Thermal characterization of Al2O3 and ZnO reinforced silicone rubber as thermal pads for heat dissipation purposes

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Abstract

Silicone rubber filled with thermally conductive, but electrically insulating Al2O3 or ZnO fillers were investigated to be used as elastomeric thermal pads, a class of thermal interface materials. The effect of Al2O3 or ZnO fillers on the thermal conductivity and coefficient of thermal expansion (CTE) of the silicone rubber were investigated, and it was found that with increasing Al2O3 or ZnO fillers, the thermal conductivity of the thermal pads increases, while the coefficient of thermal expansion (CTE) decreases. The thermal conductivity results obtained were also analyzed using the Agari model to explain the effect of Al2O3 or ZnO fillers on the formation of thermal conductive networks. Thermal gravimetry analysis (TGA) showed that the addition of either Al2O3 or ZnO fillers increases the thermal stability of the silicone rubber, while the scanning electron microscope (SEM) showed that at 10 vol.% filler loading percolation threshold has yet to be reached.

Introduction

Recent advancement in electronics technology has resulted in the miniaturization of transistors, allowing more transistors to be crammed and integrated into a single device, resulting in a higher performance device [1]. Nevertheless, integration and cramming of transistors has resulted in the escalation of power dissipation as well as an increase in heat flux at the devices. It is well known that the reliability of devices is exponentially dependant on the operating temperature of the junction, whereby a small difference in operating temperatures (in the order of 10–15 °C) can result in a two times reduction in the lifespan of a device [2]. Therefore, it is essentially crucial for the heat generated from the devices to be dissipated as quickly and effectively as possible, to maintain the operating temperatures of the device at a desired level [3], [4].

Among the various methods used to dissipate heat from the devices includes the attachment of a high thermal conductivity and low coefficient of thermal expansion (CTE) heat sink or heat spreader on the devices [5], [6], [7]. However, without good thermal contacts, the performance of a high thermal conductivity heat sink to dissipate heat is limited, due to interfacial thermal resistance arising from non-surface flatness and surface roughness of both the devices and heat sink. Non-surface flatness, are commonly observed in the form of convex, concave and wavy surfaces, resulting in as much as 99% of the interfaces being separated by air gaps [8]. Interstitial air gaps trapped due to improper mating of the surfaces significantly reduces the capability to dissipate heat, due to the low thermal conductivity value of air (kair = 0.026 Wm−1 K−1). One method that is commonly used to reduce the thermal contact resistance between the two surfaces is to include an additional material, commonly referred as thermal interface materials (TIM), to provide an effective heat path, as shown in Fig. 1 [9], [10], [11], [12], [13], [14].

TIMs are typically made up of polymer or silicone matrix reinforced with highly thermally conductive but electrical insulating fillers such as aluminum nitride, boron nitride, alumina or silicon carbide [15], [16], [17]. An ideal TIM should not only have high thermal conductivity but must also have low coefficient of thermal expansion. Besides that the material must be easily deformed by small contact pressure to contact all the uneven areas of the mating surfaces [15].

TIMs can be categorized into elastomeric thermal pads, thermal greases, solders and phase change materials [18], [19]. Of all the classes of TIMs mentioned, elastomeric thermal pads are popular for cooling of low power devices, such as chip sets and mobile processors [2]. Elastomeric thermal pads, typically 200–1000 μm thick, consists of elastomer filled with either ceramic or metal fillers. The advantages of elastomeric thermal pads is that they are easy to handle, in addition to being compressible to 25% of their total thickness, enabling the pads to absorb tolerance variances in assemblies [2].

In this study, elastomeric thermal pads were developed from silicone rubber filled with alumina (Al2O3) or zinc oxide (ZnO) fillers at various loadings up to 10 vol.%. Filler loading in the present study has been limited to 10 vol.% to avoid the hardening of pads, which could consequently result in an increase in contact resistance. The effect of Al2O3 or ZnO fillers at various filler loadings on the thermal conductivities and coefficient of thermal expansion (CTE) of the silicone rubber were studied. Experimental data obtained was fitted into a model equation, namely Maxwell–Eucken, Bruggeman, Cheng–Vochan and Agari for thermal conductivity and rules of mixture for CTE, with the values obtained, analyzed and compared. The developed thermal pads were also investigated for thermal stability using the thermal gravimetry analysis (TGA).

Section snippets

Materials

Silicone rubber used in this study, is a siloxane based polymer manufactured by Shin–Etsu silicones, while the curing agent used was 2,5-bis(tert-butyl peroxy)-2,5-dimethylhexane, also from Shin–Etsu silicones. All the above chemicals are used as received. The fillers used in this study are aluminium oxide (Al2O3) 99.9% and zinc oxide (ZnO) 99.7% from Aldrich, with an average particle size of 10 and 1 μm, respectively. Silicone rubber was chosen as the matrix for the thermal pads due to the

Morphology observations

The state of filler distribution is important, as under the percolation theory; filler units need to touch one another to form a continuous heat conduction path [27], [28]. SEM micrographs of the Al2O3 (Fig. 3) and ZnO fillers (Fig. 4), showed that Al2O3 fillers have a flat platelet shape, while ZnO fillers are rhombohedral in shape. Fig. 5(a) is a SEM micrograph showing the cross-section of the Al2O3 filled thermal pads while Fig. 5(b) is that of ZnO filled thermal pads. Both the samples are

Conclusion

Elastomeric thermal pads were successfully prepared from silicone rubber and thermal conductive Al2O3 or ZnO fillers. The addition of either Al2O3 or ZnO fillers into the silicone rubber increases both its thermal stability and thermal conductivity but reduces its CTE. Relationship between fillers and the silicone rubber matrix obtained from the Agari model, showed that Al2O3 filled thermal pads have a higher likelihood for the formation of conductive chains, due to its larger particle size, as

Acknowledgements

The authors thankfully acknowledge the research grant awarded by Intel Technologies Sdn. Bhd. One of the authors, Mr. Sim Lim Chong a recipient of Intel fellowship grant, thankfully acknowledges Intel Technologies Sdn. Bhd. for the award.

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