Materials Letters 66 (2012) 135–137

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Materials Letters

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Textured growth of Cu6Sn5 grains formed at a Sn3.5Ag/Cu interface

Mingyu Li a,b,⁎, Ming Yang b, Jongmyung Kim c,⁎⁎a State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin 150001, People's Republic of Chinab Shenzhen Graduate School, Harbin Institute of Technology, Shenzhen 518055, People's Republic of Chinac Jeonnam Provincial College, Jeonnam 517-802, Republic of Korea

⁎ Correspondence to: M. Li, State Key Laboratory of AHarbin Institute of Technology, Harbin 150001, People+86 755 26033463.⁎⁎ Corresponding author. Tel.: +82 61 3808603; fax:

E-mail addresses: myli@hit.edu.cn (M. Li), Kimjm@d

0167-577X/$ – see front matter © 2011 Elsevier B.V. Adoi:10.1016/j.matlet.2011.08.014

a b s t r a c t

a r t i c l e i n f o

Article history:Received 23 June 2011Accepted 4 August 2011Available online 9 August 2011

Keywords:Lead-free solderCu6Sn5

Crystal growthTexture

The growth orientations of Cu6Sn5 grains formed at a Sn3.5Ag/polycrystalline Cu interface were investigated.Similar as reported on Cu single crystals, strong textures in Cu6Sn5 layers can also form on polycrystalline Cu,but the texture formation mechanisms differ. The texture formation on polycrystalline Cu occurs during theripening growth and results from the differences in stability of the interfacial grains with various orientationsat different temperatures. A reaction temperature of 240 °C causes the Cu6Sn5 layer to form [0001] texture inthe direction normal to the substrate, and a special morphology of interfacial Cu6Sn5 grains can be formed onthis layer to reinforce joint properties.

dvanced Welding and Joining,'s Republic of China. Tel./fax:

+82 61 3808604.orip.ac.kr (J. Kim).

ll rights reserved.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The trend toward the miniaturization of electronic products leadsto the need for a shrinkage of joint size, resulting in a high volumefraction of Cu6Sn5 intermetallic compounds (IMCs) generated at theSn-based solders/Cu interface [1]. Due to the different electronicmobility and atomic diffusivity along various crystal orientations [2],the growth orientations of interfacial Cu6Sn5 grains may have a greatinfluence on the electrical and mechanical characteristics of solderjoints. Therefore, an understanding of the growth orientation ofCu6Sn5 at the interface is necessary.

Recently, some new findings indicate that the Cu6Sn5 with strongtexture would form on some Cu single crystals [3,4]. This textureforms in nucleation stage, caused by the low misfit between Cu6Sn5

and Cu in some crystal lattice directions [3,4]. On the other hand,Cu6Sn5 with strong texture was also observed, in some documents, onpolycrystalline Cu where the nucleation behavior of Cu6Sn5 is quitedifferent from that on Cu single crystals [5–8]. This phenomenon,however, has not been systematically studied and explained appro-priately, let alone its potential application. Here, the growthorientations of Cu6Sn5 grains formed at the Sn3.5Ag/Cu (polycrystal-line Cu unless otherwise specified) interface were investigated interms of reflow temperatures and exposed times using electronbackscattering diffraction (EBSD) analysis. In addition, the potential

application of the interfacial Cu6Sn5 grain growth characteristic wasevaluated.

2. Experimental

To prepare EBSD samples, ~150 mg of Sn3.5Ag beads were placedon polycrystalline Cu foil (5 mm×5 mm×0.3 mm) and then reflowedby hot air at peak temperatures of 240 °C and 280 °C for 30 min and600 min, respectively. The reflow parameters were determined basedon Ref. [8]. All samples were cooled in water after soldering.Subsequently, solder alloys of all of the samples were etched outwith 10% HNO3 to reveal the interfacial Cu6Sn5. To facilitate EBSDcharacterization, these samples with interfacial Cu6Sn5 exposed weremounted and after that, the tips of interfacial Cu6Sn5 grains werecarefully grinded and polished to faceted surfaces (Fig. 1). During theEBSD acquisition, a step size of 1–2 μm was chosen.

3. Results and discussion

Since the Cu6Sn5 layer was quickly cooled down after soldering,the η-Cu6Sn5 with a hexagonal structure was used in our EBSDanalysis [5–9]. To obtain the texture of the IMC, numerous grains froman area of at least 500 μm×500 μm were analyzed. Fig. 2(a)–(d)shows the pole figures (PFs) of Cu6Sn5 grains formed at the Sn3.5Ag/Cu interface. Both the interfacial grains formed at 240 °C and thoseformed at 280 °C show clear textures after reflow for 30 min and theirorientation distributions become more concentrated with increasedreflow time. This change can be seen more clearly from themisorientation chart (Fig. 3). Thereinto, the misorientation is denotedby θ, the angle between [0001] direction of Cu6Sn5 and the normaldirection of the Cu substrate [Fig. 2(g)]. As shown in Fig. 3, after reflow

Fig. 1. Top-polished Cu6Sn5 layer, which is parallel to the substrate and has aninclination of 70° to the observation plane.

Fig. 3. Misorientation of Cu6Sn5 layer with polycrystalline Cu substrate toward [001]direction.

136 M. Li et al. / Materials Letters 66 (2012) 135–137

for 600 min, the θ for about 70% of the interfacial grains formed at240 °C falls within 30°, and that for about 70% of the interfacial grainsformed at 280 °C falls anywhere between 45° and 60°.

Based on the above results, we hypothesized the texture formationof the Cu6Sn5 layer formed on polycrystalline Cu. Due to the largedriving force for the chemical reaction between Cu and Sn atoms,Cu6Sn5 crystallites can form very fast by the heterogeneousnucleation, and cover the entire Cu substrate as soon as the soldermelts [1,6,10]. This indicates that the initial nuclei for the samplesreflowed at 240 °C and those reflowed at 280 °C are similar, implyingthe texture should form during the Cu6Sn5 growth process. Thegrowth of Cu6Sn5 grains at the interface is dominated by a ripeningprocess: a Cu6Sn5 grain grows at the expense of its nearest neighborsand therefore the interfacial grains become bigger but fewer withtime [11]. This means that a grain appearing after reflow for a while isderived from an initial nucleus with the same orientation. Onpolycrystalline Cu, which has lots of nucleation sites, such as grainboundaries, surface defects, and Cu grains with different orientations,random nucleation will become dominant [3]. That is to say, theorientations of Cu6Sn5 nuclei are random at initial stage. A thermalgrooving process due to simultaneous dissolution and growth of theCu6Sn5 makes those nuclei with different orientations grow into asimilar scalloped structure [8,12]. In addition, the Cu6Sn5 crystal isanisotropic. Thus, the surface energies for the scallops with variouscrystal orientations should be different in the same system. Inripening, the driving force is the reduction of total energy, indicatingthat when their sizes are similar in the earlier stage, the scallops withhigher surface energy are easier to dissolve and those with lowersurface energy grow faster. The latter become bigger than the formerwith reflow time increasing, and “swallow” the former more easilydue to the bigger size [11]. Consequently, the texture gradually formsand becomes stronger with reflow time. In the light of the textures

Fig. 2. Pole figures of Cu6Sn5 formed at Sn3.5Ag/Cu interface at 240 °C for (a) 30 min and (280 °C for (e) 30 min and (f) 2 min. (g) Diagram for the orientation of Cu6Sn5 at the int

formed in molten Sn3.5Ag solder (Fig. 3), it is basically concluded thatthe surface energy for the grains with [0001] direction approximatelynormal to the substrate is lower at 240 °C, while that for those with θbeing about 45°–60° is lower at 280 °C, although the exact values oftheir surface energies are hard to determine.

As mentioned earlier, a grain appearing after reflow for a while isderived from an initial nucleus. Therefore, no new crystal would appearduring the ripening growth. This implies that the growth orientations ofinterfacial IMCs may have some “heredity”: the growth orientationevolution of interfacial grains is based on the grain orientations formedin the previous reflows. As shown in Figs. 2(e) and 3, the orientationdistributions for the interfacial grains formed at 240 °C for 600 min afterreflow at 280 °C for 30 min are still within the range of the grainorientations appearing after the first reflow, but become moreconcentrated to a smaller θ. In consideration of the orientationdistributions of the interfacial grains formed only at 240 °C and 280 °C(Fig. 3), it is reasonable to believe that a grain with a smaller θ is morestable at a lower temperature. In addition, it should be pointed out thatthe texture for the grains formed at 240 °C for 600 min after reflow at280 °C for 2 min has been quite different from that for the grains onlyformed at 240 °C for 600 min [Fig. 2(f)]. The differences in the twotextures indicate that the initial nuclei with [0001] direction approx-imately normal to the interface are very unstable at 280 °C, most ofwhich will be “swallowed” after reflow for less than 2 min.

Based on the above findings, the growth of interfacial Cu6Sn5 canbe intentionally controlled to improve joint proprieties, combinedwith the growth behavior of Cu6Sn5 at the interface. In previous work,it was found that if the precipitated Cu is enough in quantity duringsolidification, long prismatic grains will form on the existing

b) 600 min, 280 °C for (c) 30 min and (d) 600 min, 240 °C for 600 min after reflow aterface.

Fig. 4.Morphologies of Cu6Sn5 formed on Cu substrate at (a) 240 °C and (b) 280 °C for 1 min, and on (c) [0001] textured Cu6Sn5 substrate at 280 °C for 1 min. The joints were cooledin air. (d) Shear strength of solder joins with the corresponding morphologies of Cu6Sn5.

137M. Li et al. / Materials Letters 66 (2012) 135–137

interfacial grains [Fig. 4(a)–(b)] [8]. The crystal planes of the sidesurfaces on these prismatic grains belong to the {10–10} family ofcrystal planes [8], indicating that the elongated orientation ofprismatic grains is along [0001] direction of the existing interfacialgrains. Another study shows that the long prismatic grains extrudeddeeply into solder matrix would inhibit the plastic deformation andcrack propagation during shear testing, and therefore reinforce thejoint strength [13]. If these interfacial grains extend along thedirection normal to the substrate, more long prismatic grains willextrude deeply into soldermatrix, and the strengthening effect shouldbe more effective. According to the “heredity” of the interfacial graingrowth orientations, this special morphology of Cu6Sn5 grains canform on the Cu6Sn5 layer with [0001] texture in the direction normalto the interface, as shown in Fig. 4(c). Thereinto, the Cu6Sn5 layer isthe interface of Sn3.5Ag/Cu solder bump whose solder alloy wasetched out after reflow at 240 °C for 30 min.

In consideration of the good wettability of Sn-based solder onCu6Sn5 films [14], this [0001] textured Cu6Sn5 substrate may beapplied in the future. To evaluate the strength of the joints formed onthe Cu6Sn5 layer, ball shear tests were conducted at 10-μm shearheight with 200-μm/s shear speed using a bonding tester [13]. Theshear force values were estimated by averaging 30 trials. As shown inFig. 4(d), the strength of joints formed between Sn3.5Ag solder balls(with a diameter of 762 μm) and the Cu6Sn5 pads (with a passivationopening diameter of 600 μm) at 280 °C is greater than that formed onCu pads under the same conditions. In addition, the electricalcharacteristics of joints formed on the Cu6Sn5 layer may also havesome advantages, which need further studies.

4. Conclusion

The growth orientations of Cu6Sn5 grains formed at a Sn3.5Ag/Cu interface were investigated. The results show that Cu6Sn5 layers

with strong textures can form on polycrystalline Cu. Unlike thatformed on Cu single crystals, the texture on polycrystalline Cuforms during the interfacial Cu6Sn5 ripening growth, probablycaused by the different stabilities of the interfacial grains withvarious orientations at different temperatures. At 240 °C, theCu6Sn5 layer formed at the Sn3.5Ag/Cu interface shows [0001]texture in the direction normal to the substrate, and a specialmorphology of interfacial Cu6Sn5 grains can be formed on this layerto reinforce joint properties.

Acknowledgments

This work was supported by National Nature Science Foundationof China under Grant nos. 50875063 and 51011140350, theGuangdong Province Lead Free Roadmap Project under Grant no.2009A08024009-5, and the National Research Foundation of Korea(NRF) Grant (Grant no. NRF-2010-D00008).

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