National Science Foundation
Cooling Technologies Research Center
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CTRC Breakthroughs


Thermal Interfaces


TIM Design for Optimized Thermal and Mechanical Properties

While, in general, increasing particle loading is required to meet the expected target of effective thermal conductivities, the increased effective elastic modulus due to increased particle loading, which together with the thinned dies has the potential to cause warping or fracture of the dies. Thus, there is a need to carryout systematic thermal-mechanical design trade-off studies of the particle filled thermal interface materials (TIMs). Thermal conduction behavior and flow behavior are directly related to particle filler loading. Microstructures with ultrahigh volume fractions are needed to effectively predict the behavior of the particulate systems. In this project, we have successfully generated microstructures with ultrahigh volume fractions and analyzed the effective thermal conductivity and elastic modulus of these TIMs with high particle loading. The analysis has also been extended to TIMs with ellipsoidal fillers.


Polymer-Metallic Hybrids for Thermal Interface Applications

Polymer-based thermal interface materials (TIMs) filled with thermally conductive particles are generally used in electronic devices. However, mismatch in the coefficient of thermal expansion (CTE) is a common problem, causing thin film delamination during a system’s lifecycle. In order to develop polymer-metallic TIMs, this project emphasized the balance between mechanical and thermal performance. We investigated various physical effects of polymer-based TIMs to access mechanical and thermal reliability. We also achieved various TIMs with different types of materials, which were nanoparticles and microporous foam as conductive networks.


Dry Contact Thermal Conductance Enhancement

Thermal interface materials (TIMs) are commonly used to enhance thermal conductance across interfaces. However, wet TIMs and high compressive pressures are disallowed for ‘pluggable’ applications where the devices need to frequently slide into contact with the heat sinks. In this project, we developed a compliant dry TIM which can enhance the thermal conductance between nonflat and rough mating surfaces at a very low pressure of ~20 kPa. This TIM consists of an array of metallized polymer micro-springs which are fabricated using cost-effective micro-stereolithography (μSL) based techniques and electroless metal plating. Even for rough and nonflat interfaces, the dry TIM is able to achieve a thermal resistance equivalent to contact between highly polished and very flat surfaces (~300 mm2·K/W). The mechanical robustness of the dry TIM was evaluated, and the performance demonstrated when placed between pluggable optical module and its riding heat sink.


Porous Metal Thermal Interface Material

Metal foam structure has been shown to have good mechanical compliance and relatively higher thermal conductivities making it an ideal candidate for thermal interface material in electronics cooling applications. Analytic models were developed in order to predict apparent thermal and mechanical performance of the material generalized with respect to raw material properties and porous geometries such as porosity and pore size. Model predictions were validated through numerical simulations and correlated to the experimentation, with which we refined the ASTM standard measurement setup and additionally developed a new setup to take into account the deformation information. Additionally, multiple composite designs with phase change material and thermal greases were tested with regards to their effective thermal and mechanical properties were analyzed.



Three-Dimensional Graphene Networks Facilitate Efficient Heat Conduction at Thermal Junctions

Graphene Networks breakthroughs figure Efficient heat dissipation becomes a crucial issue due to increase in generated power density in modern electronic devices. Therefore, high-performance TIMs are required to achieve efficient heat conduction across thermal junctions (heat sources/sinks) by filling in surface irregularities. In this project, graphene networks prepared by chemical vapor deposition method were investigated as thermal interface materials. The thermal resistance was characterized between graphene networks and copper. The result indicates that graphene networks can improve the heat transfer at interfaces between dissimilar materials. Researchers also conducted systematic studies on thermal interface resistance of graphene networks with various densities and demonstrated their feasibility as thermal interface materials. In addition, researchers investigated mechanical properties such as compressive strength.


Impedance-Based Visualization of Voids in Thermal Interface Materials

Impedance TIMs breakthroughs figure Thermal interface materials are an essential component of thermal management of modern electronics systems. These materials tend to degrade over time, reducing their effectiveness at conducting heat. The application of capacitance imaging to the size scale of a thermal interface material (~1 cm x ~1 cm x ~0.01 cm) represents an innovative development for experimental instrumentation. The technique may be used by researchers to better understand the behavior of thermal materials subjected to various stress conditions or as a quality control technique. Embedded electrical sensors have been successfully used in a lab environment to image either a thin layer of thermal grease or a thick thermal pad while it is situated between two opaque surfaces. Bubbles and other defects in the material may be detected and imaged using image reconstruction algorithms.


Designing Transient Liquid Phase Sintering Systems for Power Electronics

Liquid Sintering breakthroughs figure In transient liquid phase sintering, a low melting temperature metal powder and an organic flux are mixed with high melting point alloys in particle form. When the temperature is increased above the melting temperature of the low melting point phase, a new compound begins to form at the interface between the liquid and the solid. This project developed model paste formulations for the development of new electrical and thermal technology based on transient liquid phase sintering for attaching cooling systems to chips, semiconductor dies, and substrates in power electronics. Thermodynamic software and down-selection criteria for specific applications were developed and applied to evaluate novel alloy formulations for these uses.


Flexible Foam New Thermal Interface Material

Flexible Foam breakthroughs figure This project developed a new set of analytic models to predict the performance of porous aluminum structure (foam) as a thermal interface material. The models include the mechanical compliance and the effective thermal conductivity of the foam by a combination of physical model validated with numerical and experimental data. The developed model aids in selecting the best geometry with the combination of materials in foam structure, which will ultimately enable the industry with the most cost effective solution for electronics cooling. The chart including the microscope image of the sample foam shows the example of placement and method to utilize the aluminum foam in the electronic cooling.


Graphene-Based Thermal Interface Materials

Graphene TIMs breakthroughs figure Graphene, a single atomic layer of graphite with honeycomb lattice structure, exhibits very high thermal conductivity (~3000-5000 W/m-K), which renders it an outstanding candidate for Thermal interface materials (TIMs) applications. Graphene composite shows significant increase of thermal conductivity when the volume percent of graphene is increased. Vertically grown graphene shows superior performance, with a thermal resistance in the order of 1 mm2 K/W, which is better than available commercial thermal interface materials. This figure is the synthesis of graphene composite, which includes the preparation of graphene solution, the drying of the powders, and the hot pressing to form nanocomposite. Shown in the right lower corner is the vertical graphene petals prepared using chemical vapor deposition (CVD).


Performance and Reliability of Thermal Interface Materials

Matrix Modeling breakthroughs figure This project has developed a test procedure to simultaneously characterize thermal and non-Newtonian flow behavior of polymeric thermal interface materials (TIMs). Also, reliability characterization of representative polymeric TIMs using the developed test procedure after exposure to high temperature bake as well as thermal cycling of the materials. The picture depicts the custom built equipment meant for the reliability testing of thermal interface materials. Simultaneous non-Newtonian flow and thermal conductivity characterization can be conducted using this setup. Such a capability allows monitoring of flow response as well as thermal conductivity change of TIMs subjected to complex environmental conditions such as bake or thermal cycle.

Impedance-Based Visualization of Voids in Thermal Interface Materials

NonTransparent Materials breakthroughs figure This method provides a new way to test the effectiveness of the thermal pastes used for keeping computer chips at appropriate temperatures during operation.The project utilizes parallel arrays of electrodes to form a square grid around the interfacial gap. Capacitance measurements taken at the crossing points of the grid allow for anomalies in the thermal interface material to be detected. The figure shows the measuring device using copper rods imbedded into two clear panels. Rods on one panel  criss-cross with the rods of the other panel. Electrical measurements can determine whether there are bubbles or cracks in the material between the panels.


Performance and Reliability Impact of Compliant Thermal Interface Materials

Compliant TIMs breakthroughs figure Present work is aimed at the development of multiple cross-validated test procedures to identify time-dependent viscoelastic behavior of compliant TIMs and to correlate the mechanical response of the materials to experimentally measured effective thermal conductivity. Exhaustive viscoelastic characterization of the Gap pad (compliant TIM) materials, and relating mechanical cycling to measured thermal conductivity degradation of these materials have been accomplished. This research also estimated the stress transmitted to the solder joints through the gap pads as a result of applied pressures during heatsink assembly. The top figure is a cross-section of the chosen TIM as viewed under environmental mode in a scanning electron microscope showing a fiberglass pad on right side of the image. The lower two figures are the chosen material also view.


Enhanced Thermal Contact Conductance Using Carbon Nanotube Interfaces

CNT breakthroughs figure A tri-layer catalyst configuration has been developed for well anchored and vertically oriented CNT arrays direct synthesized on various substrates with plasma-enhanced chemical vapor deposition and different catalyst metals. A test rig for thermal contact resistance measurement (adapted from ASTM D 5470) has been established in a high-vacuum environment, and temperature measurements are accomplished with an infrared imaging system. The figure above summarizes thermal interfacial resistance results for a bare copper-silicon interface, a single-sided CNT array, and a double-sided array. The results demonstrate that the CNT arrays can exhibit resistances below 10 mm2K/W that are comparable to the resistance of an ultra-thin soldered joint.


Thermal and Electrical Characterization of CNT Vias

CNT Vias breakthrough figure CNTs with their unique combination of high current carrying capacity, high thermal conductivity, mechanical integrity and electron mean free paths of several microns may be ideal candidates for next-generation interconnects. In this project, stable and dense CNTs were grown from electrodeposited Pd in the bottom of porous anodic alumina (PAA) pores, using buckypaper as a plasma screen. CNT density was calculated to be up to 1.0x1010 cm-2, and the minimum current density transported in CNT vias was calculated to be 106 A/cm2, which is comparable to or higher than Cu interconnects. Modifications to CNTs by electron donating and electron withdrawing chemicals (e.g., TTF and TCNE) have been made to improve the electrical properties of CNT vias. The first figure shows a side view of as-grown CNT's using Pd as catalysts.


Validated Models for Particulate Thermal Interface Materials

TIMs breakthroughs figure Numerical modeling of realistic 3D microstructures (at high filler volume loadings) considering inter-particle interactions was performed using full-field meshless simulations and random particle network simulations. The developed models are validated with experiments on representative systems. The models can be efficiently used to accurately predict the effect of varying: i) the filler particle conductivity, ii) the base polymer matrix conductivity, and iii) size-distribution and arrangement of the filler particles on the composite thermal conductivity of TIMs.


Thermal Cycling Fatigue Effects on the Contact Resistance of Organic Interface Materials on Si

The development of an accurate constitutive model of the effect of thermal cycling to polymer interface layers will be necessary for the reliable integration of organic materials into high performance packages. Accurate measurements of heat flow are dependent upon knowing and controlling the ambient operating conditions. Mapping the development of interface defects is important for improving the materials which aid in the removal of heat from electronic packages. Finding and documenting the evolution of the defects is dependent upon a full understanding of the thermal probes response to changing environmental conditions. Changes in test operating conditions can dominate changes to local interface properties.