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Mechanical Design for Cooling

Thermal Analysis and Heat Sink Design Optimize Cooling of High-Performance Modules

Computer-driven simulation and analysis of thermal characteristics can develop optimal cooling solutions for high-performance board designs that can be verified by real-world testing to improve MTBF and shorten time-to-market.

MICHAEL HASKELL, ADVANCED THERMAL SOLUTIONS

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A leading provider of embedded computing solutions needed to determine the thermal characteristics of its new packet processor. Such devices often entail complex heat transfer and other thermal characteristics. Comprehensive thermal management analysis and design services can be tailored to help bring telecommunications, networking, embedded computing and other high-performance electronic products to market faster, ensure their reliability and reduce development costs. 

The packet processor was a high-performance 4-port Gigabit Ethernet PCI-X card that provides secure, high-speed connectivity and complex security processing with wire speed performance. It features a 16 core 500 MHz processor, which must be kept at a junction temperature below 110°C to maintain reliable operation.

A previous card model employed an active heat sink for cooling the processor. However, the card maker’s design team wanted a complete and thorough thermal examination of this component so that a passive cooling solution could be used. Passing cooling would provide higher reliability and save on both cost and implementation time. Several thermal management analysis and design services were used in developing such a solution. The process starts with a detailed thermal analysis of the PCI board design, which leads to CAD and computational fluid dynamics (CFD) simulation and analysis. The results of the analysis and simulation are used to implement the rapid design of the prototype heat sink. The design is then tested and verified using wind tunnel and chassis testing.

Advanced Thermal Solutions’ engineers studied several key factors including volumetric flow rate, pressure drop and component temperature. They started by researching and simulating the thermal characteristics of the memory DIMM and other critical components. Using this approach they produced analytical junction temperatures for each device as well as an estimate of the airflow necessary to ensure proper cooling of the PCI-X card. It also reduced the number of CFD (computational fluid dynamics) iterations that needed to be performed, and validated those CFD findings.

Based on the results of the thermal analysis, engineers used SolidWorks 3D CAD software to produce a detailed model of the board assembly (Figure 1). Then they ran multiple airflow simulations, at various flow rates, using CFD software from CFDesign. The result was a detailed set of CFD images showing the airflow patterns and providing a temperature profile of the card.

Figure 1
SolidWorks model of the PCI-X card.

Using the data from the initial analytical and CFD studies enabled the design of a high-performance heat sink based on ATS’ maxiFLOW flared fin architecture that, when installed, would adequately cool the package and ensure proper performance of the PCI-X card. maxiFLOW heat sinks feature a low-profile, spread fin array to maximize their surface area for more effective convection (air) cooling (Figure 2).

Figure 2
From left to right, SolidWorks images of the maxiFLOW heat sink, along with its CFD simulation and prototype.

In the case of the PCI-X card, the heat sink was found to keep the junction temperature of the processor at or below acceptable levels with an airflow of 200-275 LFM or greater within a PCI card slot, at an ambient air temperature of 55°C. Engineers then had two heat sink samples precision-machined at their in-house prototype and volume manufacturing facility.

As part of the design process, all heat sinks are tested in a thermal fluids laboratory, using a CWT-100 series open-loop wind tunnel, to verify any analytical or computational simulation results. When the heat sink was tested, elements in the open-loop wind tunnel were arranged to simulate the PCI slot conditions, thus validating CFD testing. Test findings also produced the heat sink’s thermal resistance and pressure drop characteristics.

The analytical design of the passive heat sink for cooling the processor was the initial phase of the PCI-X card thermal characterization. Because the heat transfer from the component to the ambient was so complex, further testing of the maxiFLOW heat sink was performed with it installed on the card, and inside an Intel SR2400 Server Chassis, under several different test conditions.

Thermocouples and ATS’ ATVS-2020 Automatic Temperature and Velocity Scanner, multi-channel, hot-wire anemometer system were used to measure case temperature as well as approach air velocity (Figure 3). For one validation test, a sample PCI-X card was installed in the chassis and subjected to variable airflow. Component temperatures were recorded over a range of airflow levels to produce a thermal resistance graph. 

Figure 3
An Intel SR2400 server chassis (left) and an ATS ATVS-2020 automatic temperature and velocity scanner (right) were used to test the ATS heat sink performance on a PCI-X Card.

In a separate test, the PCI-X card was modeled in CAD and simulated with CFD. The PCB was modeled as a standard 12-layer board with bulk thermal properties as follows: a thermal conductivity of 85.6575 W/m-K in the X and Y directions, thermal conductivity of 0.333705 W/m-K in the Z direction, 432.54 kg/m3 density, and 837.002 J/kg-K specific heat. The card and the CFD simulation are shown in Figure 4.

Figure 4
CFD Simulation of the Packet Processor PCI-X Card with the ATS maxiFLOW Heat Sink Installed, alongside a sample card.

The results of these two testing methods showed close agreement, with an error of 2-13% from 100-600 LFM velocity. The final test stage included CFD simulation of the complete PCI card with the heat sink attached. This simulation also showed close agreement with the chassis testing. CFD results showed a consistently higher thermal resistance of 2-13%, and this was mainly due to a lack of radiation heat transfer in the simulation (Figure 5). Once the base simulation was correlated with experimental results, future scenarios could be investigated within CFD to predict processor upgrade thermal performance.

Figure 5
The results of CFD simulations and chassis testing of the ATS maxiFLOW Heat Sink. The PCI-X Card manufacturer recommends an airflow of at least 275 LFM if a passive heat sink is used.

Effective thermal management study and design produced valuable analytical, computational and experimental data that allowed for a high-performance, optimized passive heat sink solution to be designed for the PCI-X card’s ideal operating conditions.

Not only did the heat sink solution offer significant cost savings, but it also provided 27 times the reliability of the active solution (Figure 6). As a result of the collaboration between the card manufacturer’s design team and ATS thermal management experts, the company was able to quickly introduce the new card to the market with cost savings and increased reliability. 

Figure 6
Comparison of cooling solutions for the PCI-X Card in terms of mean time between failures.

Advanced Thermal Solutions.
Norwood, MA.
(781) 769-2800.
[www.qats.com].