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Thermochimica Acta 542 (2012) 42– 45

Contents lists available at SciVerse ScienceDirect

Thermochimica Acta

journa l h o me page: www.elsev ier .com/ locate / tca

hermal diffusivity of Sn–Ag–Cu-based, Pb-free, micro- and nano-sized solderalls

nkoo Kima, Man Il Kanga, Sok Won Kima,∗, Eun Jungb, Sang Hyun Leec

Department of Physics, University of Ulsan, Ulsan 680-749, Republic of KoreaSchool of Materials Science and Engineering of Ulsan, Ulsan 680-749, Republic of KoreaDivision of Physical Metrology, Korea Research Institute of Standards and Science, Daejon 305-600, Republic of Korea

r t i c l e i n f o

rticle history:vailable online 12 November 2011

eywords:older balln–Ag–Cu

a b s t r a c t

In order to determine the effects of ball size and porosity on the thermophysical properties of soldermaterials, several Sn–3.0Ag–0.5Cu solder balls with average ball diameters of 170 nm, 10 �m, 29 �m,and 140 �m were prepared, and disk-type samples were formed under compaction pressures of 100,200, and 300 psi. The thermal diffusivity of each sample was then measured using a laser flash apparatusover a temperature range of room temperature to 150 ◦C. The results showed that the thermal diffusivity

all size effectaser flash methodhermal diffusivity

increased as both the diameter of the solder ball and the compaction pressure increased. On the otherhand, the thermal diffusivity decreased by as much as 28% for the same ball sizes and pressures at highertemperatures. Overall, the sample with a ball diameter of 140 �m prepared under a compaction pressureof 300 psi exhibited the highest thermal diffusivity (about 30 × 10−6 m2/s). Thus, it was found that thethermal diffusivity of a sample composed of solder balls is strongly dependent on the ball size, porosity,and preparation temperature.

. Introduction

Tin–lead (Sn–Pb)-based solders have long been the most popu-ar materials for electronic packaging because of their low costs andxcellent properties for interconnecting electronic components.n particular, of the many assembling technologies that utilizehese solders, ball grid array (BGA) technology has been used forigh-volume package productions during the last few years. BGA

oints provide both high mechanical strength and high electricalonductivity, which play significant roles in the establishment ofonnections between electronic components and printed circuitoards [1,2].

Yet, due to the toxicity of lead, conventional Sn–Pb solders areradually being replaced with Sn-based soldering alloys contain-ng additives of other metals such as Ag, Cu, Bi, Ga, In, Sb, and Zn.his shift has forced the development of Pb-free solders, and then–Ag–Cu ternary eutectic alloy is considered to be a promisinglternative [3–7]. In fact, several studies have already investigatedany of the properties of the Sn–Ag–Cu alloy, but its thermal prop-

rties remain relatively unexplored. Our previous study found that,mong the numerous Sn–Ag–Cu alloy series with varying Ag and

∗ Corresponding author. Tel.: +82 52 259 2388; fax: +82 52 259 1693.E-mail address: sokkim@ulsan.ac.kr (S.W. Kim).

040-6031/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.tca.2011.10.022

© 2011 Elsevier B.V. All rights reserved.

Cu contents, the Sn–3.0Ag–0.5Cu solder ball had the most desirablethermophysical properties [8].

In this study, in order to determine the effects of ball size andporosity on these thermophysical properties, the thermal diffu-sivity of several disk-type samples composed of Sn–3.0Ag–0.5Cusolder balls with different ball sizes and compaction pressures weremeasured using a laser flash apparatus.

2. Experimental

Disk-type samples were prepared using solder ball powderswith ball diameters of 170 nm to 140 �m under compaction pres-sures of 100, 200, and 300 psi without resin. The solder balls werecomposed of Sn–3.0Ag–0.5Cu ternary alloys. The nano-sized solderballs were fabricated using a fine solder wire-explosion process (thediameter of the fine solder wire was 0.3 mm) [9] and micron-sizedsolder balls were fabricated using a centrifugal atomization pro-cess (with a disc rpm of 60,000) [10]. All of the ball sizes were thenmeasured using a SEM and a TEM. The diameter and thickness ofeach disk-type sample was about 10 mm and 2 mm, respectively. X-ray diffraction patterns of the solder balls used in the samples wereobtained using an X-ray diffractometer (XRD; RAD-3C, Rigaku) with

Cu K� radiation of wavelength 1.54 A, and micromorphologies ofthe samples were investigated using a scanning electron micro-scope (SEM; JSM-6500F, JEOL). The masses of the solder balls weremeasured using a precise electronic balance (E04130, Ohaus).

imica Acta 542 (2012) 42– 45 43

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I. Kim et al. / Thermoch

The thermal diffusivity of a solder ball was measured using aaser flash apparatus (LFA 457, NETZSCH) [11,12] over a tempera-ure range of room temperature to 150 ◦C since the melting pointf the alloy is about 183 ◦C. A Nd-YAG laser produced a maximumulse energy of 18.5 J for a pulse duration of 0.5 ms, which was usedo heat the front surface of the sample. Note that the apparatus usedn this experiment employed an improved Cape–Lehman model toliminate the possible errors caused by transient heat loss and thenite laser pulse time effect [13].

. Results and discussion

.1. Structural properties

Fig. 1(a) shows an XRD pattern of the sample with a ball diameterf 140 �m under a compaction pressure of 100 psi. The pattern dis-lays the diffraction peaks of Sn, Ag, and Cu and reveals a diffractioneak for SnO2 in the (1 1 0) plane induced by oxidation. This result

ndicates that the solder ball was, indeed, a ternary alloy composedf Sn, Ag, and Cu. The samples with ball diameters of 10 �m and9 �m had patterns similar to those of the 140-�m ball under allressures. Fig. 1(b) shows an XRD pattern of the sample with a balliameter of 170 nm and a pressure of 100 psi. The pattern is againimilar to that of the 140-�m ball. However, the SnO2 peak in the1 1 0) plane caused by oxidation was more intense than that fromhe sample with a ball diameter of 140 �m. These results imply thatxidation was stronger when the diameter of the solder ball waseduced to several hundred nanometers.

Fig. 2 shows surface morphologies of the samples with differentall sizes under the same compaction pressure of 100 psi. Note thathe number of boundaries among the observed particles increasedith a decrease in ball size.

.2. Density and porosity

The masses of the samples were measured using a precise elec-ronic balance with an accuracy of 0.1 mg. The measured mass was

hen used to calculate the density of the sample. The results inig. 3 show that the density of the sample increased as the com-action pressure increased from 100 psi to 300 psi for every ballize, indicating that the space between the particles in the sample

Fig. 2. SEM images of the samples with ball diameters of (a) 170 nm, (b) 10

Fig. 1. XRD patterns of solder balls with ball diameters of (a) 140 �m and (b) 170 nmunder a compaction pressure of 100 psi.

contracted as the pressure rose. As the ball diameter of the sol-der balls decreased from 140 �m to 10 �m, the densities of thesamples also decreased linearly; however, when the ball diameterwas reduced to 170 nm, the density abruptly dropped from about6 g/cm3 to 4 g/cm3 for all compaction pressures. This drastic changein density was likely a response to the surge in the number ofboundaries and pores in the samples with reduced ball diameters.

The number of solder balls included in a sample can be estimated

using the following equation:

N ∼= Vsample

Vball(1)

�m, (c) 29 �m, and (d) 140 �m and a compaction pressure of 100 psi.

44 I. Kim et al. / Thermochimica Acta 542 (2012) 42– 45

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and like density, the thermal diffusivity increased according tothe increased compaction pressure. As the ball size of the solderball decreased from 140 �m to 10 �m, the thermal diffusivities of

Fig. 3. Densities for various ball sizes and compaction pressures.

here Vsample and Vball are the volumes of the sample and a sol-er ball, respectively. If the solder ball is a perfect sphere, thenball = (4/3)�r3 and the volume of the ball is proportional to r3.hen, as the radius of the solder ball increases, the number of ballsn the sample should decrease in proportion to r−3. Fig. 4 showshe results calculated using Eq. (1) – namely, as the radius of theolder ball decreased from 70 �m to 5 �m, the number of balls inhe sample increased from 9.56 × 104 to 2.64 × 108. In particular,hen the radius of the ball was reduced to 170 nm, the number of

alls dramatically increased by about 2.10 × 105 (from 2.64 × 108

o 5.62 × 1013). Note that the line in Fig. 4 was obtained by plottinghe results via Eq. (2):

= arb (2)

here N is the number of balls, and a and b are correlation coef-cients. According to the curve, the coefficients were determinedo be a = 3.36 × 1010 ± 1.50 × 106 and b = −3.01 ± 1.80 × 10−5. Thesealues imply that the change in the number of balls was approxi-ately proportional to r−3 when the radius of the solder ball was

ecreased.The porosities of the samples with various ball sizes were subse-

uently deduced from the obtained densities. The porosity (�) wasalculated using the formula for density, which can be expressed

s [14]:

=(

1 − �sample

�ball

)× 100% (3)

Fig. 4. The number of balls according to the radii of the solder balls.

Fig. 5. Porosities for various ball sizes and compaction pressures.

where �sample is the total average density of the sample, and �ball isthe density of the ball. The porosity results are illustrated in Fig. 5,which shows that, as the pressure increased, the porosities of thesamples decreased. This result follows the theory that, in the pack-ing of dry fine spheres, the dominant force between the spheresis the van der Waals force and thus, the porosity increases with adecrease in particle size [15]. Therefore, as the ball size of the solderball decreased from 140 �m to 10 �m, the porosities of the samplesincreased linearly. Yet, as before, when the size of the ball decreasedto 170 nm, the porosities increased nonlinearly from 23.0, 17.4, and13.3% to 42.1, 39.9, and 35.2% under pressures of 100, 200, and300 psi, respectively.

3.3. Thermal properties

Fig. 6 shows the averages of the measured thermal diffusivi-ties at room temperature. The experiments were performed fivetimes for each sample, and the uncertainties of all the obtainedvalues were found to be within 5.6%. The change in thermal dif-fusivity with ball size was similar to the variations in density,

Fig. 6. Thermal diffusivities for various ball sizes and compaction pressures.

I. Kim et al. / Thermochimica Acta 542 (2012) 42– 45 45

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Table 1Thermal diffusivity at various temperatures.

Temperature(◦C)

Compactionpressure (psi)

Thermal diffusivity with ballsize (10−6 m2/s)

170 nm 10 �m 29 �m 140 �m

RT100 0.92 16.78 22.88 28.58200 1.28 17.43 23.24 29.65300 1.38 18.15 27.32 30.01

100100 0.89 15.65 21.05 25.99200 1.25 16.22 21.58 27.16300 1.33 17.01 25.21 27.39

[[

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[[[15] A.B. Yu, C.L. Feng, R.P. Zou, R.Y. Yang, Powder Technol. 130 (2003) 76.[16] K.Y. Sastry, L. Froyen, J. Vleugels, E.H. Bentefour, C. Glorieux, Int. J. Thermophys.

25 (2004) 1611.

action pressure of 100 psi.

he samples decreased linearly; however, as the size of the ballecreased to 170 nm, the thermal diffusivity dropped more rapidlyrom 16.78 × 10−6 to 0.92 × 10−6 m2/s under a pressure of 100 psi.imilar results were obtained for the other pressures, as shown inigs. 3 and 5. Furthermore, as the radius of the solder ball decreased,he porosity and number of balls in the sample changed in propor-ion to r−3. In general, thermal diffusivity is inversely proportionalo porosity and the number of boundaries among the balls [16].hus, as the number of balls in the sample increased due to a reduc-ion in the solder ball size, the porosity and number of boundariesf the sample also grew, and the thermal diffusivity of the sampleecreased rapidly. This decrease can further be explained by theact that solder balls with pores have a relatively low thermal dif-usivity, especially when compared to metallic solder balls, and thecattering of thermal carriers (e.g., electrons) increases boundaryesistance, thereby resulting in a lower overall thermal diffusivityf the sample [17].

Additionally, the intense oxidation of the nano-sized particles,s shown in Fig. 1(b), could be another reason for the rapid decreasef thermal diffusivity. However, it is estimated to be within severalercents [18]; therefore it was not considered. Consequently, theext study will investigate the effects of oxidation on the thermalroperties of nano-sized particles. Until then, the results of thistudy indicate that larger ball sizes and lower porosities are theost optimal for creating solder balls with high thermal conduc-

ivity for use as electronic packaging material.Fig. 7 shows the thermal diffusivities for various tempera-

ures and ball sizes under a compaction pressure of 100 psi. Forhe ball sizes from 10 �m to 140 �m, the thermal diffusivitiesecreased linearly by about 11, 12 and 13% (i.e., from 16.78 × 10−6,2.88 × 10−6, and 28.58 × 10−6 m2/s to 14.96 × 10−6, 20.09 × 10−6,nd 24.57 × 10−6 m2/s), respectively, over temperatures rangingrom room temperature to 150 ◦C. These results indicate that thehermal diffusivity of the sample depends on its internal thermalesistance, and that the temperature-dependent boundary scat-ering of thermal carriers, such as electrons in metal, increasesuch resistance. Note that, in the case of the 170-nm ball size, thehermal diffusivity decreased more drastically, from 0.92 × 10−6 to.73 × 10−6 m2/s, than it did in the other cases with the same tem-erature variations. This phenomenon is thought to be caused byhe increased oxidation of nano-sized balls with the rising tem-

eratures; however, further quantitative analysis is needed to fullynderstand this effect.

[[

150100 0.73 14.96 20.09 24.57200 1.02 15.41 20.67 25.63300 1.13 16.17 24.12 25.96

Table 1 lists all of the thermal diffusivities measured across arange of temperatures, showing that, as the temperature increasedfrom room temperature to 150 ◦C, the thermal diffusivity decreasedslightly.

4. Conclusions

The thermal diffusivities of Sn–3.0Ag–0.5Cu alloy samples madeof solder balls with various ball sizes and compaction pressureswere measured using the laser flash method over a temperaturerange from room temperature to 150 ◦C. As the solder ball sizedecreased, the thermal diffusivity of the samples also decreasedsince the thermal diffusivity depends strongly on the increasedporosity and the number of boundaries formed in the samples. Inaddition, the thermal diffusivity was found to be proportional tothe compaction pressure of the sample. For a quantitative analysisof the rapid decrease in the thermal conductivity of the samplesmade from nano-sized particles, an investigation into the effects ofoxidation is needed.

Acknowledgment

This work was supported by the Priority Research Centers Pro-gram through the National Research Foundation of Korea (NRF),funded by the Ministry of Education, Science and Technology(2009-0093818).

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