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Journal of Alloys and Compounds 460 (2008) 594–598

Effect of Zn addition on the formation and growth ofintermetallic compound at Sn–3.5 wt% Ag/Cu interface

Chun Yu, Hao Lu ∗, Shiming LiSchool of Materials Science and Engineering, Shanghai Jiaotong University, Shanghai 200030, PR China

Received 17 April 2007; received in revised form 29 May 2007; accepted 5 June 2007Available online 13 June 2007

bstract

A small amount of Zn addition was found to be beneficial in depressing the growth of intermetallic compound (IMC) layer at Sn–3.5 wt% Ag/Cunterface as aged isothermally. The results show that Cu6Sn5 was still the original interfacial product, regardless of the concentration of Zn additionn the eutectic solder. While the formation of Cu Sn was retarded as aged at 150 ◦C, instead, a continuous or non-continuous Cu Zn IMC layer,

3 5 8

hich was determined by the Zn concentration, was formed. Since the diffusion rate of Cu in Cu5Zn8 was much lower than that in Cu6Sn5, therowth rate of IMC layer at the interface was slowed down. Furthermore, the evolution and morphology of the IMC layer were correlated with theoncentration of Zn in the solder.

2007 Elsevier B.V. All rights reserved.

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eywords: Metals and alloys; Solid-state reaction; Microstructure; Intermetalli

. Introduction

Nowadays, a lot of Sn-based lead free solders have beeneveloped for replacing of the conventional SnPb eutectic sol-er used in electronic packaging industry [1]. At the meantime,o meet some requirements in certain special applications, theeeking for new candidates has never been stopped. Since oneole of the solder alloys is serving as a structural material toustain the components, one of the major concerns in the devel-pment of electronic packaging is the reliability of the solderoints. Actually, during the reflowing process, the IMC layersre formed inevitably at the interface between the solder andubstrate. Moreover, due to the solid-state diffusion, the IMCayer becomes thicker and thicker at the thermal aging stage.he properties of the solder joints as well as the reliabilityf the whole package are very sensitive to the thickness andorphology of the IMC layers at the interface of the joints

2,3]. A thin IMC layer is necessary to keep a good joining

etween solder and substrate, while too thick IMC layer isensitive to stress and sometimes induces crack initiation andropagation.

∗ Corresponding author. Tel.: +86 21 62932661; fax: +86 21 62932661.E-mail address: shweld@sjtu.edu.cn (H. Lu).

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925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2007.06.031

Sn–3.5 wt% Ag eutectic solder is regarded as a prospectiveubstitution for Sn–37 wt% Pb alloy in certain applications, dueo its good mechanical properties and high reliability [4,5]. Inrder to further improve the properties and reliabilities, alloy-ng elements, such as Cu [6], Sb [7], Ni [8], In, Bi [9], Zn10,11] as well as rare earth elements [12] have been addednto Sn–3.5 wt% Ag eutectic solder. Although a lot of worksave been done to study the formation and growth of the IMCayer at the Sn–3.5 wt% Ag/Cu interface [13], as well as theechanical properties of the joints [14], few works focused on

ow to constrain the growth rate of the IMC layer. Recently,ao et al. [13] found that 0.2 wt% Co or Ni would accelerate the

MC growth rate at the Cu/solder interface. Whereas, Wang etl. reported that a small amount of Zn is beneficial in suppress-ng the growth of IMC at the Sn–0.7 wt% Cu/Cu interface [15].his is an interesting report. Zn is a valuable element for lead

ree solder due to its low eutectic temperature with Sn. Actually,s early as 10 years ago, McCormack et al. [10] found that 1%n were shown to improve the solidification microstructure ofn–3.5 wt% Ag eutectic solder by eliminating the large �-Snendritic grain and introducing a finer and more uniform two-

hase distribution throughout the alloy. In addition, Fawzy [16]ound that 1 wt% Zn addition could increase the yield stress andltimate tensile stress of Sn–3.5 wt% Ag solder. However, thereere few reports about the investigations of the interfacial char-

C. Yu et al. / Journal of Alloys and Co

Table 1Chemical compositions of the solders (wt%)

Zn Ag Sn

Sn3.5Ag 0 3.48 Bal.Sn3.5Ag0.5Zn 0.52 3.43 Bal.SS

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n3.5Ag1Zn 1.03 3.45 Bal.n3.5Ag2Zn 1.98 3.41 Bal.

cteristics between the improved Sn–Ag solder and substrate,hich are of great importance for the applications of the solder

17]. Therefore, in the present work, we investigated the effectsf Zn addition on the formation, evolution and growth behav-ors of the IMCs at the Sn–3.5 wt% Ag/Cu interface by addingsmall amount of Zn addition into the eutectic solder, in order

o look for a kind of IMC-depressed lead free solder for specialpplications, such as high temperature application.

. Experimental materials and methods

Table 1 shows the chemical compositions of the solders used in this study.he as cast samples of bulk alloy were prepared by melting the high purity tin,ilver, and zinc at 600 ◦C for several hours in a crucible protected by eutecticCl + LiCl solution, casting into a stainless steel mold of 6 mm in diameter, and

hen cooling in air to room temperature with a rate of 10 K/s. The solders wereade into disk shape with dimensions of 6 mm in diameter and 2 mm in height

nd dipped into rosin mildly activated (RMA) flux for soldering. The Cu plate25 mm × 25 mm × 0.2 mm) with 99.9% purity was used as the substrate. Beforeoldering, the Cu surface was polished with grade no. 3 metallographic sandpa-er and burnished, and then degreased with acetone using ultrasonic vibration.he joint samples were obtained by melting the solder disk on the top face ofu substrate in an IR-reflow machine under atmosphere environment. The peak

eflow temperature was 260 ◦C and the time above the melting point of the sol-ers was about 60 s. Immediately after the reflow process, the four kinds of solder

◦

oints were subjected to high-temperature aging at 150 C for 5, 10, 15, and 20ays, respectively. The as-soldered and as-aged samples were mounted with coldpoxy, ground by using SiC paper, polished with 0.05 �m Al2O3 powder andhen lightly etched for the cross-sectional observation. The microstructures ofhe joints were observed in a scanning electron microscopy (SEM), equipped

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Fig. 1. Evolution of the IMCs at the Sn–3.5 wt% Ag/Cu interface a

Fig. 2. Evolution of the IMCs at the Sn–3.5 wt% Ag–0.5 wt% Zn/Cu inte

mpounds 460 (2008) 594–598 595

ith energy dispersive spectrometer (EDS) and an electron probe micro-analyzerEPMA) to analyze the chemical compositions of each phase at the interface. Theverage thickness of the IMC layer at the interface was determined by dividinghe overall area of the IMCs by the horizontal length of the interface.

. Results and discussions

.1. Evolutions of the IMCs at the interface

Fig. 1 shows the microstructural evolutions of the Sn–3.5 wt%g/Cu interface. It is seen from Fig. 1a that a continuous scal-

op like Cu6Sn5 IMC layer was formed at the solder/Cu interfacefter the reflowing process. The morphology of the Cu6Sn5 IMCayer was changed from a rounded scallop shape to an elongatedcallop shape after 10 days aging, as shown in Fig. 1b. Mean-ime, a thin IMC layer was formed between the Cu substratend Cu6Sn5 IMC layer, EDS analysis identified that the newayer consists of Cu3Sn. As the aging time was prolonged to 20ays, the scallop like Cu6Sn5 was elongated further, while littlehange was found for the morphology of Cu3Sn IMC layer, aseen in Fig. 1c. The thickness of these IMC layers was found toe increased greatly with the aging time.

Fig. 2 presents the microstructural evolutions of the IMC lay-rs at the Sn–3.5 wt% Ag–0.5 wt% Zn/Cu interface. Accordingo Fig. 2a, it can be seen that the continuous scallop like Cu6Sn5MC layer was still the only original product at the solder/Cunterface. The thickness of the Cu6Sn5 IMC layer was close tohat of the Cu6Sn5 layer formed at the Sn–3.5 wt% Ag/Cu inter-ace. It indicates that 0.5 wt% Zn addition has little effect on theormation of the initial IMC at the interface. Likewise, it is seenrom Fig. 2b that the scallop like Cu6Sn5 IMC layer was slightlylongated as aged for 10 days. However, the Cu3Sn IMC layer

as not formed at the interface between the Cu substrate andu6Sn5 IMC layer. EDS analysis indicated that the concentra-

ions of Zn and Sn at the Cu/Cu6Sn5 interface near Cu side were.2 wt% and 0.2 wt%, respectively. Therefore, it can be deduced

ged at 150 ◦C. (a) As soldered, (b) 10 days and (c) 20 days.

rface aged at 150 ◦C. (a) As soldered, (b) 10 days and (c) 20 days.

596 C. Yu et al. / Journal of Alloys and Compounds 460 (2008) 594–598

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The Gibbs free energy of Cu6Sn5 was less than that of Cu3Sn[18]. Hence, the above reactions ensured that the growth rateof Cu6Sn5 IMC layer was faster than that of Cu3Sn. At theCu/Cu3Sn interface, due to the concentration of Cu was much

Fig. 3. Evolution of the IMCs at the Sn–3.5 wt% Ag–1.0 wt% Zn/C

hat Cu(Zn) solid solution layer was formed instead of Cu3Sn.fter a longer aging process, the Cu(Zn) layer was found to be

ransformed to Cu5Zn8 IMC layer step by step. Fig. 2c shows theicrostructure of the interface as aged for 20 days, it is seen thatnew flat IMC layer was formed at the Cu/Cu6Sn5 interface,DS analysis identified the IMC layer as Cu5Zn8. It is found the

hickness of the IMC layers was also increased with the agingime, while the growth rate was slightly slowed down, this woulde discussed in the latter section.

Fig. 3 depicts the microstructural variation of the Sn–3.5 wt%g–1 wt% Zn/Cu interface. Still, the initial IMC layer was

callop like Cu6Sn5 after the reflowing process. However, theomposition and morphology of the IMC layers changed greatlyith the aging time. It is seen from Fig. 3b that a mixed IMC

ayer was formed at the reaction interface of the joint as agedor 10 days. EDS identified the light gray IMC as Cu6Sn5 andark gray IMC as Cu5Zn8. Both the two IMC layers were notontinuous. Moreover, the formation of Cu3Sn IMC layer waslso depressed. As the aging time was prolonged to 20 days, its seen from Fig. 3c that the IMC layers grew up, meantime, theroportion of the Cu5Zn8 in the mixed IMC layer was obviouslyncreased.

.2. Thickness of the IMC layer

The mean thickness of the total IMC layers at the reactionnterface was determined by dividing the total area of the IMCegion by the length of the region parallel to the interface onhe cross-section. The relation curve of thickness versus agingime is plotted in Fig. 4. As shown in Fig. 4, it is seen that thehickness of the IMC layers at the Sn–3.5 wt% Ag/Cu interfaceas increased greatly with the aging time, while a small amountf Zn addition was beneficial in decreasing the growth rate ofhe IMC layers. Moreover, the decreasing effect was correlatedith the morphology of the IMC layer. The thickness of the

MC layers was decreased by about 2.8 �m with a continuousu5Zn8 IMC layer (0.5 wt% Zn addition) after 20 days thermalging, while it was decreased by 2.2 �m with a non-continuousne (1 wt% Zn addition).

.3. Discussions

Fig. 5 shows the sketch map of the evolution behaviors of IMCayer at Cu/solder interface, with or without Zn addition. Forhe Sn–Ag/Cu interface, Cu6Sn5 IMC was initially formed afteroldering. Therefore, there existed two interfaces at the reaction

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rface aged at 150 ◦C. (a) As soldered, (b) 10 days and (c) 20 days.

one, namely, Cu/Cu6Sn5 and solder/Cu6Sn5 interfaces, as seenn Fig. 5a. At the aging process, atoms diffused into the twonterfaces under the driving force from the chemical potential.ccording to Sn–Cu binary phase diagram, Cu and Cu6Sn5 wereot in equilibrium state at the solid-state aging process, so did Snnd Cu3Sn, while Sn and Cu6Sn5 were in equilibrium. There-ore, as the concentration of Zn addition was zero, Cu wouldeact directly with Cu6Sn5 at Cu/Cu6Sn5 interface, it was:

Cu + Cu6Sn5 → 5Cu3Sn (1)

his reaction resulted in the formation of Cu3Sn IMC layernd initial dissolution of Cu substrate. Therefore, Cu/Cu6Sn5nterface disappeared. Instead, Cu/Cu3Sn and Cu3Sn/Cu6Sn5nterfaces were formed, as seen in Fig. 5b1. Meantime, Cu dif-used into the solder/Cu6Sn5 interface, and reacted with Sn inhe solder:

Cu + 5Sn → Cu6Sn5 (2)

onsequently, Cu6Sn5 IMC layer grew up. Likewise, Sntoms diffused into Cu3Sn/Cu6Sn5 interface, and then reactionccurred as:

Sn + 2Cu3Sn → Cu6Sn5 (3)

ig. 4. Relationship of the total thickness of IMCs and aging time, concentrationf Zn.

C. Yu et al. / Journal of Alloys and Compounds 460 (2008) 594–598 597

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Fig. 5. Evolution process of IMC layers at the Cu/solde

arger than that of Sn, meanwhile the equilibrium of Cu/Cu3Snnterface should be kept by Cu3Sn, hence, Cu and Sn directlyeacted and yielded Cu3Sn IMC, it was:

n + 3Cu → Cu3Sn (4)

his reaction also resulted in great dissolution of Cu substratet the aging process.

The Gibbs free energies for Cu6Sn5, Cu3Sn, and Cu5Zn8 methe relationship, �G(Cu5Zn8) < �G(Cu6Sn5) < �G(Cu3Sn)18,19]. Therefore, Cu5Zn8 would be formed firstly at theu/Sn–Ag–Zn interface. In fact, Lee et al. verified that Cu5Zn8as the primary product at Sn–9 wt% Zn/Cu interface [20].owever, the first product was Cu6Sn5 after Ag was added inton–9 wt% Zn solder as an addition [21], it indicated that Ag

ould depress the formation of Cu5Zn8. Our experimental resultsre in good agreement with this experimental phenomenon.oreover, the formation of Cu5Zn8 required that the compo-

ition of Zn reached to 64.9 wt%. Meantime, the solid solubility

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rface for Zn addition of 0, 0.5 and 1 wt%, respectively.

imit of Zn in Cu at 150 ◦C was as high as 20 wt%. Therefore,ccording to EDS analysis, it could be demonstrated that thereas not enough Zn to form Cu5Zn8 IMC at the early aging stage,ut to form Cu(Zn) solid solution. There existed two sourceshich provided Zn for the Cu/Cu6Sn5 interface, namely, theissolved Zn in the Cu substrate near Cu/Cu6Sn5 interface dur-ng the soldering process, and another was the solder. These twoources contributed to the formation of Cu5Zn8 IMC layer athe Cu/Cu6Sn5 interface. However, as Zn addition was 0.5 wt%,he amount of Zn dissolved into Cu substrate at soldering stageas very small; meantime, the diffusing atomic flux towards theu/Cu6Sn5 interface at the aging process was low due to the lowoncentration gradient. Therefore, since the concentration of Znould reach the requirement in a very long time, the formationf Cu5Zn8 has been delayed. Whereas, the formation of Cu(Zn)

olid solution at this stage was also expected to decrease the freenergy of Cu/Cu6Sn5 interface, reactions (1) and (4) were thenestricted, and therefore the formation of Cu3Sn, as shown inig. 5b2.

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98 C. Yu et al. / Journal of Alloys a

The diffusion of Zn also impacted the morphology of the IMCayer. As the aging time was prolonged, Zn atoms continuallyiffused into Cu(Zn)/Cu6Sn5 interface. For the diffusion rate ofn in Cu6Sn5 was very slow, it spent a long time for Zn diffusing

hrough the Cu6Sn5 IMC layer. In addition, the morphology ofu6Sn5 IMC layer became flatter and flatter at the aging period.ence, the distribution of Zn atoms at Cu(Zn)/Cu6Sn5 interfaceas uniform at some extend. As the composition requirementas met, Cu5Zn8 IMC was formed at the same time along the

nterface. Therefore, a flat like Cu5Zn8 IMC layer was formed athe Cu(Zn)/Cu6Sn5 interface, as seen in Fig. 5c2. At 150 ◦C, theiffusion rates of Cu in Cu6Sn5 and Cu5Zn8 were 4.3 × 10−13

nd 2.95 × 10−15 cm2/s [22]. Therefore, the Cu atomic flux inhe IMC layer was decreased with the existence of Cu5Zn8 layer.ince Cu was the dominant species contributing to the growthf IMC layer at the interface [23], the growth rate of Cu6Sn5MC layer was then bound to be slowed down.

However, as the concentration of Zn addition in Sn–3.5 wt%g solder reached to 1 wt%, the concentration of dissolved Zn

n Cu substrate was increased. In addition, the diffusion drivingorce for Zn was also increased due to a relative large con-entration gradient. Therefore, a larger amount of Zn atomicux moved towards the Cu(Zn)/Cu6Sn5 interface. The abovehanges would shorten the required time for the formation ofu5Zn8. Since Cu6Sn5 IMC layer usually kept scallop shape

or a very long time at the aging state, the Zn atoms whichiffused along the grain boundary of Cu6Sn5 IMC layer firstlyeached to the Cu(Zn)/Cu6Sn5 interface, as shown in Fig. 5b3.nd therefore, the content of Zn at the zone just below the grainoundary firstly reached the required composition of formingu5Zn8 IMC. Consequently, block and non-continuous Cu5Zn8

MC was formed there, as seen in Fig. 5b3. The depression effectf Cu5Zn8 IMC layer on the growth of total IMC layer was weak-ned slightly due to the non-continuous shape and non-uniformhickness of Cu5Zn8 IMC layer. The Cu6Sn5 IMC layer kept aelative higher growth rate at the following aging process. Mean-hile, block Cu5Zn8 IMC grew up and coalesced together, thisould be benefit to depress the growth rate of Cu6Sn5 IMC layer

t the late aging stage.

. Conclusions

A small amount of Zn was added into the Sn–3.5 wt% Agutectic solder to investigate the impacts of Zn addition on

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mpounds 460 (2008) 594–598

he formation, growth and evolution of the IMC layers at theu/solder interface. It was found that the formation of Cu3Sn

MC was depressed, regardless of the concentration of Zn in theolder. As the Zn concentration was 0.5 wt%, the formation ofu5Zn8 IMC experienced a long incubation period at the thermalging process, and finally a continuous Cu5Zn8 IMC layer wasormed at the Cu/Cu6Sn5 interface, this continuous IMC layeras found to be beneficial in depressing the growth of Cu6Sn5

MC layer. As the Zn concentration was increased to 1 wt%,u5Zn8 IMC was formed quickly, while both the Cu5Zn8 IMC

ayer and the Cu6Sn5 IMC layer were non-continuous, and there-ore the retarding effect of Cu5Zn8 IMC layer on the growth ofMC layer was weakened slightly.

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