Microelectronics Reliability 53 (2013) 1968–1978

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Microelectronics Reliability

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Recent advances of nanolead-free solder material for low processingtemperature interconnect applications

0026-2714/$ - see front matter � 2013 Published by Elsevier Ltd.http://dx.doi.org/10.1016/j.microrel.2013.04.005

⇑ Corresponding author. Tel.: +852 3943 1188.E-mail address: cpwong@cuhk.edu.hk (C.P. Wong).

Hongjin Jiang a, Kyoung-sik Moon b, C.P. Wong c,⇑a Intel Corporation, 5000 W. Chandler Blvd., Chandler, AZ 85226, United Statesb School of Materials Science & Engineering, Georgia Institute of Technology, 771 Ferst Dr., NW, Atlanta, GA 30332, United Statesc Faculty of Engineering, Chinese University of Hong Kong, Shatin, Hong Kong, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 19 September 2008Received in revised form 8 April 2013Accepted 9 April 2013Available online 8 July 2013

Recent advances of nanolead-free solder materials for microelectronic packaging is presented. The syn-theses of Sn, SnAg and SnAgCu nanoparticles and their size dependent melting temperature are dis-cussed. Capping nanoparticle surfaces with organic molecules for antioxidation and particle sizecontrol is studied as well. An in-house made nanosolder pastes is formulated and its metallurgical jointonto a Cu substrate is demonstrated.

� 2013 Published by Elsevier Ltd.

1. Introduction

Tin/lead (SnPb) solder alloy has been the de facto interconnectmaterial in most areas of electronic packaging, including intercon-nection technologies such as pin through hole (PTH), surfacemount techniques (SMT), ball grid array (BGA) (Fig. 1), chip scalepackaging (CSP), flip-chip etc. Lead, a major component in solder,has long been recognized as a health threat to human beings andthe major concern is that such lead from discarded consumer elec-tronics in landfills could leach into underground water and eventu-ally into drinking water system [1].

Nowadays most of electronic products have very short productlife cycles (e.g., smartphones, electronic toys, etc.) due to their fre-quent model changes, which often end up in landfills in just a fewmonths or years. Recycling lead in electronics should pay morecost and efforts than that in batteries or cathode ray tubes (CRT)due to the difficulty in removing it from the components.

In fact, the batteries occupying over 80% of lead consumptionare not considered a critical environmental issue due to its highrecycling rate. However, about 0.5% of world’s annual lead produc-tion (�5 million tons) has been employed in electronics devices,which brings critical concerns on human health.

Thanks to worldwide efforts on eliminating lead in electronicsfor decades, lead based solder in any current consumer electronicshas been replaced with lead-free materials. However, there hasbeen found no drop-in replacement for lead containing solder yetdue to its excellent wettability and mechanical properties. Thus,lead solder is still allowed in high reliability microelectronics,which includes server level computing systems, engine control

units (ECU) near the engine room of automobiles, military elec-tronics, etc.

Material scientists have been dedicating tremendous efforts ondeveloping alternative lead-free solders, based on the followingrequirements.

(1) Low melting point: The melting point of the lead-free alloyjoints should be low enough to avoid thermal damage tothe packages, yet their mechanical properties are highenough to survive the operating temperature cycles. Themelting point is required to be very close to that of theeutectic SnPb (183 �C).

(2) Wettability: Solder should readily wet the bond pads to pro-vide reliable bonding between components.

(3) Availability and cost: There should be adequate supplies withlow cost. The microelectronics industry is extremely costconscious. The electronic manufactures are unlikely tochange to an alternative solder material with an increasedcost unless it has demonstrated higher performance or thereis legislative pressure to do so.

Lead-free candidates and their respective melting points arelisted in Table 1. The melting points of the SnAg3.0Cu0.5 andSnAg3.5 alloys are 30–40 �C higher than that of the eutectic SnPballoy, and the higher melting point requires a higher reflow tem-perature. The increased reflow temperature leads to a number ofundesirable consequences such as a higher residual stress of com-ponents and substrates, which adversely affects their reliabilityincluding an increased tendency of the ‘‘pop-corning’’ in the plasticencapsulated ICs during the reflow process and increased warpagein organic substrates. Furthermore, heat-sensitive componentssuch as electrolytic capacitors might not survive the high processtemperatures. Therefore, industry’s attentions have been paid to

Fig. 1. Schematic of PTH, BGA and SMT packages using solder interconnects.

Table 1Lead-free alloys.

Alloy Melting point

Sn96.5Ag3.5 221 �CSn96Ag3.5Cu0.5 217 �CSn20Au80 280 �C (mainly used in interconnects for optoelectronic

packaging)Sn99.3/Cu0.7 227 �CSnAgCuX(X = Sb,

In)Temperature ranges according to compositions, usuallyabove 210 �C

SnAgBi Temperature ranges according to compositions, usuallyabove 200 �C

Sn95Sb5 232–240 �CSn91Zn9 199 �CSnZnAgAlGa 189 �CSn42Bi58 138 �C

H. Jiang et al. / Microelectronics Reliability 53 (2013) 1968–1978 1969

lead-free solders of low melting point for the compatible process-ing temperature to the lead containing solder.

The melting and freezing behaviors of finite systems have beenof theoretical and experimental interests for many years. As earlyas 1888, J.J. Thomson suggested that the freezing temperature ofa finite particle depends on the physical and chemical propertiesof the surface [19]. It was not until 1909, however, that an explicitexpression for a size-dependent solid–liquid coexistence tempera-ture first appeared. By considering a system consisting of small so-lid and liquid spheres of equal mass in equilibrium with theircommon vapor, it was shown that the temperature of the triplepoint was inversely proportion to the particle size [44]. A similarconclusion was later reached based on the conditions for equilib-rium between a solid spherical core and a thin surrounding liquidshell. Systematic experimental studies of the melting and freezingbehaviors of small particles began to appear in the late 1940s andearly 1950s [45,46]: first in a series of experiments on the freezingbehavior of isolated micrometer sized metallic droplets, and later

in an electron diffraction study of the melting and freezing temper-atures of vapor-deposited discontinuous films consisting of nano-sized islands of Pb, Sn, and Bi. These studies demonstrated thatsmall molten particles could often be dramatically undercooled,and that solid particles melted significantly below their bulk melt-ing temperatures. To date, the melting point of substances can bedramatically decreased when their size is reduced to nanometersize [2–12]. This is believed to be due to the high surface area tovolume ratio for nanoparticles and the surface premelting processhas been suggested as one of sources of the melting point depres-sion of the nanoparticles [13].

Transmission electron microscopy (TEM) and nanocalorimetryhave been used to study the melting behavior of a single Sn nano-particle or its cluster. At the melting point, the diffraction patternof the crystal structure in TEM exhibits the order–disorder transi-tion [14]. Lai et al. investigated the melting process of supported Snclusters by nanocalorimeter [3] and discovered the melting pointdepending nonlinearly on the inverse of the cluster radius R, whichwas in contrast to the traditional description of the melting behav-ior of small particles (Fig. 2a). They first reported a particle-size-dependent reduction of the latent heat of fusion (DHm) for Snnanoparticles (Fig. 2b). Bachels et al. studied the melting behaviorof isolated Sn nanoparticles or clusters by nanocalorimeter as well[4], where the melting point of the Sn clusters was lowered by125 K and the DHm per atom was reduced by 35% as comparedto that of bulk Sn. However, for both the TEM and nanocalorimetricmeasurements, the single element of Sn nanoparticle or the clusterwas prepared by the deposition method inside the environmen-tally controlled chambers and in situ observed in order to preventthe particles from oxidation.

Various approaches to synthesize the single element nanoparti-cles have been reported, which can be categorized into ‘‘bottom-up’’ (chemical reduction) and ‘‘top-down’’ methods (physicalmethod). The chemical reduction methods include the inert gascondensation, sol–gel, aerosol, micelle/reverse micelles, and

Fig. 2. (a) Size dependence of the melting points of Sn nanoparticles. (b) Sizedependence of the normalized heat of fusion. [3].

Fig. 3. (a) TEM and inset HRTEM images; (b) XRD of Sn nanoparticles synthesizedby using 2.1 � 10�4 mol tin (II) acetate as a precursor in the presence of 0.045 molsurfactants.

1970 H. Jiang et al. / Microelectronics Reliability 53 (2013) 1968–1978

irradiation by UV, microwave, etc. [15–23]. For bimetallic or mul-ticomponents nanoparticles, the physical ‘‘top-down’’ method hasbeen used as well. The chemical method uses the co-reduction ofdissimilar metal precursors or successive reduction of two metalsalts, which is usually carried out to prepare a core–shell structureof bimetallic nanoparticles [47–50]. The binary alloys in the formof nanoalloys or core–shell structures such as Ag–Au are an alloysystem that forms the solid solution and the order of the coreand shell can be controlled by the reduction order. On the otherhand, in the case of alloys that do not favor the formation of solidsolutions such as eutectic alloys (Sn-based alloys), limited researchresult has been reported. The physical method can be used to syn-thesize both monometallic [24] and bimetallic nanoparticles [25]by which nanoparticles can be synthesized in gram quantities di-rectly from the bulk materials without complex reactionprocedures.

Hsiao and Duh has reported Sn–3.5AgxCu (x = 0.2, 0.5 and 1.0)nanoparticle synthesis for the lead-free solder application [26],where the differential scanning calorimetry (DSC) profile showedthe endothermic peak of the SnAgCu alloy nanoparticles at�215 �C, which is the melting point of the micron meter sizedSnAg3.0Cu0.5 alloy. The report exhibited no obvious melting pointdepression from their particles.

Collaborative research work between Intel Corp. and our groupat the Georgia Tech has been carried out regarding fundamentalunderstanding of nanosolder particle syntheses and their thermalproperties for low temperature interconnect technology. Also, a re-search project, namely Pb-free Nano Solder Project funded byInternational Electronics Manufacturing Initative (iNEMI), has beenperformed during 2005–2009 by Motorola, Inc., Indium Corp., Pur-due Univ., Delphi, etc. [42]. In addition, Chalmers and Shanghai

Universities have also collaborated on nanosolder projects [43].They have demonstrated reduced melting and solidification tem-peratures of nano Sn particles in DSC but the oxidation of Sn par-ticles was an unresolved issue.

In this article, we report recent results of our efforts in creatingnanosolder particles including synthesizing oxide-free Sn nanopar-ticles and Sn based alloys and their thermal properties, finallydemonstrating their wetting performance of an in-house made sol-der paste.

2. Syntheses and thermal properties of Sn nanoparticles

Fig. 3a shows the TEM and inset HRTEM images of Sn nanopar-ticles synthesized by using 2.1 � 10�4 mol tin (II) acetate as a pre-cursor in the presence of 0.045 mol of an organic surfactant withan amine functional group. The average particle size calculatedfrom the TEM picture is around 61 nm and the obvious latticefringes in the HRTEM image implied the crystalline structure. Theinterplanar spacing is about 0.29 nm which corresponds to theorientation of (200) atomic planes of the tetragonal structure ofSn. In the XRD pattern (Fig. 3b), all the peaks can be indexed to atetragonal cell of Sn with a = 0.582 and c = 0.317 nm. The relativeintensity of the peaks is consistent with that of Sn nanoparticlesreported elsewhere [27]. No obvious peaks at 26.8�, 33.8� and51.8� are found, which match the crystal planes of SnO2: (110),(101), (200) and (211), respectively [28]. However, without thesurfactant the obvious oxidation peaks of SnO2 are observed from

H. Jiang et al. / Microelectronics Reliability 53 (2013) 1968–1978 1971

the XRD pattern of the Sn nanoparticles. The XRD and HRTEM re-sults shows that the Sn nanoparticles synthesized are well cappedwith the surfactant and prevented from oxidation.

Fig. 4 shows TGA (a) and DSC (b) curves of the as-synthesized Snnanoparticles with the amine functional group surfactant. Theweight loss at below 275 �C is attributed to the evaporation of ab-sorbed moisture and the decomposition of the surfactants, whilethe weight gain above 275 �C is due to thermal oxidation of theSn nanoparticles. The surfactant on the Sn nanoparticles is by�4 wt% to the particle. Fig. 4b shows the thermal profiles of theas-synthesized Sn nanoparticles obtained from DSC where themelting point (Tm) was observed at 223.5 �C. Compared to the Tm

of micron sized Sn particles (232.6 �C), the Sn nanoparticles exhib-ited the Tm depression by �9.1 �C.

Fig. 5a, c and e shows TEM images of the Sn nanoparticles syn-thesized by using different ratios of a tin (II) acetate precursor vs. asurfactant and their DSC profiles are (b), (d) and (f), respectively.For the precursor 1.75 � 10�4 mol/the surfactant 0.045 mol, theparticle size is around 26 nm and the melting onset point is around�190 �C, which is 42.6 �C lower than that of micron sized Sn parti-cles. The width of the endothermic curve ranges across about 70 �C,which is much wider than other three samples. This phenomenonis attributed to a broadening of the phase transition due to thefinite size effect [29] and wide size distribution of the Sn nanopar-ticles. Schmidt et al. also found a melting temperature range of60 K for cluster size with the number of atoms between 70 �Cand 200 �C [8]. This broadening also reflects that melting is a

Fig. 4. TGA (a) and DSC (b) profiles of the as-synthesized Sn nanoparticles.

cooperative phenomenon that is not well defined for a small num-ber of members of ensemble [41].

From Table 2, it is seen that the larger molar ratios between thesurfactant used in this study and the precursors, the smaller theparticle size is. This is because a larger amount of surfactants canfurther restrict the growth of Sn nanoparticles. The surfactant mol-ecules coordinate with the nanoclusters, resulting in the cappingeffect to restrict the particle growth. This capping effect was alsofound by Pal et al. on their gold nanoparticle syntheses, whereincreasing the concentrations of surfactants limits the particle sizethrough the restriction of particle growth [30]. The DSC resultsshow size dependent melting depression behavior and size depen-dent latent heat of fusion.

Fig. 6 shows the size dependence of the melting points of thesynthesized Sn nanoparticle powders, which is compared withLai et al.’s model [3]. Lai et al. obtained this solid line based onthe model of Hanszen [31] in which it was assumed that the solidparticle was embedded in a thin liquid overlayer and the meltingtemperature was taken to be the temperature of equilibrium be-tween the solid sphere core and the liquid overlayer of a given crit-ical thickness. Our experimental results are in reasonableagreement with the Lai et al.’s model and the melting point de-pends nonlinearly on the cluster radius.

It is also discovered that the DHm of the different sized Sn nano-particles is smaller than that of micron sized Sn powders. Ercolessiet al. have found that DHm of gold nanoparticles decreases steadilyfrom 114 meV/atom (bulk) to 23 meV/atom (N = 879) and 10 meV/atom (N = 477) by molecular dynamic simulation [32]. It is alsofound by experiments that the normalized DHm of Sn nanoparticlesdecreases markedly from the bulk value (58.9 J/g) by as much as70% when the particle size is reduced, which can be interpretedas a solid core melting following the gradual surface melting forsmall particles [3]. The DHm values of Sn particles in our studyare smaller than those of Lai et al.’s model as well.

3. Syntheses and thermal properties of 96.5Sn/3.5Ag alloynanoparticles

Fig. 7a shows the TEM image of the SnAg3.5 alloy nanoparticlessynthesized by using 7.4 � 10�4 mol tin (II) 2-ethylhexanoate and3.0 � 10�5 mol silver nitrate in the presence of 5.6 � 10�4 mol sur-factants at 0 �C. The average diameter of the particles is smallerthan 24.0 nm. The XRD patterns of the as-synthesized SnAg3.5 al-loy nanoparticles are shown in Fig. 7b. In addition to the peaks in-dexed to a tetragonal cell of Sn with a = 0.582 and c = 0.317 nm, theAg3Sn phase (�39.6�) is found in the XRD patterns, indicating thesuccessful alloying of Sn and Ag is made via the reduction process[26,33]. No prominent oxide peak was observed in the XRD pat-terns due to the successful protection by the surfactant [34,35].

Fig. 8 shows the HRTEM image of the as-synthesized SnAg3.5alloy nanoparticles, where the particles are in the form of core–shell structures. The dark core and the brighter shell correspondto the crystalline metal and the amorphous organic surfactants,respectively.

Fig. 9a shows the TGA curve of the dried SnAg3.5 alloy nanopar-ticles under a nitrogen atmosphere. The weight loss at below180 �C is due to the evaporation of physically absorbed moistureand surfactants. The weight gain above 180 �C is attributed to ther-mal oxidation. Fig. 9b displays the DSC profile of the dried SnAg3.5alloy nanoparticles. The melting onset point of the SnAg3.5 nano-particles is found at �199.3 �C, about 23 �C lower than that ofthe micron sized SnAg3.5 particles (222.6 �C). This is an obvioussize dependent melting point depression. The DHm of the SnAg3.5alloy nanoparticles (24.2 J/g) is smaller than that of micron sizedSnAg3.5 powders (68.6 J/g). The recrystallization onset point of

Fig. 5. TEM image and thermal behavior of Sn nanoparticles which were synthesized by using 4.2 � 10�4 mol (a and b), 1.1 � 10�3 mol (c and d), 1.75 � 10�4 mol (e and f) tin(II) acetate as a precursor in the presence of 0.045 mol surfactants.

Table 2Onset temperature and heat of fusion of different sized Sn nanoparticles.

Samples Surfactants/precursor

Average size(nm)

Onset temperature(�C)

DH (J/g)

1 N/A Micron size 232.6 60.02 0.045/1.1 � 10�3 50.6 ± 10 228.60 32.13 0.045/2.1 � 10�4 61 ± 10 225.96 24.54 0.045/4.2 � 10�4 52 ± 8 223.72 28.75 0.045/

1.75 � 10�426 ± 10 190.36 27.4

1972 H. Jiang et al. / Microelectronics Reliability 53 (2013) 1968–1978

the as-synthesized SnAg3.5 alloy nanoparticles is found at �115 �C,which is �85.4 �C lower than that of the micron sized SnAg3.5 par-ticles (200.4 �C). Fine particles freeze at different temperaturesthan the bulk and undergo significantly higher supercooling [36].

The TEM image and the corresponding DSC curves of SnAg3.5alloy nanoparticles which are synthesized by using 7.4 � 10�4 moltin (II) 2-ethylhexanoate and 3.0 � 10�5 mol silver nitrate as pre-cursors in the presence of 1.1 � 10�3 mol surfactants at � �20 �Care shown in Fig. 10a and b. The average diameter of the as-synthe-sized nanoparticles is below 10 nm (Fig. 10a). Fig. 10b shows themelting onset temperature of 172.6 �C, which is about �46.9 �Clower than that of micron sized SnAg3.5 particles (219.5 �C). The

Fig. 6. Size dependence of the melting points of Sn nanoparticles (Y error bars standfor the melting temperature from the onset point to the peak point of DSC curves).The solid line is calculated from an equation in a Ref. [3].

Fig. 7. TEM image (a) and XRD patterns (b) of the SnAg3.5 alloy nanoparticlessynthesized by using 7.4 � 10�4 mol tin (II) 2-ethylhexanoate and 3.0 � 10�5 molsilver nitrate as precursors in the presence of 5.6 � 10�4 mol surfactants.

Fig. 8. HRTEM image of SnAg3.5 alloy nanoparticles synthesized by using7.4 � 10�4 mol tin (II) 2-ethylhexanoate and 3.0 � 10�5 mol silver nitrate asprecursors in the presence of 5.6 � 10�4 mol surfactants.

Fig. 9. TGA (a) and DSC (b) profiles of the SnAg3.5 alloy nanoparticles synthesizedby using 7.4 � 10�4 mol tin (II) 2-ethylhexanoate and 3.0 � 10�5 mol silver nitrateas precursors in the presence of 5.6 � 10�4 mol surfactants.

H. Jiang et al. / Microelectronics Reliability 53 (2013) 1968–1978 1973

melting takes place over a temperature range of about 21.7 �C,which is much wider than the micron sized particles (3.1 �C). Asmall peak at 226.0 �C was also observed in the heating scan, whichresults from the melting of a small amount of larger sized particlesexisting in this sample.

Table 3 shows the peak temperatures and DHm of differentsized SnAg3.5 alloy nanoparticles. The peak temperatures andDHm vs. the corresponding particle radii are plotted in Fig. 11where both the particle size dependent melting point and DHm

are observed. It has been reported that surface premelting of small

particles occurs in a continuous manner over a broad temperaturerange, whereas the homogeneous melting of the solid core occursabruptly at the critical temperature Tm [38,39]. For tiny particles,the surface melting is strongly enhanced by curvature effects.Therefore with the decreasing of particle size, both the meltingpoint and DHm decrease as well.

4. Syntheses and thermal properties of 96.5Sn/3.0Ag/0.5Cu alloynanoparticles

Fig. 12 shows the TEM image of the as-synthesized SnAg3.0-Cu0.5 alloy nanoparticles with the average diameter of �22 nm,

Fig. 10. TEM image (a) and DSC curves (b) of the SnAg3.5 alloy nanoparticlessynthesized by using 7.4 � 10�4 mol tin (II) 2-ethylhexanoate and 3.0 � 10�5 molsilver nitrate as precursors in the presence of 1.1 � 10�3 mol surfactants.

Table 3Peak temperatures and heat of fusion of the as-synthesized different sized SnAg alloynanoparticles.

No. Surfactants(mol)

Reactiontemperature(�C)

Averagediameter(nm)

Peaktemperature(�C)

DHm

(J/g)

1 5.6 � 10�4 25 64 220.0 44.72 5.6 � 10�4 0 24 209.5 24.23 8.0 � 10�4 ��10 17 206.0 15.14 1.1 � 10�3 ��20 10 194.3 9.95

The melting point and latent heat of fusion for micron sized 96.5Sn3.5Ag are222.6 �C and 68.5 J/g respectively.

Fig. 11. Peak temperatures and heat of fusion in DSC with radius of as-synthesizedSnAg3.5 alloy nanoparticles.

Fig. 12. TEM image of �22 nm SnAg3.0Cu0.5 alloy nanoparticles.

Fig. 13. XRD pattern of �22 nm SnAg3.0Cu0.5 alloy nanoparticles.

Fig. 14. TGA curve of �22 nm SnAg3.0Cu0.5 alloy nanoparticles.

1974 H. Jiang et al. / Microelectronics Reliability 53 (2013) 1968–1978

and its XRD patterns are shown in Fig. 13. In addition to the peaksindexed to a tetragonal cell of Sn with a = 0.582 and c = 0.317 nm,the Ag3Sn and Cu6Sn5 intermetallic phases are found in the XRDpatterns, indicating the successful alloying of Sn and Ag after thereduction process [26,33,40]. Well protected nanoparticles fromoxidation show no prominent oxide in the XRD patterns [34,40].The HRTEM characterization shows that the surfactants cover theparticle surface and form a core–shell structure [40]. The core isfrom the crystalline metal particles and the shell from the amor-phous surfactants. The amorphous surfactant shells on the particlesurface protect the alloy particles from the diffusion of oxygen.

Fig. 15. DSC profile of �22 nm SnAg3.0Cu0.5 alloy nanoparticles during heatingand cooling.

Fig. 16. TEM image (a) and DSC curves (b) of 10–13 nm SnAg3.0Cu0.5 alloynanoparticles.

Table 4The endothermic peak and recrystallization points, heat of fusion of different sizedSnAgCu alloy nanoparticles.

Size Peak point (�C) DH (J/g) Recrystallization (�C)

Micron 217 72.2 145�28 nm 210.4 42.7 112.3�22 nm 207.3 31.0 109.9�18 nm 206.1 24.7 108.7�10–13 nm 199 15.9 103.6

Fig. 17. The composition of solder pastes.

H. Jiang et al. / Microelectronics Reliability 53 (2013) 1968–1978 1975

Fig. 14 shows the TGA curve of the as-synthesized SnAg3.0Cu0.5alloy nanoparticles dried under a nitrogen atmosphere. Above180 �C, the weight gain was observed, which is attributed to ther-mal oxidation of the SnAg3.0Cu0.5 alloy nanoparticles.

The thermal properties of the as-synthesized SnAg3.0Cu0.5 al-loy nanoparticles are studied by DSC (Fig. 15). In the first heatingscan, a melting onset is observed at �198 �C, which is about

19–21 �C lower than the melting point of micron sized SnAg3.0-Cu0.5 (217–219 �C) particles. In the cooling scan, the much re-duced supercooling temperature is found. Such a super coolingeffect in the recrystallization of the melted Sn and SnAg alloy nano-particles has been reported as well [36,37,40].

Fig. 16a shows the TEM image of 10–13 nm sized SnAg3.0Cu0.5alloy nanoparticles. From DSC studies (Fig. 16b), the melting onsettemperature is 177 �C, which is 37.5 �C lower than micron sizedparticles (214.5 �C). Like other nanoparticles in this study, thisSnAgCu nanoparticle also shows melting point depression andbroadening of the melting region.

Table 4 shows the melting peaks, DHm and recrystallizationpeak temperatures of different sized SnAg3.0Cu0.5 alloy nanopar-ticles, indicating the size dependency. From DSC study we ob-served marked size dependent melting points and DHm of notonly a single element metal (Sn) but also two and three compo-nents alloys (SnAg3.5 and SnAg3.0Cu0.5).

5. Solder paste and wetting properties of nanoalloy solderpastes

A solder paste consists of solder alloy powders and flux vehicle,which is used for forming metallurgical bonds at a given reflowcondition (Fig. 17). The flux vehicle is a carrier of the solder pow-der, providing desirable rheology, tackiness and protecting solderand bond pads from re-oxidation, which includes fluxing agentsand various chemical additives. The fluxing agent removes the sur-face oxides such as SnO and CuO from the solder particles and bondpads, respectively, during the reflow. Typical fluxing chemistry isshown in the below.

SnOþ 2R3COOH ¼ SnðR3COOÞ þH2O

CuOþ 2R3COOH ¼ CuðR3COOÞ þH2O

The cleaned metal particles are brought to coalescence, meltand act as glue between components’ lead and the cleaned bondpads resulting in metallurgical joints (Fig. 18). There are severaltypes of fluxes:

� Type R – rosin flux, the weakest, containing only rosin withoutthe presence of activator.� Type RMA (mildly activated rosin) is a system containing both

rosin and activator.

Fig. 18. Schematic of fluxing agent function in the solder paste.

Fig. 19. The chemical structures of the acids used in rosin.

Fig. 20. The chemical structures of BHT and hydroquinone.

1976 H. Jiang et al. / Microelectronics Reliability 53 (2013) 1968–1978

� Type RA is a fully activated rosin and resin system, having ahigher flux strength than the RMA type.� Type OA is an organic acid flux and possessing high fluxing

activity and is generally considered corrosive.� Type SA (synthetic activator) is designed to improve fluxing

activity on hard-to-solder surfaces. The SA flux displays betterwetting ability than RA flux and is equivalent to OA flux.

As major components of unmodified rosin are used abietic acid,isopimaric acid, neoabietic acid, pimaric acid, dihydroabietic acidand dehydroabietic acid. The chemical structures of these acidsare shown in Fig. 19.

The flux vehicles we used in this study consisted of tackifier,solvent, anti-oxidation agent and surfactants.

The tackifier is of typically medium-to-high viscosity, high sur-face tension liquid serving to wet the printed circuit board and thecomponent, retaining the component in a position during handlingand reflow soldering. The tackifier usually comprises one or moreglycols, aromatic hydrocarbon solvents, aliphatic hydrocarbon sol-vents or polymers.

A flux vehicle includes solvent that is chemically inert withother ingredients in the flux. Three important parameters for sol-vents are boiling point, viscosity and polarity. Alcohols are usuallyused as solvents. The more hydroxyl groups the solvents have, thebetter activity the flux has.

Antioxidation agents are used for protecting solder particlesfrom oxidation, for which butylated hydroxytoluene (BHT) andhydroquinone (Fig. 20) have been typically used. Oxygen reactspreferentially with BHT or hydroquinone rather than oxidizing Snalloy, thereby protecting them from oxidation.

The surfactant is a compound which improves the solder wet-ting rate and enables better and more uniform spreading of moltensolder across the surface to be soldered. Suitable surfactants in-clude polybasic acids, e.g., polycarboxylic acids such as dicarbox-ylic and tricarboxylic acids. The dibasic acids typically have fourto ten carbon atoms. Suitable tricarboxylic acids typically compriseacids having six to seven carbon atoms. Other suitable surfactantsinclude hydroxyl substituted polybasic acids, such as tartaric acidand citric acid. The selected surfactant is present in the flux mix-ture in an amount of at least one weight percent of the resultantflux mixture.

The solderability, flow properties, wetting properties and solid-ification are important physical properties for the solder pastes.Solderability is the ability to achieve a clean, metallic surface onsolder powder and on substrates during the dynamic heating pro-cess so that a complete coalescence of solder powder particles andgood wetting of molten solder on the surface of the substrates canbe formed. Solderability depends on fluxing efficiency by the solderpastes and the quality of surface of substrates.

When heat is applied to the pastes through any means, thepaste tends to spread or slump due to gravity and to thermal en-ergy generated. The surface energies of the liquid and the solidsubstrates are key factors in determining the spreading and wet-ting properties. For a system with liquid to wet the solid substrate,the spreading occurs only if the surface energy of the substrate tobe wetted is higher than that of the liquid to be spread.

The solidification of liquid metal occurs by nucleation and crys-tal growth. Dendritic growth during solidification is a commonphenomenon in pure metals and alloys. The heat flow during thesolidification and the crystal structure of the alloy are crucial fac-tors to the properties and structure of the solidified alloy.

Application techniques for the solder pastes usually includeprinting and paste dispensing. Stencil printing is a viable methodto economically produce accurate and reproducible transfer ofpaste onto the designed pattern, but only suitable for flat surface.However, paste dispensing can be used for irregular surfaces orhard-to-reach area.

Table 5The composition of flux and vehicle made by our group.

Fluxing agent

Tackifier

Surfactant

Solvent

Antioxidation BHT

Fig. 21. SEM image of the soldered interface of SnAg3.5 alloy nanoparticles (sample#1 in Table 3) based paste on the cleaned copper foil after reflow.

Fig. 22. SEM image of the soldered interface of the 50 nm SnAg3.0Cu0.5 alloynanoparticles based paste on copper foil after reflow.

H. Jiang et al. / Microelectronics Reliability 53 (2013) 1968–1978 1977

Conduction reflow is most suitable for the assemblies with flatsurfaces, composed of thermal conductive materials as the sub-strates and with single-side component/device populations (fastheating rate and operational simplicity). Infrared reflow is a dy-namic process. The precise temperature profile depends on solderalloy composition, properties of the pastes and assembly involved.Some other reflow methods, such as vapor phase reflow, convec-tion reflow, hot gas reflow, resistance reflow, laser reflow, induc-tion reflow are also being used.

Flux residues after soldering consist of polar organics, nonpolarorganics, ionic salts and metal salts of organics. The cleaning sol-vents include trichlorotrifluoroethane, 1,1,1-trichloroethane, ace-tone, methylene chloride, low carbon-chain alcohols, water etc.Basic techniques: vapor degreasing, liquid spray, liquid immersion,high pressure spray and liquid immersion with ultrasonic acid.

A certain amount of fluxing agents, solvent, anti-oxidationagents, surfactants and a tackifier are mixed together to preparethe flux and vehicle for the SnAg3.5 or SnAg3.0Cu0.5 alloy nano-particles. Table 5 shows the composition of the flux vehicle in thisstudy and the chemical structures of ingredients used.

The SnAg3.5 nanoparticle (sample 1 in Table 3) is mixed with anacidic type flux to form nanosolder pastes at room temperature. Acopper foil is cleaned by hydrochloric acid to remove the oxidelayer and then rinsed with DI water for 4 times. Thereafter, thenanosolder paste is dispensed on top of the copper foil and then re-flowed at 230 �C in an air atmosphere for 5 min. The cross-sectionof the sample after the reflow is shown in Fig. 21 and it is seen thatthe alloy forms metallic joint to the cleaned copper foil surface. Theenergy dispersive spectroscopy (EDS) results revealed the forma-tion of typical scallop-like intermetallic compound (IMC) of Cu6Sn5

with �4.0 lm.The SnAg3.0Cu0.5 paste is also prepared with �50 nm (average

diameter) alloy nanoparticle and the wetting test is performed at220 �C with the same way as for the SnAg3.5 nanosolder. TheEDS result reveals the formation of the intermetallic compounds(Cu6Sn5) in Fig. 22 which thickness is approximately 10.0 lm. Fur-ther studies on the wetting properties of different sized alloy nano-particles at different reflow temperatures will be performed.

6. Concluding remarks

Low temperature processible lead-free solder is desirable forenhanced reliability of microelectronics as well as green electronicpackaging. Many efforts to reduce the reflow temperature of lead-free solders by using the size effect of nanomaterials have beendedicated by collaborative work between industry members andacademia.

Nanoparticles of Sn, SnAg3.5 and SnAg3.0Cu0.5 are successfullysynthesized and their size dependent melting temperatures andDHm are demonstrated. Marked size dependency of melting pointsand DHm of not only a single element metal (Sn) but also two andthree components alloys (SnAg3.5 and SnAg3.0Cu0.5) is observed.

As nanosized metal particles have a strong tendency to formoxide shell layers or turn into entire oxides, capping the Sn basednanoparticles with organic molecules plays an important role.The amine functional group containing surfactant is found to bevery efficient in not only protecting the Sn based nanoparticlesfrom oxidation but also controlling the particle size.

Further studies on surfactant chemistry for capping during par-ticle synthesis and decapping behavior during reflow are also cru-cial for the successful nanolead-free solder paste development.Wetting study for nanosolder pastes of SnAg3.5 and SnAg3.0Cu0.5nanoparticles on Cu pads exhibited the successful IMC formationon the Cu pad.

1978 H. Jiang et al. / Microelectronics Reliability 53 (2013) 1968–1978

Nanolead-free particle research is increasingly important fornanosolder as well as for metal filled polymer composites and itsprintable ink applications to be employed for ultra-fine pitch inter-connections for 2D/3D printed electronics. Therefore, a variety ofrelated interesting research reports are to be expected in nearfuture.

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