion fluids alter the electromagnetic environment around PCB traces, connectors, and cables: their dielectric constant (Er) is higher than air (Er=1), typically falling in the range Er=1.78–1.9 for fluorochemical fluids and Er=2.0–2.2 for hydrocarbon/oil fluids. This shift in permittivity changes transmission line impedance, propagation velocity, and crosstalk characteristics, potentially degrading signal integrity at high data rates.

The OCP Signal Integrity for Immersion Cooling workstream, co-led by Andy Young (aDoas/Asperitas) and Kai Wang (Intel), was launched in mid-2023 and has produced the first systematic characterization of immersion SI effects using the OAI 2.0 platform as a common test vehicle. Key finding: across all interfaces tested (PCIe 5.0, DDR5, Ethernet 56G PAM4, UPI 4), immersion effects are small — typically <2% eye diagram difference vs. air — and temporally stable at 24 months. This does not eliminate SI as a concern, but establishes a quantitative baseline and narrows the open questions to high-speed interfaces at next-generation data rates (112G PAM4 for PCIe Gen 6, OAM-to-UBB interconnects).