termodinamika

Introduction

Concerns about the health and envi-ronmental hazards of lead, and legislativeactions around the world drove the re-search community to find replaceable sol-der alloys for the traditional Sn-Pb alloys(Refs. 1–3). Among the lead-free candi-dates that have been developed, such asSn-Ag, Sn-Cu, Sn-Bi and Sn-Zn alloy sys-tems, Sn-Zn alloy has been receiving spe-cial attention due to its low cost, wide rawmaterial sources, superior strength, andlow melting point near to eutectic Sn-Pbsolder (Ref. 4). Nevertheless, there arestill several problems that need to be ad-dressed in order to facilitate the practicaluse of this solder alloy, such as its inferiorwettability and easy oxidation (Refs. 5, 6).Heretofore, a lot of research has been

done to improve the performances of theSn-Zn solders (Refs. 6–8). Recent studiesalso pointed out that Sn-Zn-Ag-Ga-Al-Cealloy presented good solderability and an-tioxidiation (Refs. 9, 10). These efforts areexpected to promote the use of Zn-con-taining lead-free solders in the electronicsindustry.

Solders are typically melted on a metal-lic substrate, such as Cu or Ni. Formationof intermetallic compounds (IMCs) is es-sential in the manufacturing (Refs. 11, 12).IMCs formed at the interface are a pre-requisite for good solderability. It is re-ported that interfacial reaction may be adominant factor in promoting the wetting,compared with the side effect of surfaceroughness (Ref. 13). Moreover, IMCs

formed at the interface also have a signif-icant effect on the mechanical propertiesand reliability of the soldered joints (Refs.14, 15). This is because the brittle natureof the IMCs and the joining of the two ma-terials with dissimilar properties, such asthermal expansion coefficient, hardness,and Young’s modulus, would degrade theinterface integrity between solder and sub-strate (Ref. 14). Thus, a comprehensiveknowledge of the intermetallic phasesformed at the interface and its mechanicalproperties is extremely important.

It is known that Cu-Zn intermetalliccompounds form between Sn-Zn soldersand Cu substrate (Ref. 16); however, thereaction products may change when otherelements are added to the solder or vari-ous substrates are used (Refs. 15, 17). Inthe literature, most of the studies werecarried out to find the mechanical proper-ties of the Cu-Sn, Ni-Sn, and Cu-Ni-Snbased intermetallics that are associatedwith Sn-Pb, Sn-Ag, and Sn-Ag-Cu solders(Refs. 12, 18–21). However, to the best ofour knowledge, the micromechanicalproperty data for the IMCs associatedwith Sn-Zn solders and different sub-strates were rarely reported in the litera-ture survey conducted. The aim of thisstudy is to investigate the solderability andthe intermetallic compounds formed be-tween Sn-9Zn-0.25Ag-0.2Ga-0.002Al-0.15Ce solder and three types of widelyutilized substrates: Cu, Au/Ni/Cu, and Sn-plated Cu.

Experimental Procedures

Material Preparation

Pure Sn, Zn, Ag, Al, Ga, and Ce(99.95% pure) were used in the present in-vestigation. The raw materials were firstmelted in a ceramic crucible to prepare Sn-9Zn, Sn-9Zn-0.1Al, Sn-9Zn-1Ag, Sn-9Zn-2Ga, and Sn-9Zn-4Ce as master alloys. In

SUPPLEMENT TO THE WELDING JOURNAL, DECEMBER 2010Sponsored by the American Welding Society and the Welding Research Council

Investigations of Sn-9Zn-Ag-Ga-Al-Ce SolderWetted on Cu, Au/Ni/Cu, and Sn-plated Cu

Substrates

The solderability of Zn-containing, lead-free alloys was examined to determinewhether they are good candidates for use in the electronics industry

BY H. WANG, S. XUE, W. CHEN, X. LIU, AND J. PAN

KEYWORDS

Lead-Free SolderSn-ZnSolderabilityNanoindentationInterfacial ReactionIntermetallic Compounds

H. WANG is with faculty of Materials Science &Chemical Engineering, Ningbo University, andthe College of Materials Science and Technology,Nanjing University of Aeronautics and Astronau-tics, P.R. China. S. XUE and W. CHEN are withCollege of Materials Science and Technology,Nanjiing University of Aeronautics andAstronau-tics, P. R. China. X. LIU and J. PAN are with fac-ulty of Materials Science & Chemical Engineer-ing, Ningbo University, P. R. China.

ABSTRACTInterfacial reaction products between Sn-9Zn-0.25Ag-0.2Ga-0.002Al-0.15Ce solder

and Cu, Au/Ni/Cu, Sn-plated Cu substrates were investigated by scanning electron mi-croscope (SEM) and X-ray diffraction (XRD), while the hardness and indentation mod-ulus of the interface reaction products were studied by the nanoindentation technique(NIT). The SEM and XRD results indicated that interfacial reaction products betweensolder and substrates were Cu5Zn8, AgZn3, AuZn3, and Ni5ZSn21. A Cu5Zn8/ Sn/Cu5Zn8 sandwich structure formed at the interface between solder and Sn-plated sub-strate. The hardness and modulus values of Cu5Zn8 and Ni5Zn21 obtained through NITwere noticeably high, while that of Sn-9Zn-0.25Ag-0.2Ga-0.002Al-0.15Ce solder waslow, exhibiting significant plasticity. Moreover, the wetting balance test and mechanicalproperty test indicated that Sn-9Zn-0.25Ag-0.2Ga-0.002Al-0.15Ce solder exhibited goodsolderability on Sn-plated Cu substrate, and the application of Au/Ni/Cu substrate mayenhance the soldered joints.

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the melting process, KCl+ LiCl molten salt,with the mass ratio of 1.3:1, was used overthe surface of the liquid alloys to preventoxidation during smelting. Then the exper-imental alloys were melted in a quartz cru-cible at 300°C by using the master alloys. Inorder to avoid oxidation and ensure the ac-tual compositions of the alloy elementsmatch the designed value, the entire melt-ing process was carried out in a nitrogen at-mosphere. Finally, the contents of Zn, Ag,Ga Al, and Ce in the alloy were tested bythe inductively coupled plasma auger elec-tron spectroscopy (ICP-AES) method. Theresult is shown in Table 1.

Pure Cu, Au/Ni/Cu, and Sn-plated/Cuflakes, respectively, were employed as wet-ted substrates.The Au/Ni/Cu flakes wereconstructed by electroplating Au/Ni overunderlying Cu flakes, while the Sn-

plated/Cu flakes wereelectroplated Sn on Cusubstrate. The size oftest flakes was 0.3 × 5× 30 mm.

The quad flat pack-age (QFP) device with48 leads (the size of theQFP leads was 0.2 ×0.5 mm) and ceramicresistors (CR)(the sizeof the CR pad is 1.25× 0.5 mm) were sol-dered on FR-4 printedcircuit boards (PCB).The pads of PCB aremade of Cu, Sn-plated/Cu, and Au/Ni/Cu, respectively.

Wetting Balance Test

Wetting experiments were performedby a SAT-5100 wetting tester (Rhesca Co.Ltd., Japan) according to Japanese Indus-try Standard JIS Z 3198-4, Test methods forlead-free solders — Part 4: Methods for sol-derability test by a wetting balance methodand a contact angle method. Figure 1 showsa typical wetting curve in the wetting bal-ance test. According to Fig. 1, the moltensolder climbs up on the sample flake (e.g.,Cu) due to the wetting force exerted on itwhen it is dipped into the solder bath. Thisis similar to the situation in wave solder-ing, where a wave of molten solder isbrought into contact with the substrate.Wetting that occurs in a short time (t0)with a high peak wetting force (Fmax) isconsidered to be good.

The wetting balance test was per-

formed at 235°C in air atmosphere appliedwith middle active rosin (MAR) flux. PureCu, Au/Ni/Cu, and Sn-plated/Cu flakeswere employed as substrates. The flakeswere immersed into the molten solder for10 s, and the immersion depth was 2 mm.

Microstructure of the Interface

In order to investigate the reactionproducts on the interface, pure Cu,Au/Ni/Cu, and Sn-plated/Cu substrateswere dipped in Sn-9Zn-0.2Ga-0.002Al-0.25Ag-0.15Ce solder for 30 s at 235°C toachieve reaction layers. After that, someof the wetted flakes were cross sectionedfor scanning electronic microscope (SEM)analysis, and the other dipped substrateswere immersed into a solution, 99%CH3OH + 0.5% HCl + 0.5% HNO3, toremove the unreacted solder. Then the ex-posed IMCs were further characterized byX-ray diffraction (XRD).

Nanoindentation Test

The nanoindentation technique ex-plored in this work is an attractive tech-nique for extracting Young’s modulus ofthe IMCs because of the relatively smallvolume tested. Indeed, the propertiesmeasured from nanoindentation are thetrue properties of the IMC layer.

A nanoindenter SHIMADZU DUH-W201S equipped with a Berkovich 115-degdiamond-probe tip, three-sided pyramidalindenter was employed. After the area of in-terest was focused, the nanoindentation testwas conducted under a 50-mN load. Theloading and unloading rates were both 2mN/s and held at 50 mN for 10 s.

In order to achieve thick IMC layers,the substrates were first dipped into themolten solder for 10 min at 260°C, andthen annealed at 150°C for 300 h, so thatat least 20 μm IMC layers would form.Thus, the size of the IMC layers in thesejoints was sufficient to be analyzed for thenanoindentation test. After the test, an op-

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Table 1 — Chemical Composition of the Prepared Solder (wt-%)

Element Zn Ag Ga Al Ce Sn

Content 8.895 0.194 0.197 0.002 0.135 Balance

Fig. 1 — Wetting curve in the wetting balance test.

xFig. 3 — Solderability of Sn-9Zn-0.2Ga-0.002Al-0.25Ag-0.15Ce solderwetted on different substrates.

Fig. 2 — Schematic illustrations of the mechanical property test of micro-joints.


tical microscopy was employed to identifythe indentation traces. The mechanicalproperties were obtained by averaging fiveexperimental indents, which were selectedby optical microscopy to avoid the bound-ary effects. All the tests were carried outout at 20°C.

Mechanical Property Tests

In order to reflect the actual instancesin the electronic assembly industry, themechanical property tests of the microjoints were carried on according to Japan-ese Industry Standard JIS Z 3198-6, Testmethods for lead-free solders — Part 6:Methods for 45-deg pull test of solder jointson QFP lead, and Part 7: Methods for sheartest of solder joints on chip components,with an STR-1000 joint strength tester, asschematically shown in Fig. 2.

First, the flat packages and ceramic re-sistors were soldered on PCB by Sn-9Zn-0.2Ga-0.002Al-0.25Ag-0.15Ce solder at250°C. Subsequently, the pull and sheartests of the soldered joints were carriedout at room temperature. The pull speedand shear speed were set as 2 and10mm/min, respectively.

Results and Discussions

Solderability of Solder

Figure 3 shows the solderability of thesolder wetted on different substrates. It isfound that the solder exhibits better solder-ability on Sn-plated/Cu substrate, withhigher wetting force and shorter wettingtime, than on Au/Ni/Cu substrates. The wet-ting time using Cu substrate and Sn-platedCu substrate are similar; however, the Fmaxon the Sn-plated Cu is higher than that onthe Cu substrate. The higher Fmax may beattributed to the Sn-plated layer, which canreduce the interfacial tension between sol-der and substrate. It is reported that Sn-

plated pads can improve the solderability ofSn-Ag-Cu and Sn-Cu solders (Ref. 22). Theresults shown in Fig. 3 also indicated thatthe Sn-plated/Cu substrate can improve thesolderability of Zn-bearing solder.

Interfacial Reactions between the Solderand Substrates

Figure 4 shows the backscattering elec-tron image associated with EDS and XRDanalysis of the interface between Sn-9Zn-0.2Ga-0.002Al-0.25Ag-0.15Ce solder andCu substrate. According to Fig. 4A, it is no-table that the interfacial IMCs can be clearlydivided into two portions, a planar layer,and an additional scallop-like layer. Ac-cording to the EDS analysis, the planar one


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Fig. 4 — A — Interface between Sn-9Zn-0.2Ga-0.002Al-0.25Ag-0.15Ce solder and Cu substrate; B — XRD pattern of IMCs.

Fig. 5 — A — Interface between Sn-9Zn-0.2Ga-0.002Al-0.25Ag-0.15Ce solder and Sn-plated Cu substrate; B — EDS scanning along the IMC layer; C — XRDpattern of the IMCs.

Table 2 — Indentation Modulus and Hardness of the Materials Tested

Material Indentation Modulus (GPa) Hardness (GPa)

Cu5Zn8 162.1 ± 8.6 4.92 ± 0.50Ni5Zn21 159.3 ± 7.2 5.12 ± 0.52AgZn3 124.4 ± 2.5 3.52 ± 0.21AuZn3 118.2 ± 2.2 3.40 ± 0.20Cu 115.7 ± 1.2 1.70 ± 0.36Solder 59.9 ± 3.2 0.36 ± 0.07


is mainly composed of Cu and Zn, while thescallop-like one contains much more Ag.The XRD pattern shown in Fig. 4B indi-cates that these IMCs are Cu5Zn8 andAgZn3. The formation of AgZn3 is consid-ered that, due to the formation of Cu5Zn8,zinc atoms transfer from the liquid solder tothe substrate and enrich at the interface.Meanwhile, silver atoms also segregate atthe interface and react with zinc to form

Ag5Zn8. The Gibb’s free energyof Ag5Zn8 is smaller than thatof AgZn3 and AgZn IMCs at235°C (Ref. 23) by heteroge-neous nucleation on the pre-formed Cu5Zn8 interface, sinceCu5Zn8 and Ag5Zn8 exhibitidentical structure and their lat-tice constants do not differgreatly. Furthermore, the sub-sequent peritectic reaction: L +γ -Ag5Zn8 → ε –AgZn3 con-tribute to the AgZn3 observedat the interface (Ref. 24).

Figure 5 shows the in-terface between the solder andSn-plated/Cu substrate, associ-ated with the line scanning andXRD pattern of the IMClayer. The XRD pattern indi-cated that the IMC is Cu5Zn8.According to the line scanning,a thin Sn layer still exists in the

middle of the Cu5Zn8 layers, forming asandwich structure. The formation of thesandwich structure is mainly because that,during the wetting period, Zn atoms dif-fuse through the Sn layer and then reactwith Cu atoms, while the Cu atoms alsodiffuse through the Sn layer to the moltensolder and react with Zn atoms to formCu5Zn8. At the same time, the Sn layer

may melt to the molten sol-der as the wetting tempera-ture is 235°C, a little higherthan the melting point ofSn. Before the substratewas taken out of the solder,the Sn layer had not meltcompletely, as a result theaforementioned sandwichstructure formed.

Figure 6 shows thebackscattering electronimage of the interface be-tween the solder andAu/Ni/Cu substrate, associ-ated with the EDS and XRDanalysis of the IMCs layer.The XRD pattern shown inFig. 6C indicates that theIMCs are AuZn3 and a littleAuAgZn2. However, the Nilayer and the Ni-Zn IMCslayer were covered by theAuZn3 and the remnantssolder, thus they were notdetected by the XRD analy-sis. The EDS indicated thatthe Ni layer was composedof Ni and Zn, according tothe report in Ref. 17. Wesupposed that this layer wasNi5Zn21.

Indentation Test

The indentation testwas carried out on both Sn-9Zn-0.25Ag-0.2Ga-0.002Al-0.15Ce solder, Cu sub-strate, and the four IMCs: Cu5Zn8,AgZn3, AuZn3 and Ni5Zn21. Figure 7shows the indentation traces on differentmaterials tested. However, a thick AgZn3layer can be hardly achieved, according tothe report in Ref. 22, massive AgZn3 ex-ists in Sn-9Zn-2Ag solder, and thus the in-dentation test on AgZn3 phase was carriedon Sn-9Zn-2Ag solder as shown in Fig.7D. According to Fig. 7, it is found that theindentation trace in the IMCs is smallerthan that in the solder. This is because thatthe IMCs are much harder than the solder.

Figure 8 shows the load-displacementplots obtained by the load indentations per-formed on the aforementioned materials.From the load-displacement curves, a hard-ness and elastic modulus are measured thatare useful in describing the deformation be-havior of the solder and IMCs.

Analysis of load-displacement data wascarried out according to the Oliver andPharr method (Refs. 24, 25). At the maxi-mum load, Pmax and hardness, HNI is de-termined by HNI = Pmax / A(hc), whereA(hc) is the projected contact area. Fromthe slope of the unloading curve, a re-duced modulus Er is measured that ac-counts for elastic recovery of the sampleand the indenter:


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HFig. 6 — A — Interface between Sn-9Zn-0.2Ga-0.002Al-0.25A-0.15Ce solder and Au/Ni/Cu substrate; B — EDS analysis of pointA; C — EDS analysis of point B; D — EDS analysis of point C;E — XRD pattern of the IMCs.

A B

C D

E


where E is the Young’s modulus and ν isPoisson’s ratio. With the properties of thediamond indenter known (E = 1140 GPa,and ν = 0.07), the indentation modulus,ENI, is defined as ENI = E/1–ν2|sample. Indentation modulus is primarily what will

be reported here; however, with knowl-edge of Poisson’s ratio of the test material,the Young’s modulus, E, can be determined.

Table 2 shows the results calculated ac-cording to the data in Fig. 8. In examiningthe results obtained, the maximum pene-tration of the indenter for Sn-9Zn-0.25Ag-0.2Ga-0.002Al-0.15Ce solder isapproximately four times that measured for

Cu5Zn8. The solder is found to be very soft,with a hardness of 0.36 GPa, exhibiting sig-nificant plasticity. In contrast to the solder,the IMCs, Cu5Zn8, and Ni5Zn21, are signif-icantly harder while the hardness of AgZn3and AuZn3 is similar to that of Cu. The in-termetallic compound had a higher modu-lus than either of the two components. Thehigher modulus of the intermetallic overthat of either of the metallic components

EE Er sample indenter

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Fig. 7 — Indentation traces on the following: A — Cu5Zn8; B — Ni5Zn21;C — AuZn3; D — AgZn3; E — solder.

Fig. 8 — Plots of load vs. depth for 50-mN maximum load indentations per-formed on Cu, solder, and IMCs.


can be attributed to the combination ofionic/covalent bonding in intermetallic com-pounds, which may result in a higher mod-ulus than that obtained from a simple ruleof mixtures of the components (Ref. 26). Itshould be noted that although AgZn3 andAuZn3 are intermetallic compounds, theydo exhibit some extent of plasticity.

Hardness is often used to explain thebrittleness of a material. The indication isthat Cu5Zn8 and Ni5Zn21 have the poten-tial for brittle behavior (crack initiation)when the soldered joints were deformed bystress. AgZn3 and AuZn3, with a lowerhardness and modulus, are actually rathersoft and ductile and not a likely source ofcrack initiation. Actually, cracks were foundlocated at the Cu5Zn8 layer, and the inter-face between AuZn3 and Ni5Zn21 layer, asshown in Fig. 9.

Mechanical Properties of Soldered Joints

Figure 10 shows the results of the me-chanical properties of the QFP and CR mi-crojoints. According to Fig. 10, it is foundthat the joints on the Au/Ni/Cu substrate ex-

hibit higher shear andpull force than that onthe Cu substrate, whilethe joints on the Sn-plated/Cu substrateshow lower shear andpull force than that onthe Cu substrate.It is known that Sn (Zn

for Sn-Zn solders) at thesolder/Cu interface re-acts rapidly with Cu toform Cu-Sn (Cu-Zn forSn-Zn solders) IMCs,which weaken the sol-dered joints (Refs.24–25). Therefore, Ni isused as a diffusion bar-rier layer to prevent therapid interfacial reac-

tions between the solder and Cu layer inelectronic devices. In this study, it was foundthat the application of Au/Ni/Cu substratecan really enhance the soldered joints. Itwas also found that Sn-plated/Cu substratecan improve the solderability however, itmay deteriorate the soldered joints.

Conclusions1) Sn-9Zn-0.25Ag-0.2Ga-0.002Al-0.15

Ce solder shows better solderability on Sn-plated Cu substrate than that of pure Cuand Au/Ni/Cu; while the application ofAu/Ni/Cu pads may deteriorate the sol-derability. A mechanical property test in-dicated that the application of Au/Ni/Cusubstrate enhanced the soldered joints.

2) Cu5Zn8 and AgZn3 intermetalliccompounds form at the interface betweenSn-9Zn-0.2Ga-0.002Al-0.25Ag-0.15Ce sol-der and Cu substrate, while AuZn3 andAuAgZn2 were present at the interface be-tween the solder and Au/Ni/Cu substrate.When the Sn-plated Cu substrate was used,the Cu5Zn8 layers and the remnants Sn

layer constituted a sandwich structure at theinterface.

3) Cu5Zn8 and Ni5Zn21 may be thecrack initiation point when the solderedjoint was deformed by stress due to thehigh hardness and modules. On the otherhand, Sn-9Zn-0.25Ag-0.2Ga-0.002Al-0.15Ce solder is soft and exhibits signifi-cant plasticity, while AgZn3 and AuZn3also exhibit a little plasticity.

Acknowledgments

The paper was sponsored by the K.C.Wang MagnaFund, Ningbo University. Theauthors would also like to acknowledge thesupport from the Talent Project Fund (019-B00228104700), and Dr. Jun Huang for thenanoindentation measurements.

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Fig. 9 — A — Cracks in the interface between solder and Cu substrate; B —cracks in the interface between solder and Au/Ni/Cu substrate.

Fig. 10 — Mechanical properties of the QFP and CR microjoints.


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