Roy W. Knight

Heat sink optimization with application to microchannels

The equations governing the fluid dynamics and combined conduction/convection heat transfer in a heat sink are presented in dimensionless form for both laminar and turbulent flow. A scheme presented for solving these equations permits the determination of heat sink dimensions that display the lowest thermal resistance between the hottest portion of the heat sink and the incoming fluid. Results from the present method are applied to heat sinks reported by previous investigators to study effects of their restrictions regarding the nature of the flow (laminar or turbulent), the ratio of fin thickness to channel width, or the aspect ratio of the fluid channel. Results indicate that when the pressure drop through the channels is small, laminar solutions yield lower thermal resistance than turbulent solutions. Conversely, when the pressure drop is large, the optimal thermal resistance is found in the turbulent region. With the relaxation of these constraints, configurations and dimensions found using the present procedure produce significant improvement in thermal resistance over those presented by all three previous studies. >

Optimal thermal design of air cooled forced convection finned heat sinks-experimental verification

D.B. Tuckerman and R.F.W. Pease (1981) showed that microchannels with water flow could be used to cool VLSI systems. Their work required the flow to be laminar, and the channel system, or fin array, was optimized analytically. Recently, it has been shown that, for some geometries and fluid pressure drops, a lower thermal resistance can be found if the channels are designed to allow turbulent flow. The current work uses the optimization scheme developed by R.W. Knight et al. (1991 and in this issue) to design three air-cooled aluminum finned arrays, which were built and tested experimentally. The thermal performances of the fin array designs, one containing 5 fins, one with 11 fins, and one with the predicted optimum of 8 fins, are compared. All arrays had turbulent flow and pressure drop across them, and all fins were the same length and width. The best thermal performance was obtained with the design predicted to be optimal. The scheme can be applied to a variety of heat sink design applications, including water-cooled microchannels. >

A Closed-Form Multiscale Thermal Contact Resistance Model

All surfaces are rough to some extent and therefore only a small portion of surfaces actually comes into contact when they are brought together. Therefore heat flow from one object to another is retarded by this incomplete contact, resulting in thermal contact resistance (TCR). Minimizing the TCR is important for many different applications where dissipating heat is important, such as in micro- and high-power electronics. This paper presents a simplified closed-form method for modeling TCR while considering the multiscale nature of surfaces in the contact mechanics and heat transfer theory. When modeling the contact between surfaces, it is important to consider the multiple scales of roughness that exist. Many rough surface contact models exist in the recent literature, but they can be difficult to implement and use for TCR predictions. This paper derives and presents a simplified closed-form multiscale model of TCR. The results are then compared with experimental measurements of the TCR for copper samples and with other existing models. The comparison shows relatively close agreement with the closed-form multiscale model.

System Design Issues for Harsh Environment Electronics Employing Metal-Backed Laminate Substrates

Designing harsh environment electronics continue to increase in difficulty to a rapid increase in feature content while electronics packaging technologies are often providing less reliability. In addition, restricted under-the-hood airflow and integrated (mechatronic) designs are significantly increasing operating temperatures toward their maximum operating capability. To provide a cost effective design, automotive electronics designers are pursuing circuit board assemblies directly attached to a metal plate. For cost purposes, this metal plate can also be used as part of the module housing to provide protection, as well as thermal efficiency. Unfortunately, the metal backing can often further reduce component reliability due to increases in substrate coefficient of thermal expansion. The paper investigates the impact of metal attachment on component reliability as it investigates the use of several board attachment options. These analyses are compared to finite element modeling to further understand the causes of earlier failure. In addition, the impact of additional component encapsulants and conformal coatings are investigated. Because all attachment materials must meet a certain thermal performance (both initial design and long-term performance) the thermal efficiencies of these design options are investigated, as well as the delamination due to product life. Finally, failure analyses are presented and ensure that failures match expected characteristics.

Numerical investigation of the effects of thermal creep in physical vapor transport

It has been recently recognized that the nonisothermal conditions present in physical vapor transport ampoules can give rise to a slip flow of gas over the side walls of the ampoule. This phenomenon, known as thermal creep, is usually insignificant relative to buoyancy-induced flows under similar nonisothermal conditions, and has therefore been neglected in previous PVT numerical models. However, thermal creep can, in principle, become a dominant convection mechanism in buoyancy-free environments such as those encountered in microgravity experiments. We present here a numerical investigation of the effects of thermal creep on the growth process in axisymmetric, binary component PVT systems. A continuum-based model, which includes buoyancy and Soret diffusion, is developed. We show that thermal creep can result in recirculating bulk flows within the ampoule. For relatively high values of the Schmidt number and large wall temperature gradients, these flows can result in significantly nonuniform distributions of mass flux at the crystal interface, and can also be comparable to or exceed the flow velocities generated by buoyancy under normal gravity. The effects of thermal creep on buoyant convection, and on the Soret transport of the vapor, are examined.

Investigation and characterization of a high performance, small form factor, modular liquid immersion cooled server model

As the demand for power consumption increases with an expanding global economy, the need for innovative solutions to meet the cooling demands of next generation servers looms large. Particular attention must be paid to ensuring that cooling mechanisms employed are not only efficient from a power consumption perspective, but tightly packed to avoid needlessly expansive server rooms resulting in increased construction costs and energy waste from cooling superfluous spaces. With these concerns in mind, a small form factor, high input power solution is presented that involves no surface modifications, i.e. bare silicon die. In this 150 mm × 300 mm × 38 mm (H × L × W) space, two-phase heat transfer results in roughly 400 Watts rejected from the four-die arrangement and at a level well below the Critical Heat Flux.

Performance of low melt alloys as thermal interface materials

This paper focuses on using low melt alloys (LMAs) containing In, Ga, Sn, and Bi as compliant high performance thermal interface material. The investigation described herein involves in-situ thermal performance of LMAs as a function of applied pressure as well as performance evaluation after accelerated life cycle testing which includes thermal cycling from -40°C to 80°C. Testing methodologies follow ASTM D5470 protocols. Measurements show that LMAs can offer thermal interfacial resistances as low as 0.006 cm2°C/W, and their performance is highly contingent upon the quality of the mating surfaces. The issue of LMA containment and dewetting are discussed along with the solutions to mitigate them. Finally, to compare the performance of LMAs, some commercial TIMs (grease, phase change material, heat spring and thermal pads) were also tested using the same methodology and apparatus.

Local thermal measurements of a confined array of impinging liquid jets for power electronics cooling

The local surface temperature, heat flux, heat transfer coefficient, and Nusselt number were measured for an inline array of circular normal jets of single phase liquid water impinging on a copper block. An experimental 2-D surface map was obtained by translating the jet array relative to the sensors. The effects of variation in jet height, nozzle length, and Reynolds number were investigated. The local maximum heat transfer coefficients were observed within the stagnation region for all configurations with secondary peaks occurring halfway between the jets. The maximum local heat transfer coefficient of 15,600W=m²K and local Nusselt number of 80.5 were observed for H/D_n = 1, L_n/D_n = 2, and ReD_n = 14,000; while the maximum surface average heat transfer coefficient of 13,700W=m²K and average Nusselt number of 71.0 were observed for H/D_n = 3, L_n/D_n = 0, and ReD_n = 14,000.

Cooling of High-Performance Server Modules Using Direct Immersion

An alternative to air-cooling of high performance computing equipment is presented. Heat removal via pool boiling in FC-72 was tested. Tests were conducted on a multichip module using 1.8 cm × 1.8 cm test die with multiple thermal test cells with temperature sensing capability. Measurements with the bare silicon die in direct contact with the fluid are reported. Additional testing included the test die directly indium-attached to copper heat spreaders having surface treatments. A screen-printed sintered boiling-enhanced surface (4 cm × 4 cm) was evaluated. Tests were conducted on an array of five die. Parameters tested include heat flux levels, dielectric liquid pool conditions (saturated or subcooled), and effect of neighboring die. Information was gathered on surface temperatures for a range of heat flux values up to 12 W/cm2. The highest heat dissipated from a circuit board with five bare die was 195 W (39 W per die). Addition of the heat spreader allowed heat dissipation of up to 740 W (from a five-die array). High-speed imaging was also acquired to help examine detailed information on the boiling process. Numerical modeling indicated that placing multiple boards in close proximity to each other did not degrade performance until board spacing was reduced to 3 mm.© 2012 ASME

Non-uniform thermal properties of an alumina granule/epoxy potting compound

In harsh environments, including high shock and vibration, magnetic devices such as transformer coils are potted to enhance thermal performance and provide mechanical protection. One potting compound frequently used is epoxy containing alumina particles. A nominally isotropic and uniform potting compound consisting of about 70 to 80% by volume 14–28 mesh (0.6 to 1.2 mm across) alumina granules in low viscosity epoxy was tested to determine its thermal properties. Examination by optical microscopy revealed that there was significant variation in volume fraction of alumina particles by location. The specific heat and thermal conductivity of the compound were measured using a Differential Scanning Calorimeter and guarded heater method based on the ASTM D5470-06. The thermal properties were found to vary with time, location, and temperature; with the specific heat ranging from 1.00 J/g°C ± 14% at 25°C to 1.22 J/g°C ± 12% at 125°C and an apparent thermal conductivity of 2.56 W/m·K ± 23%. Users of such compounds should be aware that the thermal properties are not necessarily constant in time or uniform, and assuming that they are could lead to significant errors when modeling their performance.