Ross K. Wilcoxon

A compliant thermal spreader with internal liquid metal cooling channels

This paper describes a 1 mm thick substrate with integrated flow channels to circulate liquid metal that is pumped with an electromagnetic pump. The substrate was fabricated using conventional circuit board assembly methods. Tests were performed with pumping current of up to 10 amps, which corresponded to a pumping power of 450 mW. This testing showed that the substrate had an effective thermal conductivity of more than 6000 W/mK. The thin geometry and organic circuit board materials of the substrate allow it to be mechanically compliant and therefore can reduce the need for thermal gap fillers to connect dissipating components to the thermal spreader.

Thermal management of an led light engine for airborne applications

Rockwell Collins has recently developed a light engine, which uses four light emitting diodes (LEDs) as the light source, for a cockpit display system. Under worst case operating conditions, the local heat flux from these LEDs can be more than 1000 W/cm2. This paper describes the thermal management approach for the light engine and presents the results of finite element modeling. This modeling was used to assess various design aspects of the light engine to understand their effects on the overall thermal resistance. Thermal testing of a set of light engines was eventually found to be quite comparable to the modeling results, although a discrepancy between testing and modeling results did help to identify a manufacturing defect that occurred in some early prototype devices, which significantly increased their thermal resistance

The effects of geometry and dielectric material on stripline and microstrip internal temperatures

The geometry and loss characteristics of 50 /spl Omega/ striplines operating at a frequency of 1.09 GHz were determined using fundamental transmission line theory. These results were then combined with finite element based thermal modeling to estimate the temperature rise within a stripline. The finite element thermal predictions were validated with measurements on a circuit board used in an avionics power amplifier. The results of this analysis showed that higher dielectric materials have higher power loss compared to traditional lower dielectric materials. But, due to the higher thermal conductivity of these materials, they can be used to produce smaller striplines that have a similar internal temperature rise to standard materials. Thermal testing and analysis indicate that, while the approach was based on a stripline with characteristic impedance of 50 /spl Omega/ and a frequency of 1.09 GHz, the thermal analysis that was developed is applicable to both striplines and microstrips of any impedance or frequency.

Water-Based Microchannel and Galinstan-Based Minichannel Cooling Beyond 1 kW/cm

Microchannel heat sinks are a relevant thermal management technology because the combination of surface area enhancement and small length scales results in low wall-to-bulk temperature differences. Previously, a thermal resistance of 0.09°C/W was achieved when a heat flux of 790 W/cm2 was imposed on a 1 cm × 1 cm footprint portion of a 400-μm-thick Si substrate utilizing single-phase water-based microchannel cooling and a 214 kPa pressure difference to drive the flow. Galinstan, a gallium, indium, and tin eutectic, may be utilized for single-phase liquid metal cooling of microelectronics due to its subambient melting temperature and high thermal conductivity. This paper describes the fabrication and assembly of water-based microchannel and Galinstan-based minichannel heat sinks and the flow sheets utilized to characterize them under the aforementioned constraints. The prefix mini rather than micro is used to describe Galinstan-based heat sinks because optimal channel widths are hundreds as opposed to tens of micrometers. The aforementioned thermal resistance of 0.09 °C was experimentally reproduced. Unprecedentedly low thermal resistance and high heat flux in single-phase water-based microchannel cooling, i.e., 0.071°C/W and 1003 W/cm2, respectively, were achieved. The first experimental data on Galinstan-based minichannel heat sinks are also reported. A thermal resistance as low as 0.077°C/W was achieved at a heat flux of 1214 W/cm2 and a maximum heat flux of 1504 W/cm2 was reached.

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Liquid metals, such as Galinstan, a gallium, indium and tin eutectic, may be exploited for enhanced cooling of microelectronics due to their favorable thermophysical properties. Hodes et al. [1] discussed the properties of Galinstan compared with those of water and provided a first-order model for minimizing the total thermal resistance of Galinstan-based heat sinks under form factor and pressure drop constraints. This paper, as a follow up, describes the fabrication and assembly of water-based microchannel and Galinstan-based minichannel heat sinks.1 The authors also discussed the flow loop has been designed and commissioned to characterize the heat sinks and report flow and thermal resistance data and compare them to theory.

Heat Flux

We describe the use of direct liquid cooling to improve the performance of an X-band GaN HEMT based power amplifier Monolithic Microwave Integrated Circuit. With improved cooling, the operating drain bias was increased from 20V to 44V and the PA MMIC delivered 5.1W/mm power density while dissipating 8.9W/mm of thermal power under continuous wave operating condition.

Thermo-fluid characteristics of a minichannel heat sink cooled with liquid metal

Thermal test vehicles were fabricated with three different commercially available die attach materials: two Nano-Sintered Silver (NSS) materials and a Silver Filled Die Attach (SFDA). The thermal resistances of individual components were determined after their initial assembly and as they accumulated a total of 1500 thermal shocks from -55°C to +125°C. Initial testing showed that the package-level thermal resistance of test vehicles with NSS was approximately half that of the test vehicles with SFDA. This improvement was maintained during the course of the thermal cycling, in which the average thermal resistance of all of the packaged devices increased. Finite element modeling indicated that the effective thermal conductivity of the NSS materials was considerably better than the SFDA, but also indicated that the thermal cycling degraded their performance to a much greater extent. Test vehicles with the NSS materials were also more likely to exhibit poor adhesion of the encapsulant, which indicates that these materials may require alternate processing steps to ensure high reliability.

Experimental evaluation of direct liquid cooling on GaN HEMT based power amplifier MMIC

This paper describes a family of composite materials that are based on Alkali Silicate Glass (ASG), which can be processed and cured at temperatures that are compatible with conventional electronics packaging processes. The focus of this paper is on ASG composite materials with high thermal conductivity filler materials for use as thermal encapsulants and adhesives in electronics packaging. Data are presented for testing on a thermal test die encapsulated with an ASG-diamond composite as well as an evaluation of using the material to adhere an inductor coil to a circuit board. These results indicated that the thermal conductivity of the material exceeded 10 W/mK and it remained robust after 1000 thermal shocks.

Thermal performance and reliability assessment of nano-sintered silver die attach materials

Electronic components, such as ball grid array (BGA), chip scale packages (CSP) and bottom terminated components (BTC) used in harsh use environments often require the use of conformal coatings to meet reliability requirements. In certain coating application methods, the conformal coating materials can flow underneath the component and cause solder joint failure during thermal expansion and contraction of the electronic assembly. In this study, BGA components were coated with an acrylic conformal coating material using two application methods and subjected to two different thermal cycling profiles to assess the integrity of SnPb and Pb-free BGA components. To better understand the observed failure modes, Finite Element Analysis (FEA) was performed on the conformally coated BGA packages. Material characterization was performed using Dynamic Mechanical Analysis (DMA) and Thermal Mechanical Analysis (TMA) to capture the temperature dependent properties of the conformal coating to better correlate simulation and experimental results. Failure modes were found to greatly depend on the conformal coating material properties around the glass transition temperature (Tg) rather than temperature cycle range. Significant differences in the failure mode were found between the Pb-free and SnPb BGA components with acrylic conformal coating materials and temperatures profiles.

Alkali Silicate Glass based thermal coatings

Soldiers and Marines rely on electronics for communication, navigation and situational awareness. Increasing these capabilities, such as with longer range and higher bandwidth communication, generally comes at a price of greater power consumption and a corresponding increase in system power dissipation/thermal load. This increased power dissipation can limit the functional capabilities of systems when used in thermally challenging environments. One of these thermally challenging environments is inside the packs used by dismounted troops. This paper presents the results of testing to quantify the thermal resistance associated with operating electronics within packs that are typical of those that may be used by soldiers and Marines. Thermal mock-up systems were tested in two types of backpacks under various conditions. Steady state and transient temperature test results are compared to predictions generated with a spreadsheet-based analysis.