Microelectronics Reliability

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1 Microelectronics Reliability 51 (2011) Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: Effect of additions of ZrO 2 nano-particles on the microstructure and shear strength of Sn Ag Cu solder on Au/Ni metallized Cu pads Asit Kumar Gain a, Y.C. Chan a,, Winco K.C. Yung b a Department of Electronic Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong b Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong article info abstract Article history: Received 10 July 2009 Received in revised form 16 January 2011 Accepted 31 March 2011 Available online 8 June 2011 Nano-sized, nonreacting, noncoarsening ZrO 2 particles reinforced Sn 3.0 wt%ag 0.5 wt%cu composite solders were prepared by mechanically dispersing ZrO 2 nano-particles into Sn Ag Cu solder. The interfacial morphology of unreinforced Sn Ag Cu solder and solder joints containing ZrO 2 nano-particles with Au/Ni metallized Cu pads on ball grid array (BGA) substrates and the distribution of reinforcing particles were characterized metallographically. At their interfaces, a Sn Ni Cu intermetallic compound (IMC) layer was found in both unreinforced Sn Ag Cu and Sn Ag Cu solder joints containing ZrO 2 nano-particles and the IMC layer thickness increased with the number of reflow cycles. In the solder ball region, AuSn 4,Ag 3 Sn, Cu 6 Sn 5 IMC particles and ZrO 2 nano-particles were found to be uniformly distributed in the b-sn matrix of Sn Ag Cu solder joints containing ZrO 2 nano-particles, which resulted in an increase in the shear strength, due to a second phase dispersion strengthening mechanism. The fracture surface of unreinforced Sn Ag Cu solder joints exhibited a brittle fracture mode with a smooth surface while Sn Ag Cu solder joints containing ZrO 2 nano-particles ductile failure characteristics with rough dimpled surfaces. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Environmental and health concerns with lead and lead-containing compounds in microelectronic devices attract more and more attentions in academia and industry [1 4]. Rapid switching to lead-free solder has come to replace lead-based solders in the packaging process of electronic devices and components. The development of new solders and their composites is also driven by the continual miniaturization of integrated circuits and the quest for better performance and reliability from interconnection joints [5]. Now, several types of Sn-based lead-free solders such as Sn Ag, Sn Cu, Sn Au, Sn Ag Cu and Sn Zn have been developed and applied in the electronic packaging industry [6 8]. Among them the Sn Ag Cu solder has been proposed as one of the most promising substitutes for lead-containing solder because of its good basket of properties such as superior solderability as well as good compatibility with current components, and is regarded as the most promising substitute for conventional Sn Pb solder [9 12]. Microelectronic components evolved to become smaller, lighter and more functional. There are also strict performance requirements for solder materials. In general, it must fulfill the expected level of electrical and mechanical performance and have a low Corresponding author. Tel.: ; fax: address: eeycchan@cityu.edu.hk (Y.C. Chan). melting temperature. Reliability of the solder joints is mainly dependent on the yield strength, elastic modulus, shear strength, fatigue and creep behavior [13,14]. 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 a ceramic, metallic or intermetallic, to a solder matrix so as to form a composite [15 17]. The foreign dispersoid second phase particles which are induced as a reinforcement with in the solder matrix, should not coarsen easily. In addition to strengthening the solder against creep deformation, the dispersed particles can serve as obstacles to grain growth and coarsening of the solder microstructure. Mavoori and Jin used nano-sized TiO 2 and Al 2 O 3 particles as reinforcements for a conventional Sn Pb solder and reported significant improvements of creep and mechanical properties [18]. Gao et al. [19,20] studied the enhanced creep resistance of a Sn3.5Ag solder by introducing micro-sized Ag, Ni, or Cu particles. It was found that solder joints reinforced with Ni particles were about five times more creep resistant than composite solder joints reinforced by Cu-particles, and about 30 times more creep resistant than the plain Sn3.5Ag solder joints and those reinforced with Ag-particles. Shen et al. [21] studied eutectic Sn Ag solder alloys with nano-sized ZrO 2 reinforcement particles and significantly improved hardness as well as refined Ag 3 Sn IMC particles. Mohan et al. [22] reported that a Sn 3.8Ag 0.7Cu composite solder reinforced with single wall carbon nano tubes (SWCNTs) had significantly improved hardness /$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi: /j.microrel

2 A.K. Gain et al. / Microelectronics Reliability 51 (2011) and ultimate tensile strength. In the current study, ZrO 2 particles of a nanometer size were used. The main advantage of ZrO 2 nano-particles are; (a) a similar density to Sn Ag Cu, q (Sn 3.0Ag 0.5Cu) = 7.11 and q (ZrO 2 ) = 5.83 g/cm 3 as compared to other ceramic particles such as q (Al 2 O 3 ) = 3.97 g/cm 3, q (SWCNT) = 1.3 g/ cm 3, and (b) a higher hardness as compared to a Sn 3.0Ag 0.5Cu matrix. However, the result of a literature search revealed that no studies have been reported so far on lead-free Sn Ag Cu solder joints containing ZrO 2 nano-particles. Accordingly, the aim of the present study is to synthesize a Sn Ag Cu solder with different percentages of ZrO 2 nano-particles. Unreinforced Sn Ag Cu solder joints and solder joints containing ZrO 2 nano-particles were characterized in terms of interfacial microstructures and shear strengths on Au/Ni metallized Cu pads on BGA substrates as a function of the number of reflow cycles and the content of ZrO 2 nano-particles. 2. Experimental procedures Composite solders were prepared by mechanically dispersing the ZrO 2 nano-particles (0, 0.5, 1 and 3 wt%) into the eutectic Sn 3.0Ag 0.5Cu ((AMTECH, USA) solder powder with particles size about lm. The mixture was blended manually for at least 30 min to achieve a uniform distribution of ZrO 2 nano-particles with a water-soluble flux (Qualitek Singapore (PTE) Ltd.). 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-70N) at 250 C to prepare about 0.76 mm diameter solder balls. A solder mask-defined copper bond pad on the flexible substrate of a BGA package was used as a base for electrodeposition of Ni and Au. The solder mask-opening diameter was 0.6 mm and a7lm thick Ni layer was deposited in these openings. 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 the prefluxed Au/Ni/Cu bond pads of the substrates and reflowed at a temperature of 250 C with a belt speed 10 cm/min in a convection reflow oven (BTU VIP-70N). The melting characteristics of the Sn Ag Cu composite solders which depended on the content of ZrO 2 nano-particles 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 270 C at a rate of 10 C min 1 under a nitrogen atmosphere. To characterize the microstructures, the reflowed samples were cross sectioned and mounted in resin, then ground by different grit sized emery papers and polished with 0.5 lm Al 2 O 3 powder. Finally, the interfacial morphology at the solder alloy/bga substrate interface was observed using a scanning electron microscope (SEM, Philips XL 40 FEG) using the back-scattered 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 (IMCs). Before SEM observation, the samples were sputter coated with Au to avoid the effects due to charging. Transmission electron microscopy (TEM, CM 20, Philips) was used for the observation of ZrO 2 nano-particles. 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 twenty 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 3.1. Characterization of ZrO 2 nano-particles Fig. 1a and b shows bright field TEM images of ZrO 2 nano-particles and (c) a selected area diffraction pattern of ZrO 2 nano-particles. In the TEM images, the spherically shaped ZrO 2 nanoparticles, about nm, in diameter are clearly observed. In the low magnification TEM observation (Fig. 1a), some agglomeration also clearly observed. However, when the sample was tilted and observed at a higher magnification, there was often some space between the particles as shown in Fig. 1b Melting point analysis Fig. 2 shows the differential scanning calorimetry (DSC) results of Sn Ag Cu solders depending on the content of ZrO 2 nano-particles (a) 0 wt%, (b) 0.5 wt%, (c) 1 wt% and (d) 3 wt%. DSC results showed that the melting point of unreinforced Sn Ag Cu solder and solder joints containing ZrO 2 nano-particles ranged from 217 C to C with only a eutectic peak. There was an elevated melting temperature less than 1 C for the Sn Ag Cu composites solders doped with ZrO 2 nano-particles. From these DSC profiles, 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 Ag Cu composite solders doped with ZrO 2 nano-particles Microstructure characterization Fig. 3 shows backscattered scanning electron micrographs of Sn Ag Cu solder joints with different weight percentages of ZrO 2 nano-particles after one reflow cycle at 250 C. The inset Fig. 3 was the etched (5% HCl) Sn Ag Cu based composite solder joints. The backscattered electron image mode of the SEM was utilized to help identify distinguishable interfacial phases between unreinforced Sn Ag Cu or Sn Ag Cu composite solders containing ZrO 2 nano-particles with Au/Ni metallized Cu pads on BGA substrates. At their interfaces, a Sn Ni Cu IMC layer with a dark contrast was clearly observed in both unreinforced Sn Ag Cu solder and Sn Ag Cu solder joints containing ZrO 2 nano-particles. According to the EDS analysis (e), the composition of the ternary IMC layer marked P consisted of a 58.5 at.% Sn, 22.0 at.% Ni and 19.5 at.% Cu phase. During the reflow process the topmost thin (0.5 lm) Au layer in the Au/Ni metallized Cu pads dissolved into the molten solder, leaving the Ni layer exposed to the molten unreinforced Sn Ag Cu solder and Sn Ag Cu composite solder containing ZrO 2 nano-particles. Yoon et al. [23] reported that there is a small atomic size difference between Cu and Ni, and since both have the same FCC lattice structure, the substitution of Ni into Cu 6 Sn 5, without causing lattice distortion or the formation of a new phase, was reasonable. On the other hand, spherically shaped ZrO 2 nano-particles were clearly observed in the interfacial region as indicated with arrow heads in Fig. 3b d from Sn Ag Cu solder joints containing ZrO 2 nano-particles. Fig. 4 shows backscattered scanning electron micrographs of Sn Ag Cu solder joints with different weight percentages of ZrO 2 nano-particles after one reflow cycle at 250 C which were taken from the solder ball region. In the unreinforced Sn Ag Cu solder joint, Ag 3 Sn, Cu 6 Sn 5 and AuSn 4 IMC particles were clearly observed in the b-sn matrix. In the solder ball region AuSn 4 IMC particles formed due to the diffusion of Au atoms from the Au/Ni metallized Cu pads. However, in the Sn Ag Cu solder joints containing ZrO 2 nano-particles, Ag 3 Sn, Cu 6 Sn 5 and AuSn 4 IMC particles as well as ZrO 2 nano-particles were found to be uniformly distributed in

3 2308 A.K. Gain et al. / Microelectronics Reliability 51 (2011) Fig. 1. Bright field TEM images (a and b) of ZrO 2 nano particles and (c) selected area diffraction pattern of ZrO 2 nano particles. Fig. 2. DSC curves Sn Ag Cu ZrO 2 composite solder alloys. the b-sn matrix. From EDS analysis (e) it was clear that the dark contrast particles appeared with ZrO 2 phase. As expected the amount of ZrO 2 nano-particles in the b-sn matrix increased with an increase in the weight percentage of ZrO 2 nano-particles in the Sn Ag Cu solder. Fig. 5 shows backscattered scanning electron micrographs of Sn Ag Cu solder joints with different weight percentages of ZrO 2 nano-particles after eight reflow cycles at 250 C. After eight reflow cycles, at their interfaces, a Sn Ni Cu IMC layer with a dark contrast was clearly observed in both the unreinforced Sn Ag Cu solder and Sn Ag Cu solder joints containing ZrO 2 nano-particles with is the same as after one reflow cycle. However, the IMC layer thickness was substantially increased with reflow cycles as shown by comparing Figs. 3 and 5. Fig. 6 shows backscattered scanning electron micrographs of Sn Ag Cu solder joints with different weight percentages of ZrO 2 nano-particles after eight reflow cycles at 250 C which were taken from the solder ball region. After increasing the number of reflow cycles from one to eight cycles, the formation of IMC particles was not significantly changed in both the unreinforced and composite solder joints. In the unreinforced Sn Ag Cu solder joint, Ag 3 Sn, Cu 6 Sn 5 and AuSn 4 IMC particles were clearly observed in the b-sn matrix the same as after one reflow cycle. On the other hand, in the Sn Ag Cu solder joints containing ZrO 2 nano-particles, Ag 3 Sn, Cu 6 Sn 5 and AuSn 4 IMC particles as well as ZrO 2 nano-particles were uniformly distributed in the b-sn matrix. Fig. 7 shows backscattered scanning electron micrographs of (a) unreinforced Sn Ag Cu solder and solder joints with different weight percentage of ZrO 2 nano-particles of (b) 0.5 wt%, (c) 1 wt% and (d) 3 wt% after 16 reflow cycles at 250 C. The inset Fig. 7 was the etched (5% HCl) Sn Ag Cu based composite solder joints. After 16 reflow cycles, at their interface, Sn Ni Cu IMC layers with dark contrast were clearly observed in both unreinforced Sn Ag Cu solder and Sn Ag Cu solder joints containing ZrO 2 nano-particles the same as after one reflow cycles. From Figs. 3, 5 and 7, it was confirmed that the IMC layer thickness was substantially increased and appeared with a dense structure. In addition, spherically-shaped Sn Ni Cu IMC particles were clearly observed at the top surface of the Sn Ni Cu IMC layer. Fig. 8 shows the change of IMC layer thickness of unreinforced Sn Ag Cu and solder joints containing ZrO 2 nano-particles as a function of the number of reflow cycles. The average thickness of the IMC layer was calculated by using the following equation: T =(t 1 + t 2 + t t n )/n where t 1, t 2, t 3,..., t n are the thicknesses of the IMC layer at various positions on the interface. It may be seen that the IMC layer thickness of all solder joints increased with

4 2309 A.K. Gain et al. / Microelectronics Reliability 51 (2011) (a) (c) P (b) (e) (d) Element Wt % At % CuL SnL NiK Total Fig. 3. SEM micrographs of Sn Ag Cu solder joints after one reflow cycle with ZrO2 nano-particle contents: (a) 0 wt%, (b) 0.5 wt%, (c) 1 wt%, (d) 3 wt% and (e) EDS and elemental analysis as marked region P in (a). The arrowheads in (b d) point to ZrO2 nano-particles. (a) (e) (b) P (c) (d) At % Element Wt % OK ZrL SnL Total Fig. 4. SEM micrographs of Sn Ag Cu solder joints after one reflow cycle with ZrO2 nano particle contents: (a) 0 wt%, (b) 0.5 wt%, (c) 1 wt%, (d) 3 wt% and (e) EDS and elemental analysis as marked region P in (b). The arrowheads in (b d) point to ZrO2 nano particles. an increase in the number of reflow cycles. The IMC layer thickness of unreinforced Sn Ag Cu solder joints after one and sixteen reflow cycles were about 2.8 lm and 6.7 lm, respectively. The IMC layer thickness of Sn Ag Cu solder joints containing 3 wt% ZrO2 nano-particles after one and sixteen reflow cycles were about 2.0 lm and 5.2 lm, respectively. From this result, it is confirmed that the ZrO2 nano-particles inhibited the formation of the IMC layer. The reason may be that the second phase ZrO2 nano-particles change the driving force for the growth of the IMC layer as well as the diffusivity of the elements involved in its growth. Usually, the

5 2310 A.K. Gain et al. / Microelectronics Reliability 51 (2011) Fig. 5. SEM micrographs of Sn Ag Cu solder joints after eight reflow cycles with ZrO 2 nano particle contents: (a) 0 wt%, (b) 0.5 wt%, (c) 1 wt% and (d) 3 wt%. The arrowheads in (b d) point to ZrO 2 nano-particles. Fig. 6. SEM micrographs of Sn Ag Cu solder joints after eight reflow cycles with ZrO 2 nano particle contents: (a) 0 wt%, (b) 0.5 wt%, (c) 1 wt% and (d) 3 wt%. The arrowheads in (b d) point to ZrO 2 nano particles. plane with maximum surface tension grows fastest, while its adsorption amount of surface-active material is maximized. However, as increasing in adsorption elements decreases its surface energy and therefore decreases the growth velocity of this plane. The addition of ZrO 2 nano-particles may be adsorbed at the grain boundary and changes relative relationship of the growth velocities between crystalline directions of the IMC particles, which reduces the IMC particles size. Li et al. [24] reported that rare earth

6 A.K. Gain et al. / Microelectronics Reliability 51 (2011) Fig. 7. SEM micrographs of Sn Ag Cu solder joints after 16 reflow cycles with ZrO 2 nano particle contents: (a) 0 wt%, (b) 0.5 wt%, (c) 1 wt% and (d) 3 wt%. The arrowheads in (b d) point to ZrO 2 nano particles. shear strength of unreinforced Sn Ag Cu and Sn Ag Cu solder joints containing ZrO 2 nano-particles as a function of the number of reflow cycles. The shear strength of unreinforced Sn Ag Cu solder joints after one reflow cycle was about 38 MPa while the shear strength after 16 reflow cycles was about 31.6 MPa. On the whole, the shear strength of solder joints containing ZrO 2 nano-particles changed very little as a function of the number of reflow cycles, perhaps because of the uniform distribution of ZrO 2 nano-particles. However, the Sn Ag Cu solder joints with high percentage of ZrO 2 nano-particles exhibited consistently higher strengths than that of solder joints with a lower ZrO 2 nano-particle content for each number of reflow cycles due to the uniform distribution of ZrO 2 nano-particles. Due to a second phase dispersion strengthening Fig. 8. IMC layer thickness of Sn Ag Cu composite solder joints as a function of the number of reflow cycles. elements reduce the rate of formation of the IMC layer in two ways i.e. by changing the diffusion coefficient and thermodynamic parameters of the elemental affinity Shear tests Since solder joints are often subjected to mechanical loading during processing and system use, the mechanical properties of solder joints, such as shear strength and the creep resistance, are crucial to their reliability. Therefore, ball shear tests are essential to evaluate the measure the strength of solder joints as a function of the number of reflow cycles. Fig. 9 shows the variation of ball Fig. 9. Shear strength of Sn Ag Cu ZrO 2 composite solder joints as a function of the number of reflow cycles.

7 2312 A.K. Gain et al. / Microelectronics Reliability 51 (2011) Fig. 10. SEM images of fracture surfaces of Sn Ag Cu solder joints depending on the ZrO 2 nano particle content: (a) 0 wt%, (b) 0.5 wt%, (c) 1 wt% and (d) 3 wt%. mechanism [25] of ZrO 2 nano-particles, the Sn Ag Cu solder joints containing ZrO 2 nano-particles showed a higher strength than that of unreinforced Sn Ag Cu solder joints. The shear strength of 0.5 wt% and 3 wt% ZrO 2 nano-particle content solder joints after one reflow cycle were about 40.7 MPa and 43.4 MPa, respectively. However, the shear strength of solder joints containing 0.5 wt% and 3 wt% ZrO 2 nano-particles after 16 reflow cycles were about 39.2 MPa and 40.9 MPa, respectively. For the Sn Ag Cu solder joints containing ZrO 2 nano-particles, the fracture occurred through the bulk of the solder and the strength of the solder joint thus depends on that of the bulk solder. Due to a second phase dispersion strengthening mechanism, the solder retained its high strength even after multiple reflows. The detailed fracture behavior of the unreinforced Sn Ag Cu solder and solder joints containing ZrO 2 nano-particles may be revealed by fractographic observations after shear tests. Fig. 10 shows SEM images of fracture surfaces of (a) the unreinforced Sn Ag Cu solder and Sn Ag Cu solder joints containing different weight percentages of ZrO 2 nano-particles (b) 0.5 wt%, (c) 1 wt% and (d) 3 wt%. The fracture surface of an unreinforced Sn Ag Cu solder joint exhibited a brittle fracture mode with a smooth surface as shown in Fig. 10a. On the other hand, Sn Ag Cu solder joints containing ZrO 2 nano-particles showed ductile failure with very rough dimpled surfaces due to the uniform distribution of ZrO 2 nano-particles in Fig. 10b d. 4. Conclusions The impact of addition of ZrO 2 nano-particles in Sn Ag Cu solder studied in this paper by IMC thickness evaluation and shear strength as a function of the number of reflow cycles as well as the content of ZrO 2 nano-particles. After the reflow process, the topmost Au layer dissolved very quickly into the molten solder, a Sn Ni Cu IMC layer was formed at their interfaces and the IMC layer thickness size was increased from 2.8 lm to 6.7 lm for unreinforced Sn Ag Cu solder joints and solder joints containing 3 wt% ZrO 2 nano-particles was increased from 2.0 lm and 5.2 lm with an increase in the number of reflow cycles from one to sixteen. During the reflow process Au atoms diffuse and form AuSn 4, Ag 3 Sn, Cu 6 Sn 5 IMC particles which were clearly observed in the b-sn matrix in the unreinforced Sn Ag Cu solder. On the other hand, ZrO 2 nano-particles as well as AuSn 4,Ag 3 Sn, Cu 6 Sn 5 IMC particles were found in the Sn Ag Cu solders containing ZrO 2 nano-particles. The solder joints with a high percentage of ZrO 2 nano-particles showed a consistently higher strength than that of solder joints with a lower content of ZrO 2 nano-particles at all numbers of reflow cycles. It is reasonable to suggest that the strength of the bulk solder was enhanced by the addition ZrO 2 nano-particles because the ZrO 2 nano-particles dispersion strengthened the solder. The shear strengths of unreinforced Sn Ag Cu solder and solder joints containing 3 wt% ZrO 2 nano-particles after one reflow cycle were about 38 MPa and 43.4 MPa, respectively and after 16 reflow cycles their shear strengths were about 31.6 MPa and 40.9 MPa, respectively. In addition, the fracture surfaces of Sn Ag Cu solder joints containing ZrO 2 nano-particles appeared to be ductile with very rough dimpled surfaces. Acknowledgments 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 (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] Kang HB, Bae JH, Lee JW, Park MH, Lee YC, Yoon JW, et al. Control of interfacial reaction layers formed in Sn 3.5Ag 0.7Cu/electroless Ni P solder joints. Scr Mater 2009;60:

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