PCIe Card Heat Pipe Case Study
Aavid, Thermal division of Boyd Corporation, took on the challengingtask of cooling a high power PCIe card housed within a rack-mountedsatellite tracking, telemetry and telecommand (TT&C) unit. This productfunctions as a critical bridge between the satellite control center and thesatellite itself.
The PCIe card in question needed to dissipate 40-50W of power,depending on operating conditions. A customer limitation on additionalactive cooling required that Aavid find a passive solution that utilizedthe existing fan array in the system.
The primary focus of the cooling solution was to maximize MTBFof the PCIe card by keeping BGA device temperatures at a minimum.
For this level of power dissipation, a heat pipe / radiator fin design wasthe most effective choice. Today’s high end computer graphics cardsutilize a similar system to dump their heat; however, most of these areactively cooled with a small onboard fan. In order to keep the systempassive, Aavid decided to look at a dual slot design where a larger finstack would occupy the second slot.
Aavid created a detailed CFD base model which included FPGA, PHY,DDR, DDS, Omap, Bridge and LTM packages. The PCB thermal propertieswere defined by the number of ground and power layers specified bythe customer. Other components in the system, such as the RF and HeatSink boards, were modeled as heat dissipating PCB boards. The systemwas then analyzed at an ambient temperature of 55°C with an altitudepressure of 750 hPa and a flow rate of 300LFM.
From the base CFD simulation, Aavid was able to determine flow and temperature maps over theboard, component junction temperatures and case thermals at the above conditions. This basemodel was then altered by the following to finalize the thermal solution:
-For components at or above their thermal limit, calculations were done to find the minimum required heat sink thermal resistance to bring them into safe operating ranges.
-Designed the thermal solution for all the components exceeding the minimum junction
-Optimized the heat sink for DFM, cost and performance.
A complete thermal report and sample units were provided to the customer for testing. These sampleunits, produced at Aavid’s rapid prototyping facility at our headquarters in Laconia, NH, allowed thecustomer quickly and effectively validate the thermal solution. After the successful sample tests, Aavidset forth on a full production run of the PCIe card cooling solution for use in the final product. Fromconcept to production - and everything in between - Aavid was able to deliver a cost effective andtimely solution for this difficult thermal problem.
-At the above tested conditions (55°C, 750hPaand 300LFM), all of the components stayedbe low the maximum junction temperaturelimits.
-Side baffles were recommended because,if included, they would prevent air bypass andimprove the junction temperatures further.
-The strategic 3D routing of heat pipes allowedheat to spread across the surface of the pcbas well as up into the fins.