Study on Liquid Crystal Polymer-Hexagonal Boron Nitride Composites

Article pubs.acs.org/IECR

Study on Liquid Crystal Polymer-Hexagonal Boron Nitride Composites for Hybrid Heat Sinks Siu N. Leung,† Omer M. Khan,† Hao Shi, Hani E. Naguib,*,† Francis Dawson,‡ and Vincent Adinkrah§ †

Department of Mechanical & Industrial Engineering, University of Toronto, 5 King’s College Road, Toronto, ON, Canada M5G 3G8 ‡ Department of Electrical & Computer Engineering, University of Toronto, 10 King’s College Road, Toronto, ON, Canada M5G 3G4 § AEG Power Solutions Inc., 2680 Fourteenth Avenue, Markham, ON, Canada L3R 5B2 ABSTRACT: Designers of electronic devices and telecommunications equipment have used different methods (e.g., threedimensional chip architecture, comprised of a vertically integrated stack of chips) to increase the number of transistors on integrated circuits. These latest chips generate excessive amounts of heat and thus can reach unacceptably high temperatures. In this context, this research aims to develop thermally conductive liquid crystal polymer (LCP)-hexagonal boron nitride (hBN) composite films to replace the traditionally used Kapton films that satisfy the electrical insulation requirements for the attachment of heat sinks to the chips without compromising the heat dissipation performance. Experimental simulations show that the maximum temperature of the heater mounted with a hybrid heat sink reduced with increased hBN content. Furthermore, a sufficient slow cooling process in the fabrication stage of the LCP/hBN composite films promotes the fibrillation of LCP matrix, leading to highly ordered structure and promoted the composite’s heat dissipation ability.

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INTRODUCTION The emerging three-dimensional chip (3D) architecture has not only served as a promising solution to mitigate the interconnect problem in modern microprocessor designs but also enables much higher memory bandwidth for future 3D microprocessors.1 Through the integration of a very large System on a Chip (SoC) in multiple tiers, the average distance between system components is reduced, leading to improved performance. This also means more electrical power is needed to run the SoC. A large portion of the power would turn into heat. The increased amount of heat generation and the reduced spacing among components would result in rapid heat up of the latest chips. Overheating chips reduce their reliability, potentially leading to computer crashes, mangled files, graphical glitches, and even permanent damage. In other words, the future of 3D Integrated Circuits crucially hinges on the development of practical solutions for heat removal. This makes thermal management a key enabling technology in the development of microelectronic systems in the future.2 In general, the objective of a thermal management program in electronic packaging is the efficient removal of heat from the component junction to the ambient environment. This process can be subdivided into three major phases. First, heat being generated during the operation of the electronic device would be transferred within the semiconductor component package. Second, the heat would then be transferred from the electronic package to a heat dissipater. Finally, the heat dissipater would remove the heat to the ambient environment. The first phase of the heat management programs prompts the needs to develop advanced composite and monolithic materials that are tailored to meet the multifunctional requirements, which include high thermal conductivity and electrical insulation, of electronic packaging. Moreover, the ease of manufacturing © XXXX American Chemical Society

three-dimensional, net-shape enclosures, and/or heat management assembly would be another important attribute of the desired materials. In light of this, extensive research has been conducted in recent years to develop thermally conductive but electrically insulative polymer-matrix composites (PMCs).3−10 Taking advantage of the polymer-matrix, these PMCs also have good processability, meaning that these PMCs are injection moldable into different shapes. The kef f of thermally conductive but electrically insulative PMCs filled with ceramic particulates or platelets reported to range between 0.5 and 2.0 W/mK when no more than 33.3 vol.% of ceramic particulates were added into the polymer matrix.3−7 Although composites with much higher thermal conductivity (i.e., 5.0−32.5 W/mK) were achieved in some of the previous research, it required either the addition of 60−90 vol.% of ceramic fillers or the uses of electrically conductive metal fillers.8−10 In the second and third phases, an engineer’s goal is to design an efficient thermal connection from the package surface to the heat spreader and on to the ambient environment. In this context, heat sinks are the most common, cost-effective hardware employed for the thermal management of microelectronic equipment. They promote the heat dissipation by extending the surface area through the use of fins. Their design and analysis is one of the most extensive research areas in electronics cooling.11 Some of the main research areas include the minimization of interfacial contact thermal resistance and the improvement of heat spreading technologies. Nakayama and Bergles12 found that the component to heat sink interfacial contact Received: May 15, 2012 Revised: May 15, 2013 Accepted: May 24, 2013

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dx.doi.org/10.1021/ie3012775 | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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thermal resistance can be comparable to the actual heat sink thermal resistance. Thus, researchers continuously investigate strategies to improve thermal interface materials12−14 as well as to minimize heat sink base thermal spreading resistance.15 Traditionally, thermally enhanced Kapton films would be sandwiched between the heat sinks and the chips, providing the required electrical insulation. Nevertheless, despite the thermal enhancement to these films, which tripled the thermal conductivity (k) of conventional Kapton, the value of k is only 0.37 W/mK. Such low thermally conductive layers significantly restrict the dissipation and management of heat in electronic assemblies. Therefore, the drive for improved cooling efficiency requires the optimization of not only the heat sink but also the insulation pads. In this context, this research aims to develop thermally conductive thermotropic liquid crystal polymer (LCP)-based composites to fabricate the insulating pads. These films would be attached to the bottom faces of the heat sinks, resulting in a hybrid heat sink structure, to satisfy the electrical insulation requirements without compromising the heat sinks’ heat dissipation performance. The ease of fibril formation of LCP has been exploited in the fabrication of the so-called in situ composites.16,17 Although extensive studies16−24 have been conducted to elucidate the use of LCP fibrillar domains as the reinforcing component in polymer-matrix composites (PMCs), other physical properties such as thermal conductivity have received much less attention.17 It is believed that the potential LCP chain alignment during the molding process would suppress the phonon scattering, which is detrimental to heat conduction, in the PMCs’ matrix. A series of parametric studies were conducted to investigate the effects of LCP-based composites’ morphology as well as the filler contents on the cooling performance of the hybrid heat sinks.

Table 2. Physical Properties of hBN (Momentive Performance Materials, PolarTherm, PTX60)

MATERIALS AND SAMPLE PREPARATION Materials. Commercially available LCP (Ticona, Vectra A950) was used as the matrix material of the PMCs fabricated in this work. Vectra A950 is a semicrystalline random copolyester of 4-hydroxybenzoic acid (HBA) and 2-hydroxy6-napthoic acid (HNA) with its HBA-to-HNA ratio equal to 73:27.17,20 Hexagonal boron nitride (hBN) (Momentive Performance Materials, PolarTherm, PTX60) was used as the thermally conductive filler to fabricate the PMCs. LCP was chosen as the matrix material because of its high service temperature (i.e., >200 °C), and its possibility to enhance chain orientation. It is believed that the extended LCP chain would suppress the phonon scattering, leading to higher kef f of the resulting PMCs. For the filler, hexagonal BN spherical agglomerate was chosen for two reasons: (i) it is electrically insulating and (ii) it resembles the layered structure of graphite, making it extremely soft, and thereby easier to be compounded at high loading. All materials were used as received without any further chemical modification. The physical properties of the polymers and fillers are summarized in Tables 1 and 2.

Figure 1. Mold temperature (at center) versus time of different cooling schemes.

Table 1. Physical Properties of LCP (Vectra A950) physical properties

values 1400 kg/m3 280 °C ∼0.20 W/m-K 47 kV/mm

values 2280 kg/m3 300+ W/m-K 53 kV/mm spherical agglomerates 60 μm

Sample Preparation. LCP pellets were ground into fine powders using a mill freezer (SPEX CertiPrep Group, 6850 Freezer/Mill). The LCP powders were dried at 60 °C for at least 12 h before being dry-blended and subsequently meltcompounded, at 300 °C and 100 rpm for 6 min, with various hBN contents (i.e., 0 vol.% to 33.3 vol.%). The LCP/hBN mixtures were compression-molded, at 310 °C, into 10 mm thick discshaped samples of 20 mm diameter as well as 100 μm thick films. These samples were water-cooled after the molding processes. Furthermore, five different cooling schemes were employed to cool and solidify the samples filled with 33.3 vol.% hBN for keff measurements. These cooling schemes include the following: (1) Ice-cooling: the sample, inside the mold, was taken out the compression molding machine; the mold assembly was cooled in an ice-bath (i.e., 0 °C). (2) Water-cooling: the sample was taken out of the compression molding machine; the mold assembly was cooled with flowing tap water (i.e., 17.5 °C). (3) Air-cooling: the sample, inside the mold, was taken out of the compression molding machine; the mold assembly was left in air (i.e., 22 °C) during cooling. (4) Thermal-gradient air-cooling: the sample, inside the mold, was left on the compression molding machine with its bottom heater set at 100 °C; the top surface of the mold assembly was opened to the surrounding during cooling (i.e., 22 °C). (5) Clamped-air-cooling: the sample, inside the mold, was left in the compression molding machine; the heater was turned off so that the mold assembly was air-cooled while it was clamped in the molding machine. Figure 1 illustrates a plot that shows the qualitative differences in the temperature profiles of the various cooling schemes.

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density (ρ) melting temperature (Tm) thermal conductivity (k) dielectric strength

physical properties density (ρ) thermal conductivity (k) dielectric strength shape average diameter

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CHARACTERIZATION Effective Thermal Conductivity (kef f). The effective thermal conductivity (keff) of each sample was studied in this B



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Figure 2. The thermal conductivity analyzer: (left) interior components; (middle) insulation layer; and (right) exterior components.6

Figure 5. Temperature distribution of a hybrid heat sink assembly mounted on the heat source (base-plate is a LCP-hBN composite film filled with 33.3 vol.% of hBN fabricated by clamped-air-cooling).

Figure 3. A schematic of the hybrid heat sink prototype tester.

Figure 6. Effect of cooling schemes on the hybrid heat sink performance (filled with 33.3 vol.% of hBN; Tmax is the maximum temperature at the heater-heat sink interface).

work. The values of kef f were measured at 150 °C by a thermal conductivity analyzer that was designed and installed6 in accordance to ASTM E1225. The thermal conductivity analyzer is illustrated in Figure 2. This method measures the sample’s keff by comparing the temperature gradient across the sample to that across the reference bars with known thermal conductivity. Temperatures at 1 mm below the top surface, in the middle, as well as at 1 mm above the bottom surface of the two reference

Figure 4. Effect of cooling schemes on kef f of LCP/hBN composites (filled with 33.3 vol.% of hBN). C



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Figure 7. SEM micrographs of compression-molded LCP/hBN composites filled with 33.3 vol.% of hBN: (a) ice-cooled; (b) water-cooled; (c) aircooled; (d) air-cooled under a temperature gradient; and (e) clamped-air-cooled.

bars and the sample are measured by ultrafine thermocouples with diameters of 0.076 mm.6 The average and standard deviation of kef f were determined from three samples for each material composition and each cooling method. Heat Dissipation Performance. A series of parametric studies were conducted to simulate the heat dissipation performance of the hybrid heat sinks by measuring the temperature distribution of the hybrid heat sink assembly and the heat source as indicated in Figure 3. The hybrid heat sink, which consists of an anodized aluminum heatsink (Digi-Key Coporation, ATS1236-ND, 29.01 mm × 29.01 mm × 19.50 mm) and a 100 μm thick thermally conductive film fabricated in this research. Heat sink silicone compound (CHEMPLEX 1381) was used to attach the thermally conductive film to the heat sink as well as to attach the hybrid heat sink to a 10 W square-faced (i.e., 10 cm × 10 cm) heater as indicated in the figure. The temperature distributions of the heat sink and tester assemblies were captured by an infrared camera. Composite Morphologies. The dispersion of the filler systems in the compression molded samples was examined on a scanning electron microscope (SEM, JEOL, model JSM6060) operated at 20 kV. Sample cross sections were obtained by cooling and fracturing the composites in liquid nitrogen. The cross sections were sputter coated with platinum prior to the SEM analyses.

Figure 8. Effect of filler contents on kef f of LCP/hBN composites (water-cooled).

PTX60, fabricated by using different cooling schemes. With the addition of 33.3 vol.% of hBN spherical agglomerates, kef f of the LCP/hBN composites exhibited dramatic improvements (i.e., 2.85 to 3.29 W/mK or 14-folded to 16-folded) over neat LCP (i.e., ∼0.20 W/mK). Comparing to our previous work of polyphenylene sulfide (PPS)/hBN composites,6 the kef f of LCP/hBN composites filled with 33.3 vol.% hBN, regardless of the cooling schemes, were higher than that of PPS/hBN

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RESULTS AND DISCUSSION Effect of Cooling Rate on kef f . Figure 4 shows the kef f measurements for LCP/hBN composites, loaded with 33.3 vol.% of D



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Figure 9. SEM micrographs of LCP/hBN composite filled with 33.3 vol.% of hBN (water cooled): (a) a hBN spherical agglomerate and (b) hBN platelets broken from hBN spherical agglomerates.

composites (i.e., 1.83 to 1.89 W/mK) with the same filler content by over 50%. In other words, experimental results clearly indicated that LCP served as a better matrix material than PPS for the development of thermally conductive polymer-hBN composites. Experimental results also revealed that cooling rates would influence LCP/hBN composite’s kef f. Measurements of kef f indicated that clamped-air-cooled composites yielded the highest kef f, while the composites prepared by the other four cooling schemes resulted in similar kef f. A one-way ANOVA test about the significance of the cooling scheme effect on the kef f revealed a P-value of 0.069. This indicates that the differences were significant with higher than 90% confidence. Therefore, it is believed that an extremely slow cooling process would enhance the composite’s kef f. For the clamped-air-cooled sample, the slow and uniform cooling promoted the uniformity of the crystal structures and the crystal perfection25 in the LCP matrix. This led to a larger amount of the highly ordered and directional structure and is believed to suppress the phonon scattering in the LCP matrix. As a result, clamped-air-cooling yields a sample with promoted kef f. The air-cooled LCP/hBN composite under a temperature gradient was originally expected to promote the crystalline orientation26 and to enhance fibril formation. However, kef f measurements suggested that the keff had not been improved when comparing to composites fabricated by other cooling schemes. Effect of Cooling Schemes on Hybrid Heat Sink Performance. The temperature distributions of the hybrid heat sink and tester assemblies were obtained by an infrared camera. Figure 5 displays the equilibrium temperature distribution of a hybrid heat sink assembly, with its base-plate made of LCP/hBN composite containing 33.3 vol.% of hBN and being fabricated by clamped-air-cooling. Similarly, the maximum temperatures (i.e., around the central region at the surface of the heat source) on the heat source and hybrid heat sink assemblies with base-plates made of LCP/hBN composites with different hBN contents or fabricated by different cooling schemes were measured. The results are plotted in Figure 6. Experimental results reveal that the hybrid heat sink with its base-plate fabricated by clamped-air-cooling performed better than the cases with the base-plate fabricated by other cooling schemes. The maximum temperature was reduced by about 3 °C when using the hybrid heat sink with its base-plate fabricated by clamped-air-cooling. This could be attributed to the improved kef f of the clamped-air-cooled LCP/hBN composite. A one-way ANOVA test about the significance of the cooling scheme effect on the maximum temperature revealed a P-value of 0.30. This indicates that the differences were

Figure 10. Effect of filler contents on the hybrid heat sink performance (water-cooled; Tmax is the maximum temperature at the heater-heat sink interface; dotted line represents Tmax measured when the heat sink was directly clamped onto the heat source without the LCP-based composite baseplate).

significant with about 70% confidence. Although the differences were not as statistically significant as the effect of cooling scheme on the kef f, it is promising enough to deserve additional investigation in future studies. For comparison, the maximum temperature on the heat source mounted with a heat sink without any insulating base-plate was measured and was found to be 123 °C. In other words, the uses of the thermally conductive and electrically insulating pad did not compromise the heat dissipation performance of the heat sink. Effect of Cooling Schemes on Composite Morphology. Figure 7a−e illustrates SEM micrographs of the compression-molded LCP/hBN composite fabricated by the aforementioned five different cooling schemes. Fibrils were formed in the LCP matrix regardless of the cooling conditions. Nevertheless, it was evident that the morphologies of the LCP matrices were not identical. The SEM micrographs revealed that the clamped-air-cooled LCP/hBN composites consisted of evenly distributed fibrils throughout the entire sample. Although fibrils could also be readily found in other samples, there were either fewer fibrils or the fibrils were slightly less uniformly distributed. From the different composite morphologies caused by various cooling schemes, it could be deduced that a sufficiently slow and uniform cooling process was required to promote the crystal perfection process. This seemed to be a potential explanation to the different LCP fibril amount and uniformity in the clamped-air-cooled compared to composites cooled by other schemes. Effect of Filler Contents on kef f . Using the water-cooling scheme, LCP/hBN composites filled with various amounts of E



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Figure 11. SEM micrographs of compression-molded LCP/hBN composites filled with various loadings of hBN: (a) neat LCP; (b) 5 vol.% hBN; (c) 10 vol.% hBN; (d) 20 vol.% hBN; and (e) 33.3 vol.% hBN.

hBN were fabricated. Figure 8 shows the effect of filler contents on the LCP/hBN composite’s kef f. Measurements showed that kef f increased with filler content as expected. A one-way ANOVA test about the significance of the hBN content effect on the kef f revealed a P-value of 5.11 × 10−9. This indicates that the differences were significant with higher than 99% confidence. When 33.3 vol.% of hBN had been added to the LCP, the average kef f was measured to be 2.94 W/mK, which represented a 15-folded increase in kef f compared to neat LCP. Despite the significant increase in the kef f of LCP/hBN over the thermal conductivity of neat LCP, the value was still significantly lower than that of the hBN fillers (i.e., >300 W/mK along the hBN platelet direction). Moreover, a clear transition from an insulting-like behavior to a conducting-like behavior at the percolation threshold that typically observed in the research of electrically conductive composite was absent. These results suggest the full potential of using hBN to promote kef f had not been achieved. It is believed that the interfacial phonon scattering, which can be mathematically related to thermal contact resistance at the LCP/hBN interfaces as well as the hBN/hBN interfaces, limited the potential increase in kef f in the fabricated composites. Furthermore, as shown in Figure 9a,b, most of the hBN agglomerates had been broken into individual platelets during the compression molding process. Figure 9a illustrates the shape of a hBN spherical agglomerate. However, Figure 7a−e as well as Figure 9b shows that most of these agglomerates had been broken into individual platelets by the shear field during the melt-compounding

and the applied compressive force during the compression molding process. The random orientation of the anisotropic hBN platelets also limited the potential increase of kef f along the heat flow direction. Effect of Filler Contents on Hybrid Heat Sink Performance. Figure 10 illustrates the effect of hBN content in the electrically insulating base-plate in the hybrid heat sink assembly on their heat dissipation performance. With the increase in keff of LCP/hBN composites as the hBN content increased, the maximum temperature measured in the heat source-hybrid heat sink assembly decreased substantially. A one-way ANOVA test about the significance of the hBN content effect on kef f revealed a P-value of 0.008. This indicates that the differences were significant with over 99% confidence. It is believed that these thermally conductive and electrically insulative films would serve as a partial solution to enhance the cooling of future chips with higher CMOS performance. Effect of Filler Contents on Composite Morphology. Figure 11a−e shows the SEM micrographs of LCB/hBN composites loaded with different hBN contents. It is apparent that the interconnectivity of thermally conductive hBN platelets was promoted with increasing hBN contents. The formation of such network attributed to the increase on PMCs’ kef f and improved the heat dissipation performance of the hybrid heat sinks.

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CONCLUSIONS Liquid crystal polymer (LCP)-based composites filled with hexagonal boron nitride (hBN) were found to promote the F



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effective thermal conductivity (keff) by as much as 16-folded when compared to the thermal conductivity of the neat LCP. Experimental results obtained in this work revealed that there were correlations among the cooling scheme, composite morphology, and the thermophysical property (e.g., kef f) of LCP/hBN composites. A sufficiently low cooling rate will not only maintain the highly ordered structure in the LCP’s melt phase but also promote the crystal perfection process, leading to the formation of uniformly distributed fibrils in the LCP matrix. In this context, clamped-air-cooled LCP/hBN composites exhibited a uniform distribution of numerous LCP fibrils throughout the composite. In contrast, if the cooling rate was too fast, the rapid solidification process would be detrimental to the formation of highly ordered nematic structure in the LCP matrix. Moreover, since the highly ordered LCP matrix would promote kef f by reducing the degree of phonon scattering during heat transfer, an optimal cooling process would be critical to maximize LCP-based composite’s kef f. The ability of LCP to form highly ordered fibrils in the matrix seemed to be a potential explanation to the higher kef f of the LCP/hBN composite prepared in this work than the polyphenylene sulfide/hBN composite prepared in our previous study.6 The heat dissipation performance of the hybrid heat sinks, with a LCP/hBN film served as thermally conductive but electrically insulative pad, was experimentally simulated by measuring the temperature distribution of the hybrid heat sink mounted to a 10 W square-faced (i.e., 10 cm by 10 cm) heater. Experimental simulation showed that the maximum temperatures on the heater were reduced with higher hBN loading as well as the adoption of clamp-air-cooling scheme to fabricate the film sample. It is believed that these thermally conductive and electrically insulative films would serve as a partial solution to enhance the cooling of future SoCs, thus allowing chip designers to achieve higher CMOS performance and efficiency trends in the future.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 1 416 978-7054. Fax: 1 416 978-7753. E-mail: naguib@ mie.utoronto.ca, mie.utoronto.ca/laboratories/sapl. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors of this study gratefully acknowledge the financial support of AEG Power Solutions Inc. and Natural Sciences and Engineering Research Council (NSERC) of Canada. The authors are grateful to Ticona for the supply of LCP Vectra A950.

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ABBREVIATIONS HBA = 4-hydroxybenzoic acid hBN = hexagonal boron nitride HNA = 2-hydroxy-6-napthoic acid LCP = liquid crystal polymer PMCs = polymer-matrix composites SoC = System on a Chip G



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Study on Liquid Crystal Polymer-Hexagonal Boron Nitride Composites

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