Optimizing Signal Integrity in Immersion Cooled IT Platforms

Author: Kai (Intel), T.T. Little (Foxconn Interconnect), Atin (FIT, CTO) Collection: 2024 OCP Global Summit - Cooling Environments & Immersion

Summary

This presentation from Intel, Foxconn Interconnect, and FIT reports on the OCP Signal Integrity (SI) Working Group's ongoing effort to characterize and mitigate the impact of single-phase immersion cooling fluids on high-speed interconnects. The core problem: immersion fluids have a dielectric constant (Dk) of 2.0–2.2 (vs. ~1.0 for air), which lowers connector impedance and increases insertion loss — effects that become critical at 112G PAM4 and become potentially disqualifying at 224G PAM4. The working group's goal has been to identify minimum-cost design modifications that allow existing platforms to work in immersion environments.

The presentation covers three linked workstreams: (1) characterizing fluid dielectric properties using the liquid cavity resonator method, (2) developing isolation test fixtures that expose only the device under test (DUT) to fluid while keeping SMAs (SubMiniature version A connectors) and PCB traces dry, and (3) measuring immersion-induced SI margin degradation across DDR, PCIe, Ethernet, and optical interfaces. Key results show DDR5600–DDR8800 and PCIe 5.0/4.0 exhibit minimal immersion impact after 24 months; Ethernet 56G PAM4 passes; but 112G PAM4 BER (bit error rate) degrades from ~1e-8 in air to 1e-6 to 1e-5 in fluid — still within spec for some configurations but at the boundary. 224G PAM4 is identified as the next critical challenge.

The group has also established that fluid temperature affects impedance: increasing from 28°C to 55°C reduces connector impedance by ~3 ohms. A key design insight is that immersion-specific connector designs — redesigning geometry to retune impedance for Dk≈2.0 environments — offer the clearest path forward for 112G and beyond. Foxconn's ExMax ARK connector was cited as an example of a connector redesigned specifically for immersion, targeting PCIe 6 compatibility.