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The use of heat sinks and extended surfaces by using card guides and the containment system to act as an added surface for heat transfer have been used extensively in microelectronic applications. However, the optimization of these surfaces and the ability to calculate the actual cooling effectiveness of these surfaces is not clearly understood. The MHTL will develop analytical models for calculating the effectiveness of different types of heat sinks and extended surfaces. The fin models will then be integrated into existing software to allow the effects of heat sink enhancements to be reflected in package and PCB thermal analyses. The algorithms can also be used as a stand alone tool for designing and optimizing heat sinks and extended surfaces.
Manufacturing constraints associated with extruding aluminum limit the size and shape of fins that can be easily incorporated in conventional rectangular heat sinks. Some manufacturers choose to avoid these manufacturing constraints by gluing fins to a base plate. Unfortunately, glued joints can impose a severe penalty on the thermal performance of heat sinks, restricting the overall effectiveness of any added heat transfer surface area. An alternate manufacturing procedure, incorporating a swagging procedure that forms a mechanical bond between the fins and base plate, eliminates the need for glues or epoxies thereby preserving both the manufacturing and the thermal advantages associated with high aspect ratio, corrugated fins.
A comprehensive research program is being undertaken by the MHTL to investigate the thermal integrity of the mechanical bond formed between the fins and the base plate. Experimental studies will be used to quantify the thermal resistance at the fin/base plate interface and to establish its relevance in the overall heat flow path between the source and the sink.
The development of simple and accurate models and algorithms for optimizing single phase, automotive-style heat exchangers is essential in the design and manufacture of effective, reliable oil coolers and radiators. Design tools of this type can be immediately incorporated into the design process of most heat exchanger manufacturers.
Three fundamental areas have been identified by the MHTL for detailed analysis:
Successful thermal management of microelectronic systems requires an accurate determination of individual thermal resistances at the various levels in the system, ranging from the device level to the system containment unit. Characteristic dimensions for thermal contact resistance can vary from submicron levels at or near the device level to hundreds of centimeters at the cold plate level. The associated interface heat flux may be as high as 100 watts per square centimeter.
Design simulation tools and correlations will be developed that provide thermal design engineers the ability to accurately predict:
Domain discretization schemes, common in most numerical techniques, such as finite difference methods, finite elements and finite volume methods, require a full three dimensional mesh where the dimensions of each cell are of similar dimension to the smallest significant dimension in the system. In the case of a printed circuit board, this could be a copper sublayer which typically would have a thickness of 0.03 mm. Even with the most optimistic allowances for cell aspect ratios, a full three dimensional solution for a typical 250 mm by 250 mm by 2 mm circuit board would require in excess of one million elements to resolve localized temperature effects. The setup and run time associated with problems of this nature severely restricts domain discretization techniques for design applications.
An analytical algorithm is being developed based on variational methods where a quadratic functional equation can be minimized to obtain the unknown Fourier coefficients in a separated variable solution of the three-dimensional Laplace equation. The principal advantage of this method lies in the fact that the Fourier coefficients needed to satisfy the proposed functional must satisfy only the boundary conditions, since the governing equation is inherently satisfied by the Fourier series. This is contrary to finite differencing techniques where the differencing equations must satisfy both the governing equation and the boundary conditions.
Existing heat transfer models developed by the MHTL cannot be used for the wide range of applications where heat is dissipated by means of a recirculating fluid within a sealed environment. Microelectronic applications which exhibit these traits can vary in size between the small, hermetically sealed ``enclosure'' created around the chip in a typical ceramic package to the larger system level ``enclosures'' found in sealed microelectronic cabinets containing printed circuit boards.
A complex enclosure problem, consisting of one or more printed circuit boards contained within a sealed cabinet, can be modelled as a thermal resistance network consisting of series and parallel paths established through the dissipation of heat by conduction, convection and radiation.
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