Research Papers

Analysis of Using Ferrofluid as an Interface Material in a Field Reversible Thermal Connector

[+] Author and Article Information
Ahmed S. Yousif

University of Technology,
Baghdad 00964, Iraq
University of Missouri,
Columbia, MO 65211
e-mail: AhmedYousif86@gmail.com

Gary L. Solbrekken

University of Missouri,
Columbia, MO 65211
e-mail: SolbrekkenG@missouri.edu

Contributed by the Electronic and Photonic Packaging Division of ASME for publication in the JOURNAL OF ELECTRONIC PACKAGING. Manuscript received May 20, 2014; final manuscript received October 22, 2014; published online November 17, 2014. Assoc. Editor: Madhusudan Iyengar.

J. Electron. Packag 137(2), 021003 (Jun 01, 2015) (9 pages) Paper No: EP-14-1056; doi: 10.1115/1.4028958 History: Received May 20, 2014; Revised October 22, 2014; Online November 17, 2014

The electrical functionality of an avionics chassis is limited due to heat dissipation limits. The limits arise due to the fact that components in an avionic computer boxes are packed very compactly, with the components mounted onto plug-in cards, and the harsh environment experienced by the chassis limits how heat can be dissipated from the cards. Convective and radiative heat transfer to the ambient are generally not possible. Therefore, it is necessary to have heat transferred from the components conducted to the edge of the plug-in cards. The heat then needs to conduct from the card edge to a cold block that not only holds the card in place but also removes the generated heat by some heat transfer fluid that is circulated through the cold block. The interface between the plug-in card and the cold block typically has a high thermal resistance since it is necessary for the card to have the capability to be reworkable, meaning that the card can be removed and then returned to the chassis. Reducing the thermal resistance of the interface is the objective of the current study and the topic of this thesis. The current design uses a pressure interface between the card and cold block. The contact pressure is increased through the addition of a wedgelock, which is a field-reversible mechanical connector. To use a wedgelock, the cold block has channels milled on the surface with widths that are larger than the thickness of the plug-in card and the unexpanded wedgelock. The card edge is placed in the channel and placed against one of the channel walls. A wedgelock is then placed between the card and the other channel wall. The wedgelock is then expanded by using either a screw or a lever. As the wedgelock expands, it fills in the remaining channel gap and bears against the other face of the plug-in card. The majority of heat generated by the components on the plug-in card is forced to conduct from the card into the wall of the cold block, effectively a single sided, dry conduction heat transfer path. Having started as a student design competition named RevCon Challenge, work was performed to evaluate the use of new field-reversible thermal connectors. The new design proposed by the University of Missouri utilized oil based iron nanoparticles, commonly known as a ferrofluid, as a thermal interface material. By using a liquid type of interface material, the channel gap can be reduced to a few micrometers, within machining tolerances, and heat can be dissipated off both sides of the card. The addition of nanoparticles improves the effective thermal conductivity of base fluid. The use of iron nanoparticles allows magnets to be used to hold the fluid in place, so the electronic cards may be easily inserted and removed while keeping the ferrofluid in the cold block channel. The ferrofluid-based design which was investigated has shown lower thermal resistance than the current wedgelock design. These results open the door for further development of electronic cards by using higher heat emitting components without compromising the simplicity of attaching/detaching cards from cooling plates.

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Fig. 1

Wedgelock design setup. (a) Wedgelock in expanded mode and (b) wedgelock in loose mode.

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Fig. 2

Wedgelock in action

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Fig. 4

The final setup for the card

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Fig. 5

(a) Thermocouple attached to card using solder paste. (b) Thermocouple attached to cooling block using same solder paste.

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Fig. 6

(a) A single neodymium magnet. (b) Magnets installed in the cooling block.

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Fig. 8

Card before and after being wiped

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Fig. 9

Cooling block dimensions

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Fig. 10

Lower half of the cooling block with copper pipes installed

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Fig. 11

Cooling system setup

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Fig. 12

Heater electric setup

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Fig. 13

The device with the ferrofluid design card attached

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Fig. 14

Stability check for the wedgelock design at 36.5 W

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Fig. 15

Stability check for the ferrofluid design at 36.5 W

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Fig. 16

Thermal resistance versus time for wedgelock design for aluminum card

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Fig. 17

Thermal resistance versus power (wedgelock design)

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Fig. 18

Thermal resistance versus time for ferrofluid design for aluminum card

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Fig. 19

Thermal resistance versus power (oil based ferrofluid)

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Fig. 20

Oil based ferrofluid design wedgelock design

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Fig. 21

Model with boundary conditions

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Fig. 22

Temperature distribution for ferrofluid design

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Fig. 23

Heat flux distribution for ferrofluid design

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Fig. 24

Temperature distribution for the wedgelock design

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Fig. 25

Heat flux direction

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Fig. 26

Heat flux distribution

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Fig. 27

Thermal conductivity of wedgelock versus the thermal resistance




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