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(a) A nanostructured interface material is used around the module; (b) a nanostructured interface material is used between the thermoelectric material and metal leads.

Semiconductor Research Corporation (SRC) and researchers from Stanford University have developed a novel combination of elements that yields a nanostructure material for packaging. The advance could mean improvements for semiconductors in the form of packaging for devices. Presently, manufacturers must rely on tiny pins or thick solder to bond sections of the semiconductor in order for the device to perform. However, current solder materials tend to degrade and fail due to heat and mechanical stress. In order to continue the scaling of integrated circuits, SRC and Stanford have researched materials that provide a high thermal connectivity — comparable to copper — with the flexible compliance of foam, the researchers say. The answer has been created through a nanostructured thermal tape that conducts heat like a metal while allowing the neighboring materials to expand and contract with temperature changes (metals are too stiff to allow this). This ability to reduce chip temperatures while remaining compliant is a key breakthrough for electronic packaging.

Read the paper.

This webinar will provide an introduction to the most commonly used Thermal Interface Material (TIM) types and their corresponding testing techniques. Basic TIM properties such as bulk or effective thermal conductivity, interfacial thermal resistance and bond line thickness will be explained. At the beginning, an overview of the existing industrial and experimental test methods aimed at measuring the thermal conductivity of these materials. The ASTM D-5470 steady state TIM testing standard will be explained and evaluated. In the second part of the presentation newly developed test setups will be shown which mean an enhancement to the existing standard, including a possible in-situ solution. These new approaches will be explained in detail and measurement results will be shown. In the final part of the presentation the presenter will discuss the importance of the evaluation of long term reliability of these materials beside the highly precise measurement of their thermal conductivity.

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Some time ago I devoted my editorial to the problems caused by the veeerrryyy slow adherence to the use of SI units to which the U.S. committed itself in 1872. When talking globalization we should speak the same scientific language, and when there are conflicts of interest, logical reasoning should prevail to decide upon the best solution, and when this turns out to be difficult some standardization body should produce the verdict. One example is the incorrect use of thermal impedance by some vendors to describe the thermal resistance of a Thermal Interface Material (TIM). The logical reasoning to get rid of this habit is the following,
“Thermal impedance,” with unit m2K/W is not the right word to indicate unit area thermal resistance, because it violates the electrothermal analogy commonly in use. First, the term is historically reserved to describe time-dependent thermal resistance with unit K/W. In limiting cases, for frequency zero or large enough times approaching steady state, the impedance becomes equal to the resistance. Second, in the electrical world ‘electrical resistance’ and ‘electrical impedance’ have the same unit, namely Ohm. Consequently, ‘thermal impedance’ should have the dimension K/W, not m2K/W. But the real problem is that sticking to the current definition of “thermal impedance” will cause a lot of confusion in the future, because the use of dynamic test methods is the obvious choice for application-specific tests, one output of which is a thermal impedance. It should be added that regarding TIMs, in situ testing is the only way for a designer to achieve a value that makes sense in real life. When quoting the performance of a TIM per area, we propose to use ‘unit area thermal resistance’ or ‘unit thermal resistance’ or ‘area thermal resistance’. The final word is to a standardization body.
Let us summarize the pros and cons.
Pro: The use of the word is generally accepted by some vendors and users in the TIM branch of industry.
Cons: (assumption: we agree on the usefulness of the electrothermal analogy)
•    Thermal impedance (m2K/W) is not the right word to describe the output of steady state TIM testers, for reasons outlined above.
•    An increasing use of dynamic test methods is expected, one output of which is a thermal impedance (K/W), leading to a lot of confusion.
•    Conflict with the established use of dynamic thermal impedance (Zth) in the power device industry.
Alternatively, we could decide to refrain from using m2K/W. However, using m2K/W has its merits:, because the unit area Rth is directly comparable to the inverse of the heat transfer coefficient, or alternatively, equal to the ratio of thickness over effective thermal conductivity, a metric to compare the thermal resistance of various TIMs with different areas. As an example, take the generally accepted feeling that future TIMs should exhibit an Rth < 5 mm2K/W. Let’s assume the Rth of the cooling solution should be less than 20% of the Rth of the TIM, otherwise improving the cooling would not be effective. What does this mean for heff? heff> 1.000.000 W/m2K ! (Recall that we talk effective h, including the area enlargement by a heat sink.)
How to improve upon this situation? Customers unite! Refuse to buy from steady state thermal impedance believers!
Following an example of a letter you could write:
Dear TIM vendor,
I hope you agree with your valued customer who is considered king by your sales department that your choice of continuing to use ‘thermal impedance’ to describe what is essentially a unit area thermal resistance, with dimensions m2K/W, is not in my interest. It leads to unacceptable confusion because thermal impedance is a historically defined parameter to describe the thermal transient response with units of K/W.
Thanks for your understanding.

Murata Electronics North America recently launched an ultra-thin waterproof piezoelectric speaker. With a thickness of only 0.9mm, this 19.5mm x 14.1mm speaker enables greater design freedom for the rapidly growing and evolving mobile market. The speaker achieves IPX7 grade waterproof protection without the need of a waterproof acoustic membrane. Using acoustic mesh and double-sided tape to seal the speaker to the front cavity, this waterproof speaker application allows for decreased application costs, thin size, and good sound performance. The high torque nature of the speaker’s piezoelectric motor also makes it ideal for operation in very small and thin back cavities where dynamic speakers struggle to operate. As such, these features make the speaker ideal for mobile phones, music players, digital still cameras, digital video cameras, IC recorders, e-books and other mobile equipment.

Source: Murata

New TIM Catalog Released

Fujipoly has released its new Thermal Interface Material and Elastomeric Connector product catalog. The free 52-page product overview and technical design guide includes installation suggestions, as well as detailed thermal performance and electrical conductivity data points.

Several new pages of high-performance and low-cost thermal materials have been added to complement the company’s current product assortment. Fujipoly’s new expanded catalog also features a complete section dedicated to high density, low resistance, electrically conductive silicone connectors.

Source: Fujipoly

Light Tack Adhesive Eases Fitting of Thermal Interface Pads

MH&W International now provides U 90 silicone-free thermal interface materials with a new light tack adhesive to provide high thermal conductivity where contamination threats prevent the use of silicone-based thermal pads, and allow their easy positioning between components and heat sinks.

MH&W’s Keratherm U 90 thermal interface material is a ceramic-filled polyurethane film with a thermal conductivity of 6.0 W/mK and thermal impedance of 0.05 Kin2/W. A lower cost version, Keratherm U 80, also silicone-free, provides 1.8 W/mK of thermal conductivity and 0.11 Kin2/W of thermal impedance.

Source: MH&W

Product Line for Cooling Applications Requiring Fan Guards

Orion Fans now offers a complete line of wire mesh, plastic, louvered, push-on, metal and filtered fan guards and kits, providing design engineers multiple guard options when evaluating fans for their specific applications. Whether the fan application requires finger protection and/or additional filtering to prevent dirt and dust from entering equipment, Orion Fans offers every type of guard option for applications including enclosures, industrial automation, network equipment, medical equipment, truck and automotive.

Get more details from Orion Fans.
Source: Orion

Testing Station Allows Thermal Analysis of Electronics Chassis

Advanced Thermal Solutions, Inc. (ATS) introduces a new testing station that allows in-house, low cost, thermal analysis and testing of electronics chassis and PCBs. The iTHERM-200™ is an integral system of instruments for precisely measuring and recording airflow velocity and temperature data at multiple points inside electronics housings and on circuit cards. The new system includes a freestanding wind tunnel, an automated wind tunnel controller, sensors and a temperature and air velocity scanner. The system’s large test chamber and eight candlestick-style sensors allows characterization testing on active or prototype boards and racks, including single ATCA, MicroTCA, cPCI and AMC.

Source: ATS

New CP3000NV UPS Series (60-160 kVA)

Targeting medium-sized data centers, light industrial systems, and other mid-range power protection applications, Chloride introduces its CP3000NV series of three-phase UPS with module ratings from 60 kVA to 160 kVA.

In line with Chloride’s green commitment, the CP3000NV Series supports environmental objectives by featuring AC-AC operating efficiencies of 95%, a flat load/efficiency curve, lower heat rejection to reduce cooling loads, and Green Battery Management™ technology that minimizes power consumption through intelligent battery charging, resulting in a longer battery life.

To minimize direct electrical and cooling costs, the CP3000NV series is transformer-less at 208/208V and 480/480V, a benefit that also translates into lower overall weight, a smaller footprint, and faster installation. The advanced IGBT front end assures low input harmonics and simple generator interface.

Source: Chloride

Thermal Management Printed Circuit Board for LED Cooling Applications

SinkPAD Corporation is a thermal management company addressing thermal challenges facing the solid state lighting industry specifically in aluminum PCB (LED PCB) applications.

IMS PCBs with efficient through plane thermal conductivities are important for high heat generating LEDs. Although high thermally conductive metals are used, IMS PCB performance is dependent upon the contact area, material thickness and thermal resistance of each material located between heat source and the atmosphere.

SinkPAD™ technology for aluminum IMS PCB significantly improves LED thermal management in all LED systems but is most effective in high-power and high-bright surface mount LED systems that can’t efficiently dissipate heat rendering them unviable for commercialization.

SinkPAD™ conducts heat out of the LED system (LED cooling) by enabling a direct thermal path between the LED and surrounding atmosphere, which eliminates thermal resistance introduced by the dielectric material in a traditional IMS PCB or MCPCB. The SinkPAD™ design completely removes the substance with the lowest thermal conductivity/highest thermal resistance from the structure. SinkPAD™ still uses a dielectric, but this dielectric isolates the metal base electrically and leaves it thermally connected.

Source: SinkPAD

Silicone Elastomer Handbook Released

The Silicone Elastometer Handbook, A guide to applied silicone elastometer technology, explores all up-to-date aspects of silicone technology and offers case histories to reinforce principles being relayed and display real-world problem solving. Written by David M. Brassard, founder and technical director of Silicone Solutions, the book is based on a short course the author teaches at the University of Akron, College of Polymer Science and Polymer Engineering. The handbook can help companies explore the possibilities of entering the silicone market, including, but not limited to formulations, raw materials, current suppliers, as well as necessary equipment.

Source: Silicone Solutions

Thermal Gap Fillers Combine High Performance, Low Pricing

New TP-S30 thermal interface pads from MH&W International provide 3.0 W/mK of thermal conductivity between hot components and heat sinks at lower costs than competing gap filler materials. Pads of TP-S30 thermal gap fillers are soft and compliant for easy compression and filling of air gaps between mounting surfaces to optimize heat transfer. Applications for these gap fillers include alternative energy, consumer electronics, telecommunications, power supplies, flat panel displays, and portable electronics.

Source: MH&W International

ATS Thermal Labs has published a white paper, “Long term Thermal Grease Reliability.” It considers how long term heat sink reliability is affected by the use of thermal greases as thermal interface materials. The implications are very important as they will affect the MTBF of electronic equipment. 

Read the paper from ATS Thermal Labs.

Read Pages 28-29 of Fujipoly’s catalog for more information.

Because of substantial increases in the power density of electronic packages over the past few decades, thermal interface resistance can comprise more than 50% of the total thermal resistance in current high-power packages [1]. Unless advanced thermal interface materials (TIMs) that achieve order-of-magnitude improvements in performance quickly emerge in the market, the portion of the thermal budget spent on interface resistance will continue to grow because die-level power dissipation densities are projected to exceed 1 W/mm² (100 W/cm²) within the next 10 years [2]. Fortunately, improved understanding of heat transfer at nanometer scales, combined with increased ability to design new materials at the atomic level, has enabled a broad range of technological advances that can be applied to develop TIMs with performance characteristics that keep pace with cooling demands as electronics continue to evolve along Moore’s law.

Carbon nanotubes (CNTs) are honeycomb-like (i.e., hexagonally shaped) arrangements of carbon atoms that are rolled into cylindrical tubes with diameters as small as a few atoms wide and aspect ratios as high as 10⁵. Because of these unique structural features and strong carbon-to-carbon bonding, CNTs possess many exceptional vibrational, optical, mechanical, and thermal properties that have been utilized in myriad applications. CNTs can be produced from a wide variety of processes, such as arc-discharge, pyrolysis of hydrocarbons over metal nanoparticles (e.g., in Chemical Vapor Deposition (CVD) or plasma-enhanced CVD processes), and laser vaporization of graphite targets, to name a few prominent methods. Considerable attention has been focused on developing advanced TIMs that utilize the extraordinarily high axial thermal conductivity of CNTs – theoretical predictions suggest values as high as 3000 W/mK [3] and 6600 W/mK [4] for individual multiwalled CNTs and single-wall CNTs, respectively. Early studies focused on dispersing CNTs in a compliant polymer matrix to enhance the effective thermal conductivity of the composite structures [5]. Yet, only modest improvements in thermal performance were achieved because enhancement of thermal conductivity in such structures is hindered by thermal interface resistance between CNTs and the matrix and mechanical stress at CNT-matrix boundaries that reduces the speed at which phonons propagate in the CNTs (i.e., the surrounding elastic medium alters phonon dispersion and reduces the intrinsic thermal conductivity in CNTs) [6]. While limited in comparison to dry CNT TIM structures as discussed below, CNT-polymer composites remain an active research focus and several companies are developing products based on this technology as highlighted in a recent article [7].

Over the past five years, significant attention has shifted to vertically oriented CNT arrays (a.k.a. CNT forest, mats, or films) as promising TIM structures that have been demonstrated to produce contact resistances that compare favorably to state-of-the-art materials [8]. Such configurations possess a synergistic combination of high mechanical compliance and high effective thermal conductivity — in the range of 10-200 W/mK [9-11]. The conformability feature is particularly advantageous in addressing mismatches in coefficients of thermal expansion that can cause TIM delamination and device failure. Also, in contrast to polymer-CNT composites and the best thermal greases, CNT array interfaces are dry and chemically stable in air from cryogenic to high temperatures (~ 450°C), making them suitable for extreme-environment applications [12].

It is important to note that all CNT array TIMs are not created equal; as a result, performance can vary greatly and depends on many factors, e.g., array density and height, CNT diameter, CNT quality, the adhesion of CNTs to the growth substrate, etc. However, since the first investigations of the efficacy of CNT arrays as TIMs, substantial improvements in metrology and synthesis control have led to lower thermal resistances and less scatter in reported values. The purpose of this article is to present and discuss recently published data on the performance of various CNT array TIMs that produce resistances that are near or below the range of resistances achieved by the best materials used today. The article highlights important characteristics, current performance bottlenecks, and significant technical considerations for integrating CNT array TIMs with real devices.

Heat transfer through CNT Array Interfaces

The most actively studied CNT array interface structure is the one-sided CNT array interface that consists of CNTs directly grown on one substrate with CNT free ends in contact with an opposing substrate (see Figure 1). The numerous CNT contacts at both substrates form parallel heat flow paths within the framework of the thermal resistance network illustrated in Figure 1. This network shows thermal resistances resolved at the individual nanotube level for true CNT-substrate interfaces, both at the growth substrate (with a nanotube number density of N, in contacts/area) and at the opposing interface (with a contacting nanotube number density of n). The resistance at each local CNT-substrate contact can be modeled as two resistances in series [13]: 1) a classical substrate constriction resistance (R cs) and 2) a resistance (R_b) that results from the ballistic nature of phonon transport through contacts much smaller than the phonon mean free path in the materials (~ 100 nm). The ballistic resistance (R_b) is usually orders of magnitude larger than Rcs for CNT-substrate contacts, which are typically on the order of 10 nm.

The remaining resistance (R”_array) is from heat conduction through the CNT array. This effective resistance is defined for the entire array (including void spaces) to simplify the modeling effort. Moreover, this quantity has been measured in prior work for representative samples and can be used to interpret experimental results that only measure overall thermal interface resistance. When array height is less than 50 μm, R”_array is usually negligible in comparison to the resistances at the CNT-substrate contacts [13].

Given knowledge of the contact number densities at the growth substrate (N) and the opposing substrate (n), an overall or total interface resistance can be calculated. The former density (N) can be estimated from scanning electron micrographs of synthesized arrays, and the latter density (n) can be estimated using a recent model that predicts real contact area in CNT array interfaces as a function of applied pressure and important array characteristics, such as porosity and CNT diameter [13]. The model reveals that fabricating arrays with low effective compressive modulus is critical for establishing large interfacial contact and minimizing total thermal resistance. A detailed development of the CNT array TIM resistor network model is presented elsewhere [13]. Applying the model to one-sided CNT array interfaces with a surface density of 10⁸ CNTs/mm² and CNT diameters of 20 nm suggest that total resistances of ~ 0.1 mm²K/W represent limiting values that could be achieved if the CNTs are completely and perfectly contacted and have well-matched acoustic impedances at all CNT-substrate interfaces.

Figure 1. (a) Schematic (not to scale) of an interface with the addition of a vertically oriented CNT array of thickness tarray [8]. (b) Buckled CNT contacting an opposing surface with its wall. As shown, some CNTs do not make direct contact with the opposing surface. (c) Resistance schematic of a one-sided CNT array interface between two substrates, showing constriction resistances (Rcsi), phonon ballistic resistances (Rbi), and the effective resistance of the CNT array (R”array).

Figure 2. CNT array interface structures: (a) example of one-sided interface; (b) example of two-sided interface, (c) example of CNT-coated foil interface, (d) CNT arrays on both sides of 25 μm-thick Al foil [8].

CNT Array TIMS

The three CNT array TIMs shown in Figure 2 have exhibited some of the most promising thermal performance characteristics to date. The first is the one-sided interface structure discussed above. The second configuration, i.e., the two-sided configuration, consists of CNT arrays adhered to surfaces on both sides of the interface and brought together in Velcro^TM-like contact (in this configuration CNTs mechanically entangle and are attracted to each other by van der Waals forces). The third structure comprises vertically oriented CNT arrays directly and simultaneously synthesized on both sides of thin foil substrates that are inserted into an interface. The CNT-coated foil structures are particularly attractive in that they serve as a method for applying CNT arrays to interfaces between heat sinks and devices that would experience damage from exposure to the high temperatures normally required for high-quality CNT growth (> 700°C).

Using CVD processes that are ubiquitous in the electronics industry, the CNT array TIMs in Figure 2 have been grown on various substrates such as silicon, silicon carbide, copper, and aluminum that are important for thermal management applications [8]. Based on conversations with a few companies that have demonstrated production-level growth of CNT arrays in large-scale CVD reactors, it is estimated that the CNT TIMs in Figure 2 can be made for significantly less than $1 per TIM (assuming an area of 2 cm² for each TIM), which is cost competitive with currently available TIMs; however, achieving sufficient process control in production-scale environments remains a technical barrier to market entry.

Figure 3. Room-temperature thermal resistances as a function of pressure. (a) One-sided CNT array interfaces. (b) Two-sided CNT array interfaces and CNT-coated foil interfaces. The blue-shaded region represents the range of resistance values for TIMs currently on the market.

Thermal Resistances of CNT Array TIMS

Figure 3 summarizes the performance of one-sided, two-sided, and CNT-coated foil interfaces as a function of pressure [14-19]. One-sided interfaces have achieved resistances as low as 7 mm²K/W [14], and two-sided interfaces have been demonstrated to produce resistances as low as 4 mm²K/W [15] — this value is comparable to the resistance of a soldered interface. For both of these configurations, the pressure dependence is weak in the measured range because the CNTs are compressed near their maximum extent within the measurement range [13]. Resistances as low as 8 mm²K/W were produced with the CNT-coated foil TIMs [16]. The CNT-coated foils enhance real contact area significantly, which results in low contact resistance, because deformation of the thin foil substrate “assists” CNT displacement to match the topology of the mating surfaces.

There are considerable data on the performance of CNT array TIMs at a single pressure [20-26]. These data are summarized in Table 1 along with the lowest resistances achieved in measurements as a function of pressure. To demonstrate performance at operating temperatures for a variety of devices, the resistances of the one-sided SiC­CNT-Ag interface in Table 1 were measured from room temperature to 250°C and the values were approximately steady in this range [12]. A few groups have explored techniques to improve CNT-substrate bonding and contact area, particularly at the interface created by free CNT ends. Bonding free ends [21, 22], or combining CNT arrays with traditional TIMs that wet the interface well (e.g., phase change materials) [19, 26], produced thermal resistances that were an order of magnitude lower than the resistances of one-sided interfaces in dry contact. These results are also presented in Table 1.

A few groups have measured thermal resistances of CNT array TIMs using transient techniques that allow the true CNT-substrate resistances and the resistance of the CNT array to be independently resolved [15, 21, 22]. Such measurements confirm that the resistances at CNT-substrate contacts are much larger than the intrinsic resistance of the CNT array, and that the resistance at the interface between CNT free ends and an opposing substrate is considerably larger than the resistance at the CNT-growth substrate interface — the true contact area established by weakly bonded van der Waals forces between CNT free ends. And the opposing substrate is considerably less than the contact area at well anchored CNT roots. Figure 4 illustrates a one-sided interface with local resistances at true CNT-substrate contacts highlighted. The resistance between CNT free ends and the opposing substrate is clearly the largest resistance in the network. The thermal resistances at the CNT free ends also comprise the largest percent of total resistance in the two-sided and CNT-coated foil configurations [15, 16].

CPU Burn-In with CNT Array TIMS

Recently, CNT-coated foil TIMs were characterized in an industry typical burn-in tester that used a current-generation Intel CPU [29]. The TIMs consisted of CNTs grown on one side of 25 μm-thick copper foil with CNT free ends in contact with a heat sink and the bare foil surface in contact with the die. The CNT-coated foil TIMs were tested for 1000 thermo-mechanical cycles. They produced resistances at least 30% lower than the resistances produced by a variety of bare foil TIMs (Cu, Al, etc.). These performance improvements were consistent over all tested cycles, and CNTs remained well adhered to the foils after removal from the interfaces. Compared to the resistances produced by state-of-the-art materials used for CPU burn-in, a twofold improvement in system resistance was achieved when paraffin wax was added to the CNT-coated TIMs [29].

“]”]

Figure 4. True contact resistances for a one-sided Si-CNT-Ag interface at 0.241 MPa measured at room temperature using a photoacoustic technique [15.

Table 1. Thermal resistances of CNT array TIMs measured at room temperature¹

Conclusions

To date, three CNT array TIM configurations have been developed to the point where they produce resistances that compare favorably to the best TIMs currently in use. So far, the lowest resistances produced by CNT array TIMs are on the order of 1 mm²K/W. Further improvements can be achieved by optimizing the compliance of CNT arrays to maximize the real contact area in the interface. Experimental data and theoretical predictions reveal that the resistances at CNT-substrate contacts severely limit the potential of CNT array TIMs. Improvements in bonding and thermal transport at these contacts can lead to substantial reductions in resistance, approaching estimated theoretical limits of ~ 0.1 mm²K/W.

References

1. Cola, B.A., Xu, J., Cheng, C., Xu, X., Hu, H., Fisher, T.S., “Photoacoustic Characterization of Carbon Nanotube Array Thermal Interfaces,” Journal of Applied Physics, Vol. 101, 2007, p. 054313.
2. Cola, B.A., Xu, X., Fisher, T.S., “Increased Real Contact in Thermal Interfaces: A Carbon Nanotube/Foil Material,” Applied Physics Letters, Vol. 90, 2007, p. 093513.
3. Xu, J., Fisher, T.S., “Enhanced Thermal Contact Conductance Using Carbon Nanotube Array Interfaces,” IEEE Transactions on Components and Packaging Technology, Vol. 29, 2006, pp. 261-267.
4. Xu, Y., Zhang, Y., Suhir, E., Wang, X., “Thermal Properties of Carbon Nanotube Array Used for Integrated Circuit Cooling,” Journal of Applied Physics, Vol. 100, 2006, p. 074302.
5. Xu, J., Fisher, T.S., “Enhancement of Thermal Interface Materials with Carbon Nanotube Arrays,” International Journal of Heat and Mass Transfer, Vol. 49, 2006, pp. 1658-1666.
6. Amama, P.B., Cola, B.A., Sands, T.D., Xu, X., Fisher, T.S., “Dendrimer­assisted Controlled Growth of Carbon Nanotubes for Enhanced Thermal Interface Conductance,” Nanotechnology, Vol. 18, 2007, p. 385303.
7. Tong, T., Zhao, Y., Delzeit, L., Kashani, A., Meyyappan, M., Majumdar, A., “Dense Vertically Aligned Multiwalled Carbon Nanotube Arrays as Thermal Interface Materials,” IEEE Transactions on Components and Packaging Technology, Vol. 30, 2007, pp. 92-99.
8. Panzer, M., Zhang, G., Mann, D., Hu, X., Pop, E., Dai, H., Goodson, K.E., “Thermal Properties of Metal-Coated Vertically Aligned Single-Wall Nanotube Arrays,” ASME Journal of Heat Transfer, Vol. 130, 2008, p. 052401.
9. Zhang, K., Chai, Y., Yuen, M.M.F., Xiao, D.G.W., Chan, P.C.H., “Carbon Nanotube Thermal Interface Material for High-Brightness Light­Emitting-Diode Cooling,” Nanotechnology, Vol. 19, 2008, p. 215706.
10. Wang, H., Feng, J., Hu, X., and Ng, K.M, “Synthesis of Aligned Carbon Nanotubes on Double-Sided Metallic Substrates by Chemical Vapor Deposition,” Journal of Physical Chemistry C, Vol. 111, 2007, pp. 12617­12624.
11. Liu, X., Zhang, Y., Cassell, A.M., and Cruden, B.A., “Implications of Catalyst Control for Carbon Nanotube Based Thermal Interface Materials,” Journal of Applied Physics, Vol. 104, 2008, p. 084310.
12. Cola, B.A., Hodson, S.L., Xu, X., and Fisher, T.S., “Carbon Nanotube Array Thermal Interfaces Enhanced with Paraffin Wax,” Proceedings of 2008 ASME Summer Heat Transfer Conference, Jacksonville, FL, 2008.
13. Prasher, R., “Thermal Interface Materials: Historical Perspective, Status, and Future Directions,” Proceedings of the IEEE, Vol. 94, No. 8, 2006, pp. 1571-1586.
14. Chung, D.D.L., “Materials for Thermal Conduction,” Applied Thermal Engineering, Vol. 21, 2001, pp. 1593-1605.
15. Cola, B.A., “Photoacoustic Characterization and Optimization of Carbon Nanotube Array Thermal Interfaces,” Ph.D. Dissertation, Purdue University, West Lafayette, IN, 2008.

Mounting pins only require 1.8mm diameter holes in the PCB. Heat sinks are mechanically attached to the PCB instead of the chip package or substrate. Shock and vibration loads are transferred to the PCB instead of the chip and solder balls. Heat sink location and orientation is precisely set by anchor and pin. The possibility of damaging the chip, heat sink, or thermal interface material during installation is greatly reduced as well as the possibility of installing the heat sink in the wrong orientation or location.

Read more from Alpha.

The role of thermal interface materials (TIM) to augment heat flow in electronics has been well-documented [1-4]: Higher thermal conductivity (k) and thinner bondlines proportionally reduce the interface thermal resistance (R_TH). Today’s high-performance TIMs contain micro- and nanometer-sized particles to increase the effective TIM thermal conductivity (k_eff,TIM) compared to the bulk fluid, e.g., from 0.15-0.20 W/mK silicone oil to a k_eff,TIM > 5 W/mK silicone-based TIM. The benefit in increased k_eff,TIM can be offset at high fill factors when the fluid properties prevent formation of a sufficiently thin bondline. Further TIM improvements, therefore, rely on the formation of thin bondlines at reasonable assembly pressures in highly-filled compositions. We have shown that paste interfaces can be formed with lower pressure and thinner bondlines by hierarchically nested channel (HNC) structures [5-7]. The reduction in thermal resistance and assembly loads is attractive for automotive, power electronics, optical, RF, and microprocessor packaging; enabling higher-power operation at lower junction temperature.

Design Considerations

The channels relieve pressure during the assembly squeeze, offsetting the effective non-Newtonian viscosity increase due to particle loading. The assembly dynamics are described by Stefan (squeeze) flow with a crucial modification: The square interface in electronics packages causes the fluid flow to bifurcate along the diagonal because the flow path to the edge of the interface is shorter (Figure 1). The bifurcation also causes local variation in particle fill factor that leads to spatial k_eff,TIM variations or electrical resistivity variations for electrically-conductive formulations as the particles may be suspended at the bifurcation lines.

Figure 1. Flow bifurcations due to TIM squeezing on flat (left), HNC1 (middle), and HNC2 (right) interfaces of an 18 x 18mm² chip. The flat and HNC1 images are optical, the HNC2 image is an acoustic scan.

Channel Hierarchy

The diagonal (“HNC1″) channels reduce the pressure, redistribute the flow field, and create new TIM bifurcations corresponding to flow being distributed either to the edges of the chip or the channels (middle picture, Figure 1). Additional channels (“HNC2″) further sub-divide the flow, but the cross section of these second-level channels can be smaller because the maximum local pressure is lower. This leads to the hierarchical nature of the technology � additional channels carry proportionally less TIM material and can have smaller cross sections. Bondline thickness (BLT) reduction due to the channels must offset the removal of highly-conductive solid material from the system. The hierarchical design ensures that the impact of removing solid is proportionately less for each additional channel “level”, corresponding to the proportionately lower impact of additional HNC levels on reducing BLT and assembly pressure. Nevertheless, coupling the thermal and flow impact of HNC leads to an optimum point, at which further HNC levels begin to increase rather than decrease the thermal resistance of the interface [7].

Channel Geometry

There are four characteristic length dimensions in the HNC system: the particle size distribution, the channel dimension, the bondline thickness, and the interface size. The filled TIM can be captured as a Bingham fluid with a yield strength that increases with particle loading and causes the effective viscosity of the TIM to increase with decreasing shear rate.

Previous work [8] using three-dimensional computational fluid dynamics (CFD) compiled all characteristic lengths into one model coupled to a compact thermal system to prescribe a simple design rule for HNC1. We found that thermal interface resistance is reduced most when the width ( w, Figure 2) of the square channels measure 3.4% of the total length of the interface (2�l, Figure 2). This corresponds to 600 �m HNC1 channels in a typical 18 x 18mm² microprocessor-to-lid interface or 1.7 mm channels in a 50 x 50mm² lid-to-cooler arrangement. Extending the analysis to further channel hierarchy levels is more complicated, but follows the same basic rule.

Figure 2. Flow vectors from numerical simulation mapped onto optical image of paste bifurcation, denoting the relevant geometries for the HNC1 design rule.

Results and Discussion

We have characterized over 20 materials with HNC and found > 20% BLT reduction for up to three HNC levels (HNC3). The technology was first developed as a “TIM1″ (silicon chip to lid) solution, but current work suggests even greater benefit on larger interfaces, for example, power electronics modules. On such 100 x larger areas, HNC can improve thermal performance without the effort and expense of qualifying and implementing newer paste formulations.

Figure 3. Temporal bondline assembly profile for Wacker P12 silicone-based TIM common in power electronics applications. The 14 x 20 cm interface was assembled with 2 bar pressure.

The temporal assembly profile of a typical power electronics module and paste follows the trend observed in TIM1 measurements [7]. Figure 3 shows:

1) The final BLT significantly reduces for increasing levels of HNC;
2) The squeeze time, i.e. the time to reach final BLT, is reduced with higher levels of HNC; and
3) The absolute reduction of final BLT and squeeze time decreases with additional levels of HNC.

The thinner bondlines in HNC1-HNC3 proportionally reduce R_TH,interface but even more dramatic is the reduced assembly time (Table 1). This can be significant for module burn-in procedures and may allow the bondline formation to couple to the curing time for adhesive TIM applications.

Table 1. Summary of HNC effects on BLT for selected materials on 280 cm² test stand

All reported data taken at 25�C; higher temperature (60�C) assembly reduced final BLT and squeeze time by 5-60%, depending on TIM formulation. Squeeze time defined by time required to reach 102% of final BLT.

Machining tolerances and mechanical considerations often require BLT > 50 �m in large-area modules. In these cases, HNC can be used to implement higher-k_eff,TIM pastes that could not be squeezed sufficiently thin otherwise. Although it may not be possible to eliminate 80% of R_TH,interface by thinning a 1-W/mK, 100-�m paste to 20 �m, replacing it with a 5-W/mK formulation that can be thinned to 100 �m by means of HNC attains the same R_TH,interface reduction within mechanical specifications. Even when thin bondlines can be tolerated, thermal contact resistance sets the lower bound where further bondline thinning does not improve thermal performance.

Assembly pressure is a third factor that can be addressed with HNC (Figure 4). While the final BLT is dependent on pressure for any non-Newtonian formulation (a Newtonian fluid would eventually reach zero thickness), HNC decouples this relationship. For all pressures, HNC results in lower BLT than a flat interface and the minimal BLT is reached for lower pressure. From Figure 4 it is evident that beyond one bar, additional assembly pressure yields little additional BLT reduction. Lower assembly pressures may then ease the mechanical strength requirements of the modules and tools, possibly yielding cost reductions.

Figure 4. Effect of assembly pressure on final BLT for flat and HNC3
power modules using the same TIM as in Figure 3.

Summary and Outlook

HNC improves thermal interfaces by enabling thinner bondlines with higher-performance pastes at lower assembly pressure. The effects are most dramatic in large-area applications, where a 90% or greater reduction in squeeze time can reduce assembly time and burn-in while enabling shorter cure times for adhesive TIM applications.

Practical Considerations

Some TIM formulations are not well-suited for HNC. Low-viscosity pastes with large particles prematurely stop the squeezing process. Stencil or screen printing the TIM tends to remove excess paste before assembly that the channels would otherwise handle during the squeeze, reducing the HNC benefit for current module geometries. Finally, small (TIM1) interfaces generally achieve smaller percent improvement from HNC with existing pastes. The challenge may rest in cooperation between paste and component vendors to formulate appropriate paste-HNC combinations that can reach the performance of high-cost solutions [5].

Although HNC is a low-cost solution, it is not a no-cost solution. We estimate less than 10% added cost to lids or coolers for suitable large-volume manufacturing processes. Laser cutting, chemical etching, and electrical discharge machining (EDM) are possible for high precision, high margin applications; however, we are focused on stamping, machining, extrusion, and injection molding. The cost may be at least partially recovered in the ancillary cost benefits of HNC; for example, by reducing a module’s mechanical strength specification.

Ongoing Work

One particularly attractive subject is the effect of HNC on the reliability of the thermal interface. The channel patterns in the surface absorb air voids trapped during bondline formation, prevent paste “bleeding” and nonuniformities that develop over time (Figure 5), and reduce the “pump out” seen after thermal cycling. Increased reliability could extend component lifetime and maintenance intervals, yielding further cost reduction.

Figure 5. HNC effect on phase separation of silicon (PDMS)-based TIM with approximately 65% ZnO volume fill. Left: flat TIM2 lid with well defined bleeding traces along the squeeze flow path; right: same lid size, but with HNC2 on the 48 x 48 mm interface.

Investigations are ongoing to identify the TIM composition and particle distributions that most affect HNC bondline formation to improve existing models and collaborate with component and paste vendors on commercialization. In addition, we are screening for further industries that may benefit from HNC. Electrically-conductive applications and solar concentrator photovoltaics (CPV) share many of the same high thermal and electrical performance requirements, and thus are interesting candidates.

Acknowledgements

The authors would like thank Ryan Linderman, Urs Kloter, Hugo Rothuizen and Thomas Brunschwiler for previous work on the HNC technology, and recognize the support of European Commission under the Nanopack project of the Seventh Framework Programme (2007-2013).

References

1. Gwinn, J., Webb, R., “Performance and Testing of Thermal Interface Materials,” Microelectronics Journal 34 (3), 2003., pp. 215-222.
2. Singhal, V., Siegmund, T., Garimella, S., “Optimization of Thermal Interface Materials for Electronics Cooling Applications,” IEEE Transactions on Components and Packaging Technologies, 27 (2), 2004, pp. 244-252.
3. Prasher, R., “Thermal Interface Materials: Historical Perspective, Status, and Future Directions,” Proceedings of the IEEE, 94(8), 2006, pp. 1571-1586.
4. Yovanovich, M.,”Four Decades of Research on Thermal Contact, Gap, and Joint Resistance in Microelectronics,” IEEE Transactions on Components and Packaging Technologies, 28 (2), 2005, pp.182-206.
5. Linderman, R., Brunschwiler, T., Smith, B., and Michel, B., “High-Performance Thermal Interface Technology Overview,” Proc. 13th Conf. on Thermal Investigations of ICs and Systems “THERMINIC 2007,” Budapest, Hungary, September 2007, B. Courtois et al. (Eds.) (EDA Publishing/Therminic 2007, Budapest, Hungary), pp. 129-134.
6. Brunschwiler, T., Kloter, U., Linderman, R., Rothuizen, H. and Michel, B., “Hierarchically Nested Channels for Fast Squeezing Interfaces with Reduced Thermal Resistance,” IEEE Transactions on Components and Packaging Technologies, 30 (2), 2007, pp.226-234.
7. Linderman, R., Brunschwiler, T., Kloter, U., Toy, H., and Michel, B., “Hierarchically Nested Surface Channels for Reduced Particle Stacking and Low-Resistance Thermal Interfaces,” Proc. 23rd Annual IEEE Semiconductor Thermal Measurement, Modeling, and Management Symp. “Semi-Therm 2007″, San Jose, CA (IEEE, 2007), pp. 87-94.
8. Smith, B., Rothuizen, H., Linderman, R., Brunschwiler, T., Michel, B., “Design of Thermal Interfaces with Embedded Microchannels to Control Bond Line Formation,” Proc. 11th Intersociety Conf. on Thermal and Thermomechanical Phenomena in Electronic Systems 2008 “ITHERM 2008,” Orlando, FL, May 2008 (IEEE, 2008), pp. 410-418.

Thermal Interface Materials (TIMs) play a key role in the thermal management of electronic systems by providing a path of low thermal resistance between the heat generating devices and the heat spreader/sink. Typical TIM solutions include adhesives, greases, gels, phase change materials, pads, and solder alloys. Most TIMs consist of a polymer matrix, such as an epoxy or silicone resin, and thermally conductive fillers such as boron nitride, alumina, aluminum, zinc oxide, and silver. The reader is referred to Viswanath et al. [1] and Saums [2] for more information on the different TIM options used in industry and their capabilities.

Amongst the various TIM options, thermal greases typically offer better thermal performance and reduced manufacturing cycle times. They have the ability to flow and conform to interfaces, do not require post-dispense processing such as curing, and have higher effective thermal conductivity compared to other classes of materials. In addition, thermal contacts made using thermal greases can be reworked allowing easy repair and upgrade. For these reasons, thermal greases are extensively employed in microelectronics and power module cooling applications.

However, on extended operation and over time, greases can degrade significantly, resulting in a higher thermal resistances across the grease interface. The degradation mechanisms of greases are considerably different and more complicated to characterize than those of other common TIM solutions such as adhesives and pads. The power cycling test is most representative of the end use environment and hence an accurate technique to assess the thermal and reliability performance of thermal greases. However, power cycling tests can be very time consuming and expensive for screening numerous interface materials from an end-user’s perspective or from a material developer’s perspective. Accelerated reliability tests utilizing a combination of materials, simple structures, and test conditions that simulate different stress conditions experienced by an actual module can offer significant reductions in test time, while providing meaningful data.

Failure Mechanisms of Grease Interface Layers

Thermal greases do not contain any cure chemistry, hence they will not cross-link to form a gel or a hard adhesive type material. Their uncured state that provides the improved ability to provide a low interfacial thermal resistance also makes them susceptible to a variety of failure mechanisms during their service life, some unlike those experienced by thermal gels, adhesives, and pads.

The two main causes of increase in thermal resistance of a grease layer are grease pump-out and grease dry-out. The powering up or powering down of the device causes a relative motion between the die and the heat spreader (in-plane and out-of-plane), which tends to squeeze the thermal grease out of the interface gap. This phenomenon is referred to as “pump-out” and results in increased thermal resistance due to loss of grease material from the interface [1, 3].

Grease “dry-out” occurs due to the separation of the filler from the polymer matrix at elevated temperatures. The polymer matrix tends to flow out of the interface preferentially and results in ‘drying-out’ of the thermal grease. This results in increased in-situ thermal resistance of the material [1]. Exposure to high humidity levels has also been shown to induce changes in the thermal resistance of a grease layer, primarily an effect of the filler and resin system employed and their response to high levels of moisture.

Thermal Characterization of Greases

Several steady state and transient methods exist for measuring the thermal performance of greases. The thermal resistance data presented in this article was obtained using a laser flash thermal diffusivity method, which facilitated measurement using representative surfaces, different bondline thicknesses (BLTs), different pressures, and in a very short period of time due to the transient nature of the measurement.

The in-situ thermal resistance of the greases was measured using a three-layer ‘sandwich’ sample comprising of an aluminum-grease-silicon stack. The thermal resistance determined using this method includes the bulk thermal resistance of the grease and the contact resistances at the grease-substrate interfaces. The reader may refer to Campbell et al. [4] for more information on the advantages and limitations of the laser flash technique.

Table 1. Characteristics of Greases Studied and their Thermal Performance

Table 1 shows the characteristics of five greases that were studied and their thermal performance as measured using the laser flash thermal diffusivity method. These thermal measurements were within 10% of those made within a microprocessor module. The primary filler in these greases is spherical boron nitride (BN) with an average filler size of 60 �m. Greases B1 and B2 have secondary fillers included in the base grease formulation of spherical BN fillers. First order effects and interaction effects are observed with respect to the performance of the greases. An increase in in-situ thermal conductivity is observed with an increase in filler loading for all five greases.

The increase in viscosity with increase in filler loading results in thicker BLTs for a given pressure level and hence a higher thermal conductivity composition does not always result in a lower in-situ thermal resistance interface. At very high filler loadings (these are not discussed in this article and the loading threshold varies with filler and resin system), there is a significant increase in interfacial thermal resistance as the greases do not wet the mating surfaces well. Table 1 also shows a significant reduction in thermal resistance with increasing pressure due to a combination of reduced bondline thickness and interfacial thermal resistance. More information on these greases, their properties, and their performance can be found in Gowda et al. [5].

Reliability Testing

Four types of reliability tests were performed � air-to-air thermal cycling, high temperature storage, high temperature/humidity exposure, and mechanically induced pump-out. Table 2 provides details of the reliability tests.

Table 2. Reliability Tests Performed

High temperature storage can be used to accelerate the drying-out of the thermal grease layers. Weight loss at elevated temperatures is also used as an indicator of grease dry-out [6] . Thermal cycling tests help induce grease movement, in a manner similar to power cycling, however between two isothermal temperature zones. The high temperature and humidity test induces moisture driven degradation of the grease interface layer.

For the thermal cycling and high temperature storage tests, two sample geometries were used. One, a three-layered silicon-grease-aluminum sample in a grease fixture wherein different bolt pressures can be applied was utilized to measure the thermal performance of the greases at time-zero and after reliability testing. Second, three-layer structures of aluminum -grease-silicon were utilized to monitor the degradation of the grease layers using acoustic microscopy. Different bolt pressures and surface roughness of the aluminum plates were used in the visual tests to determine if there was any effect of these parameters on the degradation of a grease interface.

The Al-grease-Si structures simulate grease movement due to the relative in-plane movement of the aluminum, silicon, and grease layers during temperature changes. However, in tests using these structures there is no cyclic change in the TIM bondline with temperature cycling, which is essential to fully simulate the warpage and pump-out mechanism that is typically seen in a flip chip module. In certain applications, though, warpage induced pump-out can be a secondary effect as compared to those driven by high temperature, high humidity, and in-plane grease movement.

To simulate the pump-out effect of grease layers due to the dimensional changes in the package, an accelerated mechanical cycling test was adopted. This mechanically induced pump -out test was reported by Chiu et al. [3], where the TIM gap is cyclically varied between two gap limits that are representative of the dimensional changes in the module under study. This test can simulate power cycling effects, but with a much faster turn-around times, and has been validated by Chiu et al. [3].

Figure 1. Mechanical cycling setup to simulate grease pump-out.

Figure 1 shows the schematic of the test set-up on an Instron® Universal Testing Machine. As shown in the figure, the test vehicle was comprised of a thermal test die (6.35 x 6.35 x 0.6 mm ) mounted on a printed circuit board. The thermal die assembly was fixed to the actuator base of the testing machine and a liquid-cooled chuck (with coolant at 30�C) was connected to the fixed head. The bondline thickness of the grease was cyclically varied between 100 and 150 �m, with junction temperature being measured every 500 cycles. For reference, Maguire et al. [7] also presents an alternative method to simulate power cycling and grease pump-out.

Reliability Test Results

Figure 2 shows the results of the reliability tests on the five greases. The percentage change in thermal resistances and bondline thicknesses for the different greases under different test conditions are shown. Data for the temperature/humidity test was available only for greases B -1 and B-2. In the Al-grease-Si sandwich structures based reliability tests (high temperature storage, thermal cycling, and temperature humidity exposure), all the greases were observed to reduce in thermal resistance. This decrease in thermal resistance was accompanied by a reduction in bondline thickness.

Figure 2. Results of the grease reliability tests.

Exposure to higher temperatures during these tests caused the greases to reduce in viscosity and under the applied clamping force, reduce in bondline thickness and wet the mating surfaces better to reduce both bulk and interfacial resistances. The reduction in viscosity at elevated temperatures had a more beneficial impact for the higher viscosity greases (A-3, B-1, and B-2).

The accelerated mechanical cycling pump-out test was more severe than the fixture based thermal cycling test and resulted in an increase in junction temperature of the device. In the Al -grease-Si fixture-based thermal cycle tests, the thermal resistance dropped for all greases.

It is important to note that in the case of the fixture-based test, the pressure on the grease layers was applied consistently across the five greases, allowing the viscosity and filler characteristics to dictate the resulting bondline thickness, while in the case of the mechanical cycling test, similar bondlines were maintained for all the greases.

In addition, the Al-grease-Si structures allow a reduction in BLT when the viscosities of the greases reduce at elevated temperatures but do not impose a cyclic change in TIM bondline during temperature cycling. In the mechanical cycling test, the BLT is maintained between the same two set limits but varied cyclically between these limits. This explains the difference in the results between the thermal cycling and mechanical cycling tests.

Figure 3 shows a result from the visual degradation studies and acoustic microscopy images of samples before and after stress testing. Greases with only spherical BN fillers (A-1, A2, and A-3) showed an increase in voiding with increasing filler content, but no significant effect of the pressure on the grease interface or the roughness of the mating surfaces. Greases with spherical BN particles and a secondary filler (B-1 and B-2) displayed a slight reduction in voiding with increasing filler content and an effect of the pressure and surface roughness. This stresses the fact that results can vary significantly based on the grease characteristics, process parameters, and the mating surfaces.

Figure 3. Visual degradation of grease interfaces for different stress test conditions.

Although there was an increase in voiding in all cases, the same structures and materials did not show an increase in thermal resistance in similar stress tests. This points to the fact that the reduction in overall thermal resistance caused by a decrease in BLT and interfacial thermal resistance can, at times, be greater than or similar to the increase in thermal resistance caused by voiding, thereby offsetting each other to some extent.

Conclusions

Characterizing the reliability of thermal greases is complicated and requires different test methods as compared to other TIM options. Although the three-layered structures used to monitor the thermal performance with reliability testing and those used to monitor visual degradation do not simulate the full range of stress conditions that is seen in a microprocessor package, they provide a means to screen thermal greases for dry-out, voiding, and in-plane grease movement during the initial stages of a grease selection process or grease development process.

These tests typically overestimate the reliability of a thermal grease layer, primarily since they do not simulate the pump-out mechanism accurately. The mechanical cycling test discussed in this article can be used complementarily to include some of the power cycling pump-out effects.

Grease characteristics (e.g. filler material, rheology, resin system, etc.), process parameters (e.g. grease application method, clamping force and method, etc.), end-use environmental conditions, and geometries (e.g. cooling structure, dimensions of die and spreader/sink, surface roughness, etc.), all have an effect on the thermal performance of a grease layer and its reliability. Hence, there may not be one grease that is the best option for all geometries, processing conditions, and use conditions.

The most accurate results will therefore be obtained when all the candidate greases are applied in the actual package and operated under actual use conditions. However, testing a large number of candidate greases in this manner is not practical and a more efficient method is needed to narrow down to a few promising candidates. A combination of accelerated stress tests using thermomechanically similar and geometrically representative test structures that are discussed in this article is a means to shorten the list of the right materials for an application and a step towards predicting their long term performance.

Acknowledgments

This work was performed under the support of the U.S. Department of Commerce, National Institute of Standards and Technology, and Advanced Technology Program, Cooperative Agreement Number 70NANB2H3034. The authors acknowledge the support of GE Silicones (now Momentive Performance Materials Inc.). The authors also acknowledge the contributions of Don Buckley and Sara Paisner.

References

1. Viswanath, R., Wakharkar, V., Watwe, A., and Lebonheur, V., “Thermal Performance Challenges from Silicon to Systems,” Intel Technology Journal, Q3, 2000.
2. Saums, D, “Developments with Metallic Thermal Interface Materials,” ElectronicsCooling, Vol. 13, No. 2, 2007.
3. Chiu, P-C., Chandran, B., Mello, M., and Kelly, K., “An Accelerated Reliability Test Method to Predict Thermal Grease Pump-Out in Flip-Chip Applications,” Electronic Components and Technology Conference, 2001.
4. Campbell, R. and Smith, S., “Flash Diffusivity Method: A Survey of Capabilities,” ElectronicsCooling, Vol. 8, No. 2, 2002.
5. Gowda, A., Paisner, S., Acharya, A., Meneghetti, P., Hans, P., Nagarkar, K., Tonapi, S., Strosaker, G., and Srihari, K., “Spherical Boron Nitride Fillers For High Performance Thermal Greases,” 7th Electronics Packaging and Technology Conference (EPTC 2005), Singapore, 2005.
6. Hunadi, R. and Wells, R., “Thermal Greases with Exceptionally High Thermal Conductivity and Low Thermal Resistance,” International Symposium on Advanced Packaging Materials, 1998, pp. 198-202.
7. Maguire, L., Behnia, M., and Morrison, G., “Systematic Evaluation of Thermal Interface Materials � A Case Study in High Power Amplifier Design,” Microelectronics and Reliability, vol. 45, 2005, pp. 711-725.

Heat is often considered the limiting factor in the advancement of electronics systems. Lower thermal resistance will drive, not follow, future electronic designs. These solutions must be cost effective, user friendly and developed quickly. Thermal interfaces are usually an afterthought to designs but play a huge factor in the performance and reliability to a device’s operation. Thermal interface materials can be used for heat dissipation thus providing a cost-effective method allowing engineers the flexibility to reduce overall size of the package. Thermal interface materials can help reduce heat sink dimensions as well as the need for larger cooling fans. Thermally conductive materials can be applied in low and high volumes with relative ease. Furthermore, they can act as a dielectric insulator between components to prevent arcing.

Considerations

1. As a start, the components on either side of the thermal joint must be identified. Is it a semiconductor, insulated gate bipolar transistor (IGBT), metal�oxide�semiconductor field-effect transistor (MOSFET), heat sink, cold plate, or something else? The actual components on the board may dictate the type of interface material. If it is a central processing unit (CPU) or graphics processing unit (GPU), a high-performance product would be used. If you are trying to interface components of varying heights, a gap filler would be used. If you are trying to insulate components to prevent arcing, an insulator would be used. Available thermal interface materials (TIM) are:

Phase Change Materials (PCM)
Thermal interface material that softens or liquefies under heat (50 to 100�C) to fill voids on surfaces and lower thermal resistance between surfaces.

Gap Fillers
Products that act to fill a sizeable gap between surfaces (usually 0.254 to 0.508 mm [0.010" to 0.200"]) and lower thermal resistance while replacing air gaps between surfaces with thermally conductive material.

Putties
Extremely soft gap fillers that can be compressed over 50 percent of their original thickness to optimize thermal performance.

Greases
Thermal greases are materials that act to reduce thermal resistance between mating surfaces and can offer lower bond line thickness between two surfaces.

Insulators
Thermal products that electrically insulate between two surfaces while lowering the thermal resistivity.

High-Performance Material
Phase change and grease products that have lower thermal resistance usually below 0.1�C-in²/Watt (0.65�C-cm²/Watt).

2. It is important to determine the amount of heat, usually in Watts, that needs to be dissipated. If you are trying to dissipate a high amount of heat, either a high-performance material or high-performance gap filler could be used. This would eliminate looking at the lower performance materials. The low thermal resistance products are much better if a large amount of heat must be removed from a package.
3. What is the type of clamping device at the thermal joint? Some common ones are bolts, spring clips, Belleville washers, etc. If bolts or screws are being used to join components, the gap fillers or insulators could be used. If it is a fixed gap, the minimum and maximum gap size must be considered. This varies by application and is determined by package design. If the use of a high-performance material is preferred, spring clips or Bellville washers are recommended so that constant, even pressure is applied.
4. What are the interface footprint and dimensions? With larger footprints, a gap filler is preferred so that all voids and surface irregularities are filled. Smaller footprints do not have this concern so high-performance products could be used. In general 25.4 x 25.4 mm (1″ x 1″) is the determining size. There are exceptions to this rule. Basically, if you have a large footprint, say 76.2 x 101.6 mm (3″ x 4″) for example, the gap fillers are better at filling the voids across the entire area without concern for surface flatness and polish. If you have a 0.508 mm (20 mils) thick gap filler, it will make contact at 0.508 mm (20 mils) in one area and 0.381 mm (15 mils) in others. With a high-performance material that is only 0.127 mm (5 mils) or thinner, you may be able to interface from 0.025 to 0.076 mm (1 to 3 mils), but will have difficulty if the gap is 0.102 to 0.127 mm (4 to 5 mils) as the PCM will not remain at 0.127 mm (5 mils) once it flows and greases tend to push out, which hampers clean contact. This is more of a concern on large areas than small areas (for example, 12.7 x 12.7 mm [0.50" x 0.50"]) where the likelihood of such a variance is remote.
5. To deal effectively with the heat at the joint, the temperature range at the interface must be known. The gap fillers are rated for -40 to +200�C. The high-performance materials are rated -25 to +125�C in most cases. Therefore, the material and type of application must run within the same parameters.
6. The clamping pressure at the thermal interface is a factor in determining which material should be used. If the clamping pressure is low 0.14 to 0.34 MPa (20 to 50 PSI), a gap filler performs well. If the pressure is higher (greater than 0.34 MPa [50 PSI]), the high-performance materials may work better. There are exceptions to this application’s specific approach. If too much pressure is applied to a silicone-based gap filler, combined with the addition of heat, a “weeping” of the silicone may occur where it can squeeze out and migrate to other components on the board. If too little pressure is used with a high-performance product, the thermal resistance across the interface will not optimize. If a design is based on a thermal resistance level of X and the actual thermal resistance is Y, heat may not be removed as expected and thermal failures and other thermal issues could result.
7. Is there a pressure limit in the thermal joint due to component attachment conditions such as Ball Grid Array (BGA) and solder limits to prevent fatigue failure? PCB bending stress is another consideration that needs to be addressed. Harder materials may cause warping of the board if placed on a BGA or another delicate component. In this case, there are several materials available that are soft and conformable. They will not stress the board as much as other materials and still can perform fairly well at very low pressures (less than 0.07 MPa [10 PSI]). Putties and very soft gap fillers are recommended for BGA’s since they lower the stress put on leads and the balls are better than traditional gap fillers.
8. The material type, surface finish and flatness of the mating surfaces in the thermal joint are important. With highly polished or machined surfaces (i.e., military, aerospace applications), either type of material, gap fillers or high performance, will work. If a casting or extrusion is used, the gap fillers are better because they can fill voids and other surface imperfections more effectively.
9. Does the interface material require electrical isolation or conduction? There are truly electrically conductive thermal materials available, such as graphitic substances. The gap fillers are electrically isolating. The phase changes and greases are neither rated isolating or conductive. If applied thickly, they will act as an insulator provided there are no voids in the application. If there are voids when applying and metal touches metal, it will provide a conductive path in some instances.
10. Will the interface material be reusable or reworkable? Although it is recommended to replace the interface material after pressure and heat have been applied to it, the gap fillers can be reused in some instances, as a general rule. High-performance materials must always be removed and replaced with new material. The PCM’s are more easily removed when they are still warm, rather than at room temperature.
11. Is the interface in a horizontal plane or vertical mounting plane? The gap fillers are normally better for vertical mounts and high-performance products would work, but consultation with the factory is recommended.
12. If vibration is a concern, such as in mobile applications, gap fillers are better at absorbing shock than the high-performance materials.
13. If it is a space application or to be used in a vacuum can some products be post baked to meet the NASA out-gassing specifications? Yes, materials that meet such requirements are available.
14. If you must meet requirements for silicone extractables, such as Bellcore, there are products available that naturally meet Belcore but are still silicone-based products.
15. In many applications a UL flammability rating is required. Many TIM products are HB and V0 rated. The insulators are 94V0 rated as are most of the high-performance products. There are so many possibilities it is advisable to contact an expert familiar with what is available.
16. Die cutting can lower the cost of the material. For example, squared corners instead of radii can share like rails on the steel rule die and this saves on losses associated with parts that have notches or corners with radii.
17. If the thermal pad requires pressure sensitive adhesive or a naturally tacky side for assembly there are several materials that are naturally tacky. All other products require the introduction of pressure sensitive adhesive to hold parts in place during assembly. Note: The adhesive is not meant for permanent bonding and must be used with some sort of clamping device as well. It is only meant to hold it in place during assembly. Adhesive is available on one side of the part to prevent misuse. Be aware that the adhesive is not thermally conductive and can impede thermal performance depending on material thickness.
18. If the thermal pad needs to stay attached to one side of a joint upon disassembly, some materials can be treated so only one side stays on the heat sink and does not stick to component during disassembly.
19. For some applications the thermal pad must “slide” into the assembly such as a card cage. There are materials that are ideal for these applications that require a sliding motion in and out of a rack because of their natural lubricity. There are also materials available that a carrier added to allow for sliding removal and insertion. The graphitic material is also excellent for sliding in and out of racks due to its naturally lubricious properties
20. Some final considerations relate to any special packaging requirements, such as kiss-cut, rolls or special liners. Special packaging and/or configurations can be supplied as needed to ease use of TIM products. If it is a price-sensitive application you may want to look at the lower performance products as this will reduce cost. If it is a low volume application, price may not be sensitive and a higher performance product may be acceptable if not desired to optimize package performance.

 

Conclusion

Although there are other factors that may come into consideration for selecting thermal interface materials this article concentrates on the top 20 most important factors. Details on product aging, compression set, thermal cycling and other factors can be provided by manufacturers if deemed important for your application.