Power dissipation associated with networking equipment is increasing as enhanced functionality is designed into various networking products. With enhanced functionality comes an attendant increase in heat load and heat density.
Some new high-capacity networking products include a bank of optical transceivers known as XFPs (an XFP is a 10 Gigabit/s version of a transceiver device typically known as a small form factor pluggable transceiver or “SFP”) placed directly in line, each dissipating 3.5 W. In normal conditions a commercial-grade SFP/XFP, which can operate up to a maximum of 70° C., is preferred. In more extreme conditions industrial-grade SFPs/XFPs are available, which can operate up to a maximum temperature of 85° C. However, industrial-grade SFPs/XFPs are substantially more expensive than commercial ones.
Past solutions to cooling a bank of SFPs or XFPs have included an individual heat sink attached to each SFP or XFP. The problem with this design is that SFPs/XFPs have been observed to operate at temperatures exceeding 85° C.
In addition, heat sinks commonly have fins which may be of the pin-fin design (round vertical fins) to allow for adequate cooling when the air flowing past the heat sink is expected to be in any orientation. The pin-fin design is suboptimal in heat transfer performance relative to parallel-fin heat sinks, which is a detriment of existing solutions. However, in some applications, the use of pin-fin heat sinks is necessary as some products may be operated in different orientations, such as horizontal and vertical modes.
A typical bank of SFPs/XFPs is located on a circuit pack and has limited space and height available in the slot occupied by the circuit pack. A typical circuit pack slot is 25 mm in height. The printed circuit board, backside wiring and components and tolerances can account for 7 to 11 mm of this height. As a typical XFP is 10 mm in height, this leaves 4 to 8 mm for the heat sink, which typically includes fins and the heat sink base. As a consequence, the height of the fins on the heat sink is 7-8 mm at most. This limits the amount of heat that the heat sink can pass to the air. For example, typical pin-fin heat sink assemblies that are 4.2 mm and 6.5 mm tall have manufacture-quoted minimum thermal resistances of 18 C/W and 11.5 C/W and are effective at dissipating heat loads of 1.5 W and 2.5 W, respectively.
Similar thermal challenges are presented by other circuit pack components. For example, networking products include arrays of three or more FBGAs (fine pitch ball grid arrays) that each dissipate 20 W.
In summary, the current solution is deficient as individual heat sinks do not have the ability to adequately dissipate heat to the air under the current space and airflow constraints found in typical circuit packs. This is particularly evident for SFPs and XFPs that are located at the downstream (relative to an incoming airflow) side of a board, where they are required to transfer heat to air that has been heated substantially by the SFPs/XFPs upstream of these components.
It is clear from the above that there is a substantial need for a more efficient means of transporting heat away from networking components (e.g. SFPs, XFPs and other heat generating components) and dissipating it to the ambient air.