Enhanced thermal conductivity of polymer composites ...

Composites: Part A 37 (2006) 727–734 www.elsevier.com/locate/compositesa

Enhanced thermal conductivity of polymer composites filled with hybrid filler Geon-Woong Leea,*, Min Parka, Junkyung Kima, Jae Ik Leeb, Ho Gyu Yoonb a

Polymer Hybrid Research Center, Korea Institute of Science and Technology, Seoul, South Korea b Division of Materials Science and Engineering, Korea University, Seoul, South Korea Received 19 January 2005; revised 29 June 2005; accepted 1 July 2005

Abstract This study aims at investigating package materials based on polymer matrix for microelectronics. The next generation package materials are expected to possess high heat dissipation capability in addition to low coefficient of thermal expansion (CTE) as the accumulated heat from high performance electronic devices should be removed for proper operation. In this study, various inorganic fillers including aluminum nitride (AlN), wollastonite, silicon carbide whisker (SiC) and boron nitride (BN) with different shape and size were used alone or in combination to prepare thermally conductive polymer composites. In case of AlN, titanate coupling agent was used for the surface treatment of fillers. The use of hybrid filler was found to be effective in increasing thermal conductivity of the composite probably due to the enhanced connectivity offered by structuring filler with high aspect ratio in hybrid filler. For given filler loading, the use of larger particle and surface treated filler resulted in composite materials with enhanced thermal conductivity. The surface treatment of filler also allowed producing the composites with lower CTE. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Hybrid; B. Thermal properties; D. Thermal analysis; E. Powder processing

1. Introduction As the demands in denser and faster circuits intensify, the heat dissipation in microelectronic packaging is becoming increasingly important. Indeed, the inability to adequately conduct heat away from the chip has imposed another engineering constraint in many new product designs [1]. Traditionally, thermal problem in encapsulated devices has been addressed by the use of high cost embedded heat sinks, which are often susceptible to thermal cracking and of limited utility in thinner packages [2]. Under this circumstance, polymers filled with thermally conductive fillers are emerging as a cost effective ways to cope with thermal management issues [3].

* Corresponding author. Address: School of Polymer, Textile and Fiber Engineering, Georgia Institute of Technology, 801 Ferst Drive, Atlanta GA 30332-0295, USA. Tel.: C1 404 385 4041; fax: C1 404 894 9766. E-mail address: [email protected] (G.-W. Lee).

1359-835X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2005.07.006

Epoxy molding compounds consisting of 30–45 vol% epoxy and 55–70 vol% fused silica are currently typical encapsulants for most large chips, small package devices. Though this cost effective encapsulant provides excellent balanced properties in terms of low coefficient of thermal expansion (CTE), high moisture resistance and mechanical characteristics, it is a rather poor thermal conductor (0.5–1.0 W/mK) due to the low thermal conductivity of the fused silica (1.5 W/mK) [1]. Therefore, the incorporation of highly thermal conductive fillers in polymers to develop high performance thermal conductive encapsulants or thermal interface materials has been mostly desired [4,5]. Bujard [6] reported thermal conductivity value of 2.3 W/ mK for epoxy resin filled with boron nitride at its maximum loading of 31 vol% which is determined by the shape and the size distribution of the filler. Using this maximum filler loading approach, he also achieved very high thermal conductivity of 4.2 W/mK for 62 vol% aluminum nitride filled epoxy [7] and 4.5 W/mK at 80 vol% filled epoxy [3]. Theoretically, the thermal resistance is caused by phonon scattering [8], thus it has to be minimized to increase thermal conductivity. Materials with high thermal conductivities can be obtained by using fillers with high intrinsic

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G.-W. Lee et al. / Composites: Part A 37 (2006) 727–734

material simulation. In addition, we also tried to minimize the interfacial phonon scattering by chemical surface treatment of the filler, which would be helpful to improve the wettability and uniform dispersion.

Relative thermal conductivity, kc/km

100 kf/km = 10

50 40 30

kf/km = 100 kf/km = 1000

20 10

2. Experimental

5 4 3

2.1. Materials

2 1 0.0

0.2

0.4

0.8

0.6

Volume fraction of filler

Fig. 1. Relative thermal conductivity as a function of filler volume fraction (Kf, thermal conductivity of filler, Km, thermal conductivity of matrix or polymer) calculated by Neilsen equation (AZ1.5, fmaxZ0.637, for randomly packed spherical particles).

conductivities. However, as discussed by Bigg [9], when the intrinsic thermal conductivity of the filler is greater than 100 times that of the polymer matrix, there is no significant improvement in the thermal conductivity of the composite. This phenomenon is illustrated in Fig. 1. On the contrary, the aspect ratio of the filler is more considerable that dictates the conductivities of a composite, because the fillers with large aspect ratios easily form the bridges between them, known as conductive network. The formation of random bridges or networks from conductive particles facilitates electron and phonon transfer leading to high conductivities. Recently, we have reported that the effect of aspect ratios of fillers on thermal conductivity of composites [10]. In addition, there are practical limits in loading maximum packing fraction, which often causes problems in terms of processibility as well as mechanical performance. This implies that hybrid filler system with large formation of networks is necessary for the preparation of the next generation heat dissipation materials. In this paper, we report that the effectiveness of hybrid filler consisting of different conductive fillers in type and shape on the fabrication of thermally conductive composites. We attempted to maximize the abundance of thermally conducting paths by using hybrid filler at its maximum packing loading, which is determined experimentally or by

Aluminum nitride (AlN), silicon carbide (SiC) whisker, boron nitride (BN) supplied by ART (USA) and wollastonite purchased from Sung-ji Int. (Korea) are used as thermally conductive fillers. The matrix is a powdered high density polyethylene of molecular weight of 300,000 supplied by Yu-Hwa (Korea). The properties of matrix polymer and fillers are listed in Table 1. The shape of AlN is roughly spherical and that of SiC whisker and wollastonite is acicular, and BN is plate-like shape. 2.2. Surface modification We used titanate coupling agent (KR-138S, Ajinomoto, Japan) with chemical structure as below in order to ensure good dispersion and improvement of interface between filler and matrix. O O Ti H 2C O C

O O

P

O O

P

O C 8H 17

2

2

OH

The surface modification was carried out as follows: (i) mixing fillers and titanate coupling agent in isopropyl alcohol at 80 8C for 2 h, (ii) vacuum drying to remove solvent at 100 8C for 24 h. The treated fillers were stored at 80 8C in order to protect the filler surfaces from water. 2.3. Composite sample preparation Both fillers and matrix were vacuum dried before mixing at 100 8C for 6 h and at 60 8C for 6 h, respectively. The composites were prepared as follows: (i) mixing fillers and HDPE using Henschel type mixer (Hung-bo Tech, Korea) at 4000 rpm for 15 min, (ii) for low to intermediate filler

Table 1 Properties of matrix and conductive fillers Materials

Matrix

Filler

HDPE AlN Wollastonite SiC BN

A-100 A-500

Density (g/cm3)

CTE (ppm/8C)

Particle diameter (mm)

Particle length (mm)

Thermal conductivity (W/ mK) at 30 8C

0.945 3.26

198 4.3

!150 4 20–25 2 0.7–1 4–5

– – – 40–50 10–15 –

0.45–0.55 150–220

2.8 3.2 2.27

5.4 4.7 4.3

2.5 85 29


Foam Metal

0.0125

Voltage (mV)

content (10, 30 and 50 vol%), injection molding using Babyplast (Spain) at 250 8C was employed for sample preparation, however, for high filler content (60 vol%) where injection molding was not feasible, compression molding at 200 8C using tetrahedron (USA) was used. The dimensions of specimen injection molded or machined after compression were 65 mm!10 mm! 3 mm. The compositions of AlN(A-100)/Wollastonite and AlN(A-500)/SiC in hybrid filler systems respectively determined by Milewski’s packing concept [11] and materials simulation by MacroPac (Oxmat, UK) were 70:30 and 83.5:16.5 by volume.

0.0120

0.0115

0.0110

0.0105 0

where, k is thermal conductivity (W/mK), r is the density (kg/m3) and Cp is the heat capacity (J/kg K). In order to ensure thermal contact between sample and the probe, thermal grease (YG-6111, Toshiba, Japan) was applied to the polished flat surface to reduce thermal contact resistance between specimens and the probe. In measuring the thermal conductivity by TC Probe, the duration time has crucial influences on the thermal conductivity of specimens. Duration time is the time period for which the probe provides heat with specimens. If duration time is too long or short, the exact value of thermal conductivity cannot be measured. The accurate duration time was obtained by the blotter method, which enables the measurement of thermal conductivity without knowing the density and heat capacity values of the sample [14–16]. After instrument was calibrated with known standards as described elsewhere, two tests are run on each sample: one with the blotter being a metal materials and the other with the blotter being an insulating foam. As the testing progresses in time, the heat wave passes through the film. During the time period of the heat wave being resident in the sample, the temperature detector is not aware of the blotting material so the slope of the data is the same for the two tests. However, after the heat wave penetrates far enough into the

1

2

3

Duration Time (sec)

2.4. Characterization The maximum packing fraction of hybrid filler of varying composition was determined by sedimentation method. The procedure was as follows. After pouring 10 ml of water into measuring cylinder, the height of water was measured. Fillers were poured into measuring cylinder until the first measured height is reached. Maximum packing fraction was obtained by measuring the increased height. The thermal conductivity was measured on TC Probe (Mathis Instrumentals Ltd) equipment which is one of modified hot wire methods (ASTM C1113) [12–14]. In TC probe, measuring area was 10 mm!30 mm with 2 mm thickness for three different samples at flatten surface after polishing. The TC probe measures the effusivity (E) of a material, which is: qffiffiffiffiffiffiffiffiffiffi E Z krCp

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Fig. 2. The determination of test duration using blotters (voltage is equivalent to temperature).

sample to reach the boundary, the measured slope increases for the insulating foam and decreases for the conductive metal (Fig. 2). By assisting blotter method, thermal conductivity can be measured as follows: kZ

0:707dE pffi t

where, d is thickness of sample, t is deviation time. CTE measurements were performed on a Thermal Mechanical Analyzer (TMA) (TMA7, Perkin–Elmer, USA). The samples mounted on TMA were heated from 30 to 100 8C at a heating rate of 5 8C/min. From the slope of the plot between thermal expansion and temperature, the CTE values were determined for at least two different samples.

3. Results and discussion 3.1. Determination of maximum packing fraction Though the use of hybrid filler consisting of the same fillers with different shape was attempted to yield very high thermal conductivity [17], the composition of the hybrid filler was not determined on basis of scientific principle. Since, for given filler system the maximum amount of thermally conductive paths would be attainable for the composites filled at or near its maximum packing fraction, their rational determination is of great importance. In this study, we determined the composition of hybrid filler system of AlN(A-100)/wollastonite and AlN(A-500)/SiC using Milewski’s packing principle and MacroPac simulation, respectively. Milewski tabulated the synergistic combination of fillers of different shape and Table 2 is the part of it [11]. In Table 2, R denotes the ratio of diameter of spherical filler to that of acicular filler. In case of AlN(A-100)/wollastonite where aspect ratio of acicular filler and R value are 15.52 and 2, the composition of 70:30

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Table 2 Maximum packing fraction and mixing composition by Milewski R value Fiber L/D 15.52

Fiber (vol%) 30 50 70

0 68.5 61.7 41.0

0.11 66.5 55.6 40.4

0.45 63.7 51.8 37.9

by volume percent was determined to yield the highest maximum packing fraction. This composition corresponds to not only the value that were experimentally determined by sedimentation method but the value from material simulation using MacroPac for which the details are described elsewhere [18]. On the other hand, in AlN(A-500)/SiC system for which Milewski’s analysis was not available, only material simulation and sedimentation method were used to determine the hybrid filler composition of 83.5:16.5 by volume percent that yields

0.94 59.9 50.7 38.2

1.95 54.6 45.5 37.3

3.71 50.3 42.0 35.7

6.96 50.5 42.4 35.5

14.30 54.1 44.3 36.0

17.40 57.5 48.1 36.8

the highest maximum packing fraction. Fig. 3 shows the plot of composition versus the maximum packing fraction obtained from simulation together with snapshots corresponding to the composition giving highest maximum packing fraction for these two hybrid filler systems. The maximum packing fraction determined by sedimentation is systematically 7–9 vol% higher than that by simulation for almost all compositions. This may be attributed to the simulation conditions employed not representative for the real systems. However, this systematic

Fig. 3. The determination of the composition in hybrid filler system that gives the highest maximum packing fraction by experiment (&) and simulation (C): (a) A-100/wollastonite, (b) A-500/SiC.


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difference between experimental and simulation values implies the usefulness of simulation approach that greatly reduces the number of laborious experimental works concerning determination of maximum packing fraction of given system once simulation condition were well chosen. 3.2. Thermal conductivity of composites containing single filler For AlN(A-100) filled samples, the comparison of experimentally determined thermal conductivity values and those predicted from Nielsen equation [19] is shown in Fig. 4. Nielsen equation was used to evaluate the thermal conductivity as a function of volume fraction of spherical filler as follows. kc 1 C ABf ; Z 1K4Bf km where B Z

kf =km K1 ð1Kfmax Þf and 4 Z 1 C kf =km C A f2max

where, AZf(geometry of particles)Z1.5, fmax(maximum packing fraction)Z0.637, for randomly packed spherical particles, respectively, [20]. Generally, it is known that while experimental values are in agreement with those predicted by Nielsen equation at low filler content [17]. However, as filler content increases then they begin to touch one another, Nielsen equation tends to overestimate thermal conductivity of the composite as the cases in Fig. 4. Considering the maximum packing fraction of A-100 lies between 0.30 and 0.38 as estimated from Fig. 3(a), this low values may be attributed to the lack of particle distribution. However, it is noted that thermal conductivity increases rapidly at filler volume fraction between 50 and 60 vol%, and it is due to critical concentration reaches at this region to highly contact each other. In addition, good wetting between AlN and polymer

Thermal conductivity (W/mK)

4 Experimental (AlN) Nielsen equation 3

2

1

0 0

10

20

30

40

50

60

70

Filler content (vol.%) Fig. 4. Thermal conductivity of the sample as a function of filler content.

Fig. 5. SEM images of the fracture surface of AlN(A-100)/HDPE composites containing (a) 60 vol%, (b) 75 vol% of AlN.

is shown in Fig. 5(a), though the composite has been filled with 60 vol% of AlN. The thermal conductivities of the composites containing various fillers for different amount of filler contents are listed in Table 3. It should be noted that the different thermal conductivity values are obtained for the samples containing the same filler content of 60 vol%, although the filler content is well beyond its maximum packing fraction as mentioned previously. The thermal conductivity of the sample containing AlN(A-500) is higher than that of composite containing AlN(A-100). This result suggests that the use of larger particle lead to improve thermal conductivity of the composites by forming thicker conductive paths reducing the interfacial phonon scattering between matrix and fillers. It is also found that the sample containing surface treated fillers (A-100) also exhibit a slightly improved thermal conductivity than the sample

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Table 3 Thermal conductivity of composites with different types of single filler Filler

Filler (vol%)


AlN(A-100)

0 10 30 50 60 70 75 60 60 50

0.35 0.46 0.75 1.08 2.01 2.31 2.27 2.14 2.42 3.66

AlN(A-100) treated AlN(A-500) BN

containing untreated fillers. Improving both dispersion and interface by filler surface treatment would lead to increased thermal conductivity by minimizing the phonon scattering at the interface, though the effect of enhancement was not so effective than the use of larger particles. For BN fillers, the maximum packing fraction is below 30 vol % as confirmed by sedimentation method. This means BN filler forms the thermally conductive networks at lower filler content than AlN. The larger difference between the maximum packing fraction and the filler content of the sample is responsible for the highest thermal conductivity value of the sample containing 50 vol% of BN which has lower intrinsic thermal conductivity than AlN. In Table 3, it is also evident that the samples containing untreated A-100 over 60 vol% suffer from the presence of voids due to poor wetting as shown in Fig. 5(b) where numerous voids can be observed as compared with Fig. 5(a). 3.3. Thermal conductivity and CTE of composites containing hybrid filler Fig. 6 shows that the thermal conductivity of composites containing AlN-wollastonite, AlN-SiC hybrid filler systems.


2.5 AlN (A-100) A-100 / wollastonite A-500 / SiC

2.0

1.5

In the case of AlN-wollastonite, thermal conductivity was increased than that of the composites with single AlN filled up to 50 vol% loading. By comparing the material conductivity between AlN and wollasonite (Table 1), this increasing is remarkable because the intrinsic conductivity of AlN is greater than that of wollastonite by more than 50 times. It is due to enhanced conductive network induced by hybrid shape fillers as depicted in Fig. 3(a). However, at the filler content of 60 vol%, the synergistic effect of hybrid filler could not be obtained. This is because when the filler content is below its maximum packing fraction the role of structuring filler in hybrid filler is more pronounced. On the contrary, at filler loading of 60 vol% which is well above its maximum packing fraction, the role of structuring filler is reduced because these already exist abundant thermal paths at this excessive filler loading level. In the case of AlN-SiC hybrid filler in Fig. 6, the trend of the thermal conductivity was similar to that in AlNwollastonite system except that the synergistic effect of hybrid filler was obtained for all filler contents. It may due to higher thermal conductivity of SiC, which has a half of that of AlN. Considering relatively low thermal conductivity of structuring filler compared to AlN, higher thermal conductivity of the samples containing hybrid filler successfully demonstrated the usefulness of the concept of synergistic effect of hybrid filler in enhancing the thermal conductivity of the composite for given filler loading. The effect of filler treatment on thermal conductivity and CTE of composites filled with hybrid fillers are listed in Table 4. In this table, all of tested composites have had the same volume contents of hybrid fillers by 60 vol%, in addition, only both of AlN particles (A-100 and A-500) were treated by titanate coupling agent not other structuring fillers (wollastonite and SiC). By introducing coupling agent, thermal conductivity has been improved in all composite samples. It is due to increasing wettability and dispersion of particle filled composites. Fig. 7 shows the SEM images of fracture surface of composites with hybrid filler by presence of filler treatment. After treatment of filler, composites show relatively good filler dispersion and wettability than the case of untreated. Especially, CTEs of composites were increased effectively by treating filler, because the property of CTE is more sensitive to their interface between filler and matric and dispersion of fillers in composites. Fig. 8 shows the CTE of the samples as Table 4 The effect of filler treatment on thermal conductivity and CTE of composites with hybrid filler in the same filler volume contents (60 vol%)

1.0

Filler

0.5

0.0 0

10

20

30

40

50

60

70

Filler content (vol.% ) Fig. 6. Thermal conductivity of composites containing hybrid filler.

AlN(A-100) A-100/wollastonite A-500/SiC


CTE (ppm/8C)

Untreated

Treated

Untreated

Treated

2.01 1.70

2.14 1.92

45.3 32.3

31.7 31.4

2.19

2.25

23.8

15.8


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Fig. 7. SEM images of the fracture surface of composites with hybrid filler, 60 vol% of filler contents, (a) AlN(A-100)/wollastonite, (b) AlN(A-500)/SiC.

a function of filler content in the case of AlN-SiC hybrid filler. The CTE of the composites decreases with increasing filler content as expected. It is noted that the CTE of composites containing treated filler was lower than that of untreated one. Conclusively, the hybrid filler is more

4. Conclusion

200 untreated AlN / SiC treated AlN / SiC

150

CTE (ppm/°C)

applicable in thermally conductive composites by introducing enhanced conducting networks, thus this system can provide synergetic effects and cost reduction simultaneously.

100

50

0 0

20

40

60

80

Filler Content (vol.%) Fig. 8. The CTE of composites containing AlN(A-500)/SiC hybrid filler.

The composites containing hybrid filler consisting of spherical and fibrous filler were found to have enhanced thermal conductivity at low to intermediate filler content. As confirmed by sedimentation method, material simulation and analysis based on packing principle proved to be quite useful tool to determine the optimal composition of hybrid filler which yields the highest maximum packing fraction. In these filler content range fibrous filler in hybrid system promotes the interaction among fillers in a way such that a structured network results which facilitates heat conduction through materials. On the other hand, at high filler content beyond the maximum packing fraction where there are already abundant thermal paths, the role of fibrous filler as structuring filler became weak. The use of titanate treated filler increased thermal conductivity by minimizing the interfacial phonon scattering. In addition, surface treatment

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of filler was also found to be effective in reducing the CTE at given filler contents.

References [1] Procter P, Solc J. Improved thermal conductivity in microelectronic encapsulants. Dexter’s technical paper 1999;February:1–8. [2] Pecht MG, Nguyen LT, Hakim EB. Plastic encapsulated microelectronics. New York: Wiley; 1995. [3] Bujard P, Kuhnlein G, Ino S, Shiobara T. Thermal conductivity of molding compounds for plastic packaging. IEEE Trans Compon Hybrids Manuf Technol Part A 1994;17(4):527–32. [4] Procter P, Solc J. Improved thermal conductivity in microelectronic encapsulants. IEEE Trans Compon Hybrids Manuf Technol 1991; 14(4):708–13. [5] Ishida H, Rimdusit S. Very high thermal conductivity obtained by boron nitride-filled polybenzoxazine. Thermochim Acta 1998;320: 177–86. [6] Bujard P. Thermal conductivity of boron nitride filled epoxy resins: temperature dependence and influence of sample preparation. In: Proceedings of the I-THERM 1988, Los Angeles, May 11–13; 1988. p. 41–9. [7] Bujard P, Ansermet J. Thermally conductive aluminium nitride-filled epoxy resin [for electronic packaging]. In: Proceedings of the SEMITHERM 1989, San Diego, February 7–9; 1989. p. 126–30. [8] Berman R. Thermal conduction in solids. Oxford: Clarendon press; 1976.

[9] Bigg DM. Metal-filled polymers. New York: Marcel Dekker; 1986. [10] Lee GW, Lee JI, Lee SS, Park M, Kim J. Comparisons of thermal properties between inorganic filler and acid-treated multiwall nanotube/polymer composites. J Mater Sci 2005;40:1259–63. [11] Milewski JV. In handbook of reinforcement for plastics. New York: VHB; 1987 p. 14. [12] Wechsler AE. The probe method for measurement of thermal conductivity. In: Maglic´ KD, Cezairliyan A, Peletsky VE, editors. Compendium of thermophysical property measurement methods. Recommended measurement techniques and practices, vol. 2. New York: Plenum Press; 1992. p. 281. [13] Voza´r L. A computer-controlled apparatus for thermal conductivity measurement by the transient hot wire method. J Therm Anal 1996; 46:495–505. [14] Mathis NE. Transient thermal conductivity measurements: comparison of destructive and nondestructive techniques. High Temp High Press 2000;32:321–7. [15] Samuels RJ, Mathis NE. Orientation specific thermal properties of polyimide film. J Electron Packaging 2001;123(3):273–7. [16] Mathis N. Eliminating density and heat capacity requirement in transient thermal conductivity measurements. In: ANTEC 1999 proceedings, New York, May 3–6; 1999. [17] Xu Y, Chung DDL, Mroz C. Thermally conducting aluminum nitride polymer–matrix composites. Compos Part A 2001;32(12):1749–57. [18] Park, M, et al. KIST report on the development of high performance package materials, 2002. Oxmat’s MacroPac Manual; 2001. [19] Nielsen LE. Thermal conductivity of particulate-filled polymers. J Appl Polym Sci 1973;17:3819–20. [20] Nielsen LE. Ind Eng Chem Fundam 1974;13:17.