Effect of 9 wt.% in addition to Sn3.5Ag0.5Cu solder on the interfacial reaction with the Au/NiP metallization on Cu pads

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1 Journal of Alloys and Compounds 396 (2005) Effect of 9 wt.% in addition to Sn3.5Ag0.5Cu solder on the interfacial reaction with the Au/NiP metallization on Cu pads M.N. Islam, Y.C. Chan, A. Sharif, M.J. Rizvi Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong, PRChina Received 25 November 2004; received in revised form 27 December 2004; accepted 3 January 2005 Available online 25 January 2005 Abstract Sn-based lead-free solders have a high melting temperature, which often cause excessive interfacial reactions at the interface. A small amount of In is added to reduce the melting temperature and to change the intermetallic compound (IMC) phases. Sn3.5Ag0.5Cu and Sn3.5Ag0.5Cu9In lead-free solder alloys have been used to identify its interfacial reactions with 2-metal layer flexible substrates. In this paper we investigate the effect of 9 at.% In addition to Sn3.5Ag0.5Cu solder during extended reflow. During reflow, Au diffuses rapidly in the molten Sn Ag Cu solder and forms AuSn 4 IMC but in the case of In-containing solder, In Sn Au IMCs form and are uniformly distributed in the solder. Some In Sn Au IMCs have been entrapped in the Sn Cu Ni In quaternary intermetallic compounds (QIMCs) due to lower diffusion rate of Au in the In-containing solder. Initially Sn Cu Ni ternary intermetallic compounds (TIMCs) and Sn Cu Ni In QIMCs form at the interface, which have higher growth rate and consume more of the NiP layer. Low-Cu QIMCs are found in the In-containing solder after 30 min reflow which are more stable in the P-rich Ni layer and significantly reduce the dissolution rate of the NiP layer. The spalling of Sn Cu Ni TIMCs in the Sn Ag Cu solder increases the diffusion rate of Sn atoms and as a consequence both the TIMCs growth rate and dissolution rate of the NiP layer also increases. In-containing solder have lower growth rate of the QIMCs and lower dissolution rate of the NiP layer than the Sn Ag Cu solder. Consumption of the NiP layer can be reduced by adding In, because of the formation of QIMCs at the interface, QIMCs are stable and are well adhering to the P-rich Ni layer during reflow Elsevier B.V. All rights reserved. Keywords: Intermetallics; Lead-free solder; Effect of In; Dissolution of electroless NiP; Sn Ag Cu 1. Introduction The ball grid array (BGA) package is an important electronic packaging technology because of high input/output terminal density, small footprint, and good reliability [1]. Au/NiP/Cu under bump metallurgy (UBM) is commonly used for its excellent solderability, corrosion resistant, uniformity, low cost, selective deposition without photolithography, and also good diffusion barrier [2]. Conventional Sn Pb solder still plays an important role in the electronic packaging. The development of lead-free solder is needed for the replacement of this solder in the electronic industry for environmental reasons. A candidate lead-free solder alloy need to Corresponding author. Tel.: ; fax: address: eeycchan@cityu.edu.hk (Y.C. Chan). fulfill the following requirements: wettability, suitable melting temperature, good mechanical properties, good resistance to mechanical and thermal fatigue, corrosion resistance, good electrical properties, good for health and environment, availability and low material cost [3,4]. According to a report on Pb-free alloys by the National Center for Manufacturing Science, Sn Ag, Sn Ag Cu and Sn Cu solders have been suggested as promising candidate lead-free solders [5]. The Sn Ag Cu ternary eutectic solder has the lowest melting temperature ( 217 C). The International Printed Circuit (IPC) Association has indicated that choices of Sn3.0Ag0.5Cu and Sn3.9Ag0.6Cu (two neareutectic alloys) are expected to be widely used by the world as a whole [6]. However, the high melting points of these alloys as compared to the Sn Pb solder ( 183 C) are the main issue in the electronic packaging. The melting temperature of solder alloys is critical, because too high a soldering /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.jallcom

2 218 M.N. Islam et al. / Journal of Alloys and Compounds 396 (2005) temperature could damage electronic devices and polymer based PCBs. When the solder alloy temperature is too low, the long term reliability of solder joints may suffer. In, Bi and Ge are used to lower the melting temperature of solders [7]. Eutectic 52In 48Sn is considered the lowest practical melting point solder ( 118 C). In-containing solders are slow in crack propagation, have good wetting behavior, increase alloy strength and fatigue resistance [7 10]. The formations of AuIn 2 compounds prevent the formation of more brittle intermetallic compounds such as AuSn 4 in the bulk solder [11]. Au-embrittlement could be curtailed for the In 49Sn solder joints because of the lower solubility of Au in the In Sn alloy [10]. It is also found that Sn Cu Ni TIMCs form during reflow in the Cu-containing solder such as Sn Ag Cu and Sn Cu [12,13]. These results indicate that IMC phases are determined by the addition of small amounts of elements such as Cu, Bi, In in the solder alloys. Therefore, the small amount of alloying element may affect the growth rate of IMCs and the consumption rate of UBM as well as the reliability of solder joints, should be studied thoroughly. In this study, 9 wt.% of indium is added to a Sn Ag Cu low melting lead-free solder for the purpose of lowering the melting point and for changing the IMC phases. The interfacial reactions such as IMC formation and consumption rate of UBM between the liquid Sn Ag Cu and Sn Ag Cu In solder with NiP substrate are investigated at 250 C for different times of reflow to find out the effect of In addition to Sn Ag Cu solder. P content of about 16 at.%, the Au layer thickness is about 0.6 m, and the Cu layer thickness is about 15 m. The solder joints were made by soldering between flexible substrates with Cu/NiP/Au coated pads and lead-free Sn Ag Cu and Sn Ag Cu In solders. The compositions of the solder were Sn3.5Ag0.5Cu9In (wt.%) and Sn3.5Ag0.5Cu (wt.%). The solidus and liquidus temperature of the Incontaining solder were about 202 and 207 C. The solidus and liquidus temperature of the Sn Ag Cu solder was about 217 C. The solder balls were placed on the prefluxed coated pad (Fig. 1a) and than reflow at 250 C in a reflow oven (BTU VIP-70N). No-clean flux was used on the pad. During reflow, solder melt and reaction occur between the molten solder and Au/electroless NiP/Cu surface. After the reflow, the samples were kept again at the same temperature for 5, 10, 30, 120, and 180 min. To investigate the microstructure of the samples, the samples were sectioned using a slow speed diamond saw and then mounted in epoxy. The cross-sectioned samples were ground and polished carefully and then gold sputter coated for examination. The chemical and microstructural analyses of the gold-coated cross-sectioned samples were obtained using a Philips XL 40 FEG Scanning Electron Microscope (SEM) equipped with energy dispersive X-ray (EDX) analysis (EDAX International, model no. DX-4). The dissolution rate of the electroless NiP layer was determined by measuring the remaining NiP layer thickness after each reflow condition. Back scattered electron mode (BSE) of SEM was used for the cross-sectional study. 2. Experimental procedure A flexible substrate with Cu pad was used to deposit the electroless NiP layer. The Cu pad is a part of internal wiring within the flexible BGA substrate. The solder mask opening diameter was mm at the ball pad. The electroless Ni was deposited on the Cu pad. Immersion Au plating was immediately layered on the top of the NiP layer to avoid oxidation of the nickel surface. The P content of the electroless Ni substrate was controlled by the P H value of the plating solution. The thickness of the plating layer was measured by SEM. The thickness of electroless Ni is about 4.6 m with a 3. Results and discussions 3.1. Dissolution kinetics and cross-sectional studies of the interface To investigate the reaction kinetics of the electroless NiP/solder joints, detailed cross-sectional studies are carried out by SEM. During reflow, molten solder react with Au/NiP layer and form different types of IMCs at the interface. These IMCs together with unreacted NiP layer provide the adhesion Fig. 1. (a) Solder ball attachment and (b) after reflow.

3 M.N. Islam et al. / Journal of Alloys and Compounds 396 (2005) between the solder and the substrate (Fig. 1b). The IMCs phases are determined by EDX analysis, as detailed below Interface after reflow In case of Sn Ag Cu solder, during reflow molten solder absorbs the entire Au layer into solution, allowing Sn, Ag and Cu from the solders to react with the NiP layer and form different types of intermetallic compounds at the interface and within the solder (Fig. 2a and b). The thickness of intermetallics is about 3.36 m. A very thin dark layer of about 339 nm is also found between the IMCs and the NiP layer. EDX analysis reveals that Sn Ni Cu TIMC has formed on the dark P-rich Ni layer [13,14]. In the In-containing solder joint, molten solder reacts with the Au layer and forms In Sn Au IMC (Fig. 2c and d). The formula composition of the IMC is In 0.48 Sn 0.25 Au The In Sn Au IMC is uniformly distributed in the solder. According to EDX analysis, asn Cu Ni In QIMC has formed on the P-rich Ni layer [16,17]. The thickness of QIMC is about 3.21 m. The formula composition of the QIMC is Sn 0.51 Cu 0.26 Ni 0.20 In A little bit of In Sn Au IMC has been entrapped into the QIMC. This is due to the lower diffusion rate of Au in the In-containing solder as compared to the Sn Ag Cu solder [10,18]. A very thin dark P-rich Ni layer of about 299 nm is also found between the QIMC and the original NiP layer (Fig. 2c). The P content in the dark P-rich Ni layer of the solder is close to the stoichiometry of the Ni 3 P [15,19]. The EDX analysis identifies the composition (at.%) of the newly appeared IMCs as Ag o.80 In 0.15 Sn 0.05, Ag 0.36 In 0.22 Sn 0.42 and Sn 0.54 Cu 0.36 In These are found in the In-containing solder (Fig. 2c and d). Sn Cu, Ag Sn and Au Sn IMCs are found in the Sn Ag Cu solder (Fig. 2b). But these are not found in the In-containing solder [17]. This is due to the addition of indium to the solder. The growth rates of Sn Cu Ni TIMCs and Sn Cu Ni In QIMCs are high and consumed more NiP layer. The morphology of the TIMCs and QIMCs are very rough. No Au-containing compound is found in the Sn Cu Ni TIMCs but small amounts of Au-containing compounds are found in the Sn Cu Ni In QIMCs Interface after long time of molten reaction A reflow time for industrial applications above 10 min may not to be realistic for practical processes. However, in this study, we have used up to 180 min. The main purpose of this unrealistic time in the molten state or reflow time is the scientific interest in establishing a database, which are then helpful for the prediction of life time and for designing new components. During 30 min reflow, it is seen that thickness of IMCs and P-rich Ni layer increase whereas the original NiP layer thickness decreases with reflow times (Figs. 3 6). In the case of Sn Ag Cu solder, the thickness of Sn Cu Ni TIMCs increases significantly with reflow time and it is about 7.51 m thick and the morphology is very rough. Sn Cu Ni Au IMCs are found in the solder. The composition of the IMCs is Sn 0.50 Cu 0.37 Ni 0.10 Au The Ni has diffused through the Sn Cu Ni TIMCs and formed Ni-containing IMCs in the Fig. 2. Interfacial reactions of NiP/solder after reflow: (a) interface of a NiP/Sn Ag Cu solder joint, (b) IMCs in the solder, (c) interface of a NiP/Sn Ag Cu In solder joint, (d) IMCs in the In-containing solder.

4 220 M.N. Islam et al. / Journal of Alloys and Compounds 396 (2005) Fig. 3. Interfacial reactions of NiP/solder after 30 min reflow: (a) interface of a NiP/Sn Ag Cu solder joint, (b) Sn Cu Ni Au and Au Sn IMCs in the solder, (c) interface of a NiP/Sn Ag Cu In solder joint, and (d) Sn Cu Ni In Au and In Sn Au IMCs in the solder. solder [12]. In case of In-containing solder, the thickness of QIMCs is about 6.93 m. At this stage, low-cu containing QIMCs are found between the QIMCs and P-rich Ni layer at the interface. The higher growth rate of QIMCs and consumption rate of NiP layer have changed after 10 min reflow (Figs. 4 and 6). This is a sign of the start of the formation of low-cu QIMCs on the P-rich Ni layer. It is due to the lower supply of Cu from the solder because of the formation of more stable Cu-containing compounds in the Incontaining solder. The growth rate of QIMCs is lower than TIMCs (Figs. 3 and 4). There may be two reasons for this: (i) formation of low-cu containing QIMCs, which have lower growth rate and lower consumption rate of NiP layer. (ii) The entrapments of In Sn Au phases in the Sn Cu Ni In QIMCs may have some effect on the growth rate of QIMCs and dissolution rate of NiP layer. The composition of low-cu QIMCs is Sn 0.60 Ni 0.27 Cu 0.08 In 0.05.Sn In Au, Ag In Sn and Sn Cu Ni In Au IMCs are found within the In-containing solder. The composition of Ni-containing IMCs is Sn 0.58 Cu 0.32 Ni 0.07 Au Sn Cu Ni In Au IMCs are more stable than Sn Cu In and Sn Cu In Au IMCs within the solder. The P content and thickness of P-rich Ni layer are lower in the In-containing solder than in the Sn Ag Cu solder (Fig. 5). It is due to the lower consumption rate of NiP layer in the NiP/In-containing solder system. Fig. 4. Thickness of IMCs with reflow time. Fig. 5. The thickness of P-rich Ni layer with reflow time.

5 M.N. Islam et al. / Journal of Alloys and Compounds 396 (2005) Fig. 6. Consumed NiP layer thickness with reflow time. In NiP/Sn Ag Cu solder system, the spalling of Sn Cu Ni TIMCs has started into the molten solder, which an increase in the diffusion rate of atoms and formation of TIMCs at the interface which is accompanied by an enhanced dissolution rate of NiP layer (Figs. 6 and 7a). There may be two reasons for such spalling of Sn Cu Ni TIMCs: (i) increased of P content in the P-rich Ni layer [20] and (ii) Ni Sn IMCs first form on P-rich Ni layer due to limited supply of Cu from the solder [13,21]. In this stage the thickness of TIMCs is not comparable to the QIMCs. The thickness of P-rich Ni layer is also increased and it is about 1.24 m, with a P content of about 33.6 at.%. Yoon et al. [15 19] reported as the thicker P-rich Ni layer grows, the more Kirkendall voids are generated, which help to increase the diffusion rate of Sn and Ni. So, the growth rate of TIMCs is increased. Due to diffusion of more Sn atoms through the broken Sn Cu Ni TIMCs, slightly brighter low-cu containing (about 8.5 at.%) TIMCs have formed on the top of the P-rich Ni layer (Fig. 7a). It is due to lower percentage of Cu within the solder. In case of NiP/Incontaining solder system, after 120 min reflow it is found that most of the scallop like low-cu containing QIMCs are stable and are well adhering to the P-rich Ni layer (Fig. 7b). The spalling of QIMCs is also observed in the In-containing solder but it is lower than the Sn Cu Ni TIMCs in the Sn Ag Cu solder. As per EDX analysis, the composition of the solder side QIMCs is Sn 0.59 Ni 0.30 Cu 0.09 In 0.02 and near P-rich Ni layer is Sn 0.57 Ni 0.33 Cu 0.08 In The atomic percentages of all atoms are almost same in all places of the QIMCs. In case of Sn Ag Cu solder, after 180 min of molten reaction it is seen that the whole NiP layer has been consumed and in some places an intermediate Ni Sn P layer is observed between the low-cu containing TIMCs and the P-rich Ni layer (Fig. 8a). The thicknesses of the slightly brighter low-cu containing TIMCs and P-rich Ni layer have increased significantly. In some places the P-rich Ni layer was broken and creating channels. These channels are the results of the coalescence of the Kirkendall voids formed during the diffusion process [15,19,22]. So, the diffusing Sn atoms come Fig. 7. Interfacial reactions of NiP/solder after 120 min reflow: (a) interface of a NiP/Sn Ag Cu solder and (b) interface of a NiP/Sn Ag Cu In solder joint. Fig. 8. Interfacial reactions of NiP/solder after 180 min reflow: (a) interface of a NiP/Sn Ag Cu solder joint and (b) interface of a NiP/Sn Ag Cu In solder joint.

6 222 M.N. Islam et al. / Journal of Alloys and Compounds 396 (2005) Table 1 Formation of IMCs during reflow Solder Interfacial IMCs IMCs within the solder During first time reflow Sn Ag Cu Sn Cu Ni TIMCs Sn Cu, Ag Sn, Au Sn Sn Ag Cu In Sn Cu Ni In QIMCs Sn Cu In, Ag In Sn, Au Sn In During extended reflow Sn Ag Cu Sn Cu Ni TIMCs, low-cu containing TIMCs Sn Cu Au, Sn Cu Ni Au, Ag Sn, Au Sn Sn Ag Cu In Low-Cu containing QIMCs Sn Cu Au In, Sn Cu Ni Au In, Ag In Sn, Au Sn In into contact to the Cu pads through these channels and react to form Ni Sn Cu and then Cu Sn IMCs. Thus, the dissolution rate of the NiP layer is increased (Fig. 6). EDX analysis reveals that Ni Sn Cu IMCs have formed under the broken P-rich Ni layer. In case of In-containing solder, it is seen that low-cu containing QIMCs are stable and well adhering to the P-rich Ni layer (Fig. 8b). The thickness of low-cu QIMCs is m. The thicknesses of the P-rich Ni layer are about 1.72 m( 37 at.% P) and 1.41 m( 32 at.% P) for Sn Ag Cu and In-containing solder, respectively (Fig. 5). Due to the increases in P content different types of Ni P compounds such as Ni 3 P, Ni 2 P, Ni 5 P 4, and NiP 2 have formed in the P-rich Ni layer [22]. About 3.45 m of the electroless NiP layer takes part in the reaction with the In-containing molten solder and has formed different type of IMCs in the solder and interface. The dissolution rate of electroless NiP layer is about m/min. In the case of Sn Ag Cu solders about 4.6 m of the electroless NiP layer has been consumed during 180 min molten reaction. The dissolution rate of the electroless NiP layer is about m/min, which is about times higher than that of the In-containing solder Effect of elements and formation of new phases The Ag Sn, Au Sn, Cu Sn, Cu Sn Au, Sn Cu Ni Au and Sn Cu Ni IMC phases are found in the NiP/Sn Ag Cu solder system. The Ag In Sn, In Sn Au, Sn Cu In, Sn Cu In Au, Sn Cu Ni Au In and Sn Cu Ni In IMC phases are found in the NiP/In-containing solder system (Table 1). The fact that the Ag Sn, Au Sn, Ag In and In Au IMCs are not found but Ag In Sn IMCs and In Sn Au IMCs are found may be due to the addition of a lower percentage of In to the solder. Many investigators have found the Ag 2 In and AuIn 2 IMCs in the In-containing solder. This was due to the addition of higher percentage of In (<20 wt.%) to the solder [10,18,23]. The Cu Sn compounds are not stable during extended reflow. So, the Cu Sn Au and more stable Sn Cu Ni Au IMCs are formed within the Sn Ag Cu solder. In the case of In-containing solder the Sn Cu In IMCs are not stable during extended reflow. So, the Sn Cu In Au and more stable Sn Cu Ni In Au IMCs are formed within the solder. An insignificant amount of Au diffuses to the interface and form Sn Cu Ni Au IMCs during extended reflow. The ternary and quaternary Au-containing compounds do not form during the initial stage of soldering because of the high solubility of Au in the molten Sn Ag Cu solder. In the case of In-containing solder, the In Sn Au IMCs form rapidly at the interface and uniformly distribute in the solder. The formations of In Sn Au compounds prevent the formation of more brittle IMCs such as AuSn 4 in the bulk In-containing solder. A higher amount of Au is found in the In Sn Au IMCs. A little bit of In Sn Au IMCs becomes entrapped in the QIMCs due to lower diffusion of Au in the In-containing solder. The growth rates of QIMCs in the In-containing solder are lower than the TIMCs in the Sn Ag Cu solder. During extended reflow, there is not much difference of Cu, Sn, Ni and In content across the thickness of the QIMCs. So, all atoms can diffuse in the QIMCs at a fast rate. Low-Cu containing QIMCs are more stable on the P-rich Ni layer and reduce the dissolution rate of the NiP layer during extended reflow. In the case of Sn Ag Cu solder, low-cu TIMCs are found at the interface in the later stage of reflow due to the previously larger supplies of Cu from the solder. It is clear that Cu-containing IMCs are more stable in the In-containing solder than the Sn Ag Cu solder. Sn Cu Ni TIMCs have higher growth rate and spalling and as well as consuming more NiP layer in the Sn Ag Cu solder. 4. Conclusions During the reaction of molten Sn3.5Ag0.5Cu and Sn3.5Ag0.5Cu9In solder the formation of IMCs at the interface and in the solder, the dissolution rates of the NiP layer and the morphology of IMCs were investigated. During reflow the Au atoms diffuse and form Au Sn IMCs in the Sn Ag Cu solder within a short time. But in the case of the In-containing solder In Sn Au IMCs form and are uniformly distributed in the solder. Some In Sn Au IMCs have been entrapped in the QIMCs due to the lower diffusion rate of Au in the In-containing solder. Due to the addition of indium to the solder, Sn Cu Ni In QIMCs are found at the interface. The growth rate of Sn Cu Ni TIMCs is higher than that of Sn Cu Ni In QIMCs. The dissolution rate of the NiP layer is also higher in the Sn Ag Cu solder than in the In-containing solder. The new IMC phases Ag In Sn, In Sn Au, Sn Cu In, Sn Cu In Au, Sn Cu Ni In Au

7 M.N. Islam et al. / Journal of Alloys and Compounds 396 (2005) are found in the solder due to the addition of indium (9 wt.%). Cu-containing IMCs are more stable in the In-containing solder than in the Sn Ag Cu solder. Low-Cu containing IMCs are more stable on P-rich Ni layer and reduce dissolution rate of NiP layer. Due to the spalling of the Sn Cu Ni TIMCs in the Sn Ag Cu solders the growth rate of TIMCs and dissolution rate of the NiP layer increases. Thus the changes of IMC phases at the interface have a significant effect on the growth rate and dissolution rate of the NiP layer. In order to reduce the melting temperature and the consumption rate of NiP UBM lead-free Sn Ag Cu solder alloy with about 9 wt.% In content is recommended. Acknowledgement The authors would like to acknowledge the financial support provided by Innovation and Technology Fund Ref. UIT/31, CityU Ref , RGC grant and Compass Technology Co. Ltd. in Hong Kong. References [1] C.E. Ho, Y.M. Chen, C.R. Kao, J. Electron. Mater. 28 (11) (1999) [2] Y.C. Sohn, Jin Yu, S.K. Kang, D.Y. Shih, T.Y. Lee, ECTC (2004) [3] N. Moelans, K.C.H. Kumar, P. Wollants, J. Alloys Compd. 360 (2003) [4] A.Z. Miric, A. Grusd, Soldering Surf. Mount Technol. 10 (1) (1998) [5] NCMS Reports 0401RE96. National Center for Manufacturing Sciences, Ann Arbor, MI, August, [6] C.M.L. Wu, D.Q. Yu, C.M.T. Law, L. Wang, Mater. Sci. Eng. R 44 (2004) [7] J.S. Hwang, Z. Guo, H. Koenigsmann, Surf. Mount Technol. 13 (2) (2001) [8] Z. Mei, J.W. Morris, J. Electron. Mater. 21 (1992) 401. [9] Z. Mei, J.W. Morris, J. Electron. Mater. 21 (1992) 599. [10] N.C. Lee, Soldering Surf. Mount Technol. 9 (2) (1997) 65. [11] D.M. Jacobson, D. Humpston, Gold Ball. 22 (1989) 9. [12] M.N. Islam, Y.C. Chan, A. Sharif, JMR, accepted for publication. [13] M.N. Islam, Y.C. Chan, A. Sharif, M.O. Alam, Microelectron. Reliab. 43 (2003) [14] K.W. Paik, Y.D. Jeon, M.G. Cho, ECTC (2004) [15] J.W. Yoon, S.B. Jung, J. Alloys Compd., in press. [16] M.J. Chiang, S.Y. Chang, T.H. Chung, J. Electron. Mater. 33 (1) (2004) [17] M.D. Cheng, S.F. Yen, S.F. Yen, T.H. Chuang, J. Electron. Mater. 33 (3) (2004) [18] J.H. Lee, Y.S. Eom, K.S. Choi, B.S. Choi, H.G. Yoon, T. Moon, Y.S. Kim, J. Electron. Mater. 33 (4) (2004) [19] Y.-D. Jeon, K.-W. Paik, ECTC, 2001, pp [20] Y.C. Sohn, J. Yu, S.K. Kang, D.Y. Shih, T.Y. Lee, ECTC, 2004, pp [21] K. Zeng, K.N. Tu, Mater. Sci. Eng. R: Rep. 38 (2) (2002) [22] M.O. Alam, Y.C. Chan, K.C. Hung, Microelectron. Reliab. 42 (2002) [23] T.H. Chuang, K.W. Huang, W.H. Lin, J. Electron. Mater. 33 (4) (2004)