Thermal Interface Materials Materials Research Articles

A Jettable and Dispensable Liquid Metal Paste as a Thermal Interface Material

Metal thermal interface materials (TIMs) have been used in the electronics industry for over 20 years. The thermal performance of any TIMs is defined by their thermal conductivity, bondline thickness, and interfacial resistance. Generally, metal TIMs have very high thermal conductivity and they can be applied in a thin layer. Most of the metals are hard and stiff, making interfacial resistance the biggest obstacle for metal TIMs to overcome. Over the years, many metal TIMs have been developed and used in applications to improve the performance of the components. Recently, liquid metal TIMs are getting more attention, especially in gaming and high-performance applications. They are excellent in terms of accommodating imperfections of the materials they are connecting, but their physical properties can make application risky. To overcome this challenge, a new metal TIMs material - liquid metal paste (LMP) TIM – has been developed. This paper will examine this new material’s thermal conductivity, viscosity, jetting, and dispensing performance compared with other existing metal TIMs, such as liquid metal. The application in mass manufacturing will be studied, with a solution to prevent leakage and provide higher thermal cycling performance (-40°C / +125°C) for LMP.

A critical review of carbon-based thermal interface materials

This review of carbon-based thermal interface materials (TIMs) covers carbon pastes, carbon nanotube (CNT)-based TIMs, graphene-based TIMs, 3D-structured TIMs and other hybrid TIMs. The TIM material design is related to the TIM performance, which is described by the thermal contact conductance of the sandwich consisting of two surfaces sandwiching the TIM. The thermal conductivity alone is a misleading and inadequate description of TIM performance, although it is considered as the sole criterion that governs TIM performance in much of the TIM literature. TIMs comprising vertically aligned carbons (CNTs, graphene sheets and carbon fibers) have received much recent attention. However, due to the cost and performance, the practical hurdles to implementation are significant. The combined use of an array and a metal cap (involving a low-melting metal) alleviates the inadequacy of the interface between the top of the array and the surface that the TIM should touch. The carbon black paste (inexpensive and effective) excels due to the solid-state conformability of the carbon black. The liquid vehicle of the pastes is commonly an oil. The solid content needs to be optimized, as the conformability decreases with increasing solid content, whereas the thermal conductivity increases with increasing solid content. The pastes are effective at small TIM thicknesses, whereas the vertically aligned carbons and 3D-structured carbons tend to be effective at large TIM thicknesses.

Influence of Ag particle shape on mechanical and thermal properties of TIM joints

PurposeThe purpose of this paper is to develop and test the thermal interface materials (TIM) for application in assembly of semiconductor chips to package. Good adhesion properties (>5 MPa shear strength) and low thermal interface resistance (better than for SAC solders) are the goal of this research.Design/methodology/approachMechanical and thermal properties of TIM joints between gold plated contacts of chip and substrate were investigated. Sintering technique based on Ag pastes was applied for purpose of this study. Performance properties were assessed by shear force tests and thermal measurements. Scanning electron microscopy was used for microstructural observations of cross-section of formed joints.FindingsIt was concluded that the best properties are achieved for pastes containing spherical Ag particles of dozens of micrometer size with flake shaped Ag particles of few micrometers size. Sintering temperature at 230°C and application of 1 MPa force on the chip during sintering gave the higher adhesion and the lowest thermal interface resistance.Originality/valueThe new material based on Ag paste containing mixtures of Ag particles of different size (form nanometer to dozens of microns) and shape (spherical, flake) suspended in resin was proposed. Joints prepared using sintering technique and Ag pastes at 230°C with applied pressure shows better mechanical and thermal than other TIM materials such as thermal grease, thermal gel or thermally conductive adhesive. Those material could enable electronic device operation at temperatures above 200°C, currently unavailable for Si-based power electronics.

Study on thermal contact resistance of low melting alloy used as thermal interface material

The development of reliable electronic equipment is a big challenge due to miniaturization and performance characteristics of high speed and high power electronic devices. Thermal control is one of the key roles to meet the above challenge. Thermal Interface Material (TIM) plays a significant role in thermal control. TIM has been extensively used in diverse electronic devices for next generation electronics. To improve or enhance heat transfer performance at the interface region between copper substrates, Low Melting Alloy (LMA) Bi-Sn-Pb is usually used as TIM. In the present work, Thermal contact resistance (TCR) and Temperature variation by TIM is experimentally investigated. In the study, Bi39.4Sn30.8Pb29.8 is used as TIM material. The variation of TCR subjected to heat flow, variation of temperature between the interface and specimen thickness is reported and discussed.

Performance evaluation of PV-TEC coupling device for power production with improved hybrid nanocarbon based thermal material interface

The waste heat harnessing from photovoltaic module (PV) with the thermoelectric device (TE) is considered as an ideal solution for effective thermal management of PV technologies. However, idealized operating conditions for lossless device coupling and reduction in thermal contact resistance need to be investigated further to expedite PV-TE device efficiency. Our simple approach to tandemly attach a low-cost thermoelectric cooling device (TEC) to PV module has improved its power output and electrical efficiency by 1.02% and 1.1% respectively (No. of TEC device-1, 720 W/m2 and 20 °C of water-cooling). Herein, we explored the TEC device for power production as a cost-effective alternative to the traditional thermoelectric (TEG) devices. To facilitates the facile heat transport among the PV-TEC interface, the thermal interface material (TIM) has been applied on TEC devices. The hybridized paste of nanocarbon and thermal grease (NC-TG) was used as a TIM material, morphologically evident the linear alignment between NC in TG matrix has ameliorated the uniform heat conduction. It was observed that the application of hybrid Carbon TIM materials enhances the power generation by 10.65% compared to PV-TG-TEC. The physicochemical analysis and evaluation results indicated that the low-cost synthesized hybrid carbon-based TIM material is efficient in the reduction of thermal contact resistance. Concomitantly, the high heat absorption capacity of nanocarbon accelerated the TEC performance by creating a sufficient temperature gradient across TEC ends. The present work proposed a simple and low-cost pathway for the improvement of TEC performance using hybrid carbon-based TIM material.

Preparation of thermal conductive Poly(methyl methacrylate)/Silicon nitride nanocomposites via click chemistry

A new polymer nanocomposite consisting of silicon nitride (Si3N4) nanoparticle as a conductive filler and poly(methyl methacrylate) (PMMA) as a polymer matrix was synthesized using “click” chemistry. The synthesis of this nanocomposite is a three-step procedure including (i) the preparation of silicon nitride nanosheet (SNNS) by mechanical exfoliation, (ii) the modification of SNNS with successive chemical modification processes, (iii) performing click reaction between alkyne functionalized PMMA and azide functional SNNS. Characterization results confirmed the functionalization of SNNS surface and the successful “grafting to” method using click reaction. Furthermore, the new thermal conductive nanocomposite was formulated as a thermal interface material (TIM) and its performance as a TIM material was investigated using Thermal Conductivity Analysis (DTC). It has been found that the thermal conductivity of the chemically modified nanocomposite (PMMA-SNNS) based TIM is considerably higher than that of TIM prepared with physically blended nanocomposite (PMMA+SNNS) having the same composition.

Analysis On Thermal Behaviour Of The Sink And Die Area With Different Thermal Interface Material For High Power Light Emitting Diodes

Its self-heating process directly affects the optical performance and reliability of light – emitting diodes (LEDs). It is important to disperse the generated heat from LED to surrounding atmosphere and keep the LED light performances same as declared by the manufacturer. Thermal interface material (TIM) is applied in between sink and source to reduce contact resistance at the junction between substrate and heat sink interface of the LED modules. This paper provides an assessment on ‘thermal interface materials’. Here different TIM materials used and the performance and problems of these commercial interface materials are discussed. From this study, one can calculate the temperature distribution in the sink area for different types of TIM materials under thermal conductivity perspective and be able to find the capability of dissipation of heat at the end surfaces of heat sinks, and design their system as well. In another process, TIMs with different thickness and input drive currents for the COB-based LED are investigated by using COMSOL simulation software. The results show that the junction temperature of the LED luminaire increases and reduce the lifetime when the input drive current and thickness of the TIM layers increase.

Study on thermal resistance of brass with and without coating of metallic surface

The thermal interface material (TIM) is basically one of the best selections to solve the thermal issues in electronic packaging applications. The basic functions of TIM are to fill micro or nano sized rough surfaces (viz., valleys, gaps etc) between two conducting materials and to improve the thermal conduction and reducing the thermal contact resistance between them. In the present study, brass substrate with and without coating of metallic substrate is considered as TIM material and thermal contact resistance of the same is assessed. Tin (Sn) is considered as a metallic coat on brass substrate. The contact pressure at the interface was varied with and without application of load (kg) to investigate the thermal contact resistance. Results indicated that Sn coated brass interface material exhibited better thermal performance than bare brass substrate.

Mechanical Characterization of Thermal Interface Materials and Its Challenges

Thermal interface materials (TIMs) play a vital role in the performance of electronic packages by enabling improved heat dissipation. These materials typically have high thermal conductivity and are designed to offer a lower thermal resistance path for efficient heat transfer. For some semiconductor components, thermal solutions are attached directly to the bare silicon die using TIM materials, while other components use an integrated heat spreader (IHS) attached on top of the die(s) and the thermal solution attached on top of the IHS. For cases with an IHS, two TIM materials are used—TIM1 is applied between the silicon die and IHS and TIM2 is used between IHS and thermal solution. TIM materials are usually comprised of a polymer matrix with thermally conductive fillers such as silica, aluminum, alumina, boron nitride, zinc oxide, etc. The polymer matrix wets the contact surface to lower the contact resistance, while the fillers help reduce the bulk resistance by increasing the bulk thermal conductivity. TIM thickness varies by application but is typically between 25 μm and around 250 μm. Selection of appropriate TIM1 and TIM2 materials is necessary for the reliable thermal performance of a product over its life and end-use conditions. It has been observed that during reliability testing, TIM materials are prone to degradation which in turn leads to a reduction in the thermal performance of the product. Typical material degradation is in the form of hardening, compression set, interfacial delamination, voiding, or excessive bleed-out. Therefore, in order to identify viable TIM materials, characterization of the thermomechanical behavior of these materials becomes important. However, developing effective metrologies for TIM characterization is difficult for two reasons: TIM materials are very soft, and the sample thickness is very small. Therefore, a well-designed test setup and a repeatable sample preparation and test procedure are needed to overcome these challenges and to obtain reliable data. In this paper, we will share some of the TIM characterization techniques developed for TIM material down-selection. The focus will be on mechanical characterization of TIM materials—including modulus, compression set, coefficient of thermal expansion (CTE), adhesion strength, and pump-out/bleed-out measurement techniques. Also, results from several TIM formulations, such as polymer TIMs and thermal gap pads, will be shared.

Theoretical and experimental evaluation of thermal interface materials and other influencing parameters for thermoelectric generator system

Thermal interface resistance of Thermoelectric Generator (TEG) plays a vital role in power production. Improving surface finish of contact surfaces, applying pressure between the contact surfaces and use of Thermal Interface Material (TIM) are few methods of reducing thermal resistance and thereby improving the efficiency of TEG. There is a need to evaluate the influence of these methods and use them optimally for TEG system. Experiments were carried out to study the influence of parameters such as thermal conductivity of TIM, contact pressure, surface roughness and heat source temperature on the voltage and power outputs from TEG. Experimental results are validated with simulations using mathematical heat transfer model and COMSOLTM Multiphysics numerical model. Appreciable agreement is seen between the experimental observations and model outputs. Experimental and model results indicate 0.6 W/mK as optimum thermal conductivity for TIM material. Hence, use of costly TIMs like MWCNT (Multi Wall Carbon Nano Tube) and copper nanoparticles may not be required for the selected application. The contact pressure and surface roughness have appreciable influence when air is used as TIM. These factors have insignificant influence for TIMs with higher thermal conductivity. Increase in heat source temperature increases voltage and power output of TEG.

Thermal Interface Materials and Cooling Technologies in Microelectronic Packaging—A Critical Review

The ever increasing demand for fast computing has led to heterogeneous integration of packages as can be seen in the latest Xeon family segments in the market. Microprocessors are now adjacent to memory chips, transceivers, field-programmable gate arrays, and even other microprocessors within a single substrate. These complex designs have instigated an increase in cooling demand for microprocessors, and hence, there has been an increased focus within the semiconductor industry on developing advance thermal solutions. From the packaging level, thermal interface materials (TIMs) play a key role in thermally connecting various components within the package and helps reduce the thermal resistance between the die surfaces and integrated heat spreaders. From the system level, cooling technology is critical to attain the desired overall thermal dissipation and performance. In this review, progress made in the area of TIMs and system cooling solutions are presented. The focus is on the evolution of TIMs and cooling technologies and their challenges in the integrated circuit packaging. Merits and demerits of various TIM materials available in the commercial market are also discussed. The article will be concluded with some directions for the future that would be potentially very beneficial.

Thermal interface materials based on graphene and silver nanopowder

PurposeThe purpose of this paper is to present results of the studies on modification of ethylene-vinyl acetate (EVA) encapsulation foil to be used as thermal interface material (TIM). It is estimated that poor thermal management in electronic devices can cause over 50 per cent of failures. As the junction temperature rises, the failure rate for electronics increases exponentially. To ensure sufficient heat transfer from its source, TIMs are used in various circuits. On the other hand, it is important to ensure high electric resistivity of the designed TIM.Design/methodology/approachThe focus of the investigation was twofold: modification of EVA with both graphene oxide (GO) and silver nanopowder (nAg); and TIM applicability through lamination of photovoltaic cells with standard and modified EVA foil. The main problem of a new type of encapsulant is proper gas evacuation during the lamination process. For this reason, reference and modified samples were compared taking into account the percentage of gas bubbles in visible volume of laminated TIM. Finally, reference and modified TIM samples were compared using differential scanning calorimetry (DSC) and laser flash analysis (LFA) measurements.FindingsThe proper parameters of the lamination process for the modified EVA foil - with both GO and organometallic nAg particles - were selected. The nAg addition results in an increase in thermal conductivity of the proposed compositions with respect to unmodified EVA foil, which was confirmed by DSC and LFA measurements.Originality/valueThe experiments confirmed the potential application of both EVA foil as a matrix for TIM material and nAg with GO as an active agent. Proposed composition can bring additional support to a solar cell or other electronic components through effective heat removal, which increases its performance.

Metallic TIM Testing and Selection for IC, Power, and RF Semiconductors

Metallic TIM Testing and Selection for IC, Power, and RF Semiconductors

Thermally Conductive-Silicone Composites with Thermally Reversible Cross-links

Thermally conductive-silicone composites that contain thermally reversible cross-links were prepared by blending diene- and dienophile-functionalized polydimethylsiloxane (PDMS) with an aluminum oxide conductive filler. This class of thermally conductive-silicones are useful as thermal interface materials (TIMs) within Information Technology (IT) hardware applications to allow rework of valuable components. The composites were rendered reworkable via retro Diels-Alder cross-links when temperatures were elevated above 130 °C and required little mechanical force to remove, making them advantageous over other TIM materials. Results show high thermal conductivity (0.4 W/m·K) at low filler loadings (45 wt %) compared to other TIM solutions (>45 wt %). Additionally, the adhesion of the material was found to be ∼7 times greater at lower temperatures (25 °C) and ∼2 times greater at higher temperatures (120 °C) than commercially available TIMs.

A HIGH THROUGHPUT, LOW COST ASSEMBLY APPROACH FOR LED IMS PACKAGES TO HEAT SINK USING ADVANCED THERMAL INTERFACE MATERIALS

The objective of this study is to design and fabricate a high reliability LED Insulated Metal Substrate (IMS) package to complex heat sink attachment using an advanced thermal interface material (TIM). The assembly consists of LED IMS parts bonded to a heat spreader/sink using an advanced TIM and a corner bond material to quickly and accurately secure the LEDs in position. The corner bond adhesive is snap cured for fast machine cycle times while the high performance, high adhesion TIM materials cure throughout the rest of the assembly operation. This approach allows high accuracy LED bonding without the need for alignment pins or fasteners to anchor to the IMS. The IMS attached to the heat sink is then electrically interconnected with a thin flex substrate on top of the IMS. This approach is expected to replace the current mechanical fastners and low strength silicone TIM materials and reduce the cycle time and overall placement cost which are key drivers especially for the automotive industry.

Comparative Test Data for TIM Materials for LED Applications

Developments in thermal interface materials (TIMs) continue across the industry with a variety of different types of materials. Thermal and mechanical design engineers are often confronted with the need to select which type or category of TIM material is the most appropriate for a specific LED module application, which can be confusing, and how to determine which materials provide the best thermal performance. The next step is understanding which TIM material types meet requirements for ease of shipping, handling, placement, cost, and rework. These are important distinctions, in addition to thermal performance. This presentation will illustrate comparative testing results for a set of thermal interface materials (TIMs) in different categories, using different TIM testing procedures. Test data prepared using three different test methods will be compared:1. ASTM D5470-06 with known temperatures and clamping forces;2. In-situ testing with industry-standard semiconductor modules, at known temperatures and estimated clamping forces;3. In-situ testing utilizing a thermal test vehicle (TTV) for TIM2 performance for a processor module. In-situ testing has been performed at an independent power semiconductor manufacturer, using both industry-standard and commonly-available modules and a custom-designed module with a relatively small footprint, capable of high operating junction temperatures. This testing data can illustrate how different types of TIM materials perform in laboratory testing conditions, for precise comparisons on thermal performance alone; and how different types of materials perform in what are termed as “in-situ” test procedures. This term is used for application-specific conditions, where additional variables are encountered in the testing (such as non-flat surface conditions and unknown clamping force values), which is significantly different from the laboratory conditions used to generate ASTM D-5470 test values. The comparative testing that has been undertaken will be described, showing that images of various power semiconductors with several different materials tend to correlate with the thermal resistance of materials measured with the ASTM D 5470-06 method. These thermal interface materials were also tested on a TTV supplied by a major processor module. The relevance of the thermal imaging, the TTV and the ASTM values will be discussed. This presentation is intended to illustrate the differences in experimental data from one TIM material to another, as well as the differences in testing procedures.

Investigations of thermal interfaces aging under thermal cycling conditions for power electronics applications

This paper presents new investigations on the aging of Thermal Interface Materials (TIMs) subjected to thermal cycling conditions. The challenge was to design a specific and original set-up in order to not only undergo avionic temperature mission profile (−50°C/+150°C) but also to perform standardized thermal characterization at always same conditions. Thermal conductivity is used as aging indicator. Several TIM materials (change phase, graphite and polymer based) have undergone more than 1500 of such cycles. As a result, only the phase change material thermal interface has been affected with a 30% increase of initial thermal resistance.

Optimization of a Flux-Less Metal Thermal Interface Material by Reflow in a Vacuum Furnace

This paper describes the work performed with a pure metal thermal interface material (TIM) for the sole purpose to improve the transfer of heat from the die to the metal cover case. A flux-less reflow process is employed in order to reflow the indium TIM material. This operation is performed in a vacuum furnace utilizing heat, vacuum, and pressure in a specific sequence in order to wet the metal lid and the backside of the flip chip die. The initial objective was to demonstrate minimal voiding of the TIM and subsequently limited flow out of molten solder from and along the sides of the die. A series of experiments were employed where acceptance criteria is evaluated by a) X-Ray, b) scanning acoustical microscopy (SAM), and c) cross-section. Acceptance criteria consists of 1) indium wetting of both lid to indium interface and indium to silicon interface die, 2) indium bond line (BLT) thickness, 3) lid tilt, and 4) lid shear strength. Acceptance is determined after a subsequent 4X ball grid array (BGA) reflow in a conventional belt reflow furnace with minimal voiding, no popcorn or blistering of the laminate substrate, and TIM thickness and solder flow out at sides of the die within the acceptable limits of the above mentioned criteria.