Microelectronics Reliability 55 (2015) 383–388

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Microelectronics Reliability

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Thermal characterization of high power LED with ceramic particlesfilled thermal paste for effective heat dissipation

http://dx.doi.org/10.1016/j.microrel.2014.10.0090026-2714/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail address: anithambigai@gmail.com (P. Anithambigai).

Nur Hasyimah Hashim, P. Anithambigai ⇑, D. MutharasuNano Optoelectronics Research Laboratory, School of Physics, Universiti Sains Malaysia, 11800 Minden, Pulau Pinang, Malaysia

a r t i c l e i n f o

Article history:Received 24 February 2014Received in revised form 12 October 2014Accepted 13 October 2014Available online 22 November 2014

Keywords:Filler dispersionLight emitting diodeThermal interface materialThermal transient measurement

a b s t r a c t

The next generation packaging materials are expected to possess high heat dissipation capability. Under-standing the needs for betterment in the field of thermal management, the present study aims at inves-tigating the package level analysis on a high power LED. In this study, commercially available thermalpaste was heavily filled with ceramic particles of aluminium nitride (AlN) and boron nitride (BN) in orderto enhance the heat dissipation of the device. Different particle sizes of AlN and BN fillers were incorpo-rated homogenously into the thermal paste and applied as a thermal interface material (TIM) for an effec-tive system level analysis employing thermal transient measurement. It was found that AlN TIM achieveless LED junction temperature by a difference of 2.20 �C compared to BN filled TIM. Furthermore, amongD50 = 1170 nm, 813 nm and 758 nm, the AlN at D50 = 1170 nm was found to exhibit the lowest junctiontemperature of 38.49 �C and the lowest total thermal resistance of 11.33 K/W compared to the other twofillers.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Light emitting diodes (LEDs) are semiconductors, and like allother solid state technology, they are getting better and cheaperon a predictable curve. The reasons LEDs have taken on the auraof inevitability; due to their high efficiency, good reliability, longlifetime, variable colour and low power consumption [1]. Thepromising features of LED technology have attracted a great dealof attention from the lighting industry. It has been stated that over-all, the solid state lighting market will enjoy a compound annualgrowth rate (CAGR) of 12% by 2017 [2].

Despite the rapid gain, high power LEDs decrease in packagesize and also the operating parameters keep on increasing. Theexcessive rise in the junction temperature causes thermal runawayand catastrophic failures which resulting in a major drawback ofthe technology [3,4]. The heat generated at the p–n junction affectsthe efficiency of the light generation process and results in ameasurable drop in LEDs brightness. Correspondingly, if the heatdissipation is ineffective, heat will accumulate inside the diesand will affect their chips, electrical and optical characteristics aswell as reliability of the device [5].

In short, heat dissipation has become a reason that affects theperformance and reliability of LEDs [6]. It provides a path to dissi-pate heat generated by LEDs so LEDs can operate at appropriatetemperature range. Therefore, a good margin for thermal dissipa-tion needs to be considered so that the junction temperature (TJ)will not exceed TJmax [7]. Thus, with the increasing demands ofthe solid state lighting industry, better thermal management solu-tions are needed.

One of the established ways to enhance the efficiency of heatdissipation in LEDs is to improve the thermal conducting proper-ties of the thermal interface materials (TIM). Since LEDs aredesigned to dissipate heat at the bottom and projected the lightabove [8], thus, enhancement in the TIM properties would be vitalin order to ensure a promising thermal management of thesepackages.

An ideal TIM must possess high thermal conductivity and lowcoefficient of thermal expansion (CTE). In addition, the materialmust be soft enough to be easily deformed by contact pressureto fill all the gaps between the mating surfaces [9]. In the past dec-ades, a wide range of polymers and conductive fillers have beencombined to form composites exhibiting useful properties of TIMs.Boudenne et al. reported on Cu particles filled polypropylene com-posites. They found the highest heat transport ability for compos-ites filled with smaller particles [10].

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A study on high filler loading of 57 vol.% of aluminium nitridefilled epoxy composites was reported to have improved thermalconductivity of 15 times higher than the polymer matrix by itself[11]. Choi and Kim reported composites of binary particles systemsof AlN and Al2O3. The composites were categorized into two sys-tems, composites filled with large-sized aluminium nitride andsmall-sized aluminium oxide particles, and composites filled withlarge-sized aluminium oxide and small-sized aluminium nitride. Itwas observed that at 58.4 vol.% total filler content, the maximumvalues of thermal conductivities of both the systems were3.40 W/m K and 2.84 W/m K, respectively [12].

In this paper, commercially available thermal paste was heavilyfilled with aluminium nitride (AlN) and boron nitride (BN) particlesof different sizes. The compounded thermal paste was tested asthermal interface material (TIM) in an LED package to study theimprovement in the heat dissipation mechanism of the system.The effective heat dissipation has been discussed extensively interms of the effect of particle sizes of the ceramic fillers.

2. Experimental work

AlN and hBN powder were purchased from Sigma Aldrich withaverage particle size of 10 and 1 lm respectively. Ball milling pro-cess was carried out employing Rocklabs Crushers. The milledpowder with corresponding milling time was categorized as 30,60 and 120 min which were represented by AlN30, AlN60 andAlN120 for AlN samples and BN30, BN60, BN120 for BN samplesrespectively.

Particle size analysis was carried out on the crushed ceramicpowder using Malvern Zetasizer Ver. 6.11. The crushed powderwas fine processed using mortar and pestle and grinded for15 min for each sample powder. The fine powder was then madeinto pellets using a Specac hydraulic pelletiser with force of12,000 N. This process was repeated for all the samples of AlNand BN milled at 30, 60 and 120 min. The particles morphologywas studied using scanning electron microscope (SEM). Sampleswere coated with platinum layer in order to avoid charging effect.

On the other hand, the milled powder was incorporated as fill-ers into commercially available thermal paste, Artic Alumina fromArtic Silver Inc which is a mid-density ceramic thermal compound.The filler to thermal paste ratio was fixed as 1:4 by weight percent-age (wt.%).

The compounded AlN and BN filled thermal paste was appliedas TIM for CREE XLamp MX-6 LED. The TIM was placed betweenthe metal core printed circuit board (MCPCB) and the external heatsink of the system with constant pressure. Fig. 1 shows the sche-matic diagram of the TIM placement in the thermal set up.

Thermal and optical properties of the samples were tested withcombined thermal and radiometric LED testing. Thermal resistanceand light output measurements were performed for a given set ofambient temperature and forward current. The ambient tempera-ture was controlled using a temperature controlled peltier fixture.T3ster Master Software was used to capture the cooling transients.The transients were fed into structure function evaluations. Thedevice under test (DUT) was heated up for 600 s and the coolingtransients were captured for 600 s as well for all measurements.

Fig. 1. Schematic diagram of the TIM placement in the thermal set up.

The DUT was pressed against the external heat sink at constantpressure in order to obtain repeatable results.

3. Results and discussion

3.1. SEM analysis of filler powder

SEM images were captured on the AlN and BN pellets asdescribed in Section 2. Fig. 2(a) and (b) shows the BN30 andBN120 respectively. As observed, the BN particles get finer as themilling time increases and the particles exhibit a hexagonal chainlike structure. Fig. 3(a) and (b) shows the AlN30 and AlN120respectively. It is seen that the AlN particles are polygonal asexpected, and the particles get smaller as the milling timeincreases. Comparing Figs. 2 and 3, it is observed that BN particlesare smaller than AlN employed in this study.

3.2. Thermal transient measurement

Thermal transient measurement enables one to determine thetemperature rise undergone by the DUT driven under specific cur-rent. Structure function evaluations on the other hand aids indetermining every region inside an LED package. Each gradient ina cumulative structure function denotes each layer in a packagewhich enables one to understand the heat transfer mechanisminside the DUT.

Fig. 2. SEM micrographs of the BN powder milled at different time, (a) BN30 and (b)BN120.

Fig. 3. SEM micrographs of the AlN powder milled at different time, (a) AlN30 and(b) AlN120.

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3.2.1. Comparison between AlN and BN, 120 minFig. 4 shows the temperature rise graph of AlN and BN; 120 min

filled TIM, maintaining the ambient temperature as 25 �C. 120 minsample was chosen for this comparison as it has been reported thatthe heat transport ability is high when the particles are smaller insize [13]. Smaller particles lead to lower inter particle distance

Fig. 4. Temperature rise of AlN and BN filled TIM.

which allow the heat transfer more efficient for a system thathas large interfacial area.

As observed from Fig. 4, the temperature rise graph actuallyexplains heat path in which the heat travelled in the system. Bothcurves moved together until 2.78 s then one curve started todiverge during the transfer of heat between thermal paste and heatsink. This suggests that, the internal resistance of the package con-tributes the largest factor of the temperature rise; followed by thedivergent point as it enters the TIM area and finally into theambient.

It is observed from Fig. 4 that junction temperature for BN ishigher than AlN. The junction temperature for AlN samples is39.26 �C while for BN is 41.43 �C which is about 10.97% higher thanAlN. This difference in the temperature is as expected due to theintrinsic thermal conductivity of the filler materials. AlN has athermal conductivity of 140–180 W/m K meanwhile BN’s thermalconductivity is approximately 33.5 W/m K.

Fig. 5 shows the differential structure function of BN and AlNfilled TIM. Each peak denotes each layer inside the MX6 XLampLED. The point of divergence is clearly shown in Fig. 5 where thejunction to board thermal resistance is approximately 9.21 K/W.The transients diverge significantly as they enter the TIM area,allowing one to determine the exact value of the thermal resis-tance contributed by the LED package. Referring to Fig. 5, the junc-tion to board thermal resistance of the LED tested with BN and AlNTIM is 13.13 K/W and 11.21 K/W respectively. Again as expected,the thermal which is resistance of AlN TIM is lowering comparedto BN TIM due to the intrinsic thermal conductivity of the fillermaterials.

In addition, the heat transfer through AlN TIM is more effectivedue to the viscosity of the TIM compounded. In present work, asmentioned above, the mixing ratio was fixed at 1:4, 1 part fillerand 4 parts thermal paste. Due to the nature of BN powder whichhas a very low density of 2.10 g/cm3, the process ability was toughas it requires more BN powder compared to AlN with a higher den-sity of 3.26 g cm�3. Thus, with less filler powder required, the vis-cosity of AlN compounded TIM was maintained within processability and the flow characteristics of the TIM was more spreadablecompared to BN compounded TIM which was too viscous. Thus,besides the intrinsic thermal conductivity factor, the process abil-ity of TIM compound is equally important to ensure a good heattransfer mechanism.

Since, AlN showed better performance with lower junction tem-perature and thermal resistance, so the subsequent experimentswere carried out using AlN filler. Characteristics of AlN filled TIMwas further studied in order to observe the influence of particle sizein enhancing the system level thermal analysis of LED packages.

Fig. 5. Differential structure function of BN and AlN filled TIM.

Fig. 6. Total junction to ambient thermal resistance against ambient temperature.

Fig. 7. Average particle size of AlN with respect to milling time.

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3.2.2. Effect of ambient temperatureIn order to investigate the behaviour of the compounded AlN

TIM with ambient temperature, another set of experiment was car-ried out with the ambient temperature varied from 25 to 85 �C atan interval of 20 �C. The driving current was held constant at

Fig. 8. Cumulative structure function of di

350 mA. Fig. 6 summarizes the total junction to ambient thermalresistance for all AlN TIM under different ambient temperature.

It was observed from Fig. 6 that, there were no clear trends inthermal resistance rise with ambient temperature. Every sampleshowed random value of thermal resistance in which no significanttrends was observed from different ambient temperature. Thissuggests that the change in ambient temperature did not affectthe overall performance of the LED with different AlN TIM.

3.2.3. Effect of particle sizeFig. 7 shows the variation of particle size of AlN with respect to

milling time obtained from particle size analysis.From Fig. 7, it is clear that the AlN particle size decreases from

1170 nm to 758 nm with increase of milling time from 30 to120 min. There is a drastic drop of approximately 31.3% in theaverage particle size of AlN and the rate of reduction has alsodropped after 60 min.

Fig. 8 shows cumulative structure function of different particlesizes of AlN filled TIM. For comparison, the unmilled AlN TIM isalso included in Fig. 8.

Unlikely as stated in Section 2, in present study, it was observedfrom Fig. 8 that the total thermal resistance of the LED packagewith AlN30 TIM is lowered compared to AlN60 and AlN120 TIM,implicating that the bigger particles have achieved higher thermalconductivity. Consequently, it has been found that larger particlesform a thicker conductive path, reducing the interfacial phononscattering between the matrix and the fillers [14] and hence forman increased thermal conductivity of the composite. Due to anincreased contact area and the interfacial thermal resistance whichbecomes increasingly dominant as the particles becomes smaller[15], a significant rise in the total thermal resistance of the LEDpackage has been recorded comparing all three cases. Thus, fromthe curve, the thermal resistances of AlN30, AlN60 and AlN120TIM are found to be 11.33, 11.43 and 11.63 K/W respectively.

The differences in these thermal resistance values can be fur-ther explained with the SEM images of the dispersion of AlN parti-cles in the thermal paste. Fig. 9(a)–(c) shows the dispersion ofAlN30, AlN60 and AlN120 particles respectively incorporated intothe thermal paste.

The polygonal grains found in Fig. 9 are the AlN particles. As themilling time increases, it is significantly noticed that the AlN parti-cles becomes smaller in size. Moreover, if observed carefully, thefiller dispersion in Fig. 9(a) and (b) shows randomly packed parti-cles in which various big and small grains are visible. This packingorientation is another reason for the total thermal resistance of the

fferent particle sizes of AlN filled TIM.

Fig. 9. Dispersion of AlN particles into the thermal paste (a) AlN30, (b) AlN60 and(c) AlN120.

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LED packages to be low compared to AlN120 TIM. It has beenreported that composites mixed with different sizes of the sametype of filler give higher thermal conductivity compared with thesame type of filler with single particle size [16]. Different sizes offiller particles when compounded together would increase thepacking density of the polymer composite [17]. As a result, a highthermally conductive composite can be produced which enhancesthe heat transfer [18].

On the other hand, as shown in Fig. 9(c), the increased millingtime has reduced the particles size more evenly in which the fillersare with single particle size after 120 min of milling. An even par-ticle distribution increases the total contact area between particlesand simultaneously increases the interfacial resistance betweenparticles. Consequently, the total thermal resistance of the LEDwith AlN120 TIM was found to be higher as compared to AlN30and AlN60.

On the contrary, the total thermal resistance value for theunmilled AlN exhibits the highest thermal resistance even thoughthe particle size is largest among other samples. This is most prob-ably because, the unmilled AlN particles were incorporated intothe thermal paste without milling process, in which the particleswere utilised as purchased. Thus, the mean particle size is moreuniform (10 lm) compared to the other crushed particles incorpo-rated into AlN30, AlN60 and AlN120 samples. As discussed above,the heat transfer is much more efficient through fillers of differentparticle sizes compared to uniform particles.

Therefore, since the AlN30 TIM performs the best among therest, the next experiment was carried out with only AlN30 TIM.

3.2.4. Effect of optical powerMeanwhile, as it is widely known, the light output of an LED

strongly depends on the operating conditions. The higher the sup-plied current, the more light is generated by LEDs. However, whenthe forward current increases or when the LEDs are driven at aconstant current source, the temperature gradient increases andeventually causes a drop in the light output. This signifies theimportance of considering optical power into the calculation ofthermal resistance of any LED package [19]. The dependence ofthermal resistance with optical power is shown in Eq. (1):

Rthreal ¼T J � TA

Pel � POptð1Þ

where Pel is the electrical power and Popt is the optical power. Con-sidering optical power in the thermal resistance calculation accord-ing to Eq. (1) yields the real thermal resistance values.

Fig. 10 shows the cumulative structure function of AlN30, withand without optical correction. It was found from Fig. 10 that a com-bined measurement of thermal and optical gave a significant differ-ence in the transients where the curves shift to the right. Thermalresistance increases approximately 5.61 K/W with optical correc-tion. The presence of optical power reduces the heat power andeventually increases the total thermal resistance of the device undertest where thermal resistance can be obtained. Thus, next experi-ment was carried out as a combined measurement of thermal andradiometric to produce more accurate thermal resistance values.

3.2.5. Effect of increasing currentFig. 11 shows the cumulative structure function of AlN30 TIM

with different current.The driving current was increased from 150 to 350 mA at an

interval of 100 mA at ambient temperature of 25 �C.As observed from Fig. 11, the transients of all three samples fol-

low the same pattern form junction to ambient denoting that theTIM has got insignificant influence on the increasing drivingcurrent.

As the input current increases, the current density at the chip ofthe LED may be about 2–3 orders of magnitude higher than in theother regions [18]. This current crowding effect produces remark-able temperature non-uniformity in the die which, in turn, affectsthe conductivity of the contact layers at the chip level. This effect isprimarily related to the device self heating. Therefore, the currentcrowding is a main mechanism responsible for the rise in thermalresistance of the LED.

Fig. 10. Cumulative structure function of AlN30, with and without opticalcorrection.

Fig. 11. Cumulative structure function of AlN30 with different current.

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4. Conclusion

A comparison between AlN and BN as fillers into commerciallyavailable thermal paste employed is reported in this paper. Thecompounded mixtures were used as thermal interface materialfor effective heat dissipation of CREE MX6 LED. Among AlN andBN fillers, AlN TIM achieved less LED junction temperature witha difference of 2.20 �C compared to BN filled TIM. This is obviouslybecause of their respective thermal conductivity values. From AlNfillers of different particle sizes, it was found that among AlN30,AlN60 and AlN120 fillers, the AlN30 TIM exhibits the lowestjunction temperature of 38.49 �C and the lowest total thermalresistance of 11.33 K/W as compared to the other two fillers.

In a nut shell, employing fillers incorporated TIM results as oneof the improved methods in thermal management of power LEDs.

Present work signifies the importance of choosing the right fillersand correct particle size of the fillers to meet the expectation ofindustrial standards for efficient heat dissipation performance.

Acknowledgement

The authors would like to thank Universiti Sains Malaysia(USM) for the PRGS (1001/PFIZIK/846074) funding.

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