As noted in a recent article from Electronic Design, the IEEE 802.3bs Ethernet Working Group published its standards for 400 Gigabit Ethernet in December and at the DesignCon conference in Santa Clara, Calif. at the end of January many of the connector companies, including industry leaders such as Molex and TE Connectivity, were already rushing to create hardware to meet the new standards.
“The 8-lane, Quad Small Form-factor Pluggable Double Density (QSFP-DD) form factor is one of four connectors that will target 400G Ethernet connections,” Technology Editor William Wong explained. “The others include OSFP, CFP8, and COBO. These new form factors are needed to handle the 25-Gb/s NRZ, 56-Gb/s PAM-4, or potentially 112-Gb/s PAM-4 signaling needed for 400G and beyond.”
Hardware specifications were also defined by the Quad Small Form-Factor Pluggable Double Density (QSFP-DD) Multi-Source Agreement (MSA) group in Sept. 2017. In addition to Molex and TE Connectivity, the 62 companies that were part of the QSFP-DD MSA included Broadcom, Cisco, Finisar, Juniper Networks, and more leaders in the communications and broadcast industries.
The report said, “QSFP-DD supports up to 400 Gb/s in aggregate over an 8 x 50 Gb/s electrical interface. The cage and connector designs provide backwards compatibility to QSFP28 modules which can be inserted into a QSFP-DD port and connected to 4 of the 8 electrical channels.” From a thermal standpoint, the group defined the operating case temperatures as 0-70°C for standard class, -5°C to 85°C for extended class and -40°C to 85°C for industrial class.
“For all power classes,” the report continued, “all module case and handle surfaces outside of the cage must comply with applicable Touch Temperature requirements.”
At DesignCon, Wong wrote, one of the biggest challenges that companies faced was how to optimize thermal management for these optical systems. This is nothing new for designers of optical transceivers, as increased density means increased heat, which can lead to performance and reliability issues.
As a recent article from Advanced Thermal Solutions, Inc. (ATS) exploring industry developments in cooling QSFP transceivers explained, “The performance and longevity of the transceiver lasers depend on the ambient temperature they operate in and the thermal characteristics of the packaging of these devices. The typical thermal management approach combines heat dissipating fins, e.g. heat sinks, and directed airflow.”
It added, “Designers can reduce thermal spreading losses by keeping the heat sources close to the thermal interface area and by increasing the thermal conductivity of the case materials.”
ATS engineers have been working with QSFP manufacturers to meet thermal needs. In a recent project, ATS engineers designed a new solution for cooling optical transceivers using a series of heat sinks in an optimized layout that took into consideration individual QSFP junction temperatures as well as the effect of airflow pushing heat onto downstream transceivers.
Test set-up of different heat sink designs on QSFP28 connector cages. Through testing, analytical modeling and computer simulations, ATS engineers optimized a design for cooling a series of QSFP. (Advanced Thermal Solutions, Inc.)
Analytical modeling and computer simulations “showed that having heat sinks with fewer fins upstream and heat sinks with more fins downstream provided a near isothermal relationship between the first and last QSFP, an important consideration for QSFP arrangements.” Overall, the innovative design showed a 30 percent improvement over QSFP heat sinks currently on the market. Using this design, there was less than a degree difference between the first QSFP in the series and the last.
“Minimizing the upstream QSFP heat sinks, which in turn minimizes the amount of preheat to the downstream QSFP and allows as much airflow to enter downstream QSFP,” the engineers remarked. “At the same time ensuring the upstream QSFP temperatures are equal to or just lower than the downstream QSFP. This keeps the downstream QSFP temperatures at a minimum, while also keeping the transceiver stack close to isothermal.”
In addition to junction temperature, ATS engineers factored in contact resistance, since QSFP transceivers require that no thermal interface material (TIM) is used. As the engineers explained, “The QSFP isn’t fixed in the cage; it can be hot swapped. After a few insertions and removals, it will gunk up the TIM.” Also, to increase the surface area, the heat sinks were extended beyond the edge of the cage and the base had to be thicker than originally planned to account for spreading resistance.
As 400G Ethernet technology continues to expand, more power is required, and transceiver-density is increasing, ATS is ready with thermal solutions to meet next-generation challenges for the QSFP market.
For more information about Advanced Thermal Solutions, Inc. (ATS) thermal management consulting and design services, visit https://www.qats.com/consulting or contact ATS at 781.769.2800 or ats-hq@qats.com.