Microstructure evolution during the aging at elevated temperature of Sn-Ag-Cu solder alloys

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1 Available online at Procedia Materials Science 1 (2012 ) th International Congress on Metallurgy & Materials SAM/CONAMET Microstructure evolution during the aging at elevated temperature of Sn-Ag-Cu solder alloys C. Morando (a,b) *, O. Fornaro (a,b), O. Garbellini (a,c) and H. Palacio (a,c) a IFIMAT- Instituto de Física de Materiales Tandil, FCE - CICPBA - MT. Universidad Nacional del Centro de la Provincia de Buenos Aires. Pinto 399 (B7000GHG) Tandil-Argentina b Consejo Nacional de Investigaciones Científicas y Técnicas CONICET c Comisión Investigaciones Científicas Provincia de Buenos Aires Abstract In this work, binary Sn-3.5%Ag and Sn-0.7%Cu and ternary Sn-3.5%Ag-0.9%Cu eutectic alloys has been used to investigate the dynamic of microstructure transformation during isothermal artificial aging process at 180ºC. The microstructure was investigated using optical microscopy (OM), scanning electron micrograph (SEM) and EDX (energy dispersive X-ray microanalysis) and followed by Differential Scanning Calorimetry technique (DSC). In the as-cast condition the binary eutectic alloys have a structure composed of Sn primary phase crystals surrounded by an irregular eutectic structure composed of intermetallic sheets of Ag 3 Sn and Cu 6 Sn 5 in an Sn matrix while the microstructure of the ternary alloy shows the following phase -Sn surrounded by Sn-Ag binary eutectic and Sn-Ag-Cu ternary eutectic. After 48 hours of isothermal aging at 180 C both alloys show a noticeable change in their microstructures. In binary alloys a larger grain structure is formed, Sn dendrites coarsen, and in the ternary alloy the intermetallic phases Ag 3 Sn Cu 6 Sn 5 are randomly distributed in a Sn matrix. There is a coarsening of Sn dendrites and Ag 3 Sn phase produced by the growth mechanism of Ostwald (Ostwald Ripening) Published by by Elsevier Ltd. Ltd. Selection and/or peer-review under under responsibility of of SAM/ 11 th CONAMET International 2011, Congress Rosario, on Metallurgy Argentina. & Open Materials access under SAM/CONAMET CC BY-NC-ND license Keywords: SnAgCu alloys; differential scanning calorimetry (DSC); lead free solder; eutectic solder alloys, microstructural changes. * Corresponding author. Tel.: ; fax: address: cmorando@exa.unicen.edu.ar, carinamorando@yahoo.com.ar Published by Elsevier Ltd. Selection and/or peer-review under responsibility of SAM/CONAMET 2011, Rosario, Argentina. Open access under CC BY-NC-ND license. doi: /j.mspro

2 C. Morando et al. / Procedia Materials Science 1 ( 2012 ) Introduction The operating environment temperature of many new electronic systems could be as high as 150ºC, for example in the automotive industry or in aviation. These demanding conditions of use, together with the need for greater reliability of all electronic systems motivate further research on the effects of high temperature aging of solder materials (Anderson and Harringa, 2004). Microstructural changes can significantly modify the mechanical properties of the joint and influence the behavior of the material during thermal fatigue (Wu and Huang, 2005). The natural and artificial agings and their effect on the microstructure and mechanical properties have been studied extensively in Pb-free alloys (LFS) used as input solder electronic processes, particularly the system Sn-Ag-Cu (SAC) as a candidate to replace eutectic Sn-Pb alloy (Islam et. al., 2005; Hsuan and Lin, 2007; Wei et. al, 2009; Seo et. al., 2009). For this reason, it is important to know the initial microstructure of the solder joint and to understand how the microstructural evolution occurs. This information will help in the development of physical models of the phenomenon and thus anticipate the changes that will occur over time. Furthermore, this knowledge may be taken into account to design standards to improve the reliability of Pb-free solder joints and to predict their useful life. Research has shown that the characteristic lifetime due to fatigue of solder joints depends on the microstructural evolution, assume that during the process occurs recrystallization and coarsening of the microstructure. Less change of the microstructure in solder joints would imply that mechanical properties remain stable and therefore the reliability and life time prediction will be more accurate using initial material data (Fix et. al., 2008). In a previous work performed by the authors (Morando et. al., 2010), microstructural and microhardness changes of binary SnAg, SnCu and ternary SnAgCu eutectic alloys were studied as a function of aging time during a heat treatment at 180 C, from the as-cast condition up to 500 hours. In all cases we have observed a reduction of up to 20% in the microhardness compared with the as-cast alloy, reaching a minimum after 48 hours of heat treatment. This softening is related to the coarsening of Sn dendrites and Ag 3 Sn phase. As the aging time exceeds 48 hours, the microhardness begings to increase and remains constant after the 100 hours. This effect corresponds to the dispersion of the Ag 3 Sn particles into Sn rich crystals wich previously didn t contain intermetallic precipitates. In this work, binary Sn-3.5%Ag and Sn-0.7%Cu and ternary Sn-3.5%Ag-0.9%Cu eutectic alloys have been used to investigate the dynamic of microstructure transformation during isothermal artificial aging process at 180ºC with Differential Scanning Calorimetry technique (DSC). The corresponding evolution of the microstructure with different times has been investigated and discussed. 2. Experimental Binary Sn-3.5%Ag, Sn-0.7%Cu and ternary Sn-3.5%Ag-0.9%Cu eutectic alloys were prepared by melting pure elements (4N) in an electric resistance-type furnace under inert Ar gas atmosphere, stirred for adequate homogeneization and cast into a refractory sand mold with a heat transfer coefficient in the metal-mold interface: hi = 0.2 x103 J/m 2.s.K. This procedure ensures the correct application of the equilibrium diagram in the system studied. Rectangular specimens, 20 mm long by 10 mm wide, were cut. The nominal compositions of the alloys are expressed as weight % of solute. The as-cast and thermal aged solder samples were polished for testing of microhardness and observations of the microstructure. Both optical microscopy (OM) and scanning electron microscopy (SEM) were used to characterize the microstructure. Energy dispersive X-ray microanalysis (EDX) was used to characterize the compositions of the compounds formed due to aging. Microstructural analysis of as-cast and thermal aged solder samples was performed on metallographic specimens that were polished by following standard

3 82 C. Morando et al. / Procedia Materials Science 1 ( 2012 ) metallographic procedures and etched with a solution of 2 of HCl in alcohol for several seconds. The microstructure was recorded by photomicrographs at regular 24 hours intervals. The observation was started one hour after the casting. The observation area was carefully selected in order to observe the microstructural change. After the photomicrographs were taken, the various specimens were aged and at the end of each 24 hours ageing stage, the specimen was polished slightly and re-etched. For calorimetric analysis, we used a differential scanning calorimeter DSC Heat Flux Rheometric Scientific SP, with a stability of line better than 1mW in the measurement range used. The samples used are 6mm in diameter and 20mg of weight. The curves were obtained for scan rates of 5 and 10K/min. 3. Results and Discussion From the microstructure examination, it was found that Sn-rich crystals are the basic microstructure of the Sn-3.5%Ag and Sn-0.7%Cu binary eutectic alloys in as-cast condition, there are intermetallic particles around the Sn-rich phase as shown in Figure 1a and 2a). From EDX microanalysis, the intermetallic compounds found were mainly Ag 3 Sn and Cu 6 Sn 5 respectively. Some coarsening of Sn-rich crystals with random orientations is observed in the annealed after 48 hours, Figures 1b and 2b). Figures 1c and 2c) show that in both binary alloys the microstructure changes significantly after 100 hours of heat treatment, showing an intermetallic particle dispersion of Ag 3 Sn and Cu 6 Sn 5 in Sn-rich crystals which initially did not contain these precipitates. Figure 1d and 2d) shows detailed views of SEM images where the precipitation of 1: -Sn, and 2: Ag 3 Sn y Cu 6 Sn 5 phases are marked. The composition of these phases was determined by EDX. Figure 3a) shows an optical micrography of the Sn-3.5%Ag-0.9%Cu as-cast alloy. The microstructure shows -Sn surrounded by Sn-Ag binary eutectic and Sn-Ag-Cu ternary eutectic. Figure 3b) shows the microstructural evolution after 48 hours of isothermal heat treatment at 180 C. Intermetallic phases Ag 3 Sn and Cu 6 Sn 5 are randomly distributed in a matrix of Sn, there is a coarsening of the Sn dendrites and Ag 3 Sn phase produced by the growth mechanism of Ostwald (Ostwald Ripening) (Snugovsky et. al., 2004). After 100 hours of heat treatment at this temperature, samples exhibit a more dispersed precipitated structure, indicating that at this aging temperature Ag rapidly diffuses from the eutectic region to Sn-rich crystals forming new precipitates, Figures 3c). The detail of this microstructure is shown in SEM micrograph of Figure 3d) in which different phases are marked whose compositions were determined by X-ray scattering -Sn, phase 2: Ag 3 Sn and phase 3: Sn-Ag 3 Sn-Cu 6 Sn 5. In every studied case there was a decrease in microhardness of up to 20% in the (Morando et. al., 2010) of that of the as-cast alloy, reaching a minimum value after 48 hours of heat treatment. This softening is related to the coarsening of Sn dendrites and Ag 3 Sn and Cu 6 Sn 5 intermetallic phases. As the aging time exceeds 48 hours, the microhardness begings to increase and remains constant after the 100 hours. Calorimetry charts are shown in Figures 4 a, b and c) for all the studied alloys and for each aging time. Figure 4a) corresponds to the Sn-Cu eutectic alloy in as-cast condition, and with artificial aging of 24, 48 and 96 hours at T = 180ºC, whilst Figure 4b) to Sn-Ag eutectic alloy and 4c) to Sn-Ag-Cu ternary eutectic alloy for similar aging time. As previously reported (Morando et. al., 2010), for aging in the range of 24 to 48 hours the samples showed a decrease in the Vickers microhardness and then an increasing starting from 96 hours of aging, reaching the original values of the as-cast condition. However, this behavior does not correlate with the data extracted from calorimetry curves of Figure 4, where there are no signs of dissolution or precipitation of new phases in the range of the studied temperatures from ºC. From the point of view of this analysis technique, the mechanism compatible with this behavior is the coarsening of existent phases and the stress relaxation during the first hour of aging.

4 C. Morando et al. / Procedia Materials Science 1 ( 2012 ) a) b) c) d) Figure 1. Microstructure of Sn-3.5% Ag eutectic alloy, a) as-cast, subjected to aging heat treatment at 180 C for b) 48 hours c) 100 hours, d) SEM micrograph, for 100hs of aging, -Sn, phase 2: Ag 3Sn. a) b)

5 84 C. Morando et al. / Procedia Materials Science 1 ( 2012 ) c) d) Figure 2. Microstructure of Sn-0.7% Cu eutectic alloy, a) as-cast, subjected to aging heat treatment at 180 C for b) 48 hours c) 100 hours, d) SEM micrograph, for 100hs of aging, in which are marked different phases: phase 1 -Sn, phase 2: Cu 6Sn 5. a) b) c) d) Figure 3. Microstructure of Sn-3.5% Ag-0.9%Cu eutectic alloy, a) as-cast, subjected to aging heat treatment at 180 C for b) 48 hours c) 100 hours, d) SEM micrograph, for 100hs of aging, in which are marked different phases: phase 1 -Sn, phase 2: Ag 3Sn and phase 3: Sn-Ag 3Sn-Cu 6Sn 5.

6 C. Morando et al. / Procedia Materials Science 1 ( 2012 ) a) 0,4 0,3 As Cast 24 hs 48 hs 96 hs Q (J/gK) 0,2 0, T ( C) b) 0,4 0,3 As Cast 24 hs 48 hs 96 hs Q (J/gK) 0,2 0, T ( C) c) 0,4 0,3 As Cast 24 hs 48 hs 96 hs Q (J/gK) 0,2 0, Figure 4. Normalized heat flux per unit mass at the rate 10 K/min, for binary eutectic a) Sn-Cu, b) Sn-Ag and c) Sn-Ag-Cu ternary eutectic alloy, during artificial aging at T=180 C, at time 0 (as-cast), 24, 48 and 96 hs. T ( C)

7 86 C. Morando et al. / Procedia Materials Science 1 ( 2012 ) Conclusions In this work we studied the dynamic of microstructure transformation of binary eutectic alloys Sn-3.5% Ag and Sn-0.7% Cu and ternary Sn-3.5%Ag-0.9% Cu during artificial aging at 180ºC at different times of heat treatment. The evolution of the microstructure was investigated using optical microscopy (OM) and scanning electron microscopy (SEM) and followed by Differential Scanning Calorimetry technique (DSC). In the as-cast condition the binary eutectic alloys has a structure composed of Sn-rich crystals surrounded by an irregular eutectic structure composed of intermetallic sheets of Ag 3 Sn and Cu 6 Sn 5 in an Sn matrix. The microstructure of the ternary alloy shows -Sn surrounded by Sn-Ag binary eutectic and Sn-Ag-Cu ternary eutectic. After 48 hours of isothermal aging at 180 C noticeable changes in the microstructures were observed. In binary alloys a larger grain structure is formed, and Sn dendrites coarsen. In the ternary alloy the intermetallic phases Ag 3 Sn Cu 6 Sn 5 are randomly distributed in a Sn matrix. There is a coarsening of Sn dendrites and Ag 3 Sn phase produced by the growth mechanism of Ostwald (Ostwald Ripening). There is no evidence of disolution or precipitation of new phases in the range of studied temperatures that can be detected by DSC calorimetry technique.as a result the acting mechanisms could be the coarsening of Sn dendrites and the relaxation of residual stresses during the first stages of isothermal aging. By means of these mechanisms the behavior of Vickers microhardness can be interpretated. Acknowledgements This work was carried out at IFIMAT (Instituto Física de Materiales de Tandil) and has been partially supported by CICPBA (Comisión de Investigaciones Científicas de la Provincia de Buenos Aires), CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas) and SeCyT-UNCPBA (Secretaría de Ciencia, Arte y Tecnología de la Universidad Nacional del Centro de la Provincia de Buenos Aires). References Anderson,I.E. and Harringa, J.L Elevated temperature aging of solder joints based on Sn-Ag-Cu: effects on joint microstructure and shear strength, Journal of Electronic Materials, 33, Nº 12, p Fix, A. R., Nuchter, W. and Wilde, F Microstructural changes of Lead-Free solder joints during long term aging, thermal cycling and vibration fatigue. Soldering & Surface Mount Technology, 20. p. 13. Hsuan, T.Ch. and Lin, K.L Effects of aging treatment on mechanical properties and microstructure of Sn 8.5Zn 0.5Ag 0.01Al 0.1Ga Solder; Materials Science and Engineering A, 456, p Islam, R.A., Wu, B.Y., Alam, M.O., Chan, Y.C.and Jillek, W Investigations on microhardness of Sn-Zn based lead-free solder alloys as replacement of Sn-Pb solder, Journal of Alloys and Compounds, 392, p Lawrence Wu, C.M. and Huang, Mingliang L Microstructural Evolution of Lead-Free Sn-Bi-Ag-Cu SMT joints during aging; IEEE Transactions on Advanced Packaging, 28, p Morando, C., Garbellini, O., Fornaro, O.and Palacio, H Evolución de la microestructura de aleaciones SnAgCu sometidas a envejecido a alta temperatura, Anales SAM/CONAMET. Seo, S.K., Kang, S.K., Shih, D.Y. and Lee, H.M The evolution of microstructure and microhardness of Sn-Ag and Sn-Ag-Cu solders during high temperature aging. Microelectronics Reliability, 49, p.288. Snugovsky, L., Perovic, D.D. and Rutter, J.W Experiments on the aging of Sn-Ag-Cu solder alloys. Materials Science and Technology, 20. p Wei, Ch., Liu, Y., Gao, Z., Xu, R. and Yang, K Effects of aging on structural evolution of the rapidly solidified Sn-Ag-Zn eutectic solder; Journal of Alloys and Compounds, 468, p. 154.