Microelectronics Reliability

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1 Microelectronics Reliability 50 (2010) Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: Effect of nano Al 2 O 3 additions on the microstructure, hardness and shear strength of eutectic Sn 9Zn solder on Au/Ni metallized Cu pads Tama Fouzder a, Asit Kumar Gain a, Y.C. Chan a, *, A. Sharif b, Winco K.C. Yung c a Department of Electronic Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong b Department of Materials and Metallurgical Engineering, Bangladesh University of Engineering and Technology, Dhaka 1000, Bangladesh c Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong article info abstract Article history: Received 7 March 2010 Received in revised form 20 June 2010 Available online 27 July 2010 Nano-sized, nonreacting, noncoarsening Al 2 O 3 particles have been incorporated into eutectic Sn Zn solder alloys to investigate the microstructure, hardness and shear strength on Au/Ni metallized Cu pads ball grid array substrate (BGA). In the plain Sn Zn solder joint and solder joints containing Al 2 O 3 nano-particles, a scallop-shaped AuZn 3 intermetallic compound layer was found at the interfaces. In the solder joints containing Al 2 O 3 nano-particles, a fine acicular-shaped Zn-rich phase and Al 2 O 3 nano-particles were found to be homogeneously distributed in the b-sn matrix. The shear strengths and hardness of solder joints containing higher percentage of Al 2 O 3 nano-particles exhibited consistently higher value than those of plain solder joint and solder joints containing lower percentage of Al 2 O 3 nano-particles due to control the fine microstructure as well as homogeneous distribution of Al 2 O 3 nano-particles acting as a second phase dispersion strengthening mechanism. The fracture surfaces of plain Sn Zn solder joints exhibited a brittle fracture mode with smooth surfaces while Sn Zn solder joints containing Al 2 O 3 nano-particles showed a typical ductile failure with very rough dimpled surfaces. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Owing to the realization of the harmful influence of lead and lead containing alloys on the environment and human health, increasing efforts have been conducted to search for suitable lead-free solders as replacement for the conventional Sn Pb eutectic alloy [1 4]. Therefore, many research groups are concerned with the development of new lead-free solders and their composites. A further factor is the continual miniaturization of integrated circuits and the quest for better performance and reliability from interconnection joints [5]. In general, new lead-free solders must meet an expected level of mechanical and electrical performance, have a suitable melting temperature, be corrosion resistant, relatively harmless to health and the environment and have a low material cost [6 8]. Reliability of a solder joint plays an important role in determining the lifetime of electronic devices. It is mainly dependent on matching the coefficient of thermal expansion, having a high elastic modulus, yield strength and shear strength together with resistance to fatigue and creep [9 11]. Studies have shown that a potentially viable and economically affordable approach to improve the mechanical properties of a solder is to add appropriate second phase particles, of ceramic, metallic or intermetallic, to a solder matrix so as to form * Corresponding author. Tel.: ; fax: address: eeycchan@cityu.edu.hk (Y.C. Chan). a composite. The formation, presence and growth of the second phase have been proposed as a potential mechanism controlling solderability, a fine microstructure and improved reliability of solder joints [12]. Lin et al. studied the influence of reinforcing TiO 2 and Cu nano-particles on microstructural development and hardness of eutectic Sn Pb solders, and the measured microhardness revealed that the addition of TiO 2 and Cu nano-particles enhanced the overall strength of the eutectic solder [13,14]. Mavoori and Jin [15] used 5 nm TiO 2 and 10 nm Al 2 O 3 particles as reinforcement for a conventional 63Sn 37Pb solder and reported significant enhancement in creep resistance and other mechanical properties. Mohan et al. [16] prepared Sn Pb composite solders by the addition of single wall carbon nano-tubes (SWCNT) as a reinforcing agent and their mechanical properties such as hardness, yield strength and ultimate tensile strength were shown to be superior to the unreinforced solder, while the melting point was not appreciably altered. Shen et al. [17] successfully prepared a ZrO 2 reinforced composite solder by mechanically dispersing ZrO 2 particles into a eutectic Sn 3.5Ag solder paste and the composite solders had an improved microhardness and a refined microstructure. Li and Gupta [18] reported that nano Al 2 O 3 reinforced 91.4Sn 4In 4.1Ag 0.5Cu alloy significantly improved the hardness, yield strength and ultimate tensile strength. However, the result of a literature search revealed that no studies have been reported so far to develop lead-free Sn Zn solder joints containing Al 2 O 3 nano-particles on Au/Ni metallized Cu /$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi: /j.microrel

2 2052 T. Fouzder et al. / Microelectronics Reliability 50 (2010) pads. Accordingly, the aim of the present scientific study is to prepare a lead-free Sn Zn solder with different weight percentages of Al 2 O 3 nano-particles. Plain Sn Zn solder joints and solder joints containing Al 2 O 3 nano-particles were characterized in terms of interfacial microstructures, shear strengths and hardness on Au/ Ni metallized Cu pads as a function of the number of reflow cycles and the content of Al 2 O 3 nano-particles. 2. Experimental procedure The Sn Zn solders containing Al 2 O 3 nano-particles were prepared primarily by mechanically dispersing Al 2 O 3 nano-particles (0, 0.5, 1, 3 wt%) into the eutectic Sn 9Zn (Showa Denko JUFFIT-E 9ZSN10 M) solder paste. The mixture was blended manually for at least 30 min to ensure a uniform distribution of Al 2 O 3 nano-particles (Inframat Advanced Materials LLC, USA). Then, the paste mixture was printed onto alumina substrates using a stainless steel stencil with a thickness of 0.15 mm and reflowed in a reflow oven (BTU VIP-70 N) at 245 C to prepare approximately 0.76 mm diameter solder balls. A solder mask-defined copper bond pad on the flexible substrate of a ball grid array (BGA) package was used as a base for the electrodeposition of Ni and Au. The solder mask-opening diameter was 0.6 mm and a 7 lm thick Ni layer was deposited on the ball pad. The average thickness of the Au layer was 0.5 lm. Lead-free solder balls with a diameter of 0.76 mm, were placed on prefluxed Au/Ni/Cu bond pads of the substrates and reflowed at a temperature of 245 C with a belt speed of 70 cm/min in a convection reflow oven (BTU VIP-70 N). The flux used in this study was a commercial rosin activated (RA) flux. To characterize the microstructures, the reflowed samples with five solder joints were cross-sectioned and mounted in resin, then ground with different grit sizes of emery paper and polished with 0.5 lm diamond powder. Finally, the interfacial morphology at the solder alloy/bga substrate interfaces was observed using a scanning electron microscope (SEM, Philips XL 40 FEG) with the backscattered electron (BSE) imaging mode and an energy dispersive X-ray spectrometer (EDX, International, Model No. DX-4) was utilized to determine the chemical composition of the intermetallic compounds (IMC). Before SEM observation, the samples were sputter coated with Au to avoid effects due to charging. Transmission electron microscopy (TEM, CM 20, Philips) was used for the observation of Al 2 O 3 nano-particles. X-ray diffraction (XRD, SIEMENS Fig. 2. DSC curves Sn Zn 3Al 2 O 3 composites solder alloys on heating. Fig. 1. Bright field TEM image (a), HRTEM image (b) and XRD profile (c) of Al 2 O 3 nano-particles.

3 T. Fouzder et al. / Microelectronics Reliability 50 (2010) D501) was used to determine the crystalline phase of Al 2 O 3 nanoparticles. The cross-sectioned polished samples were placed in a Vickers hardness tester (FV-700) to measure the microhardness in the solder ball region. The applied load was 0.5 kgf for 10 s. The average hardness of 15 solder balls was taken for each condition. The melting characteristics of the Sn Zn Al 2 O 3 composite solders were investigated using a differential scanning calorimeter (DSC Q 10). For the DSC analysis, 10 mg of each solder was placed into an aluminum pan and scanned from 100 C to 280 C at a rate of 10 C min 1 under a nitrogen atmosphere. To measure the shear strength, ball shear tests were performed on the reflow samples using a shear testing machine (PTR-1000, Rhesca Co. Ltd., Japan) with a 50 lm shear tool height and 500 lm/s shear speed. The average strength of 20 solder balls with the minimum and maximum values removed was taken for each condition. After ball shear testing, the fracture surfaces and compositions were investigated thoroughly using SEM and EDX techniques. 3. Results and discussion Fig. 1 shows bright field TEM (a), HRTEM (b) micrographs and XRD profile (c) of Al 2 O 3 nano-particles. In the TEM image (a), Al 2 O 3 nano-particles appeared an elongated shape with an average particle size of about 160 nm, but some agglomeration also clearly observed. In High Resolution Transmission Electron Microscope (HRTEM) image of Fig. 1b, some lattice distortion can be seen in the inner region as indicated with circles. However, internal defects such as twins or dislocations were not found in Al 2 O 3 nanoparticles. X-ray diffraction was performed to determine the phase composition of the Al 2 O 3 nano-particles. From XRD profile, it was clear that the Al 2 O 3 nano-particles appeared crystalline phase with sharp peaks. With the addition of 3 wt% Al 2 O 3 nano-particles into Sn Zn solder, the melting temperature of the composite solder changes slightly. As shown in Fig. 2, the melting temperature is C for Sn Zn 3Al 2 O 3 as compared with C for eutectic Sn Zn solder [19]. From these DSC profile, it was confirmed that it was not required to make any changes to the existing solder process parameters such as the reflow temperature when applying these Sn Zn solders doped with Al 2 O 3 nano-particles. Fig. 3 shows cross-sectioned backscattered SEM micrographs of Sn Zn solder joints after one reflow cycle with different Al 2 O 3 nano-particle contents; (a) 0 wt%, (b) 0.5 wt% and (c) 3 wt%. In the plain Sn Zn solder joints, a scallop-shaped AuZn 3 intermetallic compound layer is clearly observed (Fig. 3a). After the addition of Al 2 O 3 nano-particles (Fig. 3b and c), the size and shape of AuZn 3 IMC layer was not significantly changed. However, in the plain Sn Zn solder joints, some defects such as microvoids were clearly observed in the interfaces as indicated with arrowheads in Fig. 3a. In general, Kirkendall void formed in lead-free solder joint at the interface due to their high surface tension, poor wettability and high solidification temperature. In addition, the vacancy flux caused by the difference in intrinsic diffusivity species was known to be responsible for Kirkendall void formation [20]. On the other hand, after the addition of Al 2 O 3 nano-particles, no defects such as microvoids were found in the interfaces. The absence of microvoids in this case may be because the diffusion process is changed by the presence of the Al 2 O 3 nano-particles which may block the formation of Kirkendall voids. In addition, in Sn Zn solder joints containing Al 2 O 3 nano-particles, very fine Al 2 O 3 particles in the Fig. 3. SEM micrographs of Sn Zn solder joints after one reflow cycle with Al 2 O 3 nano-particle contents: (a) 0 wt%, (b) 0.5 wt% and (c) 3 wt%. The arrowheads in (a) point to microvoids whilst in (b and c) the arrowheads point to Al 2 O 3 nano-particles, (d) represents EDS profile is which taken from arrowheads region in (c).

4 2054 T. Fouzder et al. / Microelectronics Reliability 50 (2010) size range of nm were clearly observed as indicated with arrow heads in Fig. 3b and c. Fig. 4 shows cross-sectioned backscattered electron micrographs of Sn Zn solder joints with different Al 2 O 3 nano-particle contents; (a) 0 wt%, (b) 0.5 wt% and (c) 3 wt% after eight reflow cycles. In the plain Sn Zn solder joints (a), no IMC layer was found at their interfaces. However, the AuZn 3 IMC layer was found to have moved into the solder. The detachment of the AuZn 3 IMC layer was presumably due to the weak adhesion between the AuZn 3 IMC layer and the Ni layer caused by the depletion of the Au layer during the reflow process. The detachment of the AuZn 3 IMC layer enhanced the brittleness of the solder joints as well as significantly effect on mechanical properties and reliability of the solder joints [21]. On the other hand, in Sn Zn solder joints containing Al 2 O 3 nano-particles, a very uniform AuZn 3 IMC layer was found at their interfaces due to promote a high nucleation density of Al 2 O 3 nanoparticles as shown in Fig. 4b and c. The AuZn 3 IMC layer thickness was about 3 lm which is similar to that found after one reflow cycle in Fig. 3b and c. Fig. 5 shows cross-sectioned backscattered electron micrographs of Sn Zn solder joints with different Al 2 O 3 nano-particle contents; (a) 0 wt% and (b) 3 wt% which were taken from the sol- Fig. 4. SEM micrographs of Sn Zn Al 2 O 3 composite solder joints after eight reflow cycles with Al 2 O 3 nano-particle contents: (a) 0 wt%, (b) 0.5 wt% and (c) 3 wt%. The arrowheads in (b and c) again point to Al 2 O 3 nano-particles. Fig. 5. SEM micrographs (a) plain Sn Zn solder joints and (b) solder joints containing 3 wt% of Al 2 O 3 nano-particles after one reflow cycle. Again the arrowheads in (b) point to Al 2 O 3 nano-particles.

5 T. Fouzder et al. / Microelectronics Reliability 50 (2010) der ball region after one reflow cycle. In the plain Sn Zn solder joint after one reflow cycle seen in Fig. 5a, the acicular-shaped Zn-rich phase is clearly seen in the b-sn matrix. However, after addition of Al 2 O 3 nano-particles, very fine acicular-shaped Zn-rich phase particles were found in the solder ball region as compared those found in the plain Sn Zn solder joints as seen by comparing in Fig. 5a with Fig. 5b. From the SEM micrographs, it was clear that the Al 2 O 3 nano-particles inhibited the growth of the Zn-rich phase during the reflow cycles. This reason may be that Al 2 O 3 nano-particles as a second phase reinforcement promote a high nucleation density in the eutectic colony during solidification. On the other hand, in the Sn Zn solder joints containing Al 2 O 3 nano particles, very fine Al 2 O 3 particles in the size range of nm were found to be homogeneously distributed in the b-sn matrix as indicated with arrowheads in Fig. 5b. This homogeneous distribution of Al 2 O 3 nano-particles affirms the efficiency of blending used in the present study. Theoretically, when a secondary blending process with a large enough deformation is used, a homogeneous distribution of reinforcing particles should be achieved regardless of the size difference between the matrix and reinforcement [22]. A uniform distribution of reinforcing particles is an important factor in the extent of strengthening that can be realized in composite materials [18]. The measurement of hardness, especially microhardness, is a usual method to characterize the mechanical properties of solidstate surfaces. The hardness of the material is often equated with its resistance to abrasion or wear and its characteristic of practical interest since it determines the durability of a material during use and it also decides the suitability of the material for special applications. Microhardness tests are made of determines the hardness of total grains, phases and structural components of alloys. The microhardness of a solder alloy depends on the motion of dislocation and growth and configuration of grains. The processes are more sensitive to the microstructure of the solder than its chemical composition. So the mechanical property such as the microhardness depends especially on the microstructure, processing temperature, and the composition. Fig. 6 shows microhardness values of the plain Sn 9Zn solder joints and solder joints containing Al 2 O 3 nano-particles as a function of number of reflow cycles. The hardness of solder joints containing higher percentage of Al 2 O 3 nanoparticles displayed consistently higher value than those of plain solder joint and solder joints containing lower percentage of Al 2 O 3 nano-particles at each reflow cycle due to control the fine microstructure as well as homogeneous distribution of Al 2 O 3 nano-particles. The hardness values of the plain Sn Zn solder joint and solder joints containing 3 wt% of Al 2 O 3 nano-particles after one reflow cycle were about 15.3 Hv and 17.8 Hv, respectively. On the other hand their hardness values after eight reflow cycles were about 14.1 Hv and 16.3 Hv, respectively. Knowledge of the mechanical properties of lead-free alloys is crucial to their successful application in electronic packaging. One of the obvious reasons people are interested in such data is the issue of reliability. Therefore, ball shear tests are essential to evaluate the effect on solder joints of the number of reflow cycles as well as the content of Al 2 O 3 nano-particles. Fig. 7 shows the variation of ball shear strength of plain Sn Zn solder joints and solder joints containing Al 2 O 3 nano-particles with the number of reflow cycles. The shear strength of plain Sn Zn solder joints after one reflow cycle was about 42.1 MPa while the shear strength after eight reflow cycles was about 39.0 MPa. On the whole, the shear strength of solder joints containing Al 2 O 3 nano-particles did not change much as a function of the number of reflow cycles. However, the Sn Zn solder joints with a high percentage of Al 2 O 3 nano-particles exhibited consistently higher strengths than those of solder joints with lower Al 2 O 3 nano-particles content for each reflow cycle. The shear strengths of solder joints containing 0.5 wt% and 3 wt% Al 2 O 3 Fig. 6. Hardness of plain Sn Zn and Sn Zn Al 2 O 3 solder joints with the number of reflow cycles. Fig. 7. Shear strength of plain Sn Zn and Sn Zn Al 2 O 3 solder joints with the number of reflow cycles. nano-particles after one reflow cycle were about 46.1 MPa and 50.8 MPa, respectively. However, in the solder joints containing Al 2 O 3 nano-particles, the shear strengths were not significantly changed after eight reflow cycles. A plausible explanation here for the higher shear strength of solder joints containing Al 2 O 3 nano-particles is due to control the IMC layer and adhered at their interface. In addition, due to a second phase dispersion strengthening mechanism [23] of Al 2 O 3 nano-particles, the Al 2 O 3 nano-particles doped solder joints showed a higher strength than the undoped solder joint at each reflow cycle. The shear strength of solder joints containing 0.5 wt% and 3 wt% Al 2 O 3 nano-particles after eight reflow cycles were about 45 MPa and 47.8 MPa, respectively. For deeper understanding of the failure behaviors inducing the strength effect in Sn Zn solder joints and solder joints containing Al 2 O 3 nano-particles, the fracture surfaces after ball shear tests were analyzed using SEM and the results are presented in Fig. 8. In the plain Sn Zn solder joints after one reflow cycle (Fig. 8a and b), the fracture surface exhibited a brittle fracture mode with a relatively smooth surface and fracture occurred in the bulk solder [21]. By contrast, Sn Zn solder joints containing Al 2 O 3 nano-particles showed ductile failure with very homogeneous dimpled sur-

6 2056 T. Fouzder et al. / Microelectronics Reliability 50 (2010) Fig. 8. SEM fracture surfaces of Sn Zn solder joints doped with Al 2 O 3 nano-particles after one reflow cycle; (a and b) 0 wt%, (c and d) 0.5 wt% and (e and f) 3 wt% Al 2 O 3 nanoparticles. faces as shown in Fig. 8c f. In addition, the roughness of the fracture surfaces increased with an increase in the content of Al 2 O 3 nano-particles from 0.5 wt% to 3 wt% reflecting the homogeneous distribution of Al 2 O 3 nano-particles as shown in Fig. 8c f. Micrographs of fracture surfaces of Sn Zn solder joints containing different contents of Al 2 O 3 nano-particles after eight reflow cycles are given in Fig. 9. Fig. 9a and b are fractographs of the Sn Zn solder joints containing 0.5 wt% Al 2 O 3 nano-particles, while Fig. 9c and d are from Sn Zn solder joints containing 3 wt% Al 2 O 3 nanoparticles. From these SEM micrographs, it is clear that the fracture mode appeared has a typical ductile characteristic with rough dimpled surfaces. Observations performed on fracture surfaces of the solder joints revealed that the crack propagation occurred above the intermetallic phase/solder interface and the dimple formation mechanism is represented schematically in Fig. 10a and b. The fracture surfaces of Sn Zn solder joints containing Al 2 O 3 nano-particles appeared to be ductile with very rough dimpled surfaces due to the homogeneous distribution of Al 2 O 3 nano-particles. 4. Conclusions Small amounts of Al 2 O 3 nano-particles were added into a Sn Zn eutectic solder to investigate the effects of Al 2 O 3 nano-particles on the formation, growth and evolution of the IMC layer structure at the Au/Ni/Cu BGA substrate/solder interfaces as well as the hardness and shear strength of solder joints. A scallop-shaped AuZn 3 IMC layer was found at the interfaces in all the solder joints after one reflow cycle. In plain Sn Zn solder joints, the AuZn 3 IMC layer was moved into solder ball region after eight reflow cycles. How-

7 T. Fouzder et al. / Microelectronics Reliability 50 (2010) Fig. 9. SEM fracture surfaces of Sn Zn solder joints doped with Al 2 O 3 nano-particles after eight reflow cycles; (a and b) 0.5 wt% and (c and d) 3 wt% Al 2 O 3 nano-particles. Fig. 10. Fracture morphology in SEM fracture surfaces of Sn Zn solder joints depending on Al 2 O 3 nano-particles; (a) 0 wt% and (b) 3 wt%. ever, in the Sn Zn solder joints containing Al 2 O 3 nano-particles after eight reflow cycles, the scallop-shaped AuZn 3 IMC layer was adhered well at their interfaces. In addition, in the Sn Zn solder joints containing Al 2 O 3 nano-particles, fine acicular-shaped Zn-rich phase particles and Al 2 O 3 nano-particles were homogeneously distributed in the b-sn matrix. New eutectic Sn Zn solder joints with stable Al 2 O 3 nano-particles significantly improved the hardness and shear strength. After the addition of Al 2 O 3 nano-particles, the hardness value and shear strengths were consistently increased with increasing the content of Al 2 O 3 nano-particles due to the uniform distribution of Al 2 O 3 nano particles as well as the controlled fine microstructure. The shear strengths of plain Sn Zn solder joints and solder joints containing 3 wt% Al 2 O 3 nano-particles after one reflow cycle were about 42.1 MPa and 50.8 MPa, respectively while their shear strengths after eight reflow cycles were about 39.0 MPa and 47.8 MPa, respectively. The fracture surface of plain Sn Zn solder exhibited a brittle fracture mode with a relatively smooth surface while Sn Zn solder joints containing Al 2 O 3 nano-particles showed typical ductile failures with very rough dimpled surfaces. Acknowledgements The authors acknowledge the financial support provided by City University of Hong Kong for the Project CERG Grant of Hong Kong Research Grants Council and RGC Ref. No

8 2058 T. Fouzder et al. / Microelectronics Reliability 50 (2010) (Development of a nano-activator doped surface modifier for Sn Zn based lead-free soldering). Professor Brian Ralph is thanked for proof reading the manuscript. References [1] Rette M, Lambeacht P, Kemp B, Graff M. High melting Pb-free solder alloys for die-attach applications. Adv Eng Mater 2005;7(10): [2] Gain AK, Chan YC, Sharif A, Wong NB, Yung WKC. Interfacial microstructure and shear strength of Ag nano particle doped Sn 9Zn Solder in ball grid array packages. Microelectron Reliab 2009;49: [3] Islam MN, Chan YC, Sharif A, Rizvi MJ. Effect of 9 wt% in addition to Sn3.5Ag0.5Cu solder on the interfacial reaction with the Au/NiP metallization on Cu pads. J Alloys Compd 2005;396: [4] Gain AK, Chan YC, Yung WKC. Effect of nano Ni additions on the structure and properties of Sn 9Zn and Sn Zn 3Bi solders in Au/Ni/Cu ball grid array packages. Mater Sci Eng B 2009;162:92 8. [5] Cho SW, Han K, Yi Y, Kang SJ, Yoo KH, Jeong K, et al. Thermal oxidation study on lead-free solders of Sn Ag Cu and Sn Ag Cu Ge. Adv Eng Mater 2006;8(1 2): [6] Ghosh G. Interfacial reaction between multicomponent lead-free solders and Ag, Cu, Ni, and Pd substrates. J Electron Mater 2004;33(10): [7] Islam MN, Chan YC, Rizvi MJ, Jillek W. Investigations of interfacial reactions of Sn Zn based and Sn Ag Cu lead-free solder alloys as replacement for Sn Pb solder. J Alloys Compd 2005;400: [8] Ichitsubo T, Matsubara E, Fujiwara K, Yamaguchi M, Irie H, Kumamoto S, et al. Control of compound forming reaction at the interface between Sn Zn solder and Cu substrate. J Alloys Compd 2005;392: [9] Duan LL, Yu DQ, Han SQ, Ma HT, Wang L. Microstructural evolution of Sn 9Zn 3Bi solder/cu joint during long-term aging at 170 C. J Alloys Compd 2004;38: [10] Date M, Shoji T, Fujiyoshi M, Sato K, Tu KN. Ductile-to-brittle transition in Sn Zn solder joints measured by impact test. Scripta Mater 2004;51: [11] Yao P, Liu P, Liu J. Effects of multiple reflows on intermetallic morphology and shear strength of SnAgCu xni composite solder joints on electrolytic Ni/Au metallized substrate. J Alloys Compd 2008;462:73 9. [12] Guo F, Lee J, Choi S, Lucas JP, Bieler TR, Subramanian KN. Processing and aging characteristics of eutectic Sn 3.5Ag solder reinforced with mechanically incorporated Ni particles. J Electron Mater 2001;30(9): [13] Lin DC, Wang GX, Srivatsan TS, Al-Hajri M, Petraroli M. Influence of titanium dioxide nanopowder addition on microstructural development and hardness of tin lead solder. Mater Lett 2003;57: [14] Lin D, Wang G, Srivatsan TS, Al-Hajri M, Petraroli M. The influence of copper nanopowders on microstructure and hardness of lead tin solder. Mater Lett 2002;53: [15] Mavoori H, Jin S. New, creep-resistant, low melting point solders with ultrafine oxide dispersions. J Electron Mater 1998;27(11): [16] Mohan KK, Kripesh V, Tay AAO. Influence of single-wall carbon nanotube addition on the microstructural and tensile properties of Sn Pb solder alloy. J Alloys Compd 2008;455: [17] Shen J, Liu YC, Han YJ, Tian YM, Gao HX. Strengthening effects of ZrO 2 nanoparticles on the microstructure and microhardness of Sn 3.5Ag lead-free solder. Mater Sci Eng A 2006;441: [18] Li ZX, Gupta M. High strength lead-free composite solder materials using nano-al 2 O 3 as reinforcement. Adv Eng Mater 2005;7(11): [19] Wei X, Huang H, Zhou L, Zang M, Liu X. On the advantages of using a hypoeutectic Sn Zn as lead free solder material. Mater Lett 2007;61: [20] Yu J, Kim JY. Effects of residual S on Kirkendall void formation at Cu/Sn 3.5Ag solder joints. Acta Mater 2008;56: [21] Yoon JW, Jung SB. Solder joint reliability evaluation of Sn Zn/Au/Ni/Cu ballgrid-array package during aging. Mater Sci Eng A 2007; : [22] Tan MJ, Zhang X. Powder metal matrix composites: selection and processing. Mater Sci Eng A 2004;244:80 5. [23] Zhang GJ, Sun YJ, Niu RM, Sun J, Wie JF, Zhao BH, et al. Microstructure and strengthening mechanism of oxide lanthanum dispersion strengthened molybdenum alloy. Adv Eng Mater 2004;6(12):943 8.