Thermal expansion of Cu6Sn5 and (Cu,Ni)6Sn5

Dekui Mu,a) Jonathan Read, Yafeng Yang, and Kazuhiro NogitaSchool of Mechanical and Mining Engineering, The University of Queensland, Brisbane, Australia

(Received 23 May 2011; accepted 12 August 2011)

Cu6Sn5 is a common intermetallic compound formed during electrical packaging. It has an allotropictransformation from the low-temperature monoclinic g’-Cu6Sn5 to high-temperature hexagonalg-Cu6Sn5 at equilibrium temperature 186 °C. In this research, the effects of this allotropictransformation and Ni addition on the thermal expansion of g’- and/or g-Cu6Sn5 were characterizedusing synchrotron x-ray diffraction and dilatometry. A volume expansion during the monoclinic tohexagonal transformation was found. The addition of Ni was found to decrease the undesirablethermal expansion by stabilizing the hexagonal Cu6Sn5 at temperatures below 186 °C and reducingthe overall thermal expansion of Cu6Sn5.

I. INTRODUCTION

Cu6Sn5 is an important intermetallic compound (IMC),which commonly forms during interface reactions betweenmost Sn-based solders and Cu substrates.1 The continuousperformance demands and minimization of modern elec-tronic products have led to an increased current and acco-mpanying Joule heating.2 As a result, the operatingtemperature of lead-free solder joints and the volume fra-ction of IMCs have increased. The thermal expansion ofCu6Sn5 therefore plays an important role in the thermalfatigue of solder joints. Jiang et al.3 investigated the coe-fficient of thermal expansion (CTE) of Cu6Sn5 between 60and 160 °C by step heating dilatometry. Wang et al.4 alsostudied the CTE of Cu6Sn5 formed at the Sn/Cu interface attemperatures from 100 to 250 °C. Zhou et al. suggesteda nonlinear thermal expansion and a CTE value higher thanthe experimental result for monoclinic Cu6Sn5 by firstprinciple computation.5 In the Sn–Cu phase diagram, thereis a monoclinic–hexagonal allotropic transformation at atemperature of 186 °C. Both monoclinic and hexagonalCu6Sn5 can form in a solder joint depending on the alloycomposition and soldering conditions. However, the effectof this allotropic transformation on the thermal expansionbehavior of Cu6Sn5 has not been considered in previousstudies.

Ni is an important alloying element in Sn–Cu lead-freesolders. The addition of Ni has been associated with suchbenefits as altered solidification microstructures, an in-creased volume fraction of the eutectic phase, and a lowerpropensity for interfacial IMCs to crack during service.6,7

Ni is also actively involved in the interface reaction andhas a remarkable influence on the formation and propertiesof Cu6Sn5.

8 It has been reported that an increase in Ni

concentration reduces the enthalpy of Cu6Sn5 and that(Cu,Ni)6Sn5 has a more negative heat of formation.9,10 Nican also increase the Young’s modulus and hardness ofCu6Sn5.

11,12 From a crystallographic point of view, theelectron diffraction patterns of monoclinic and hexago-nal Cu6Sn5 have been reported.13 Recently, Nogita andNishimura14,15 found that Ni stabilizes hexagonal Cu6Sn5at temperatures down to room temperature. The effect of Nisolubility on the thermal expansion behavior of Cu6Sn5,however, remains unknown.

In this research, the effects of phase transformation andNi addition on the thermal expansion of Cu6Sn5 wereinvestigated by dilatometry. X-ray diffraction (XRD) polefigures were also plotted to investigate the presence orabsence of any crystallographic texture in the dilatometerysamples. Subsequent to this, the dilatometry results wereconfirmed with the lattice parameters and unit cell volumesof Cu6Sn5 and (Cu,Ni)6Sn5 measured by synchrotronXRD.

II. EXPERIMENT

Cu6Sn5 and (Cu,Ni)6Sn5 powders were prepared bypreferential dissolution of Sn from Sn–4Cu and Sn–4Cu–0.05Ni ingots supplied by Nihon Superior Co., Ltd.(Osaka, Japan) Approximately 60 g of the alloy cubeswith a side length about 5 mm were placed in a solution ofortho-nitrophenol (50 g) and NaOH (75 g) in 1.5 L of waterat 80 °C for 24 h, during which the Sn phase dissolved. Theremaining eutectic Cu6Sn5 and (Cu,Ni)6Sn5 IMCs werecollected using vacuum filtration and rinsed, first withwater, then ethanol, and dried in air. The dried Cu6Sn5and (Cu,Ni)6Sn5 were crushed in an agate mortar.

For dilatometry measurements, the crushed IMCs werepressed into a rod shape in a die of 10-mm diameter. Thelinear thermal expansion of Cu6Sn5 and (Cu,Ni)6Sn5 rodswas measured using a NETZSCH 402-C (NETZSCH,Selb, Germany) dilatometer with a temperature range of25 –250 °C, a heating/cooling rate of 1 °C/min, and 30min

a)Address all correspondence to this author.e-mail: d.mu@uq.edu.au

DOI: 10.1557/jmr.2011.293

J. Mater. Res., Vol. 26, No. 20, Oct 28, 2011 �Materials Research Society 20112660

holding time at the end of heating or cooling. In total, threethermal cycles were measured, and the thermal dilationsduring second and third cycles were used for analysis,using the NETZSCH Proteus software. According to theNETZSCH 402-C user manual, thermal dilation during thefirst heating cycle was not used for the analysis due tothe unstable initial condition. Synchrotron XRD measure-ments were done at the powder diffraction beam line at theAustralian Synchrotron. The Cu6Sn5 and (Cu,Ni)6Sn5powders were loaded into a quartz capillary sample cell(0.3 mm in diameter) and heated with a hot air gun.Synchrotron XRD was performed using x-ray with wave-lengths of 0.0825 nm for Cu6Sn5 and 0.0773 nm for(Cu,Ni)6Sn5, and 10° to 60° 2h scan angles at temperaturesof 30, 170, 200, 220, and 250 °C with a heating rate of6 °C/min. Once the desired temperature was reached, thesample was kept at that temperature for 11 min, with 1 minfor thermal stabilization and 10 min for data collection.A Si standard (NIST640C) cell was measured at roomtemperature for 3 min and an empty capillary cell wasmeasured for 3 min at each experimental temperature forcalibration. Further detailed experimental procedures aredescribed elsewhere.16 The lattice parameters and unit cellvolume of Cu6Sn5 and (Cu,Ni)6Sn5 were calculated usingx-ray peak data obtained at each temperature usingRIETAN-FP Rietveld analysis software.17 The refinement paramet-ers were optimized to minimize the residual Rwp, Rp, andS factors. As a reference crystallography and atomic coor-dination, International Center for Diffraction Data (ICDD)number of 045-1488 (for monoclinic) and 047-1575 (forhexagonal) were used in association with RIETAN-FP.

III. RESULTS AND DISCUSSIONS

The hexagonal (1 0 1) pole figure of Cu6Sn5 and(Cu,Ni)6Sn5 samples after dilatometry measurement isshown in Fig. 1. The Normal Direction (ND) is parallel tothe cylindrical axis of the sample; while Rolling Direction(RD) and Transverse Direction (TD) are perpendicular tothis axis. For both Cu6Sn5 and (Cu,Ni)6Sn5, the intensitypeaks along both RD and TD directions are randomlydistributed and no specific grain orientations exist. Hence,the isotropy of Cu6Sn5 and (Cu,Ni)6Sn5 rods during thermalexpansion can be confirmed, and the dilatometry datarepresents an average thermal expansion, rather than onein a specific crystal direction.

The normalized thermal dilations, expressed as apercentage of initial sample length, during second andthird heating/cooling cycles and their differentiations areplotted in Fig. 2. The average linear CTEs were thencalculated by linear regression. In doing so, an implicitassumption has been made that only linear thermalexpansion occurred during the dilatometry test, whichsimplifies the comparison of data between Cu6Sn5 and(Cu,Ni)6Sn5. In Figs. 2(a) and 2(c), the thermal dilations

during the second and third heating stages are almostidentical, while thermal dilation during the cooling stagesshows some variations such as the disturbed thermal dila-tions at highest temperature during cooling stages as shownin Fig. 2(c). This is likely due to the combined effects oftemperature change and the allotropic transformation;however, Cu6Sn5 and (Cu,Ni)6Sn5 demonstrated similaroverall thermal expansion behaviors during both heatingand cooling stages.

The CTEs (per °C) were calculated as 32 ppm forCu6Sn5 and 24.5 ppm for (Cu,Ni)6Sn5 by the linear reg-ression of thermal dilation data during the heating stages.The calculated CTEs of Cu6Sn5 are higher than the valuesreported in previous works,3,4 which is partly due to theincrease in thermal dilation at approximately 180 °C asshown in Fig. 2(a). In the differentiated thermal dilation forCu6Sn5, a peak can be clearly observed in Fig. 2(b), startingat 170 °C and finishing at 210 °C, which is associated withthe increase in thermal dilation around 180 °C. Consideringthe monoclinic to hexagonal allotropic transformation ofCu6Sn5 occurs at an equilibrium temperature of 186 °C, it isreasonable to conclude that a volume expansion occursduring the monoclinic to hexagonal transformation. On thedifferentiation curve of the thermal dilation during coolingstages, as indicated by the arrow in Fig. 2(d), there isanother transformation peak at almost the same temperatureas observed during heating stages. Moreover, the increaseon the differentiation of thermal dilation during the coolingstage also indicates that the hexagonal to monoclinictransformation results in a volume shrinkage at a temperaturearound 180 °C. This volume shrinkage is undesirable duringlead-free soldering because it may result in strain-relatedmechanical damage of intermetallics during the cooling ofsoldered joints.6,7

From Figs. 2(a), 2(b), and 2(d), there is a volumeexpansion during the monoclinic to hexagonal transforma-tion of Cu6Sn5. This is in contrast to a previous theoreticalstudy18 in which the monoclinic to hexagonal transformationresulted in a 2.15% volume shrinkage at room temperature.One possible reason is that the CTE of Cu6Sn5 is stronglydependent on temperature and the room temperature“quenched” hexagonal Cu6Sn5 has a similar or a smallerCTE value than monoclinic Cu6Sn5, but as the temperatureincreases, the CTE of the hexagonal Cu6Sn5 exceeds that ofits monoclinic allotrope. Unfortunately, direct dilatometryevidence is not available yet due to the difficulty inpreventing the hexagonal to monoclinic transformation of“quenched” hexagonal Cu6Sn5 at the evaluated temperatures.

It has been reported that the high-temperature hexagonalCu6Sn5 phase is stabilized by Ni at around 9 at.%14 or4–5 at.%.6,7 In this study, the Ni content was measured asaround 4 at.% in the dissolved Cu6Sn5 and the stabilizationof hexagonal Cu6Sn5 is supported by comparing the datain Figs. 2(b) and 2(d), in which the differentiation curvesof Cu6Sn5 demonstrate a volume change that does not

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J. Mater. Res., Vol. 26, No. 20, Oct 28, 2011 2661

occur in (Cu,Ni)6Sn5 and also show more variability attemperatures above 100 °C. In Figs. 2(a) and 2(b), it canalso be observed that the (Cu,Ni)6Sn5 shows less thermalexpansion and has smaller CTE values than the Cu6Sn5.This is supportive of previous work6,7,14 that Ni can reducethe mechanical damage by preventing the volume changeassociated with the allotropic phase transformation andreducing the thermal expansion of Cu6Sn5.

The dilatometry data represents the thermal behaviorof both monoclinic and hexagonal Cu6Sn5. To directly

investigate the thermal expansion of the monoclinic and/or hexagonal Cu6Sn5 and (Cu,Ni)6Sn5 phases, the IMCswere also investigated by synchrotron XRD as shown inFig. 3. From this XRD data, it can be seen in Figs. 3(a)and 3(b) that all Cu6Sn5 samples above 170 °C and all(Cu,Ni)6Sn5 samples at all temperatures investigated arehexagonal. It can also be seen in Fig. 3(a) that attemperatures of 170 °C and lower, the Ni free samples(i.e., Cu6Sn5) have a predominantly monoclinic struc-ture. However, previous differential scanning calorimetry

FIG. 1. Hexagonal (1 0 1) pole figure of pressed intermetallic compound samples: (a) Cu6Sn5 and (b)(Cu,Ni)6Sn5.

FIG. 2. (a) Thermal dilation during heating stages, (b) differentiation of thermal dilation during heating stages, (c) thermal dilation during coolingstages, and (d) differentiation of thermal dilation during cooling stages.

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results18 have shown a cooling rate of around 1 °C/min isrequired to ensure the completion of hexagonal to mono-clinic transformation. As our cooling rate was faster thanthis, some metastable hexagonal phases may be present,and the Cu6Sn5 samples at 170 °C and lower are a mixtureof metastable hexagonal and low-temperature monoclinicphases.

Assuming the hexagonal structure (ICDD 047-1575),the lattice parameters of Cu6Sn5 and (Cu,Ni)6Sn5 alonga and c axes are plotted in Figs. 4(a) and 4(b). (Cu,Ni)6Sn5shows less thermal expansion than Cu6Sn5 along botha and c axes, which is in agreement with the dilatometryresults from the bulk sample. Moreover, Ni reduces thermalexpansion more along the a axis; while the thermalexpansion of Cu6Sn5 and (Cu,Ni)6Sn5 along the c axis issimilar. It has been reported that the misfit between Cuatoms in single-crystal Cu substrates and Cu6Sn5 affectsthe morphology of Cu6Sn5.

19 Hence, Ni may modify themorphology of Cu6Sn5 formed during lead-free solderingby changing the lattice parameter and nucleation of Cu6Sn5.

The measured unit cell volumes of Cu6Sn5 and (Cu,Ni)6Sn5 at temperatures of 30, 170, 200, 220, and 250 °Cand average volume CTEs (per °C, calculated by takingunit cell volume at 30 °C as the initial condition) attemperatures 170–250 °C are plotted in Figs. 5(a) and 5(b).The unit cell volume is measured as 0.0787 nm3 at 170 °Cfor monoclinic Cu6Sn5 and 0.0792 nm3 for hexagonalCu6Sn5 at 200 °C. Hence, the volume expansion ofCu6Sn5 during the monoclinic to hexagonal allotropic

transformation observed by dilatometry can be confirmed.The volume CTEs are measured as 56.3 ppm for themonoclinic Cu6Sn5 and 64.0 ppm for hexagonal Cu6Sn5 at170 °C, which is remarkably lower than the volume CTEof 83.2 ppm for hexagonal Cu6Sn5 at 200 °C. This maysuggest that the thermal expansion of hexagonal Cu6Sn5 issensitive to temperature and there is an increase of volumeCTE of Cu6Sn5 as the temperature rises to 200 °C orabove.

Moreover, the effect of Ni on reducing the thermalexpansion of hexagonal (Cu,Ni)6Sn5 can be observedfrom the XRD results. In Fig. 5(a), the unit cell volumes of(Cu,Ni)6Sn5 are smaller than Cu6Sn5 at temperatures30–250 °C, which is in agreement with the dilatometryresults. In Fig. 5(b), the average volume CTE of (Cu,Ni)6Sn5 is 70.4 ppm at 170 °C, which is slightly higherthan Cu6Sn5 (64.1 ppm). At temperatures 200 °C or above,the converted volume CTEs of (Cu,Ni)6Sn5 are lower thanCu6Sn5 as shown in Fig. 5(b). Overall, the CTEs directlymeasured using dilatometry show that Ni reduces thethermal expansion of Cu6Sn5 for all temperatures in-vestigated and this is supported by XRD results fortemperatures above 170 °C.

FIG. 3. X-ray diffraction peak profile for the samples heated from 30 to250 °C for (a) Cu6Sn5 and (b) (Cu,Ni)6Sn5.

FIG. 4. Lattice parameter along (a) a- and (b) c-axes of hexagonalCu6Sn5 and (Cu,Ni)6Sn5.

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J. Mater. Res., Vol. 26, No. 20, Oct 28, 2011 2663

IV. CONCLUSIONS

In summary, the monoclinic to hexagonal transforma-tion of Cu6Sn5 at 186 °C results in a volume expansion,which is in contrast to previous computational results. TheXRD results also show an increase of the volume expa-nsion coefficient of hexagonal Cu6Sn5 as the temperatureincreases from 170 to 200 °C. Assuming a hexagonalstructure, Ni is found to reduce the thermal expansion ofCu6Sn5, particularly along a axis. By stabilizing thehexagonal Cu6Sn5 at temperatures below 186 °C andreducing the thermal expansion of Cu6Sn5, the addition ofNi may reduce thermal stresses generated in solder jointsduring thermal cycling.

ACKNOWLEDGMENTS

Nihon Superior Co. Ltd. (Osaka, Japan) supplied the testsamples under the University of Queensland-Nihon Superiorcollaborative research program. X-ray diffraction (XRD)experiments were performed at the Australian Synchrotron(project IDs: FI_2009/1_ FI1077, 2010/1_ P2249) under theQueensland Foundation Investor beamtime scheme. Theauthors would like to thank Dr Q. F. Gu of the AustralianSynchrotron for XRD experiments, Prof. H. Yasuda in

Osaka University and FrontierLab@OsakaU program forthe access to pole figure XRD equipment, and Dr. S. D.McDonald and Dr. Y. Q. Wu in the University of Queens-land for stimulating discussions and suggestions. D. Mu isfinancially supported by an Australian Postgraduate Award.

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FIG. 5. (a) Unit cell volume and (b) normalized coefficient of thermalexpansion of Cu6Sn5 and (Cu,Ni)6Sn5.

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