INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY … · 2020. 12. 20. · composites’ thermal and...

INTERNATIONAL JOURNAL OF SCIENTIFIC & TECHNOLOGY RESEARCH VOLUME 9, ISSUE 12, DECEMBER 2020 ISSN 2277-8616

18 IJSTR©2020 www.ijstr.org

Influence Of Aluminum Particles On Thermal Interface Material Hardness And Thermal

Conductivity

R. Kamarudin, M. Z. Abdullah, Z. Bachok, M. S. Abdul Aziz, F. Che Ani

Abstract : Electronic device heat dissipation is crucial and challenging because it limits the performance of the device. The electronic device is processing data at high speed, significantly generate heat and requires to be dissipated into the environment. Therefore, this experiment aims to investigate the influence of thermal interface material hardness on thermal conductivity. The results were analyzed by using conductivity measurement set-up and scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDX). In the present study, thermal interface material (TIM) is hardened by aluminum oxide (Al@Al2O3) and thermal coefficient are measured for different hardness and the heat rate of 3W and 7W. The results show TIM hardness is significantly improved the thermal conductivity coefficient up to 10 times compare to pure TIM. The results also show that the Al@Al2O3 particles distribution inside the TIM is well distributed. The outcomes of this paper can be a guideline to the semiconductor industries especially for the usage of TIM in product development. Keywords: Thermal interface material, thermal conductivity, aluminum particles, hardness, heat rate

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1. INTRODUCTION Due to demanding high performance and highly integrated functionalities of electronic package ranging from manufacturing to consumer product, the power of the electronic component is rising over the years. Such electronic devices process data at high speed, generating heat significantly that has to be dissipated into the ambient. Miniaturization and lightweight nature of electronic system components, higher demands on materials with high thermal conductivity and low density are constantly raised and become more challenging [1][2]. Thus, electronic device thermal management becomes very critical to ensuring functional, improved reliability and longer life span. The thermal interface material (TIM) between silicon die and heat spreader or heat sink and the use of an external heat sink is to provide more efficient thermal conductivity. The TIM solution can be in various forms, such as adhesive, grease, gels, phase change material and sheets. Majority of TIM are of polymer matrix such as an epoxy or silicone resin and thermally conductive filler such as boron nitride [3], zinc oxide [4], aluminium nitride and alumina [5]. The TIM mixed with ultra-thin full carbon films with extraordinarily high thermal conductivity (k) and promising mechanical power, but the fragility (approximately 0.5-3.0 percent break elongation) limits their applications. In addition to the high k, superior TIM required excellent durability, low thickness and low hardness in order to ensure good thermal management [6]. Thermally conductive and insulating thermal interface materials composed of core-shell structured aluminium/aluminium oxide (Al@Al2O3) and epoxy resin. The composite exhibit 0.92W/mK thermal conductivity at 60wt percent filler content, which is about 4.2 times higher than the epoxy resin content. The oxidation of Al particles results in the formation of dense nanoscale Al2O3 shell isolating which effectively restricts the transfer of electrons, resulting in very high electrical resistivity and composite puncture voltage. The polymer composites filled with Al@Al2O3 particles have excellent insulation property, high thermal conductivity and outstanding thermal-mechanical performance, which could be a potential thermal interface material in advanced electronic packaging techniques [7].

The thermal and electrical conductivity of epoxy composites

(TIM), creating epoxy composites containing graphite (G),

expanded graphite (EG), and copper powder (Cu) hybrid

fillers. EG was plated electroless in silver (Ag) to enhance

the composites’ better conductivity properties. Ag-Eg, G

and Cu were mixed into the epoxy to form hybrid (Ag-

EG/G/Cu) composites. The results indicated that the

composites’ thermal and electrical conductivities were

improved by incorporating the hybrid fillers with the

development of a conductive interconnecting pathway [8].

The fabricated paper based thermal interface material on a

composite of cellulose and graphene has significantly

enhanced in-plane thermal conductivity [9]. The anisotropic

thermal properties for epoxy resin-based TIM reinforced

with various of thermal conductive fillers. The thermal

properties of graphene nanoplatelet-based thermal

interface material can be improved significantly by

incorporating Al@Al2O3 particles into its matrix material.

Synergistic effect of graphene nanoplatelets and Al@Al2O3

particles are giving high thermal properties for TIM [10].

Particulate distribution and particle size at high volume

fraction, high thermal conductivity, low thermal contact

resistance and essentially total particle size of hybrid fillers

significantly affected optimal effective thermal conductivity.

Particulate distribution and particles size however, show no

important effect on the importance of the ideal thermal

conductivity ratio between two fillers [11]. Chen et al. [12]

specified that the combination of fillers that result in a

higher thermal conductivity than the use of a single filler for

the same total load. For epoxy composites filled with single-

walled carbon nanotubes and graphite nanoplatelets, the

optimal ratio of hybrid filler was demonstrated to be correct.

In addition, the thermal conductivity of polymer composites

can be strengthened by adding nano fillers to the polymer

matrix containing the micro fillers as shown in [13]. Interface

resistance can be due primarily to differences between

microscale surface roughness and irregular surface profiles



but contact thermal resistance can be minimized by

applying high compressive pressures to mechanically

deform surfaces and interface contact region [14], [15]. An

efficient way to reduce thermal contact resistance without

using high pressure is to fill the holes with wet thermal

interface materials, such as greases and pastes [16].

Another useful approach for these liquidation TIMs is to

build high-performance thermal interfaces using nano-

structured materials [17], [18]. Particle-laden polymers

(PLPs) [19] have the same characteristics as the polymer

matrix, while nano-filling particles of high thermal

conductivity enhance the thermal conductivity of the

polymer matrix. Higher filler particle ratio results in better

thermal conductivity, but with the mechanical compliance

cost [18]. Compliant metal nano-springs [20] and nanowires

[21] can be quickly deformed and soldered onto a rough

surface in a conformal manner. These micro and nano-

structured TIMs provide feasible solutions that greatly

improve the thermal interfacial conductance of surfaces

with microscopic roughness, but these TIMs bind surfaces

permanently or plastically deform to achieve conformal

contact with the mating surfaces, which are intended for

applications where the mating surfaces are not moving

through [22]. Thermal interface materials, such as thermal

conductive gap filling materials are commonly used in

electronic packaging to minimize the thermal contact

resistance of interfacial air, the volume and thermal

conductivity (k) of which can be as high as 99% and as low

as 0.026 W/mK, respectively [23]. To rising the overall

thermal resistance, high performance TIMs should possess

high k, low thickness and high conformability (low hardness

and low modulus). Therefore, in the present study on the

TIM is hardened by Al@Al2O3 particles (filler) with different

weight percentage has been made. The influence of

Al@Al2O3 particles on the thermal conductivity, mixture

homogeneity and hardness have been considered. The aim

of this works is to obtain the relationship between hardness

and thermal conductivity of the hardened TIM for better

electronic device heat dissipation applications.

2. EXPERIMENTAL SET-UP As shown in Figure 1, a bar heater (1) is used as a heat

source in the right and the cooling water flow in (2) and flow

out (3) is used as a heat sink in the left to ensure a steady

state heat flow along longitude axis of the test specimen

(4). Several TIM hardness values are made and tested

varied from 45 to 70 shore for the heat rate of 3W and 7W.

A set of thermocouples (5) is mounted at the brass bar

together with temperature and power measurement

systems. Heat sink compound is applied to reduce the

contact resistance between heater and chiller. Hollow

cylindrical brass block acted as a chiller (7). The cooling

water tank (2) and water circulation pump has been used to

obtain a steady state condition during experimental

measurement. The experimental rig setup and specimen

are shown in Figure 2. The experimental rig is developed

for ambient environment experiment. The apparatus

consists of two brass bars and specimen (TIM) at the

middle. Four different specimens insulated with insulation

material in the experiment, i.e. TIM is hardened with

different weight percentage of Al@Al2O3 particles. The

dimension of specimen as shown in Figure 3 with diameter

of 25mm and length of 10mm. The specimen hardness is

measured by a shore-hardness durometer.



3. RESULTS AND DISCUSSION The result of temperature for different hardness is taken at

steady state condition where it took about 30 minutes to

reach steady state after the experimental rig is switched

‘on’. Six locations are measured on the experimental rig

using thermocouples and have been used to obtained

thermal coefficient of TIM with different hardness. The

thermal conductivity coefficient of TIM is measured at

different heat rate. Figures 4 and 5 show the typical

temperature measurements across the heat conduction

apparatus and specimens for 3W and 7W respectively. The

thermal conductivity coefficient can be obtained from the

Fourier equation of heat transfer as given by:

⁄ (1)

where is the conductivity coefficient, Q is the heat rate

used, A is the cross section area of specimen, ⁄ is the

temperature gradient across specimen. The value of dT/dx

for different hardness of the specimens are found to be in

the range of 622.99-735.02K/m for Q=3W (Figure 4), and

1402.9-1765.78k/m for Q= 7W (Figure 5). The results show

that the lower temperature gradient obtained at higher TIM

hardness has increased the thermal conductivity coefficient

as expected from the Fourier equation (1).

Figure 4: Temperature distribution along axial distance at Q= 3W

Temparature versus Distance

Distance (mm)

0 20 40 60 80

Te

mp

ara

ture

(d

eg

C)

28

30

32

34

36

38

40

42

44

46

45 shore

54 shore

65 shore

70 shore

Specimen

Temperature versus Distance

Tem

pe

ratu

re (

deg

C )



Temperature versus Distance

Distance (mm)

0 20 40 60 80

Te

mp

era

ture

(d

eg

C)

25

30

35

40

45

50

55

60

65

45 shore

65 shore

70 shore

Specimen

Figure 5: Temperature distribution along axial distance at Q= 7W

In the present study all the specimens (total of 16) have been group for the hardness ranges of 45-52 shore (Category I), 54-60

shore (Category II), 61-66 shore (Category III) and 67-70 shore (Category IV) as illustrated in Table 1. The results obtained

show that the thermal conductivity increased significantly with the hardness up to nearly 26%. The Al@Al2O3 particles increased

the hardness, thermal conductivity of the TIM and could improve heat dissipation of the electronic device. Moreover, the TIM

hardened with Al@Al2O3 particles has increased about 10 times compare to the TIM without filler.

Table 1: Average thermal conductivity of TIM specimens

No. Specimen Category

Hardness Range

Thermal Conductivity Q= 3W

(Average)

Thermal Conductivity Q= 7W

(Average)

1. I 45-52 shore 8.319 W/mK 8.080 W/mK

2. II 54-60 shore 8.512 W/mK 8.140 W/mK

3. III 61-66 shore 9.346 W/mK 9.670 W/mK

4. IV 67-70 shore 9.815 W/mK 10.170 W/mK

Figures 6(a)-(d) is the SEM images of TIM with different hardness to observe the homogeneity of fillers. As overall, the

homogeneity of Al@Al2O3 particles distributions are improved with the increase of percentage filler. Figure 6(d) shows the

particles are well distributed inside silicone and provide higher hardness of the TIM as shown in Table 1. Moreover, Figures 7(a)-

(d) show the EDX results of different hardness. The figures clearly show the elements consists of silicone, aluminum and

oxygen. The increase of specimen’s hardness is expected as the aluminum percentage increases and the grain size of

aluminum are smaller and compactly formed as shown in Figures 6(d) and 7(d). The produced microstructure improves

conductivity of heat in the specimen which eventually increases the thermal conductivity.



Figure 6: SEM images of (a) Specimen I with hardness 45 shore (b) Specimen II with hardness 54 shore (c) Specimen III with

hardness 65 shore and (d) Specimen IV with hardness 70 shore

(c) Specimen III with hardness 65 and (d) Specimen IV with hardness 70

(a)

Al

(b

) Si Al

Si

Si K

Al K O K

Al K

O K

Si K

Al K O K

Si K

Al K

O K

Si K



Figure 7: SEM images and EDX results of (a) Specimen I with hardness 45 shore (b) Specimen II with hardness 54 shore (c)

Specimen III with hardness 65 shore and (d) Specimen IV with hardness 70 shore

Figure 8 shows the overall results obtained from the study. The thermal conductivity of hardened TIM for different heat rate also

show similar trend. Both heat rates of 3W and 7W are almost linearly increased with the hardness and the maximum variation of

the thermal conductivity is about 3.5%. The results also show that the TIM is dense with Al@Al2O3 particles for the higher

hardness value.

Figure 8: Thermal Conductivity at different heat rate and hardness for specimen categories I-IV

(d

)

Al Al

(c)

Al

Si

Al K

O K Si K

Al K O K

Si K

Al K

O K

Si K

Al K

O K Si K



4. CONCLUSION

TIM reinforced with Al@Al2O3 particles was successfully

hardened the TIM and has increased the thermal

conductivity coefficient up to almost 10 times compared to

the pure silicone. Whereas the hardness has increased up

to 27%. Thus, additional of Al@Al2O3 particles are

significantly improved the hardness and thermal

conductivity of TIM. The thermal coefficient is varied almost

linearly up to 10.17W/mK for the hardness of 70 Shore. The

Al@Al2O3 particles used as filler are found good

homogeneity at higher hardness. The results also can be a

guideline to the semiconductor industries especially for the

usage of TIM in product development. Moreover, with

successful development of experimental set-up, various

TIM and filler materials could be investigated and analysed

further on the thermal resistance via experiment in future

study.

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