Microelectronic Engineering

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1 Microelectronic Engineering 85 (2008) Contents lists available at ScienceDirect Microelectronic Engineering journal homepage: Effects of Co addition in eutectic Sn 3.5Ag solder on shear strength and microstructural development Jung-Sub Lee a, Kun-Mo Chu a,1, Rainer Patzelt b, Dionysios Manessis b, Andreas Ostmann b, Duk Young Jeon a, * a Department of Materials Science and Engineering, KAIST, 335 Gwahangno, Yuseong-gu, Daejeon , Republic of Korea b Department of Chip Interconnection Technology and Advanced Packages, Fraunhofer IZM, Gustav-Meyer-Allee 25, D Berlin, Germany article info abstract Article history: Received 24 December 2007 Accepted 9 March 2008 Available online 15 March 2008 Keywords: Sn 3.5Ag Co addition Composite solder In this study, the approach of composite solder using eutectic Sn 3.5Ag solder and Co was tried. Co particles and Sn 3.5Ag solder paste were mechanically mixed at Co weight fractions from 0.1% to 2.0%. For the Co-mixed Sn 3.5Ag solder pastes, their melting temperatures and spreading areas were measured. The solder pastes were stencil printed on test substrates and reflowed to form solder bumps. Ball shear test was performed to examine shear strength of Co-reinforced Sn 3.5Ag solder bumps. As a result, Co addition up to 2 wt.% did not alter the melting temperature under heating but reduced undercooling. The maximum shear strength of Co-reinforced Sn 3.5Ag solder bumps increased by 28% compared to normal ones. The increase in shear strength can be attributed to the (Cu,Co) 3 Sn 2 intermetallic compounds. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Solder bumps used in flip chip bonding of microelectronic devices play an important role of mechanical joining as well as electrical interconnection and heat dissipation from devices. The use of Pb-free solders has already become unavoidable due to health and environmental safety concerns of Pb-containing materials [1]. Nowadays, it has become a very significant issue to improve properties and reliability of Pb-free solders for microsystems with miniaturized size and high performance [2]. Especially, solders under severe service environments are required to have high reliability and excellent mechanical properties. For example, when solder bumps are used in flip chip bonding of optoelectronic devices, they are required to have dimensional stability for long period of time in order to maintain coupling efficiency [3 5]. However, heat generated from the devices can lead to dimensional change of the solder bumps by creep phenomena. In the case of solders used in automotive parts, they are subjected to severe temperature variation as well as mechanical vibration induced by the engine. Also they may experience occasional shocks from road conditions [6]. Therefore, solders under such severe environments should be strengthened to endure creep deformation, thermal fatigue, and mechanical shock. * Corresponding author. Tel.: ; fax: address: dyj@kaist.ac.kr (D.Y. Jeon). 1 Present address: Micro Systems Lab, Samsung Advanced Institute of Technology, Mt. 14-1, Nongseo-dong, Giheung-gu, Yongin-Si , Gyeonggi-do, Republic of Korea. In order to improve the mechanical properties of solders, the composite approach has been tried. Composite solders generally contain fine second-phase reinforcing particles dispersed uniformly in the solder matrix. These particles impede movements of dislocations and pin grain boundaries so as to prevent the solder matrix from being deformed. This resistance to deformation enhances the mechanical properties of solders [7,8]. There are several examples of the second-phase reinforcing particles. For example, Marshall et al. have found superior mechanical properties of conventional 63Sn/37Pb solders by introducing Cu 6 Sn 5 intermetallic compound (IMC), Cu 3 Sn IMC, Cu, Ag, and Ni particles [9,10]. Ni 3 Sn 4 IMC also has been used as a reinforcing particle by Betrabet et al. [7]. In the case of Pb-free solders, Guo et al. have reported enhanced creep resistance of Sn 3.5Ag solder by introducing particles such as Cu, Ag, and Ni [11 15]. However none of them have been yet adopted and applied actually to soldering applications. Therefore, it is meaningful to find and develop another candidate of a reinforcing particle for Pb-free solders. Primary metals such as Cu, Ni, and Ag as well as IMCs such as Cu 3 Sn, Cu 6 Sn 5,Ag 3 Sn, and Ni 3 Sn 4 have a common feature that they bond to Sn well. Co, a transition metal element like Cu, Ni, and Ag, has sufficient capability to be used as a reinforcing particle because Co readily bonds to Sn. Recent studies about the use of Co as under bump metallurgy or diffusion barrier layer in Pb-free soldering have proven the good bondability of Co to Sn [16 19]. It is known that Co reacts with Sn to form Co-Sn IMCs such as CoSn 2, CoSn, and Co 3 Sn 2. Besides, Co has higher values of Young s modulus, rigidity modulus, and bulk modulus compared to Cu, Ni, and Ag [20] /$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi: /j.mee

2 1578 J.-S. Lee et al. / Microelectronic Engineering 85 (2008) Therefore Co is expected to exert improved reinforcing effect in the solder matrix. In this study, we have systematically formed and characterized Co-reinforced Sn 3.5Ag solder. Co-mixed Sn 3.5Ag solder pastes were prepared by mechanical mixing. It is known that particles added in a well established solder alloy should not alter the melting temperature and solderability in order to maintain the advantages of the solder alloy [21,22]. Therefore, the melting temperatures and solderability of the Co-mixed Sn 3.5Ag solder pastes were investigated by differential scanning calorimeter (DSC) analysis and spreadability test, respectively. Subsequently, Co-reinforced Sn 3.5Ag solder bumps were formed by reflow after stencil printing on test substrates. For the solder bumps, ball shear test was conducted to measure shear strength as a basic mechanical property of solder bumps. Finally, microstructures and fracture surfaces of the solder bumps were observed by using scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). 2. Experimental procedure Commercially available eutectic Sn 3.5Ag solder paste (Heraeus F510, formerly DSC ) was used in this study. The solder paste contained 89 wt.% solder particles whose size was from 5 lm to15lm (Type 6). Co powder used was also a commercial one (Aldrich) and had its particle size below 2 lm. The Co powder was mechanically mixed with the Sn 3.5Ag solder paste by using a solder paste mixer (UM-103, UNIX, Japan) to achieve uniform mixing. The weight fractions of the Co powder compared to the Sn 3.5Ag particles were 0.1%, 0.5%, 1.0%, and 2.0%. DSC measurements were performed on the Co-mixed and the normal Sn 3.5Ag solder pastes in order to examine the influence of Co addition on their melting and solidifying behaviors. The solder pastes were heated from room temperature to 300 C, maintained for 5 min, and cooled to room temperature. The heating and cooling rate was 10 C/min. Spreadability test was carried out in order to examine the influence of Co addition on solderability. Cu foil laminated on FR4 substrate was etched in 2 vol.% HCl in methanol and subsequently cleaned in ethanol to obtain clean Cu surface without oxide layer and organic contaminants. The Co-mixed and the normal Sn 3.5Ag solder pastes were stencil printed on the clean Cu surface, respectively. Thickness and aperture diameter of the stencil mask used in this work were 100 lm and 210 lm, respectively. The substrates on which pastes were stencil printed were heated in a reflow oven maintained at 250 C inn 2 atmosphere with a dwell time of 2 min. The spreading areas of the solders on Cu surface were measured by using an optical microscope. For ball shear test, solder bumps were formed by stencil printing and reflow. The Co-mixed and the normal Sn 3.5Ag solder pastes were stencil printed on a prepared FR4 substrate. Thicknesses of Cu layer and solder resist layer of the FR4 substrate were 30 lm and 15 lm, respectively. Circular openings for bond pads were patterned on the solder resist layer. Diameter of the circular opening was 80 lm. The stencil mask used was the same one used for the spreadability test. Before stencil printing, Cu opening surfaces were also cleaned in the same way as explained in the spreadability test. After stencil printing, the substrates were reflowed in a four-zone reflow oven with N 2 atmosphere. The maximum temperature maintained during reflow was 260 C. For the formed solder bumps, ball shear tests were carried out using a bond tester (Dage 4000) in order to measure shear strength. More than 40 solder bumps on each substrate were selected. For the shear testing the JEDEC standard JESD22-B117A was followed [23]. According to the standard the shear tool height should not exceed 25% of the bump height. The shear height and the speed were 20 lm and 20 lm/s, respectively. The substrates were mounted with epoxy resin, cured at room temperature, mechanically ground, and then polished up to 1 lm to obtain cross-sections of the solder/cu interfaces. Sn of the polished samples was selectively etched in 2 vol.% HCl in methanol to reveal microstructures. Fracture surfaces after ball shear test as well as cross-sections of the substrates were examined using a SEM equipped with EDS. 3. Results and discussion The Co-mixed Sn 3.5Ag solder pastes were prepared by mechanical mixing. Fig. 1 shows the SEM images of the normal and the Co-mixed Sn 3.5Ag solder pastes. The Sn 3.5Ag solder particles had maximum size of 15 lm. As a result of mechanical mixing using the solder paste mixer, Co particles which were much smaller than Sn 3.5Ag solder particles were well dispersed and mixed with the Sn 3.5Ag solder paste. In order to find out the influence of Co addition in Sn 3.5Ag solder paste on its melting and solidifying temperature, DSC analysis was conducted. Heat flow curves during heating were obtained as shown in Fig. 2a. Each curve shows an endothermic peak which indicates melting behavior of each solder paste. The melting temperatures were determined from onset values of the endothermic peaks. Melting events of all samples occurred at about 221 C, which is the melting temperature of eutectic Sn 3.5Ag solder. It seems that during heating there is no change in the melting temperature of Sn 3.5Ag solder paste when Co particles were added. During cooling, exothermic peaks which indicate solidifying of Fig. 1. Sn 3.5Ag solder pastes (a) without and (b) with Co particles.

3 J.-S. Lee et al. / Microelectronic Engineering 85 (2008) Fig. 3. Spreading areas as a function of Co content. molten solder appeared as shown in Fig. 2b. The onset values of all the Co-mixed pastes were about 218 C whereas that of normal Sn 3.5Ag solder was C. As reported elsewhere [24], undercooling was observed in all samples during cooling. The amounts of undercooling of normal and the Co-mixed Sn 3.5Ag solder were 29.1 and 3 C, respectively. The presence of Co particles in molten Sn 3.5Ag solder provides nucleation sites for solidification. Thus, the undercooling of the Co-mixed Sn 3.5Ag solder could be reduced. Solderability as well as melting behavior is also an important feature of solder pastes. To examine the solderability of the Comixed Sn 3.5Ag solder pastes, spreadability test was performed. Solder paste melts and wets on Cu surface when it is heated over its melting temperature. After cooling the sample, the solder alloy is solidified to cover an area of Cu surface. Larger spreading area means better solderability of the solder paste. Spreading areas measured in this experiment are plotted in Fig. 3 as a function of Co content. All the solder pastes showed similar spreading areas of about lm 2 and no remarkable tendency was observed. Also, spreading areas of the Co-mixed Sn 3.5Ag solder pastes were all included in the error range of that of the normal Sn 3.5Ag solder paste. As a result, it can be concluded that Co addition up to 2 wt.% in Sn 3.5Ag solder paste does not degrade its solderability. Based on the results of melting temperatures and solderability, it was confirmed that the same processing conditions of a commercial Sn 3.5Ag solder paste could be applied to the Co-mixed ones. The normal and the Co-mixed Sn 3.5Ag solder pastes were stencil printed and subsequently reflowed to form spherical solder bumps as shown in Fig. 4. Diameter, height, and pitch of the solder bumps were 140 lm, 125 lm, and 500 lm, respectively. For the solder Fig. 2. DSC curves of normal and the Co-mixed Sn 3.5Ag solder pastes during (a) heating up to 300 C and (b) cooling. Fig. 4. Solder bumps formed from (a) normal and (b) Co-mixed Sn 3.5Ag solder pastes.

4 1580 J.-S. Lee et al. / Microelectronic Engineering 85 (2008) Fig. 5. Shear strength of solder bumps as a function of Co content. bumps shear strength was measured by ball shear test. Fig. 5 shows the change of shear strength as a function of Co content. When 0.1 wt.% of Co was added in Sn 3.5Ag solder, shear strength increased by 28% compared to normal Sn 3.5Ag solder bumps. Up to 1.0 wt.% of Co content shear strength of the solder bumps had similar values. On the other hand, there was no increase in shear strength when 2.0 wt.% of Co was added. Judging from the change of shear strength, it is thought that Co particles actually exert reinforcing effect in the eutectic Sn 3.5Ag solder matrix and the optimum content of Co addition lies near 0.1 wt.%. In order to figure out the reason for the increased shear strength, interfacial cross-sections were observed by SEM equipped with EDS. Fig. 6a shows the interface between eutectic Sn 3.5Ag solder and Cu. The microstructure of eutectic Sn 3.5Ag is composed of Sn and Ag 3 Sn IMCs in the solder region. At the interface between solder and Cu, Cu 6 Sn 5 IMCs were formed toward solder region. Cu 6 Sn 5 IMCs had well known scallop-like shape. When Co was added in the solder, distinguishable IMCs which had facetted needle-like shape appeared as shown in Fig. 6b e. From the results of EDS analysis as shown in Fig. 6f, it was revealed that the IMCs consisted of Sn, Co, and Cu and the composition of the IMCs was determined as (Cu,Co) 3 Sn 2. The (Cu,Co) 3 Sn 2 IMCs generally located in the middle of Sn region with random direction. As Co content increased, size and number of the (Cu,Co) 3 Sn 2 IMCs decreased and increased, respectively. In the cases of 0.1 and 0.5 wt.% of Co content, the (Cu,Co) 3 Sn 2 IMCs were separated to Fig. 6. Interfacial cross-sections of (a) 0, (b) 0.1, (c) 0.5, (d) 1.0, and (e) 2.0 wt.% Co-added Sn 3.5Ag solder on Cu and (f) EDS results on the facetted needle-like shaped (Cu,Co) 3 Sn 2 IMCs.

5 J.-S. Lee et al. / Microelectronic Engineering 85 (2008) each other. In the cases of 1.0 and 2.0 wt.% of Co content, however, (Cu,Co) 3 Sn 2 IMCs gathered together and formed a star-like structure due to drastic increase in number of the (Cu,Co) 3 Sn 2 IMCs. Particularly, the Sn 3.5Ag solder containing 2.0 wt.% of Co showed very complicated and linked IMC structure composed of Ag 3 Sn and (Cu,Co) 3 Sn 2 IMCs. Also Sn region was much reduced compared to the other cases. It is thought that Sn has been consumed much to form the heavily grown IMCs in the case of 2.0 wt.% of Co content. Formation of (Cu,Co) 3 Sn 2 IMCs can be explained in terms of phase diagram. Three kinds of IMCs, Co 3 Sn 2, CoSn, and CoSn 2, exist between Co and Sn under about 400 C as shown in Fig. 7a [25]. When the Co-mixed Sn 3.5Ag solder pastes are heated over the melting temperature, Co particles which still remain in a solid phase are surrounded by molten Sn 3.5Ag solder. At the interface between Co particles and molten solder, Co 3 Sn 2 IMCs begins to grow because Co 3 Sn 2 is the Co-richest phase. At the same time, Cu diffuses into molten solder from the Cu layer of the substrate. Cu in molten solder combines with Co 3 Sn 2 IMCs and acts similarly as Co to form (Cu,Co) 3 Sn 2 IMCs because Co makes solid solution with Cu in the full range of composition under 422 C as shown Fig. 7. Binary alloy phase diagrams of (a) Co Sn and (b) Co Cu.

6 1582 J.-S. Lee et al. / Microelectronic Engineering 85 (2008) Fig. 8. Fracture surfaces of (a) 0, (b) 0.1, (c) 0.5, (d) 1.0, and (e) 2.0 wt.% Co-reinforced Sn 3.5Ag solder bumps after ball shear test. in Fig. 7b [26]. If there is no Cu, Co 3 Sn 2 IMCs will be turned into the Sn-richest CoSn 2 IMCs eventually. However, Cu supplied from the bottom limits the transformation of Co 3 Sn 2 to CoSn 2 and maintains X 3 Sn 2 composition where X is either Cu or Co. Fracture surfaces after ball shear test were also observed by SEM as shown in Fig. 8. The normal Sn 3.5Ag solder bumps showed ductile fracture inside the solder matrix and smooth fracture surface. For the Sn 3.5Ag solder bumps containing 0.1, 0.5, and 1.0 wt.% of Co, fracture also occurred inside the solder matrix. However, the fracture surfaces were not smooth as those of the normal Sn 3.5Ag solder bumps. As shown in Fig. 8b d, scratches on the fracture surfaces and pieces of the (Cu,Co) 3 Sn 2 IMC at the end of some scratches were observed. On the other hand, the Sn 3.5Ag solder bumps containing 2.0 wt.% of Co showed somewhat different fracture mode as shown in Fig. 8e. The fracture surface showed both intermetallic interface and remained solder matrix. Besides, the scratches on the fracture surface were finer than those of ones containing 0.1, 0.5, and 1.0 wt.% of Co. It is thought that those fine scratches were due to the (Cu,Co) 3 Sn 2 IMCs whose average size and number decreased and increased, respectively, when Co content increased. Based on the results of interfaces and fracture surfaces, we can explain the shear strength increase in the Sn 3.5Ag solder bumps containing 0.1, 0.5, and 1.0 wt.% of Co. The facetted needle-like shaped (Cu,Co) 3 Sn 2 IMCs which are distributed in the solder with random direction reinforce the Sn 3.5Ag solder matrix. The (Cu,Co) 3 Sn 2 IMCs act as obstacles in the fracture path and make scratches on fracture surface during shearing. That means the (Cu,Co) 3 Sn 2 IMCs impede the generation of fracture surface and produce frictional path once a fracture occurs. Therefore, it can be concluded that the (Cu,Co) 3 Sn 2 IMCs formed by Co addition in Sn 3.5Ag solder strengthen the Sn 3.5Ag solder bumps by reinforcing effect. On the other hand, there was no increase in shear strength of the Sn 3.5Ag solder bumps containing 2.0 wt.% Co. The heavily grown (Cu,Co) 3 Sn 2 and Ag 3 Sn IMCs sustain the solder matrix so as to increase fracture strength of the solder matrix. Therefore, when a shear stress is applied, cracks prefer to grow through the Cu/solder interface rather than paths inside the solder matrix. The solder matrix has been torn apart almost entirely showing little plastic deformation once a fracture occurs. Too much (Cu,Co) 3 Sn 2 IMCs by excessive addition of Co particles in Sn 3.5Ag solder result in change of fracture path from inside the strengthened solder matrix to weak Cu/solder interface. 4. Conclusion Formation and characterization of Co-reinforced Sn 3.5Ag solder were conducted in this work. Co powder and a commercial Sn 3.5Ag solder paste were mixed mechanically. Co addition in Sn 3.5Ag solder paste did not alter melting temperature during heating. However, undercooling phenomenon during cooling the molten solder was reduced due to increased nucleation sites by

7 J.-S. Lee et al. / Microelectronic Engineering 85 (2008) the presence of Co particles. There was no degradation in solderability of the Co-mixed Sn 3.5Ag solder pastes. Using the pastes, solder bumps were successfully formed through stencil printing and reflow. When Co particles were added up to 1.0 wt.% shear strength increased by the existence of the (Cu,Co) 3 Sn 2 IMCs, which exerted reinforcing effect. When Co particles were added by 2.0 wt.%, there was no increase in shear strength because the heavily grown (Cu,Co) 3 Sn 2 and Ag 3 Sn IMCs caused interfacial fracture. Our experimental results have proven that Co can be effectively used as a reinforcing particle in Sn 3.5Ag solder. However, several problems such as Co particle size, uniform dispersion of IMCs, IMC growth behavior, and other mechanical tests still remain unsolved. In order to optimize the process and obtain more plausible interpretations, further experimental study is currently underway. Acknowledgments This work was supported by the Center for Electronic Packaging Materials (ERC) of MOST/KOSEF and the Joint Laboratory Program for CEPM-Fraunhofer IZM. References [1] S.K. Kang, D.Y. Shih, K. Fogel, P. Lauro, M.J. Yim, G.G. Advocate Jr., M. Griffin, C. Goldsmith, D.W. Henderson, T.A. Gosselin, D.E. King, J.J. Konrad, A. Sarkhel, K.J. Puttlitz, IEEE Trans. Electron. Pack. 25 (2002) 155. [2] K. Mohankumar, A.A.O. Tay, in: Proc. of the Sixth Electronics Packaging Technology Conference, Singapore, 2004, p [3] H. Mavoori, S. Jin, J. Electron. Mater. 27 (1998) [4] D.C. Lin, S. Liu, T.M. Guo, G.X. Wang, T.S. Srivatsan, M. Petraroli, Mat. Sci. Eng. A-Struct. 360 (2003) 285. [5] D. Lin, G.X. Wang, T.S. Srivatsan, M. Al-Hajri, M. Petraroli, Mater. Lett. 53 (2002) 333. [6] J.P. Liu, F. Guo, Y.F. Yan, W.B. Wang, Y.W. Shi, J. Electron. Mater. 33 (2004) 958. [7] H.S. Betrabet, S.M. McGee, J.K. McKinlay, Scripta Metall. Mater. 25 (1991) [8] R.C. Reno, M.J. Panunto, J. Electron. Mater. 26 (1997) 11. [9] J.L. Marshall, J. Calderon, J. Sees, G. Lucey, J.S. Hwang, IEEE Trans. Compon. Hybr. 14 (1991) 698. [10] J.L. Marshall, J. Calderon, Solder. Surf. Mt. Technol. 9 (1997) 22. [11] F. Guo, S. Choi, J.P. Lucas, K.N. Subramanian, Solder. Surf. Mt. Technol. 13 (2001) 7. [12] F. Guo, J. Lee, K.N. Subramanian, Solder. Surf. Mt. Technol. 15 (2003) 39. [13] F. Guo, J. Lee, S. Choi, J.P. Lucas, T.R. Bieler, K.N. Subramanian, J. Electron. Mater. 30 (2001) [14] F. Guo, J. Lee, J.P. Lucas, K.N. Subramanian, T.R. Bieler, J. Electron. Mater. 30 (2001) [15] F. Guo, J.P. Lucas, K.N. Subramanian, J. Mater. Sci.-Mater. Electron. 12 (2001) 27. [16] R. Labie, E. Beyne, P. Ratchev, Proc. of the 53rd Electronic Components and Technology Conference, New Orleans, 2003, p [17] R. Labie, E. Beyne, R. Mertens, P. Ratchev, J. V. Humbeeck, in: Proc. of the Fifth Electronics Packaging Technology Conference, Singapore, 2003, p [18] P. Ratchev, R. Labie, E. Beyne, Proc. of the Sixth Electronics Packaging Technology Conference, Singapore, 2004, p [19] L. Magagnin, V. Sirtori, S. Seregni, A. Origo, P.L. Cavallotti, Electrochim. Acta 50 (2005) [20] E.A. Brandes, G.B. Brook, Smithells Metals Reference Book, seventh ed., Elsevier, Amsterdam, pp , 15-2, [21] K.J. Puttlitz, K.A. Stalter, Handbook of Lead-Free Solder Technology for Microelectronic Assemblies, Marcel Dekker, New York, pp [22] F. Tai, F. Guo, Z.D. Xia, Y.P. Lei, Y.F. Yan, J.P. Liu, Y.W. Shi, J. Electron. Mater. 34 (2005) [23] JEDEC Standard JESD22-B117A, [24] S.K. Kang, M.G. Cho, P. Lauro, D.Y. Shih, J. Mater. Res. 22 (2007) 557. [25] Thaddeus B. Massalski, Hiroaki Okamoto, P.R. Subramanian, Linda Kacprzak, second ed., Binary Alloy Phase Diagrams, vol. 2, ASM International, Ohio, 1990, pp [26] P.R. Subramanian, D.J. Chakrabarti, D.E. Laughlin, Phase Diagrams of Binary Copper Alloys, ASM International, Ohio, 1994, pp