THERMAL LIVE 2015
October 6 – 8, 2015
Online Event

Featuring practical thermal management techniques and topics, this event includes roundtables, webinars and videos; and there’s no cost to attend.

Program Highlights:

• LED Design
• Data Center Management
• Heat Sink Selection & Design
• JEDEC Update
• Fan Technologies
• Liquid Cooling
• Thermal Interface Materials
• PCB Design Strategies
• Basic Thermal Calculations
• and more…

More Information

The post THERMAL LIVE 2015 appeared first on Electronics Cooling Magazine - Focused on Thermal Management, TIMs, Fans, Heat Sinks, CFD Software, LEDs/Lighting.

Abstract— The focus of this paper is on the development of a novel thermal interface material (TIM) called Vertical Carbon TIM (VCTIM). This composite material consists of a soft polymer matrix and carbon flakes aligned in the direction of the heat flow in the semiconductor package. The VCTIM has high thermal conductivity (measured > 30 W/mK) and low interface thermal resistance. Two different VCTIM formulations were investigated and their thermal performance reliability was evaluated with a set of semiconductor industry standard reliability tests, in particular- bake and highly accelerated stress tests. The first formulation showed significant thermal performance degradation following the reliability stress tests. The mechanism of degradation was understood by evaluating the thermal interfacial resistance and material micro-hardness. This understanding guided the matrix improvements and improved VCTIM formulation development.

Introduction

Flip chip microelectronics packaging includes a variety of connections to the board: land grid array, ball grid array and pin grid array. These packages may have either a single chip or multiple chip processors. For many applications an integrated heat spreader (IHS) is attached to the package to manage thermal performance as shown in Figure 1 [1]. In such applications a thermal interface material is placed between the die and the IHS (TIM1) and between the IHS and the heat sink (TIM2) to ensure adequate heat transfer between the components. A variety of TIMs are used. Typical TIM1 materials include indium solder, particle- filled polymer and elastomeric materials. Typical TIM2 materials include particle-filled greases, phase-change pads and carbon fiber pads [2]-[3]. Package thermo-mechanical needs, surface treatments, reflow temperatures, mechanical load and performance targets all play a role in choosing an adequate TIM [2].

Figure 1 Architecture typically used in desktop and server applications. Legend: I – Heat Sink, II – TIM2, III – IHS, IV – TIM1, V – Die, VI – Underfill, and VII – Package Substrate [1]

A single TIM that could address the package thermo-mechanical requirements is desired by the industry. This TIM should have good thermal performance for use on high power processors and maintain adequate thermal performance through reliability conditions. To achieve this goal, significant efforts have been made to develop TIMs comprising of various forms of carbon based fillers [4]-[9]. One such endeavor was the development of vertically aligned graphitic TIM by Hitachi Chemical, called VCTIM, which will be the subject of this paper.

Material Properties

VCTIM is a highly conductive graphitic carbon based TIM where the orientation of graphitic fillers is achieved through extrusion of graphitic fillers with a matrix material. Although many variants of VCTIM have been developed, this article will focus on two specific formulations, called VCTIM and improved VCTIM, which contain acrylic rubber, polybutadiene rubbers, epoxy and other polymers. Through the manufacturing process the highly thermally conductive graphitic filler is oriented in the thickness direction of the preform (Figure 2) allowing efficient heat transfer in the vertical direction.

Figure 2 SEM cross-section of 250 µm VCTIM preform and a schematic of the carbon fillers alignment.

To characterize the fundamental thermal properties of the VCTIM, data was collected using a metrology based on the ASTM D5470 test method [10]. The total thermal resistance of the TIM can be divided into two components; bulk and interfacial (contact) thermal resistance. The high vertical direction bulk thermal conductivity (Figure 3) is the result of the vertical carbon fiber alignment in VCTIM. The reliability of VCTIM performance under the effects of high temperature was investigated using bake test at 125°C for 100 hours. The samples were baked ex-situ, suspended in an elevated ambient environment, and then analyzed in the tester under the loads of 138 kPa (20psi), 345 kPa (50psi) and 621 kPa (90psi). Figure 3 shows that upon bake stress, the bulk thermal conductivity was decreased.

Figure 3 Bulk thermal conductivity of VCTIM as a function of applied pressure, measured as per the ASTM D5470 standard [9].

The polymer matrix plays a crucial role in ensuring efficient heat transfer through formation of good interfacial contact between the graphitic fillers and the die or IHS. The properties of the matrix material that most impact the thermal performance of a TIM in a package are thermal conductivity, hardness, elastic deformation or compressibility, adhesion, wetting, and stability under reliability testing. VCTIM forms a weak van der Waals interaction with the IHS on contact. Thus, the integrity of the interface is sensitive to package thermo-mechanical changes. During exposure to temperature cycling, the package undergoes dynamic warpage which causes the gap between the die and IHS (which the TIM occupies) to expand and contract in non-enabled configurations (no heat sink enabling load). If the TIM does not expand and contract with the gap changes, this can lead to TIM-IHS delamination at cold temperature and re-formation of the interface at high temperature [3]. Any changes to the exposed surface of VCTIM would be reflected in the degradation of the thermal contact resistance (Figure 4) after exposure to thermal stress (bake at 125°C for 100 hours). The 4x increase in the VCTIM thermal contact resistance at low pressures indicates a lowered ability to effectively transfer the heat across the interface between the thermal interface material and the adjacent surface. Application of higher pressures allows for improved interfacial contact between VCTIM and the adjacent surfaces to occur.

Figure 4 Measured contact thermal resistance of VCTIM for pre and post-bake for various pressures.

It is understood from the thermo-mechanical models and validation experiments [11]-[12] that a softer material has better thermal contact through higher contact area and creates an interface with lower thermal contact resistance. To evaluate the hardness of the VCTIM material, micro-hardness measurements were conducted on VCTIM samples using a Vickers diamond indenter. Figure 5 shows the hardness change of VCTIM formulations during bake at 125° for 100 hours and highly accelerated stress testing at 110°C, 85% relative humidity for 100 hours. VCTIM exhibits an increase of more than 2X post-bake and about 10X post-HAST.

Figure 5 Average micro-hardness measurements comparing improved VCTIM to VCTIM at end-of-line (t0), post-bake and post-HAST.

VCTIM Improvements

The thermal performance degradation of VCTIM upon exposure to different reliability test conditions can be explained by the hydrolysis and oxidation of the acrylic rubber matrix [12]. The moisture and oxygen induced cross-linking of the matrix leads to a harder matrix, as shown in Figure 5, by the increase in the material micro-hardness. Thermal interfacial resistance is due to surface irregularities of the two mating surfaces causing localized regions of good and poor contact. Bulk thermal resistance directly depends on the thermal conductivity of the material and the thermal resistance between the filler and the matrix polymers. Since it is possible that degradation in total thermal resistance of the TIM can be due to a degradation of either or both interfacial and bulk thermal resistance, understanding of which component degradation occurs is important in developing a solution. To address the VCTIM hardening induced thermal performance degradation, alternative matrix polymer formulations were prepared, called improved VCTIM.

Improved VCTIM formulation consists of graphitic fillers vertically aligned in a soft polymer matrix. The effect of the matrix polymer change was evaluated by the micro-hardness measurement and the thermal contact resistance. Micro-hardness analysis (Figure 5) indicates that improved VCTIM formulation exhibits minimal degradation after bake at 125°C for 100 hours and a 2x degradation after highly accelerated stress testing at 110°C, 85% relative humidity for 100 hours. Figure 6 shows the minimal thermal contact resistance and bulk thermal conductivity degradation at low pressures upon the samples ex-situ exposure for 100 hours to 125°C bake stress. Modification of polymer matrix allowed for a significantly reduced thermal performance degradation as compared to VCTIM (Figure 4).

The difference in the reliability performance between the different VCTIM formulations can be explained by the different degradation mechanisms due to the changes to the matrix composition [13]. Improved VCTIM formulation maintains the softness of the matrix to a greater extent, as compared to VCTIM formulation. The greater reliability of the improved VCTIM formulation can be observed for the bulk thermal conductivity, which remains constant at 25 W/mK. Thus, the polymer matrix change allows for the maintenance of adequate thermal performance through reliability conditions.

(a)

(b)

Figure 6 (a) Contact thermal resistance of improved VCTIM samples comparing at end-of-line and post-bake for various pressures. (b) Bulk thermal conductivity of improved VCTIM samples comparing at end-of-line and post-bake for various pressures.

Summary

In this paper, development efforts of vertically aligned carbon-based thermal interface materials were discussed. Innovative manufacturing to vertically orient graphitic fillers in a polymer matrix led to a high bulk thermal conductivity material. The thermal performance and the stability of the same were evaluated for VCTIM. It was determined that thermal performance degradation was due to the polymer matrix hardening upon exposure to reliability testing. Improvements to the VCTIM were achieved by the modification of the polymer matrix which was less susceptible to hydrolysis and oxidation induced hardening, leading to improved reliability of the thermal interface material. The good thermal performances through reliability conditions make VCTIM a promising material to meet the requirements of semiconductor packages.

References

• Mahajan, “Thermal Interface Materials: A Brief Review of Design Characteristics and Materials”, Electronics Cooling, Feb 2004
• C.Tong, “Thermal Management Fundamentals and Design Guides in Electronic Packaging”, Springer Series in Advanced Microelectronics, Volume 30, 2011
• Viswanath, V. Wakharkar, A. Watwe, V. Lebonheur, “Thermal Performance Challenges from Silicon to Systems”, Intel Technology Journal Q3, 2000
• J. Biercuk, M. C. Llaguno, M. Radosavljevic, J.K. Hyun, A. T. Johnsond, and J. E. Fischer, “Carbon nanotube composites for thermal management,” Applied Physics Letters 80, 2667-2769, 2002.
• Xu and T. S. Fisher, “Thermal Contact Conductance Enhancement with Carbon Nanotube Arrays,” Proc. 2004 ASME International Mechanical Engineering Congress & Expo, IMECE2004-60185, 2004.
• Xu, and T. S. Fisher, “Enhanced Thermal Contact Conductance Using Carbon Nanotube Arrays,” ITherm 2004 Proceedings: Ninth Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, Las Vegas, NV, pp 549-555, 2004.
• Berber, Y. K. Kwon, and D. Tomanek, “Unusually high thermal conductivity of carbon nanotubes,” Physical Review Letters 84, 4613-4617, 2000..
• W. Che, T. Cagin, and W. A. Goddard, “Thermal conductivity of carbon nanotubes,” Nanotechnology 11, 65-69, 2000.
• L. Seaman, T. R. Knowles, “Carbon Velvet Thermal Interface Gaskets,” 39^th AIAA Aerospace Sciences Meeting, Reno, NV, 2001
• ASTM Standard D5470-06(2011), 2006, “Standard Test Method for Thermal Transmission Properties of Thermally Conductive Electrical Insulation Materials,” ASTM International, West Conshohocken, PA, 2006, DOI: 10.1520/D5470-06R11, astm.org
• B. Mikic, “Thermal Contact Conductance; Theoretical Consideration,” Int. J. Heat Mass Transfer, Vol.17, pp. 205-214, Pergamon Press, 1974.
• M. Yovanovich, “Four Decades of Research on Thermal Contact, Gap and Joint Resistance in Microelectronics,” IEEE Transactions on Components and Packaging Technologies, Vol. 28, No.2, June 2005, USA.
• Sauciuc, et al., “Carbon Based Thermal Interface Material for High Performance Cooling Applications”, ITHERM May 2014 Proceeding, Orlando FL, USA

Intel^® is a registered trademark of Intel Corporation or its subsidiaries in the United States and other countries.

The post Carbon Based Thermal Interface Material for High Performance Cooling appeared first on Electronics Cooling Magazine - Focused on Thermal Management, TIMs, Fans, Heat Sinks, CFD Software, LEDs/Lighting.

Fujipoly America Corporation has announced its new die-cut thermal interface gaskets. The gaskets easily transfer heat from electronic devices and components to the environment. The die-cut shape allows the gaskets to fit mostly any application.

“When placed between a heat source and a heatsink, the gaskets conform to all uneven surfaces and fill any air gaps. This significantly increases surface contact with the heatsink thereby increasing cooling efficiency. The physical characteristics of this product also allow it to act as a low-pressure mounting cushion to prevent deformation,” according to the company.

The gaskets provide a thermal conductivity between 0.9 and 17.0 W/m°K and a thermal resistance between 0.03 and 3.47 °Cin2/W. The gaskets can be made with many different formulations and users can choose from over 12 thermal interface materials.

The post New Thermal Gaskets Easily Transfer Heat appeared first on Electronics Cooling Magazine - Focused on Thermal Management, TIMs, Fans, Heat Sinks, CFD Software, LEDs/Lighting.

Fujipoly America Corporation introduces its new Sarcon 250G-HF2d thermal interface material. This TIM is made of reinforced nylon mesh and a low-tac surface treatment to prevent die-cut holes from collapsing. The Sarcon 250G-HF2d improves cooling performance “by filling unwanted air gaps between board components and processors thereby increasing surface contact with a heatsink,” according to the company.

The TIM transfers heat with “a thermal conductivity of 1.5 W/m°K per ASTM D2326 and a thermal resistance of 2.61°Cin²/W at 14.5 PSI (16.81°Ccm²/W at 100Kpa),” according to the company.  The Sarcon 250G-HF2d is 2.5 mm thick, flame retardant, and reduces elongation, damage and tearing.

The material can be die-cut or trimmed to fit many different components and different thicknesses are also available from 0.5mm-5.0mm.

On February 5, which was World Nutella Day, Cool Master celebrated the hazelnut-chocolate spread by using Nutella as a thermal paste.

The firm heated the Nutella first and cleaned the CPU before applying the spread. Surprisingly, the Nutella kept the test processor running at a maximum of 50 degrees Celsius for a while. The firm urges not to try this at home, since Nutella would eventually dry out and damage the processor.

Honeywell Electronic Materials (HEM) will make a new product announcement during SEMI-THERM 2015. Honeywell also recently announced its TIMS are being integrated into the production of Video Graphic Array cards to help them stay cool and function better.

Visit Honeywell at booth 404 to hear the announcement.

In addition, Honeywell Electronic Materials and Juniper Networks will host a workshop on Tuesday, March 17, from 4 to 5 p.m. titled “Thermal Interface Material Performance Characterization for High Power Networking Applications.”

For more information click here.

To see the full SEMI-THERM program click here.

Fujipoly America Corporation introduces the Sarcon SG-26SL, a new thermal interface material.

The Sarcon SG-26SL is easy to apply, thermally conductive, silicone-based, and electrically non-conductive. This TIM offers thermal conductivity of 2.6 W/m°K, an operation temperature range of -55 to 205 degrees Celsius, and is ideal for use in thin bond line applications. If the TIM needs to be re-worked, it can easily be removed by wiping it with a cloth.

A new book titled “Thermal Interface Materials 2015-2025: Status, Opportunities and Market Forecasts” has been released.

 “The state of the market in 2015, a geographic breakdown of the market, and forecasts to 2025, are separated by TIM type and by application. These have been compiled after an extensive interview program with thermal interface material manufacturers making a variety of materials, and many different applications, and using financial data published by public companies. Thermal Interface Materials 2015-2025 includes profiles of 29 companies working in this industry.”

The book states that if a thermal interface material (TIM) is used appropriately with a device, it reduces cost, eliminates the need for liquid cooling, reduces system cooling power consumption, reduces building power consumption and increases operational lifetime.

The book also notes that the need for TIMs will always be modern, especially with technologies that are constantly growing in the industry, such as faster computers and electronic devices, greener lighting, and better connectivity. With electronics becoming more portable and compact, they become hotter much faster, thus the need for TIMs may never diminish.

Also summarized in the book are the top ten TIM technologies which include: pressure-sensitive adhesive tapes, thermal adhesives, thermal greases, thermal gels, pastes and liquids, elastomeric pads, phase change materials, graphite, solders and phase change metals, compressible interface materials and liquid metals.

The book addresses current and growing opportunities for TIMs and lists the following markets: LED lighting, telecommunications equipment, automotive electronics, aerospace and defense electronics, consumer electronics, medical electronics, testing equipment, wireless sensor networks and more.

Angstron Materials Inc. has developed a family of thermal foil products ideal for smartphones, and other hand-held devices. The new thermal foil sheets have a thickness of from 5 um to 40 um and thermal conductivity between 800 W/m-K and 1,700 W/m-K. The sheets offer design flexibility and they are available in different grades; they are also cost-effective.

“Miniaturization results in less space to dissipate heat generated from today’s high-performance processors. Efficient thermal interface materials are critical for current and next generation devices due to the growing demand for greater functionality in smaller spaces like the new Apple Watch,” Ian Fuller, Angstron’s application engineering manager, said.

These thermal sheets are ideal for use in EMI Shielding, smartphones, tablets, laptops, flatscreen TVs and other applications.

Fujipoly is hosting a gift giveaway. On June 5, an engineer will win his/her choice of a free Samsung Galaxy Tab Pro, Apple iPhone 6, or a $500 American Express Giftcard, in Fujipoly America Corporation’s Free Gift Giveaway.

To enter, name one of Fujipoly’s thermal interface materials or elastomeric connector products. Fill out a short entry form before May 29 for a chance to win. Only one entry per person is allowed, but the number of people who can enter from the same company is unlimited.

Complete the entry form here.

By: Chandan K. Roy¹, Sushil Bhavnani¹, Michael C. Hamilton², R. Wayne Johnson³,

Roy W. Knight¹, & Daniel K. Harris¹

¹ Department of Mechanical Engineering, Auburn University, Auburn, AL

² Department of Electrical and Computer Engineering, Auburn University, Auburn, AL

³Department of Electrical and Computer Engineering, Tennessee Tech University, Cookeville, TN

Introduction

As the power density of microelectronics steadily increases, thermal management continues to play a key role in the continuous march towards miniaturization, performance improvements, and higher reliability demands [1]. One crucial thermal management consideration in electronics packaging involves reducing the thermal interfacial resistance between devices and their adjoining heat sink or substrate through improvements in thermal interface materials (TIMs). In a high power package, the thermal resistance of the TIM can account for as much as 50% of the overall thermal resistance [2]. Any desirable TIM candidate material would need to have a high thermal conductivity, low thermal resistance at a thin bond line thickness (BLT), excellent wetting properties, and all at a low cost while also being environmental and health friendly [3]. In addition to thermal considerations, mechanical considerations of TIMs are equally important to the performance and health of high power electronics. TIMs can be either compliant (e.g. greases and gels) or non-compliant (e.g. solders and adhesives), where the former bears no interstitial mechanical stress due to thermally induced strains. Non-compliant TIMS must be able to withstand the mechanical stresses from coefficient of thermal expansion (CTE) mismatches between the adjoining materials (e.g. silicon – copper). If the CTE strain overwhelms the mechanical properties of the TIM, the joint will ultimately fail. Therefore, high performing compliant TIMS are a desirable design option for better thermal performance and improved reliability.

Commonly used commercially available TIMs include greases, phase change materials (PCM), gels, and pads, which are polymer based materials loaded with conductive particles (metal or ceramic) to enhance the thermal conductivity. Greases are the most widely used TIMs. However, greases are known to be messy, difficult to apply and remove due to their high viscosity, and have reliability issues such as pump out, phase separations, and dryout, which can limit the use of greases as a reliable TIM [3-4]. Carbon-based materials such as carbon nanotubes [5] and graphene [6] have been investigated by many researchers for use in TIMs. The thermal performance of different commercial and emerging TIMs based on previous research works is presented in Figure 1.

Figure 1 – Thermal performance comparison of variety of commercial TIMs with emerging TIMs such as CNTs and LMAs [4,5,7,10,12,13].

Figure 2 – (a) Testing of LMA TIM using modified test rig; the copper disks assembly with the TIM at the interface was placed between the TIM tester surfaces (b) Schematic diagram of the testing methodology.

Although LMAs offer a low thermal resistance, there are several concerns such as oxidation/corrosion, intermetallic growth, dewetting and migration. Several investigators have offered different approaches to mitigating these concerns [8-10]. In this work, three alloys were chosen to test their performance as TIMs. The properties of these alloys are presented in Table 1. These alloys were selected for study because they have a wide range of MPs (from 16^oC to 60^oC) spanning the range of interests for most application in today’s markets.

Experimental

Description of the apparatus

The thermal performances of the LMAs reported here were generated using an ASTM D5470 standard TIM tester. The detailed specifications of the apparatus used can be found in reference [14]. To avoid any contamination of the TIM tester surfaces and to improve the accuracy of the test results, the LMAs were tested by placing them between copper disks (alloy 110) of thickness 3.2 mm and 33 mm in diameter. The disks are both sides polished with a surface flatness within 7-8 microns. This high level of surface finish was needed for accurate thermal performance measurement but it is not required in actual application. However, it should be noted that the quality of the mating surfaces directly affects interfacial thermal performance. The resulting disk assembly with the alloy at the interface was then placed under the tester, Figure 2. Silicone oil (Xiameter PMX-200, viscosity: 1000CS) was applied on the top and bottom surfaces of the adjoining copper disks to ensure reproducible contact between the test surfaces of the TIM tester and the copper disks. The temperature differential (ΔT) across the LMA bond line was measured using two high precision thermistor probes (1 mm dia., accuracy 0.05^oC) inserted in the middle of each copper disk. The hole was filled with thermal grease (laird tech. Tgrease 880) to ensure reproducibility of the measurements.

Sample preparation and diffusion barriers

The oxide layers on the copper disk surfaces were first chemically removed using methanol, acetone, and a 5% hydrochloric acid solution prior to application of the LMAs. This method of surface preparation was sufficient for testing purpose, but not necessary for normal applications. To enhance the wetting, the disk surfaces were mechanically scrubbed with the alloy using cotton swabs. In the case of Ga and In-Bi-Sn alloy, both the alloy and the disk were heated above the melting point of the alloy after which the molten metal was applied onto the heated disk’s surface using a brisk mechanical rubbing application technique.

Since LMAs are known to react with and form intermetallics with Cu [8-10], surface treatments were used in order to retard the diffusion of the alloy components into the disks. Hence, a thin metallic barrier layer was applied on the copper disk surfaces by sputtering. As tungsten (W) provides superior protection against gallium (Ga) and Ga-based alloys, the copper disks were coated with 2 mm W. For better adhesion of the W, a 50 nm layer of titanium (Ti) was first applied.

RESULTS

In-Situ Thermal resistance

The thermal resistances of six different substrate-alloy combinations is presented in Figure 3.

Figure 3 – In-situ thermal resistances of three alloys between Cu and W surfaces at 138 kPa.

Three samples of each combination were tested at 138 kPa. The calculated experimental uncertainties [15] are represented by the error bars. Each resistance value presented here is the average of three repeated measurements. The measured lowest resistance was as low as 0.005 cm²°C/W with W/In-Bi-Sn/W (In-Bi-Sn alloy between W coated surfaces) and as high as 0.065 cm²°C/W with Cu/In-Bi-Sn/Cu (alloy 3 between bare Cu surfaces). The variation in thermal resistance for the similar substrate-alloy combination results from the unique nature of each sample. Since LMAs are highly conductive and the joints are relatively thin (SEM cross-sectional analysis reveals that the BLT of Cu/Ga-In/Cu joint is about 37 µm), the contact resistances (mostly due to surface irregularities such as surface roughness and flatness) dominate the interfacial resistance. Even a small change in surface properties would result in observable changes in the overall thermal resistance. It was assumed that all the disks had the same degree of surface roughness and flatness, but this is not valid in reality. It should be noted that it was not possible to maintain the exact amount of LMAs at the interface for each pair of disks during testing. Another source of variation might appear from the mechanical rubbing (wetting) of LMAs onto the disk surfaces. During the wetting process, it was found that some samples were more easily wetted while others required hard scrubbing to induce wetting. If the LMAs do not wet the mating surfaces properly, small air pockets might be present at the interface, which in turn increases the thermal resistance. Other properties of the alloys such as viscosity, surface tension, and oxidation state during application may have caused some variation in the thermal resistance measurements. Considering all these factors, each disks pair is different and it is a challenge to reproduce the result even with similar substrate-alloy combination.

Figure 4 – Thermal resistances of variety of commercial TIMs and LMAs as a function of applied pressure.

The thermal performance of LMAs and some commonly used commercial TIMs are tested at different pressures using the same apparatus and similar methodology and measurements are compared (Figure 4). The results indicate that the thermal resistances of LMAs are independent of applied pressures in the range 69-345 kPa. This study concludes that LMAs offer excellent thermal bond even at a small pressure. None of the commercial TIMs were found to have a thermal joint resistance as low as the resistances of LMAs. Current high conductivity greases can offer resistances at the high end of what LMAs offer. The thermal resistances of the commercial TIMs tested were found to be as low as 0.065 cm^2oC/W and as high as 2 cm^2oC/W depending on the type of material being tested.

Isothermal aging

Accelerated aging was carried out by exposing the samples at an elevated temperature of 130^oC (followed DARPA’s Thermal Management Technologies (TMT) program’s reliability profile [16]) in an atmospheric furnace for extended periods of time with no additional pressure and/or constrain during aging. The thermal aging test results of Ga between different substrate surfaces (Cu and W) and Ga-In and In-Bi-Sn alloys between Cu surfaces are presented in Figure 5.

Figure 5 – Isothermal aging (at 130°C) of three alloys as a function of aging time.

The thermal resistances are normalized by the initial resistance values. It was found that the thermal resistance of all three alloys between bare Cu remained unchanged after long periods of exposure (1500 hours for Ga-In alloy), which indicated a stable thermal joint. Ga between W surfaces also showed a negligible change in thermal resistance up to 576 hours of aging. However, the resistance started to increase rapidly thereafter. The thermal resistance increased by about 220% after 1152 hours of aging. The superior aging performance of all three alloys between bare Cu surfaces is believed to be due to the enhanced wetting of the alloys with Cu and the diffusion of the alloy into the Cu substrate. Since the alloys contain Ga and In, they will most likely diffuse into Cu and form intermetallics [9,10]. Our results suggested that any diffusion and formation of these intermetallics that may have occurred did not negatively impact the thermal performance over time. Since the thin layer of W acted as a diffusion barrier, the alloy could not interact with the underlying Cu substrate, thereby, causing the thermal resistance to increase significantly upon aging.

Thermal cycling

The purpose of these tests is to cycle each sample above and below the melting point of the interfacial alloy. The Cu disk assembly with alloys at the interface was placed in a thermal cycling chamber to cycle from -40^oC to 80^oC [16] with no additional pressure and/or constrain. The heating and cooling ramp rates were 3^oC per minute. The samples were soaked for 20 minutes at the two extreme temperature plateaus, ensuring that the interface reached the chamber temperature. The time needed to complete a thermal cycle was two hours.  Figure 6 shows the thermal cycling results of all three alloys between bare Cu and W surfaces. The results showed that the thermal resistance of all three alloys between bare Cu did not change significantly even after 600 cycles. However, alloys placed between W coated surfaces, the resistance increased significantly just after 200 cycles. It can be noticed from the aging studies (Figure 5) that the thermal resistance of Ga between W surfaces started to increase after 576 hours. However, the resistance increased significantly just after 200 cycles (400 hours). This is because, during aging tests, the samples were always kept at a constant temperature, whereas during cycling, the samples experienced thermal expansion/contraction resulting from the heating and cooling of the joint. All these results suggest that the bare Cu-alloy surface combination makes a more reliable LMA thermal joint.

SUMMARY AND CONCLUSIONS

The application of LMAs as efficient TIMs has been investigated herein. Three alloys (Ga-In, Ga, In-Bi-Sn) with different melting points and various compositions were chosen to test the thermal performance. The thermal resistances of the alloys were tested by placing them between bare Cu and W coated Cu substrates using the ASTM D5470 standard methodology. The in-situ thermal performance shows that LMAs can offer extremely lower thermal resistances at small contact pressures compared to many commercial TIMs. Accelerated tests include isothermal aging at 130^oC and thermal cycling from -40^oC to 80^oC. It was observed that all the alloys between bare Cu surfaces survived both isothermal aging and thermal cycling. However, alloys applied between W coated surfaces failed to withstand both aging as well as cycling. The results indicate that any Cu-alloy interactions (which is primarily the diffusion and interfacial reaction) produce a more reliable thermal joint.

Figure 6 – Thermal cycling (-40°C to 80°C, 3°C/min, dwell: 20 minutes, 2 hours/cycle) of three alloys between Cu and W surfaces.

In actual processor-heat sink application, the LMAs are placed between two dissimilar materials, typically silicon (die) and copper (heat sink). In this work, LMAs are found to be effective when applied between bare Cu substrates. LMAs are thin liquids in molten form and they can accommodate any thermomechanical stress that results from the CTE mismatch of different materials. Therefore, it is presumed that they will continue to perform reliably between dissimilar material joints. LMAs are environmental and health friendly unless they contain any mercury, lead or cadmium compounds. They are easy to apply and commercially available, thus manufacturing at high volume would not be a concern. Many widely used commercial TIMs are greases or highly solid-loaded pastes that have a high viscosity and in some cases shear harden which makes these types of TIMs more difficult to apply and rework. LMAs are not only more compliant and less viscous, but do not shear harden. They are also cost-effective compared to other TIMs and have been used in many commercial applications [9]. As LMAs have very low viscosity compared to greases, only a small amount (6-9 mg/cm²) is required to wet the mating surfaces, which makes LMA interface at least 25% cheaper compared to commercial greases. However, the mechanical rubbing (wetting) of the alloys needs to be addressed considering mass application.

REFERENCES

• Bloschock Kristen P., Bar-Cohen, Avram, “Advanced Thermal Management Technologies for Defense Electronics, ” Defense Transformation and Net-Centric Systems 2012, SPIE Defense, Security, and Sensing, 23-27 April 2012 in Baltimore, Maryland
• Guenin, Bruce. “Use of the Monte Carlo method in packaging thermal calculations.” Electronics Cooling Magazine, December 2014 issue.
• Gwinn, Joshua P., and R. L. Webb. “Performance and testing of thermal interface materials.” Microelectronics Journal 34.3 (2003): 215-222
• Prasher, Ravi. “Thermal interface materials: historical perspective, status, and future directions.” Proceedings of the IEEE 94.8 (2006): 1571-1586.
• Cola, Baratunde A. “Carbon nanotubes as high performance thermal interface materials.” Electron. Cooling Mag 16.1 (2010): 10-15.
• Shahil, Khan MF, and Alexander A. Balandin. “Graphene–multilayer graphene nanocomposites as highly efficient thermal interface materials.” Nano letters12.2 (2012): 861-867
• Hamdan, A., et al. “Characterization of a liquid–metal microdroplet thermal interface material.” Experimental Thermal and Fluid Science 35.7 (2011): 1250-1254.
• Webb, R.L., Gwinn, J.P. “Low melting point thermal interface material.” ITHERM. 2002. 671-676.
• Hill, R.F., and Strader, J.L. “Practical utilization of low melting alloy thermal interface materials.” Semiconductor Thermal Measurement and Management Symposium, 2006 IEEE Twenty-Second Annual IEEE. Dallas, TX, 2006. 23-27.Macris, C.G. et al. “Performance, Reliability, and Approaches Using a Low Melt Alloy as a Thermal Interface Material.” IMAPS, 37th Int. symposium on Microelectronics. Long Beach, Ca, 2004
• Macris, C.G. et al. “Performance, Reliability, and Approaches Using a Low Melt Alloy as a Thermal Interface Material.” IMAPS, 37th Int. symposium on Microelectronics. Long Beach, Ca, 2004
• Carlberg, B. et al. “Nanostructured polymer-metal composite for thermal interface material application.” 58th Electronics Components and Technology Conference (ECTC). 2008. 191-197.
• Wasniewski, Joseph R., et al. “Characterization of metallically bonded carbon nanotube-based thermal interface materials using a high accuracy 1D steady-state technique.” Journal of Electronic Packaging 134.2 (2012): 020901.
• Blazej, D. “Thermal Interface Materials.” Electronics Cooling. 9.4 (2003): 14-20.
• Analysis Tech. Material Thermal Testers: TIM Tester 1400 Specifications. 2012. 18 April 2014.
• Roy, Chandan K., et al. “Investigation into the application of low melting temperature alloys as wet thermal interface materials.” International Journal of Heat and Mass Transfer85 (2015): 996-1002.
• DeVoto, Douglas, et al. “Thermal performance and reliability characterization of bonded interface materials (BIMs).” Thermal and Thermomechanical Phenomena in Electronic Systems (ITherm), 2014 IEEE Intersociety Conference on. IEEE, 2014.

Advanced Thermal Solutions, Inc. introduces the clipKIT – a heat sink attachment system available from Digi-Key. The clipKIT can be used with mostly any industry standard heat sink, such as straight fin, pin fin, cross cut, and slant fin heat sinks.

The clipKIT system improves thermal transfers, TIM performance, and vibration and shock resistance by 20 percent, according to the company.

“ClipKIT is a line of patented, reliable heat sink attachment assemblies featuring the widely used maxiGRIP and superGRIP frame clip and spring clip. They are available for 15 – 45mm component packages and are designed for heat sinks with a base thickness of 1.75 to 4mm,” according to the company.

The clipKIT reduces the need for epoxies and thermal tapes and eliminates the need to drill holes; thus eliminating possible damage to PCBs while also reducing time, space, and cost. It also offers a secure hold, increases reliability, and has corrosion-resistant and flame retardant plastic frame clips and stainless steel spring clips. It also passes the Telecordia shock and vibration standards.

Dow Corning introduces a next generation thermal interface material (TIM 1) called the TC-3040 – which is a thermally conductive gel that delivers double the thermal performance of any other TIM in the industry.

“TIM-1 solutions are a class of high-purity, thermal interface materials that are applied between the chip surface and a heat spreader to help dissipate damaging heat to the exterior of a semiconductor package,” according to EFY Times.

“Developed through the help of IBM, this cutting-edge new material offers more effective and reliable thermal management, reduced stress and excellent under-die coverage for demanding flip chip applications,” according to the company. It also offers a high thermal conductivity of 4W/mK.

“A long-time member of IBM’s ecosystem, Dow Corning brought decades of expertise in advanced silicone technology to help formulate this break-through TIM-1 material for high-end chip packaging. It’s the latest innovation on the roadmap of thermal management solutions Dow Corning has planned for this rapidly evolving global market,” Andrew Ho, global market segment leader at Dow Corning, added.

Techsil introduces two new thermally conductive and pressure sensitive adhesive tapes for thermal management of electronic systems, the Stokvis 202331 and 202332. The tapes are double sided and provide efficient cooling solutions by dissipating heat away from sensitive electronic components quickly. The tapes are ideal for bonding heat sinks on PCBs, LED strips and IC packages, heat sink and heat pipe assembly, and mounting of ICs and GPUs.

The tapes offer advanced heat resistance, age resistance, weatherability, high thermal conductivity and a service temperature range of -40 to +105 degrees Celsius. Stokvis 202331 and 202332 are also made of acrylic foam, and are flame retardant and halogen-free.

“Providing excellent thermal coupling between components and heat sinks – their foam-like, compliant interface means that they fill any air gaps thereby reducing the thermal resistance at the interface and can accommodate materials of different coefficients of thermal expansion,” according to the company.

For more information click here.

Aavid Thermalloy introduces three new lines of high-performance thermal interface materials – thermally conductive gap fillers. The three lines include the SuperThermal™, SoftFlex™, and WaveBlocker™. These product lines offer very high thermal conductivity without compromising workability or compliancy.

“The new lines of thermal gap fillers are designed to increase the performance and capabilities of electronics’ design. Each is specially formulated to support specific application requirements that, when combined with their high thermal conductivities, will significantly improve functionality,” according to the company.

The SuperThermal™ line offers high thermal conductivity up to 13.2 watts per meter Kelvin.

“The average thermal conductivity for SuperThermal™ gap fillers is such that utilizing these pads allows engineers to use smaller thermal solutions or considerably increase performance without increasing the volume or weight of the solution or device,” Aavid Thermalloy added.

The SoftFlex™ line is manufactured with five different materials that vary in conductivity, compliancy, cushioning ability and elasticity. Because of its softness and compressibility, the SoftFlex line is the best option for use in applications with increased shock or vibration and to decrease board strain. It was designed for applications with multiple device heights and to offer more design flexibility.

The WaveBlocker™ line offers gap fillers with high electromagnetic permeability, which dampens electromagnetic interference (EMI). These gap fillers offer high signal accuracy and are best suited for use in audio equipment, radios and testing equipment. This line is ideal for use in Aerospace and Military applications.

Fujipoly America Corporation introduces an assortment of TIMs for commercial and military vehicle ECUs and embedded systems cooling.

“The company manufactures its proprietary Sarcon® material in many different forms, thicknesses and shapes to satisfy nearly any application requirement.  These silicone based options include sheets, thin films, putty, and form-in-place compounds,” according to the company.

The new TIMs offer high performance, a low manufacturing cost, a thermal conductivity range from 0.9 to 17.0 W/m°K and a thermal resistance of .02°Cin2/W at 14.5 PSI.

Henkel introduces the first ever low-stress thermal interface material with EMI absorption capabilities, known as the Gap Pad EMI 1.0.

“The latest in its line of BERGQUIST GAP PAD products, Henkel’s GAP PAD EMI 1.0 offers electronic specialists critical heat and electromagnetic energy control in a flexible, gap filling product designed to exhibit exceptionally low stress,” the company said.

“Today’s electronics devices are smaller and higher-functioning than ever before. In addition, widely recognized industry standards defined to control applications that use multiple frequencies dictate effective EMI and heat transfer management for end product acceptance and reliability.  These facts were the driving force behind the development of GAP PAD EMI 1.0,” Doug Dixon, Henkel’s global marketing director, said.

The gap pad is ideal for use in consumer, electronics, telecommunications, LEDs and automotive applications.

“Not only does GAP PAD EMI 1.0 technology offer superb thermal and EMI performance, but the material is the softest and most compliant on the market.  Its ability to conform to various topographies and provide a high degree of flexibility ensures exceptionally low stress on solder joints,” the company reported. The GAP PAD EMI 1.0 reduces joint stress and fractures that lead to in-field failures.

The TIM also offers a thermal conductivity of 1.0 W/m-K, EMI absorption for frequencies above 1GHz, robust thermal management control, high levels of EMI protection, and more. It is also available in various sizes and thicknesses as well as in both die-cut of sheet forms. The adhesive can also be reworked easily.

Honeywell Electronic Materials has announced the availability of a new thermal management material for semiconductors – the Honeywell PTM6000 Phase Change Material (PCM). The PTM6000 was designed to beet both high performance needs and long lasting reliability.

“Honeywell’s proven PTM and PCM series of thermal management materials are based on sophisticated phase-change chemistry and advanced filler technology that was developed specifically for high-performing electronic devices. The TIMs technology transfers thermal energy from the chip to the heat sink or spreader, where it is dissipated into the surrounding environment,” according to the company.

“This functionality keeps the chip cool while allowing the heat sink module to perform optimally. The company’s patented formulations provide long-lasting chemical and mechanical stability, enabling consistently higher thermal performance compared with alternative thermal interface materials that break down or dry out,” the company added.

Fujipoly recently introduced the Sarcon® GR80A, which is a soft thermal interface material that also has a low thermal resistance.

According to the company, the thermal interface material is available in sheets from 0.3mm to 3.0mm thick and can be die-cut to fit almost any application shape.

“When placed between a heat source such as a semiconductor and a nearby heat sink, this TIM provides a thermal conductivity of 13.0 W/m°K per ASTM D5470 (8.0 W/m°K per ISO/CD 22007-2 Hot disk)  and a thermal resistance as low as 0.50°C cm²/W,” Fujipoly added.

“The silicone-based material completely fills air gaps between uneven components, board protrusions and recessed areas while exhibiting very low pressure,” the company stated.

Fujipoly America has just introduced its newest high-performance, low compression force putty-like thermal interface material, Sarcon PG80A.

The TIM’s “13 W/m°K gap filler pad gently conforms to most component shapes and uneven surfaces to transfer heat from its source to a nearby heat sink or spreader while exhibiting a thermal resistance as low as 0.08°Cin2/W at 14 PSI,” says the company.

It is recommended for “applications that have delicate or wide-variation component heights and require material compression between 30% and 90%,” and is “well-suited for environments with operating temperatures that range from -40 to +150°,” according to Fujipoly.

Sarcon PG80A is available in four sheet thicknesses (0.5, 1.0, 1.5 and 2.0mm) up to a maximum dimension of 300mm x 200mm, can be ordered in die-cut form, and exhibits a UL94 flame retardant rating of V-0, as reported by the company.