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John O | August 2017

ATS designing heat sinks to cool QSFP optical transceivers


Advanced Thermal Solutions, Inc. (ATS) released a technical discussion this week with field application engineer Peter Konstatilakis to highlight a new design that he created for cooling a series of QSFP optical transceivers. The interview ranged from where the idea for the heat sinks came from through the analysis and testing that Konstatilakis had to perform to optimize the design.

 


ATS engineer Peter Konstatilakis holds the heat sinks he designed for cooling QSFP.
(Advanced Thermal Solutions, Inc.)

 

The article explained, “After conducting an analytical analysis, running computer simulations, and testing the heat sinks in the state-of-the-art ATS labs, Peter demonstrated a new heat sink design and optimized layout sequence that showed 30 percent improvement on QSFP heat sinks currently on the market.”

 

Increased throughput into QSFP and SFP transceivers requires more power, which also means more heat. So, thermal management has become an increasing priority for engineers that are designing a board that includes QSFP stacks. The developments in thermal management of QSFP transceivers was covered in another article from ATS.

 

One of the interesting points that Konstatilakis made in his interview was that a thermal interface material (TIM) cannot be used with the heat sink because the QSFP is not fixed in the cage. He said, “After a few insertions and removals, it will gunk up the TIM.”

 

With no TIM in use, Konstatilakis added, “You have to specify a good enough flatness and surface roughness, within cost means, that will still have a low contact resistance. That was one of the challenges as well as understanding the airflow of typical QSFP arrangements because you have four in a row, so you’re going to have preheated air going into the fourth QSFP.”

 

The heat sink design had to account for the series of QSFP with the goal of making the stack isothermal. The key, according to Konstatilakis, was to make the first and last QSFP the same temperature to ensure proper performance of the laser.

 

With the upstream QSFP receiving the most and the freshest airflow, heat sinks that had fewer fins were placed on the first two QSFP. The two downstream QSFP had heat sinks with denser fin arrays to maximize heat transfer from the preheated air that was flowing into them. Fewer fins on the first two QSFP had the added benefit of not pulling as much heat into the ambient, lessening the impact of the preheated air.

 

“What happens is the front heat sinks aren’t as effective,” Konstatilakis said. “This is fine as long as the upstream QSFP case temperatures are lower than the downstream QSFP. The overall effect is that the upstream QSFP temperatures will be closer to the temperature of the downstream QSFP, keeping the stack as isothermal as possible.”

 

He also discussed the testing rig that he had to design to make sure the empirical data met the expected analysis and the CFD simulations that were run. In the end, ATS was able to create a design that put the temperature of the first QSFP within one degree of the fourth QSFP at high airflow.

 

Konstatilakis concluded, “Whenever we run into this issue, we can say we tested that in the lab and explain the solution that we found. We don’t need to do more analysis, but provide the customer with a solution.”

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