Page 66 - 2020 Interconnect Innovations eBook

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ICMs and the OCP NIC 3.0 Specification
When the OCP Server Working Group released the OCP NIC 3.0 specification in late 2019, it created an entirely new set of guidelines for Ethernet-based servers that is intended to unify the mechanical and electrical parameters for connecting server and storage systems. The network interface cards (NICs) and mezzanine adapter cards within these systems had to deviate from standard PCIe adapter card form factors to accommodate the mechanical restrictions and satisfy the high-speed I/O connectivity demands of these new systems. The new cards were originally designed around high-speed QSFP and SFP+ cage connectors, but market pressure pushed the group to include traditional RJ45 Ethernet connectivity in the standard as well. However, most 1GBASE-T and 10GBASE-T RJ45 connectors were either too tall or too wide to fit into the space allotted for OCP NIC 3.0 face plates. This drove demand for specialized RJ45 connectors that could accommodate the new, restrictive space requirements for OCP NIC 3.0 adapters.
Section 2.4 of the OCP NIC 3.0 specification defines the I/O faceplates for the small and large form factor versions. Due to airflow requirements designed to optimize heat transfer and maximize the number of cards in a system, the faceplate dimensions are relatively small and the tolerances are tightly held to ensure that every compatible card is guaranteed to fit into any system designer’s hardware. Standard RJ45 connectors typically have a half-inch (12.7mm) above-board height profile and fit nicely into standard PCIe NIC adapter faceplates. However, OCP NIC 3.0 adapters limit maximum component height to 10.13mm. As such, standard RJ45 connectors required design modifications to fit into the faceplate.
Resourceful connector engineers have developed RJ45 connector designs that allow the components to be mounted within the PCB rather than on top of it. The sunken mounting configuration of the new OCP-compatible RJ45s is called a midplane or offset jack mounting configuration and uses the PCB thickness as part of the height of the connector itself. Connectors like this have existed before, but in the case of the OCP faceplate restrictions, the offset of the RJ45 connectors had to be such that the overall height of the connector did not exceed the 10.13mm restriction and the portion that hangs below the bottom of the PCB did not interfere with the lower heat shield of the OCP NIC 3.0 adapter assembly.
IMAGE
» This comparison of OCP-compatible RJ45 connector (top) and standard RJ45 connector (bottom) heights illustrates the differences between the midplane or offset jack mounting configuration that satisfies the specification’s 10.13mm component height limitation and the standard board mounting configuration.