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

Investigations of Sn-9Zn-Ag-Ga-Al-Ce Solder Wetted on Cu, Au/Ni/Cu, and Sn-plated Cu Substrates The solderability of Zn-containing, lead-free alloys was examined to determine whether they are good candidates for use in the electronics industry

ABSTRACT Interfacial 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 microscope (SEM) and X-ray diffraction (XRD), while the hardness and indentation modulus of the interface reaction products were studied by the nanoindentation technique (NIT). The SEM and XRD results indicated that interfacial reaction products between solder and substrates were Cu5Zn8, AgZn3, AuZn3, and Ni5ZSn21. A Cu5Zn8/ Sn/ Cu5Zn8 sandwich structure formed at the interface between solder and Sn-plated substrate. The hardness and modulus values of Cu5Zn8 and Ni5Zn21 obtained through NIT were noticeably high, while that of Sn-9Zn-0.25Ag-0.2Ga-0.002Al-0.15Ce solder was low, exhibiting significant plasticity. Moreover, the wetting balance test and mechanical property test indicated that Sn-9Zn-0.25Ag-0.2Ga-0.002Al-0.15Ce solder exhibited good solderability on Sn-plated Cu substrate, and the application of Au/Ni/Cu substrate may enhance the soldered joints.

Introduction Concerns about the health and environmental hazards of lead, and legislative actions around the world drove the research community to find replaceable solder alloys for the traditional Sn-Pb alloys (Refs. 1–3). Among the lead-free candidates that have been developed, such as Sn-Ag, Sn-Cu, Sn-Bi and Sn-Zn alloy systems, Sn-Zn alloy has been receiving special attention due to its low cost, wide raw material sources, superior strength, and low melting point near to eutectic Sn-Pb solder (Ref. 4). Nevertheless, there are still several problems that need to be addressed in order to facilitate the practical use of this solder alloy, such as its inferior wettability and easy oxidation (Refs. 5, 6). Heretofore, a lot of research has been

done to improve the performances of the Sn-Zn solders (Refs. 6–8). Recent studies also pointed out that Sn-Zn-Ag-Ga-Al-Ce alloy presented good solderability and antioxidiation (Refs. 9, 10). These efforts are expected to promote the use of Zn-containing lead-free solders in the electronics industry. Solders are typically melted on a metallic substrate, such as Cu or Ni. Formation of intermetallic compounds (IMCs) is essential in the manufacturing (Refs. 11, 12). IMCs formed at the interface are a prerequisite for good solderability. It is reported that interfacial reaction may be a dominant factor in promoting the wetting, compared with the side effect of surface roughness (Ref. 13). Moreover, IMCs

KEYWORDS H. WANG is with faculty of Materials Science & Chemical Engineering, Ningbo University, and the College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, P.R. China. S. XUE and W. CHEN are with College of Materials Science and Technology, Nanjiing University of Aeronautics andAstronautics, P. R. China. X. LIU and J. PAN are with faculty of Materials Science & Chemical Engineering, Ningbo University, P. R. China.

Lead-Free Solder Sn-Zn Solderability Nanoindentation Interfacial Reaction Intermetallic Compounds

formed at the interface also have a significant effect on the mechanical properties and reliability of the soldered joints (Refs. 14, 15). This is because the brittle nature of the IMCs and the joining of the two materials with dissimilar properties, such as thermal expansion coefficient, hardness, and Young’s modulus, would degrade the interface integrity between solder and substrate (Ref. 14). Thus, a comprehensive knowledge of the intermetallic phases formed at the interface and its mechanical properties is extremely important. It is known that Cu-Zn intermetallic compounds form between Sn-Zn solders and Cu substrate (Ref. 16); however, the reaction products may change when other elements are added to the solder or various substrates are used (Refs. 15, 17). In the literature, most of the studies were carried out to find the mechanical properties of the Cu-Sn, Ni-Sn, and Cu-Ni-Sn based intermetallics that are associated with Sn-Pb, Sn-Ag, and Sn-Ag-Cu solders (Refs. 12, 18–21). However, to the best of our knowledge, the micromechanical property data for the IMCs associated with Sn-Zn solders and different substrates were rarely reported in the literature survey conducted. The aim of this study is to investigate the solderability and the intermetallic compounds formed between Sn-9Zn-0.25Ag-0.2Ga-0.002Al0.15Ce solder and three types of widely utilized substrates: Cu, Au/Ni/Cu, and Snplated Cu.

Experimental Procedures Material Preparation

Pure Sn, Zn, Ag, Al, Ga, and Ce (99.95% pure) were used in the present investigation. The raw materials were first melted in a ceramic crucible to prepare Sn9Zn, Sn-9Zn-0.1Al, Sn-9Zn-1Ag, Sn-9Zn2Ga, and Sn-9Zn-4Ce as master alloys. In

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

Fig. 2 — Schematic illustrations of the mechanical property test of microjoints.

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

plated/Cu flakes were electroplated Sn on Cu substrate. The size of test flakes was 0.3 × 5 × 30 mm. The quad flat package (QFP) device with 48 leads (the size of the QFP leads was 0.2 × 0.5 mm) and ceramic resistors (CR)(the size of the CR pad is 1.25 × 0.5 mm) were soldered on FR-4 printed circuit boards (PCB). The pads of PCB are made of Cu, Sn-plated/ Cu, and Au/Ni/Cu, respectively. Wetting Balance Test

the melting process, KCl+ LiCl molten salt, with the mass ratio of 1.3:1, was used over the surface of the liquid alloys to prevent oxidation during smelting. Then the experimental alloys were melted in a quartz crucible at 300°C by using the master alloys. In order to avoid oxidation and ensure the actual compositions of the alloy elements match the designed value, the entire melting process was carried out in a nitrogen atmosphere. Finally, the contents of Zn, Ag, Ga Al, and Ce in the alloy were tested by the inductively coupled plasma auger electron spectroscopy (ICP-AES) method. The result is shown in Table 1. Pure Cu, Au/Ni/Cu, and Sn-plated/Cu flakes, respectively, were employed as wetted substrates.The Au/Ni/Cu flakes were constructed by electroplating Au/Ni over underlying Cu flakes, while the Sn-

Wetting experiments were performed by a SAT-5100 wetting tester (Rhesca Co. Ltd., Japan) according to Japanese Industry Standard JIS Z 3198-4, Test methods for lead-free solders — Part 4: Methods for solderability test by a wetting balance method and a contact angle method. Figure 1 shows a typical wetting curve in the wetting balance test. According to Fig. 1, the molten solder climbs up on the sample flake (e.g., Cu) due to the wetting force exerted on it when it is dipped into the solder bath. This is similar to the situation in wave soldering, where a wave of molten solder is brought into contact with the substrate. Wetting that occurs in a short time (t0) with a high peak wetting force (Fmax) is considered to be good. The wetting balance test was per-

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

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formed at 235°C in air atmosphere applied with middle active rosin (MAR) flux. Pure Cu, Au/Ni/Cu, and Sn-plated/Cu flakes were employed as substrates. The flakes were immersed into the molten solder for 10 s, and the immersion depth was 2 mm. Microstructure of the Interface

In order to investigate the reaction products on the interface, pure Cu, Au/Ni/Cu, and Sn-plated/Cu substrates were dipped in Sn-9Zn-0.2Ga-0.002Al0.25Ag-0.15Ce solder for 30 s at 235°C to achieve reaction layers. After that, some of the wetted flakes were cross sectioned for scanning electronic microscope (SEM) analysis, and the other dipped substrates were immersed into a solution, 99% CH3OH + 0.5% HCl + 0.5% HNO3, to remove the unreacted solder. Then the exposed IMCs were further characterized by X-ray diffraction (XRD). Nanoindentation Test

The nanoindentation technique explored in this work is an attractive technique for extracting Young’s modulus of the IMCs because of the relatively small volume tested. Indeed, the properties measured from nanoindentation are the true properties of the IMC layer. A nanoindenter SHIMADZU DUHW201S equipped with a Berkovich 115-deg diamond-probe tip, three-sided pyramidal indenter was employed. After the area of interest was focused, the nanoindentation test was conducted under a 50-mN load. The loading and unloading rates were both 2 mN/s and held at 50 mN for 10 s. In order to achieve thick IMC layers, the substrates were first dipped into the molten solder for 10 min at 260°C, and then annealed at 150°C for 300 h, so that at least 20 μm IMC layers would form. Thus, the size of the IMC layers in these joints was sufficient to be analyzed for the nanoindentation test. After the test, an op-

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A

B

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

B

C

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 — XRD pattern of the IMCs.

tical microscopy was employed to identify the indentation traces. The mechanical properties were obtained by averaging five experimental indents, which were selected by optical microscopy to avoid the boundary effects. All the tests were carried out out at 20°C. Mechanical Property Tests

In order to reflect the actual instances in the electronic assembly industry, the mechanical property tests of the micro joints were carried on according to Japanese Industry Standard JIS Z 3198-6, Test methods for lead-free solders — Part 6: Methods for 45-deg pull test of solder joints on QFP lead, and Part 7: Methods for shear test of solder joints on chip components, with an STR-1000 joint strength tester, as schematically shown in Fig. 2. First, the flat packages and ceramic resistors were soldered on PCB by Sn-9Zn0.2Ga-0.002Al-0.25Ag-0.15Ce solder at 250°C. Subsequently, the pull and shear tests of the soldered joints were carried out at room temperature. The pull speed and shear speed were set as 2 and 10mm/min, respectively.

Results and Discussions Solderability of Solder

Figure 3 shows the solderability of the solder wetted on different substrates. It is found that the solder exhibits better solderability on Sn-plated/Cu substrate, with higher wetting force and shorter wetting time, than on Au/Ni/Cu substrates. The wetting time using Cu substrate and Sn-plated Cu substrate are similar; however, the Fmax on the Sn-plated Cu is higher than that on the Cu substrate. The higher Fmax may be attributed to the Sn-plated layer, which can reduce the interfacial tension between solder and substrate. It is reported that Sn-

plated pads can improve the solderability of Sn-Ag-Cu and Sn-Cu solders (Ref. 22). The results shown in Fig. 3 also indicated that the Sn-plated/Cu substrate can improve the solderability of Zn-bearing solder. Interfacial Reactions between the Solder and Substrates

Figure 4 shows the backscattering electron image associated with EDS and XRD analysis of the interface between Sn-9Zn0.2Ga-0.002Al-0.25Ag-0.15Ce solder and Cu substrate. According to Fig. 4A, it is notable that the interfacial IMCs can be clearly divided into two portions, a planar layer, and an additional scallop-like layer. According to the EDS analysis, the planar one

Table 2 — Indentation Modulus and Hardness of the Materials Tested Material Cu5Zn8 Ni5Zn21 AgZn3 AuZn3 Cu Solder

Indentation Modulus (GPa) 162.1 ± 8.6 159.3 ± 7.2 124.4 ± 2.5 118.2 ± 2.2 115.7 ± 1.2 59.9 ± 3.2

Hardness (GPa) 4.92 ± 0.50 5.12 ± 0.52 3.52 ± 0.21 3.40 ± 0.20 1.70 ± 0.36 0.36 ± 0.07

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Ag5Zn8. The Gibb’s free energy of Ag5Zn8 is smaller than that of AgZn3 and AgZn IMCs at 235°C (Ref. 23) by heterogeneous nucleation on the preformed Cu5Zn8 interface, since Cu5Zn8 and Ag5Zn8 exhibit identical structure and their lattice constants do not differ greatly. Furthermore, the subsequent peritectic reaction: L + γ -Ag5Zn8 → ε –AgZn3 contribute to the AgZn3 observed E at the interface (Ref. 24). Figure 5 shows the interface between the solder and Fig. 6 — A — Interface between Sn-9Zn-0.2Ga-0.002Al-0.25ASn-plated/Cu substrate, associ0.15Ce solder and Au/Ni/Cu substrate; B — EDS analysis of point A; C — EDS analysis of point B; D — EDS analysis of point C; ated with the line scanning and XRD pattern of the IMC E — XRD pattern of the IMCs. layer. The XRD pattern indicated that the IMC is Cu5Zn8. According to the line scanning, a thin Sn layer still exists in the is mainly composed of Cu and Zn, while the middle of the Cu5Zn8 layers, forming a scallop-like one contains much more Ag. sandwich structure. The formation of the The XRD pattern shown in Fig. 4B indisandwich structure is mainly because that, cates that these IMCs are Cu5Zn8 and during the wetting period, Zn atoms difAgZn3. The formation of AgZn3 is considfuse through the Sn layer and then react ered that, due to the formation of Cu5Zn8, with Cu atoms, while the Cu atoms also zinc atoms transfer from the liquid solder to diffuse through the Sn layer to the molten the substrate and enrich at the interface. solder and react with Zn atoms to form Meanwhile, silver atoms also segregate at Cu5Zn8. At the same time, the Sn layer the interface and react with zinc to form 252-s DECEMBER 2010, VOL. 89

may melt to the molten solder as the wetting temperature is 235°C, a little higher than the melting point of Sn. Before the substrate was taken out of the solder, the Sn layer had not melt completely, as a result the aforementioned sandwich structure formed. Figure 6 shows the backscattering electron image of the interface between the solder and Au/Ni/Cu substrate, associated with the EDS and XRD analysis of the IMCs layer. The XRD pattern shown in Fig. 6C indicates that the IMCs are AuZn3 and a little AuAgZn2. However, the Ni layer and the Ni-Zn IMCs layer were covered by the AuZn3 and the remnants solder, thus they were not detected by the XRD analysis. The EDS indicated that the Ni layer was composed of Ni and Zn, according to the report in Ref. 17. We supposed that this layer was Ni5Zn21. Indentation Test

The indentation test was carried out on both Sn-9Zn-0.25Ag0.2Ga-0.002Al-0.15Ce solder, Cu substrate, and the four IMCs: Cu5Zn8, AgZn3, AuZn3 and Ni5Zn21. Figure 7 shows the indentation traces on different materials tested. However, a thick AgZn3 layer can be hardly achieved, according to the report in Ref. 22, massive AgZn3 exists in Sn-9Zn-2Ag solder, and thus the indentation test on AgZn3 phase was carried on Sn-9Zn-2Ag solder as shown in Fig. 7D. According to Fig. 7, it is found that the indentation trace in the IMCs is smaller than that in the solder. This is because that the IMCs are much harder than the solder. Figure 8 shows the load-displacement plots obtained by the load indentations performed on the aforementioned materials. From the load-displacement curves, a hardness and elastic modulus are measured that are useful in describing the deformation behavior of the solder and IMCs. Analysis of load-displacement data was carried out according to the Oliver and Pharr method (Refs. 24, 25). At the maximum load, Pmax and hardness, HNI is determined by HNI = Pmax / A(hc), where A(hc) is the projected contact area. From the slope of the unloading curve, a reduced modulus Er is measured that accounts for elastic recovery of the sample and the indenter:

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B

C

D

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A

E

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

⎛ 1 − ν2 E =⎜ r ⎜ ⎝ E

sample

+

1 − ν2 E

⎞ ⎟ indenter ⎟ ⎠

−1

where E is the Young’s modulus and ν is Poisson’s ratio. With the properties of the diamond 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

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

be reported here; however, with knowledge of Poisson’s ratio of the test material, the Young’s modulus, E, can be determined. Table 2 shows the results calculated according to the data in Fig. 8. In examining the results obtained, the maximum penetration of the indenter for Sn-9Zn-0.25Ag0.2Ga-0.002Al-0.15Ce solder is approximately four times that measured for

Cu5Zn8. The solder is found to be very soft, with a hardness of 0.36 GPa, exhibiting significant plasticity. In contrast to the solder, the IMCs, Cu5Zn8, and Ni5Zn21, are significantly harder while the hardness of AgZn3 and AuZn3 is similar to that of Cu. The intermetallic compound had a higher modulus than either of the two components. The higher modulus of the intermetallic over that of either of the metallic components WELDING JOURNAL 253-s

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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.

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hibit higher shear and pull force than that on the Cu substrate, while the joints on the Snplated/Cu substrate show lower shear and pull force than that on the Cu substrate. It is known that Sn (Zn for Sn-Zn solders) at the solder/Cu interface reacts rapidly with Cu to form Cu-Sn (Cu-Zn for Sn-Zn solders) IMCs, which weaken the soldered joints (Refs. Fig. 10 — Mechanical properties of the QFP and CR microjoints. 24–25). Therefore, Ni is used as a diffusion barrier layer to prevent the rapid interfacial reaccan be attributed to the combination of tions between the solder and Cu layer in ionic/covalent bonding in intermetallic comelectronic devices. In this study, it was found pounds, which may result in a higher modthat the application of Au/Ni/Cu substrate ulus than that obtained from a simple rule can really enhance the soldered joints. It of mixtures of the components (Ref. 26). It was also found that Sn-plated/Cu substrate should be noted that although AgZn3 and can improve the solderability however, it AuZn3 are intermetallic compounds, they may deteriorate the soldered joints. do exhibit some extent of plasticity. Hardness is often used to explain the Conclusions brittleness of a material. The indication is that Cu5Zn8 and Ni5Zn21 have the poten1) Sn-9Zn-0.25Ag-0.2Ga-0.002Al-0.15 tial for brittle behavior (crack initiation) Ce solder shows better solderability on Snwhen the soldered joints were deformed by plated Cu substrate than that of pure Cu stress. AgZn3 and AuZn3, with a lower and Au/Ni/Cu; while the application of hardness and modulus, are actually rather Au/Ni/Cu pads may deteriorate the solsoft and ductile and not a likely source of derability. A mechanical property test incrack initiation. Actually, cracks were found dicated that the application of Au/Ni/Cu located at the Cu5Zn8 layer, and the intersubstrate enhanced the soldered joints. face between AuZn3 and Ni5Zn21 layer, as 2) Cu5Zn8 and AgZn3 intermetallic shown in Fig. 9. compounds form at the interface between Sn-9Zn-0.2Ga-0.002Al-0.25Ag-0.15Ce solMechanical Properties of Soldered Joints der and Cu substrate, while AuZn3 and AuAgZn2 were present at the interface beFigure 10 shows the results of the metween the solder and Au/Ni/Cu substrate. chanical properties of the QFP and CR miWhen the Sn-plated Cu substrate was used, crojoints. According to Fig. 10, it is found the Cu5Zn8 layers and the remnants Sn that the joints on the Au/Ni/Cu substrate ex-

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layer constituted a sandwich structure at the interface. 3) Cu5Zn8 and Ni5Zn21 may be the crack initiation point when the soldered joint was deformed by stress due to the high hardness and modules. On the other hand, Sn-9Zn-0.25Ag-0.2Ga-0.002Al0.15Ce solder is soft and exhibits significant plasticity, while AgZn3 and AuZn3 also exhibit a little plasticity. Acknowledgments The paper was sponsored by the K.C. Wang MagnaFund, Ningbo University. The authors would also like to acknowledge the support from the Talent Project Fund (019B00228104700), and Dr. Jun Huang for the nanoindentation measurements. References 1. Byig, B., Budi D., Hamdi, M., and Ariga, T. 2009. The effects of adding silver and indium to lead-free solders. Welding Journal 88(4): 45-s to 47-s. 2. McCormack, M., and Jin, S. 1993. Progress in the design of new lead-free solder alloys. Journal of the Minerals, Metals and Materials Society 45(7): 36 to 40. 3. Abtew, M., and Selvaduray, G. 2000. Lead-free solders in microelectronics. Materials Science and Engineering R. 27(5–6): 95 to 141. 4. Mavoori, H., Chin, J., Vaynman, S., Moran, B., Keer, L., and Fine, M. 1997. Creep, stress relaxation, and plastic deformation in SnAg and Sn-Zn eutectic solders. Journal of Electron Materials 26(7): 783 to 790. 5. Lin, K. L., and Liu, T. P. 1998. High-temperature oxidation of a Sn-Zn-Al solder. Oxidation of Metals. 50(3-4): 255 to 267. 6. Kim, K. S., Ryu, K. W., Yu, C. H., and Kim, J. K. 2005. The formation and growth of intermetallic compounds and shear strength at Sn-Zn solder/Au-Ni-Cu interfaces. Microelectronics and Reliability 45(3-4): 647 to 655. 7. Date, M., Tu, K. N., Shoji, T., Fujiyosh, M., and Sato, K. 2004. Interfacial reactions and impact reliability of Sn-Zn solder joints on Cu

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or electroless Au/Ni(P) bond-pads. Journal of Materials Research 19(10): 2887 to 2896. 8. Hirose, A., Yanagawa, H., Ide, E., and Kobayashi, K. F. 2004. Joint strength and interfacial microstructure between Sn-Ag-Cu and Sn-Zn-Bi solders and Cu substrate. Science and Technology of Advanced Materials. 5(1-2): 267 to 276. 9. Wang, H., Xue, S. B., Chen, W. X., and Zhao, F. 2009. Effects of Ga-Ag, Ga-Al, and Al-Ag addition on the wetting characteristics of Sn-9Zn-X-Y lead-free solders. Journal of Materials Science: Materials in Electronics 20(12): 1239 to 1246. 10. Wang, H., Xue, S. B., Chen, W. X., Zhao, F. 2010. Effects of Ga, Al, Ag, and Ce multi-additions on the properties of Sn-9Zn lead-free solder. Journal of Materials Science: Materials in Electronics 21(2): 111 to 119. 11. Manko, H. H. 2001. Solders and Soldering. p. 61, New York, McGraw-Hill, Inc. 12. Mohan, K. K., Shen, L., Zeng K. Y., and Tay, A. A. O. 2006. Nanoindentation study of Zn-based Pb free solders used in fine pitch interconnect applications. Materials Science and Engineering A, 423(1-2): 57 to 63. 13. Wang, H. Q., Gao, F., Ma, X., and Qian,Y. Y. 2006. Reactive wetting of solders on Cu and Cu6Sn5/Cu3Sn/Cu substrates using wetting balance. Scripta Materialia 55(9): 823 to 826.

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14. Huang, C. W., and Lin, K. L. 2004. Interfacial reactions of lead-free Sn-Zn based solders on Cu and Cu plated electroless Ni-P/Au layer under aging at 150. Journal of Materials Research 19(12): 3560-s to 3568-s. 15. Yoon, J. W., and Jung, S. B. 2005. Reliability studies of Sn-9Zn/Cu solder joints with aging treatment. Journal of Alloys and Compounds 407(1-2): 141 to 149. 16. Suganuma, K., Niihara, K., Shoutoku, T., and Nakamura, Y. 1998. Wetting and interface microstructure between Sn-Zn binary alloys and Cu. Journal of Materials Research 13(10): 2859 to 2865. 17. Yoon, J. W., and Jung, S. B. 2007. Solder joint reliability evaluation of Sn-Zn/Au/Ni/Cu ball-grid-array package during aging. Materials Science and Engineering A., 452-453: 46 to 54. 18. Yang, P. F., Lai, Y. S., Jian, S. R., Chen, J., and Chen, R. S. 2008. Nanoindentation identifications of mechanical properties of Cu6Sn5, Cu3Sn, and Ni3Sn4 intermetallic compounds derived by diffusion couples. Materials Science and Engineering A. 485(1-2): 305 to 310. 19. Deng, X., Koopman, M., Chawla, N., and Chawla, K. K. 2004. Young’s modulus of (Cu, Ag)-Sn intermetallics measured by nanoindentation. Materials Science and Engineering A. 364(1-2): 240 to 243. 20. Xu, L. H., and Pang John, H. L. 2006.

Nanoindentation on SnAgCu lead-free solder joints and analysis. Journal of Electronic Materials 35(12): 2107 to 2115. 21. Lucas, J. P., Rhee, H., Guo, F., and Subramanian, K. N. 2003. Mechanical properties of intermetallic compounds associated with Pbfree solder joints using nanoindentation. Journal of Electron Materials 32(12): 1375 to 1383. 22. Collier, P., Sunappan, V., and Periannan, A. 2002. Lead-free solder process implementation for PCB assembly. Soldering and Surface Mount Technology 14(3): 12 to 18. 23. Tsai, Y. L., and Hwang, W. S. 2005. Solidification behavior of Sn-9Zn-xAg lead-free solder alloys. Materials Science and Engineering A. 413-414: 312 to 316. 24. Song, J. M., Liu, P. C., Shih, C. L., and Lin, K. L. 2005. Role of Ag in the formation of interfacial intermetallic phases in Sn-Zn soldering. Journal of Electronic Materials 34(9): 1249 to 1254. 25. Oliver, W. C., and Pharr, G. M. 1992. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. Journal of Materials Research 7(6):1564 to 1583. 26. Meyers, M. A., and Chawla, K. K. 1999. Mechanical Behavior of Materials. p. 112, Prentice-Hall, Upper Saddle River, N.J.

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