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Journal of Alloys and Compounds 436 (2007) 112–117

The effect of crosshead speed on the joint strength betweenSn-Zn-Bi lead-free solders and Cu substrate

Ramani Mayappan a,∗, Ahmad Badri Ismail a, Zainal Arifin Ahmad a,Tadashi Ariga b, Luay Bakir Hussain a

a School of Materials and Minerals Resources Engineering, Engineering Campus, Universiti Sains Malaysia,14300 Nibong Tebal, Pulau Pinang, Malaysia

b Department of Material Science, Tokai University, Tokyo, Japan

Received 27 April 2006; received in revised form 7 July 2006; accepted 7 July 2006Available online 5 September 2006

bstract

The solder joints between Cu/Sn-40Pb/Cu lead solder and Cu/Sn-9Zn/Cu and Cu/Sn-8Zn-3Bi/Cu lead-free solders were studied. The studyocuses upon the formation of interface intermetallic (IMC), adhesive strength of the joints and the fracture morphology of the as-soldered solderedoints. For Sn-40Pb, Cu6Sn5 IMC was formed. For Sn-9Zn and Sn-8Zn-3Bi solders, two layers were detected. For Sn-9Zn solder, the first layer nexto the solder is a Cu6Sn5 phase with some Zn dissolve in it. The second layer next to the Cu substrate is �-Cu5Zn8 phase. While for Sn-8Zn-3Bi solder,he first layer is mixture of Cu-Sn + Cu-Zn phase and the second layer is �-Cu5Zn8 phase. Sn-8Zn-3Bi solder has the highest strength followed by Sn-

0Pb and Sn-9Zn, respectively, and their strength increases with crosshead speed. The fracture of the solder joints occurs in the elastic strain regionr after a small plastic strain with Sn-40Pb solder has the highest strain followed by Sn-8Zn-3Bi and Sn-9Zn solders. The fracture surface of the threeolders investigated showed ductile dimple appearance. The addition of 3% Bi increases the tensile strength and ductility of the Sn-Zn-Bi system.

2006 Elsevier B.V. All rights reserved.

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eywords: Lead-free solders; Intermetallic; Sn-Zn based solders; Tensile test

. Introduction

Concerns about lead toxicity have resulted in banning ofead based solders in the electronics industry [1]. Since cost isne of the main issue in selecting a new lead-free solder andhe avoidance of costly solder like In-based solders, Sn-Znased alloys have been considered as one alternative becausehey also possess the advantages of high strength, good creepesistance, high thermal fatigue resistance [2]. Furthermore,he Sn-Zn based solders have been highly recommended as theubstitute for Sn-Pb solder due to its melting point close to thatf a Sn-Pb eutectic alloy [3].

The solder used in electronics industry serves not only to

rovide electrical continuity but also to support the stress expe-ienced during service. To ensure the reliability of the joints,nowledge is required of the mechanical properties of the sol-

∗ Corresponding author. Tel.: +60 4 5941010; fax: +60 4 5941011.E-mail address: rasiadi@yahoo.com (R. Mayappan).

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925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2006.07.035

ers under the service conditions. Furthermore, in order totudy the stress–strain behavior of solder joints subjected toechanical loading, the stress–strain curves under various load-

ng conditions should be determined. So, the strength of theolder joints is important for this role [4]. There are not manytudies reported on the mechanical properties of the joints byn-Zn solders [5,6,7], and a few stress–strain curves have beeneported for the solders [5]. The mechanical properties of theubstrate/solder joint are greatly influenced by the interfaceeaction products between them. The formation of intermetal-ic compounds (IMC) at the interface during soldering processlays an important role in wettability and reliability of the solder8]. Therefore, the solder–substrate interactions are increasinglymportant. Although the formation of an IMC layer is importantor good wetting and bonding, but an excessively thick layer isarmful because of the brittle which make it prone to mechanical

ailure even under low load [9].

In general, the interfacial morphology of the joint is closelyependent on the soldering condition because the soldering con-ition determines the initial interface morphology right after

R. Mayappan et al. / Journal of Alloys and Compounds 436 (2007) 112–117 113

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Fig. 1. Schematic illustration of joint test for specimen.

oldering [10]. Since lead-free solders are Sn-based, the com-on intermetallic compound layer formed was Cu6Sn5. Theu6Sn5 phase composed of scallop-shaped grains, of which the

ize increases or the shape may change with further soldering11]. For Sn-Zn eutectic solder reacting with Cu, it is estimatedhat the first forming phase is �-Cu5Zn8 when soldering wasone at 250 ◦C [11]. Liu et al. [12] found Cu6Sn5 and Cu5Zn8hases when Cu was hot dipped into Sn-Zn based solder at50 ◦C for 60 s. On the other hand, when Sn-9Zn solder wasoldered to Cu at 270 ◦C, �-Cu5Zn8 and �′-CuZn phases weredentified [13].

In our previous study [14], wetting balance was used to studyhe wetting properties of the Sn-Zn solders. The present studyas focused on tensile tests carried out on the Cu/Sn-9Zn/Cu andu/Sn-8Zn-3Bi/Cu joints strength at different crosshead speed.he traditional Sn-40Pb solder was also investigated under sameonditions for comparisons purposes. The relationship betweenicrostructure and mechanical properties was explored. The for-ation of intermetallic compounds formed between the solders

nd the Cu substrate was also discussed.

. Experimental

Alloys of Sn-40Pb, Sn-9Zn and Sn-8Zn-3Bi were used in the present work.n-9Zn alloy were prepared using Sn bar and Zn granules. Sn was melted in anlumina crucible at 500 ◦C and followed by the addition of Zn granules and theixer was stirred to homogenize. Sn-40Pb are commercially available solder

nd Sn-8Zn-3Bi are supplied by Nihon Almit Co. Ltd., Japan.For microstructure analysis, Sn-40Pb, Sn-9Zn and Sn-8Zn-3Bi were

ounted in a epoxy resin. The samples surface were grinded with 320, 400,00, 1000, 1200 grit SiC coated papers, respectively, and polished with 1nd 0.05 �m alumina powders. After each step, the samples were ultrasoni-ally cleaned with ethanol. The samples were etched with nital 2% for aroundmin. The microstructure was observed by SEM and energy dispersive X-ray

EDX).Solder joints were made by a method that is conventionally used in the

aboratory to make tensile test specimens to estimate the bonding strength. Two0 mm Cu plates with 5 mm in width and 1.5 mm in thickness were separated bydistance of 1 mm in a long direction as shown in Fig. 1. Soldering was made on

he hot plate using HCl based flux and the temperature profile is shown in Fig. 2.he temperature above 200 ◦C was maintained for about 3.5 min with the highest

emperature reaching 248 ◦C. The soldered joint was left to solidify on the hotlate. For microstructure analysis of the interface, the soldered joints were cut,ounted and polished. The microstructure of the interface were observed by

EM and energy dispersive X-ray (EDX).

For the tensile test, the soldered copper plates were mechanically polishedith a 600-grit SiC paper to remove excess solder. The test was performed at

oom temperature and crosshead speed of 0.02, 0.2 and 2.0 mm/min and thealues for maximum stress and strain at fracture (failure) were recorded. Theractured surface were observed using SEM.

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Fig. 2. Temperature profile on the hot plate.

. Results and discussions

.1. Microstructure

The SEM backscattered image observation of the microstruc-ure of Sn-40Pb solder alloy is presented in Fig. 3(a). The darkhase is a Sn-rich phase and the bright phase is a Pb-rich phase.ig. 3(b) shows that SEM observation of the eutectic Sn-9Zn sys-

em with a needle-shaped Zn-rich phase. Since Zn is soluble initric acid, the etching of the polished surface with this acid willenerate etched areas at the Zn-rich phases [3]. So, the Zn-richreas will form dark grooves where originally occupied by Zn.his phase is homogenously dispersed in the matrix. Because

he solubility of Zn in Sn is as low as 0.09 wt%, part of Zn segre-ates in the Sn-9Zn matrix when cooled in air (low cooling rate)15]. The EDX and Sn-Zn binary phase diagram indicate that theatrix is �-Sn with some Zn dissolved as solid solution. Fig. 3(c)

hows the Sn-8Zn-3Bi system has Zn-rich needle-shaped phase.small amount of Bi-rich phase precipitates (white) in the

-Sn matrix. The size of the Bi phase is of the order of sub-icrometers. The �-Sn matrix has some Bi and Zn dissolved as

olid solution. The grooves are Zn-rich phase being etched by thetchand.

.2. Interface microstructure

Figs. 4–6 show the SEM micrographs of the IMC for the Sn-0Pb, Sn-9Zn and Sn-8Zn-3Bi solders, respectively. The IMCayers formed at both the upper and lower solder/Cu interfaces.or the three solders, at the interface, a reaction layer(s), of

hickness between 1.5 and 5 �m were formed. The chemicalompositions of the interface(s) are listed in Table 1 with theirespective phase identified from the phase diagrams. The exis-ence of oxygen (O) is attributed to the absorption of oxygenn the air by the specimen’s surface during melting, grind-ng and polishing. The Sn-40Pb IMC shows that the atomicercentage of Sn and Cu is 35 and 42%, respectively. Thisomposition is rectified from the phase diagram and it showsu6Sn5 phase. The morphology of this phase is rather flat andontinues.

It was reported [13] that, for Sn-9Zn solder joints with Cut 290 ◦C, the reaction layer consists of three sub-layers. Fromhe solder side, they are �-Cu5Zn8/�′-Cu-Zn/ a thin unknownu-Zn phase, respectively. In this study, we observed two IMC

114 R. Mayappan et al. / Journal of Alloys and Compounds 436 (2007) 112–117

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Fig. 4. SEM backscattered microstructure image of the interface between Sn-40Pb solder and Cu substrate.

Fig. 5. SEM backscattered microstructure image of the interface between Sn-9Zn solder and Cu substrate.

ig. 3. SEM backscattered micrograph of samples of: (a) Sn-40Pb, (b) Sn-9Znnd (c) Sn-8Zn-3Bi.

ayers were formed along the interface as shown in Fig. 5. Therst layer next to the solder is a Cu6Sn5 phase with some Znissolve in it. The second layer next to the Cu is �-Cu5Zn8hase. The morphology of the two layers was different. The firstMC layer is a discontinuous segment while the second IMCorms a continuous layer.

As shown in Fig. 6, the Sn-8Zn-3Bi/Cu system also exhibitswo layers. The first layer is a mixture of Cu-Sn + Cu-Zn andhe morphology is scallop type and the second layer close

o the Cu substrate is a flat �-Cu5Zn8 phase. The similaresults have been reported elsewhere [14]. Bi does not takeart in the interfacial reaction and not form any IMCs with Cu16].

Fig. 6. SEM backscattered microstructure image of the interface between Sn-8Zn-3Bi solder and Cu substrate.

R. Mayappan et al. / Journal of Alloys and Compounds 436 (2007) 112–117 115

Table 1Chemical composition of the IMC layers formed of (a) Sn-40Pb, (b) Sn-9Znand (c) Sn-8Zn-3Bi solders with Cu interface immediately after soldering

Chemical composition (at%)

Cu Sn Zn O C Phase

(a) Sn-40Pb 42 35 – 10 12 Cu6Sn5 [17]

(b) Sn-9ZnFirst layer 62 38 – – – Cu6Sn5 [17]Second layer 38 1 61 – – �-Cu5Zn8 [18]

(c) Sn-8Zn-3Bi

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First layer 27 10 24 26 13 Cu-Sn + Cu-ZnSecond layer 29 1 50 6 14 �-Cu5Zn8 [18]

.3. Influence of crosshead speed on ductility

Figs. 7–11 show the results of the joint test in as-solderedondition. Fig. 7 shows the joints deformation and failure underensile loading, whereby, only the solder experienced defor-

ation and not the Cu was noted. The error bars shown inigs. 8 and 9 show the maximum and minimum values of the sixeasured joint strengths for each solder condition. From Fig. 8,

he values of ductility appeared to increase with crosshead speed.or Sn-40Pb solder, there is a significant plastic deformationefore failure was observed with ductility increases from 26 to4% with increase in crosshead speed. The fracture of the Sn-Zn and Sn-8Zn-3Bi solders occurred within the elastic strainange due to stress concentration or at small plastic strain. Theuctility increases from 17 to 51% for Sn-8Zn-3Bi solder butor Sn-9Zn solder the increase is not very significant. For Sn-Zn solder, the ductility varies from 27 to 33% with maximumalue of 35% at 0.2 mm/min crosshead speed. This mode of fail-re is due to development of IMC brittle structure at Cu/soldernterface. The representative stress–strain curves are shown inig. 9. The stress–strain curve agrees well with this observation,here stress reduced vertically when the sample breaks. The

train at failure for Sn-8Zn-3Bi solder is generally higher thanhe Sn-9Zn solder. The addition of 3 wt% Bi to the Sn-Zn systemncreases the ductility of the solder.

.4. Joint strength properties

The stress–strain behavior is dependent on the solder type,he test temperature and the strain rate [6]. In this paper, thetrength of the bonding interface in solders is defined as theaximum stress of the specimen. Fig. 10 shows stress at maxi-um load for the solders which increases with crosshead speed.he error bars show the maximum and minimum value of theve measured joint strength for each solder. The strength of

he Sn-8Zn-3Bi solder is much higher than that of the Sn-9Znnd Sn-40Pb solders. The strength of the Sn-8Zn-3Bi solder is2 MPa at 0.02 mm/min crosshead speed. This value is 31 and

4% higher than that of Sn-9Zn and Sn-40Pb solders, respec-ively. Similarly, for 2.0 mm/min crosshead speed, the strengthf the Sn-8Zn-3Bi solder is 40% higher than that of Sn-9Zn andn-40Pb solders. So, it can be said that the strength of Sn-8Zn-

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ig. 7. Fracture surface of (a) Sn-40Pb, (b) Sn-9Zn and (c) Sn-8Zn-3Bi soldersor 0.2 mm/min.

Bi is 1.5 times greater than that of Sn-9Zn and Sn-40Pb soldersor all crosshead speed investigated. Since during soldering theemperature above 200 ◦C were maintained for 4.5 min, the Snnd Zn rich phases become increasingly supersaturated with Bi.he super saturation is relieved by the precipitation of Bi rich

hase within the Sn rich phase. This segregation of Bi promoteshe refinement of the microstructure of solder in solidification.his fine microstructure is effective in improving the strengthf the solder joints. Furthermore, the scallop morphology of the

116 R. Mayappan et al. / Journal of Alloys and Compounds 436 (2007) 112–117

Fig. 8. Total strain values at the fracture.

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ig. 9. Stress–strain curves of solder joints at room temperature for 0.2 mm/minrosshead speed.

u-Sn + Cu-Zn interface may have contributed in strengtheninghe Sn-8Zn-3Bi/Cu joints.

.5. Fracture morphologies

Fig. 11 shows the SEM secondary electron image of frac-ure surface of Sn-40Pb, Sn-9Zn and Sn-8Zn-3Bi solders afterhe joint test. A ductile pattern can be seen on the fracture sur-ace pattern can be seen on the fracture of Sn-40Pb alloy atll crosshead speed. For Sn-9Zn and Sn-8Zn-3Bi fracture sur-ace, ductile fracture was also observed. For Sn-9Zn solder, theimple size decreases with increasing crosshead speed. Other

esearchers reported [6,15] that the failure occurred not betweenhe solder/Cu interface but at the interface of IMC layer and theolder. For Sn-9Zn solder, Chang et al. [15] explained that theacture occurs at the interface between the solder and Cu5Zn8

Fig. 10. Stress at maximum load is correlated to strength of the solder.

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ig. 11. SEM secondary electron image of fracture surface: (a) Sn-40Pb, (b)n-9Zn and (c) Sn-8Zn-3Bi solders for 0.2 mm/min.

ayer, indicating tat the Cu5Zn8 layer has an inferior loadingtrength to the Sn-9Zn solder alloy.

. Conclusion

Microstructure, IMC formation and joint strength for Sn-9Znnd Sn-8Zn-3Bi solders were investigated and compared withhose of a Sn-40Pb near eutectic solder at various crossheadpeed. Sn-40Pb has Sn-rich and Pb-rich phases. Sn-9Zn and Sn-Zn-3Bi solders have Sn-rich and small amount of needle-liken-rich phases distributed in the Sn matrix when they are slowooled. The Sn-9Zn has a more uniform and finer microstructure.urthermore, in Sn-8Zn-3Bi system, a small amount of Bi-rich

hase segregates towards the surface.

For Sn-40Pb/Cu interface, a single Cu6Sn5 IMC layer wasbserved. For Sn-9Zn solder the first layer next to the solder is au6Sn5 phase with some Zn dissolve in it. The second layer next

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[16] M.N. Islam, Y.C. Chan, M.J. Rizvi, W. Jillek, J. Alloy Compd. 400 (2005)136.

[17] T. Massalski, Binary Alloy Phase Diagrams 2 (1992) 1481.[18] T. Massalski, Binary Alloy Phase Diagrams 2 (1992) 1508.

R. Mayappan et al. / Journal of Allo

o the Cu substrate is �-Cu5Zn8 phase. While for Sn-8Zn-3Biolder, the first layer is mixture of Cu-Sn + Cu-Zn phase and theecond layer is �-Cu5Zn8 phase.

The tensile strength of Sn-8Zn-3Bi solder is approximately.5 times higher than Sn-9Zn and Sn-40Pb solders in therosshead speed range investigated. The addition of 3 wt% ofi tends to enhance the joint strength of the Sn-8Zn-3Bi solderue to fine microstructure. Although the ductility increases withncreasing crosshead speed, the Sn-8Zn-3Bi and Sn-9Zn soldersxhibited lower ductility than Sn-40Pb. The crosshead speed hasdirect influence on the joint strength of the three solders. The

racture of the solder joints occurs in the elastic strain region orfter a small plastic strain. For Sn-8Zn-3Bi and Sn-9Zn solders,he fracture were at the interface of IMC layer and the solder andor Sn-40Pb solder the fracture occurs at the solder matrix. Theracture surface of the three solders investigated showed ductileimple appearance.

cknowledgement

The work described in this report was supported byUN/SEED-net Grant no. 6050031.

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