Interfacial reactions of Sn Cu solder with Ni/Au surface finish on Cu pad during reflow and aging in ball grid array packages

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1 Materials Science and Engineering B 117 (2005) Interfacial reactions of Sn Cu solder with Ni/Au surface finish on Cu pad during reflow and aging in ball grid array packages M.N. Islam, Y.C. Chan Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong, PRChina Received 26 November 2003; received in revised form 25 November 2004; accepted 27 November 2004 Abstract The interface between the solder and the under-bump metallization (UBM) affect the reliability of the solder joints. In this paper an investigation is reported on the interfacial reactions of Sn0.7Cu (wt.%) solder on electrolytic Ni layer for different times of reflow and solidstate aging. It is found that during reflow, the formation of Sn Cu Ni ternary intermetallic compounds (IMCs) at the interface of solder joints is much quicker, resulting in the entrapment of some Pb (which is present as impurity in the Sn Cu solder) rich phase in the ternary IMCs. The growth rate of IMCs during reflow is higher than that in aging. Less than 3 m of the electrolytic Ni layer was consumed by the higher Sn-containing Sn0.7Cu solder with 180 min molten reaction at 250 C and with 16 days aging at 175 C. Ni and Au can diffuse into the Sn Cu solder in both solid and liquid state and form stable quaternary compounds in the solder. The diffusion of Au into the high Ni-containing ternary and quaternary compounds is found to be low. The shear strength of solder joint is relatively stable from 1.98 to 1.86 kgf during long time reflow with high Ni and negligible amount of Au in the ternary IMCs at the interface. The shear strength during aging increases up to 4 days and than decreases with the increase of aging time up to 12 days. It is due to the formation of quaternary compounds with relatively higher amount of Au Elsevier B.V. All rights reserved. Keywords: Sn0.7Cu solders; Ni UBM; Extended time of reflow and aging; Interfacial reactions; Shear strength 1. Introduction Soldering is the most important method for joining mechanical components in electronics. Since solder joints are often subjected to mechanical loadings during handling and system use [1], mechanical properties of the solder joints, such as the fatigue and shear strengths and the creep resistance, are the critical issues for the solder joint reliability as well as the integrity of electronic packaging [2,3]. One of the most influential factors in the solder joint quality of a ball grid array (BGA) component is the metal surface finish on the pads. The most common surface on BGA is electrolytic Ni/Au plated over the copper pad of the flexible substrate. Interaction and interdiffusion behavior between the solder and Cu has been studied elsewhere. It is found that at Corresponding author. Tel.: ; fax: address: eeycchan@cityu.edu.hk (Y.C. Chan). the Sn-containing solder/cu interface, tin reacts rapidly with Cu to form Cu Sn intermetallic compound, which weaken the solder joints due to the brittle nature of the IMC. The strength of the solder joint decreases an increasing thickness of IMC that form at the interface and act as initiation sites for microcracks [3 5]. Electrolytic Ni layer on Cu pads creates good solderable surface and also acts as a good diffusion barrier layer. Many studies have reported that the growth rate of intermetallic compounds is lower in the Ni/solder system than in the Cu/solder system [6 8]. The reaction rate of molten eutectic SnPb on Ni is about a 100 times slower than that of molten eutectic SnPb on Cu [9]. The Sn0.7Cu solder is one of the promising lead-free alloys. The solder has the longest thermal fatigue life among all the lead-free solder/ubm interconnect structures [10]. The eutectic Sn Cu solder is the most promising lead-free replacement for the Sn Pb solder in wave soldering applications [11]. The cost of this solder is lower than the Sn Ag and /$ see front matter 2004 Elsevier B.V. All rights reserved. doi: /j.mseb

2 M.N. Islam, Y.C. Chan / Materials Science and Engineering B 117 (2005) Sn Ag Cu solder but melting temperature is slightly higher [12]. The metallurgical behavior of Sn0.7Cu solder joints with electrolytic Ni and the related joint reliabilities have not been sufficiently studied as yet. A detailed study to correlate the microstructures and mechanical properties of a solder joint with compositional changes, as a function of reflow and aging time is needed. Therefore, the present study was carried out to investigate the interfacial reaction kinetics with electrolytic Ni metallization during prolonged reflow and aging for Sn0.7Cu solder. 2. Experimental procedures The solder joints were made by soldering between twometal layer flexible substrates with Cu/Ni/Au coated pads and Sn Cu solder. The flexible substrate had ball pads of a circular area with a diameter of 650 m. The composition of the solder ball was Sn0.7Cu (wt.%). The sizes of the solder balls were mm. The flux used in this work was CLEANLINE TM LR721H2 BGW no-clean flux. The solder balls were placed on the ball pad of flexible substrate as shown in Fig. 1 and then reflowed at 250 C in a five-zone N 2 reflow oven (BTU VIP-70N). After the as-reflowed, some samples were kept again in the same temperature as mentioned above for 5, 10, 30, 120, and 180 min for long time reflow and some samples were subjected to aging at 175 C for 2, 4, 8, 12, and 16 days. To investigate the microstructure, the samples were mounted in epoxy after each reflow. The samples were ground and polished very carefully and then gold-coated. The chemical and microstructural analyses of the gold-coated crosssectioned samples were obtained using a Philips XL 40 FEG scanning electron microscope (SEM). For the compositional analysis, energy dispersive X-ray analysis (EDAX International, 91McKee Drive, Mahwah NJ 07430, USA) was performed in the SEM. Shear tests were performed on reflowed and aged samples by using a Dage Series 4000 Bond Tester. Shear tool height of 100 m and shear speed of ms 1 were used. A total of 20 solder ball joints were sheared for each condition. The fracture surfaces after ball shear test were investigated thoroughly by SEM in the secondary electron mode as well as by EDX and optical microscope. 3. Results and discussions 3.1. Mechanical strength Fig. 2 depicts the solder ball shear test results of electrolytic Ni/Au plating/solder joints. At the time of ball shear test mostly ductile fracture occurs within the solder and solder IMC interface for the reflowed samples but for aging mainly ductile fracture is observed for the samples aged up to 4 days (Fig. 3a) and for the long time aging ductile brittle fracture occurs within the IMCs and/or the solder IMCs interface, as shown in Fig. 3b. The initial average shear strength of solder joints is around 1.89 kgf. Fig. 2 shows that solder ball shear load during aging increases with an increase of aging time up to 4 days and then turn to decrease up to 12 days. The solder joints have not shown such gradual decreasing trends of shear strength during long time reflow. However, the main differences between long time aging and reflow of the solder joints are that the maximum average shear strength is found during re- Fig. 1. Solder ball attachment on two-metal layer flexible substrate. Fig. 2. Shear strength of the solder joints: (a) during reflow at 250 C; (b) during aging at 175 C.

3 248 M.N. Islam, Y.C. Chan / Materials Science and Engineering B 117 (2005) the interfaces. These IMCs together with unreacted Ni provides the adhesion between the solder and the substrate. The thickness of the intermetallic compound layer depends on a number of factors, such as temperature/time, volume of solder, property of solder alloy and morphology of the deposit [13]. Fig. 3. Fracture surfaces of the solder joints after: (a) 4 days aging; (b) 12 days aging at 175 C. flow after 180 min (1.98 kgf) and during aging after 4 days (2.07 kgf). Solder joint shows a minimum value of average shear strength (1.86 kgf) after 120 min reflow, an 6% reduction from the highest strength, whereas during aging it is found after 12 days (1.45 kgf), an 30% reduction from the highest strength. The solder gives relatively stable ball shear strength from 1.98 to 1.86 kgf over the whole duration of reflow but during aging shear strength turn to decrease after 4 days. These results demonstrate that electrolytic Ni/Sn0.7Cu solder joints have greater solder joint integrity during long time reflow as compared to the aging. The reasons for such different trends of mechanical strength for solder joints could be explained by in-depth study of the interfaces, as detailed below Interface after as-reflowed Dissolved Au forms a binary Au Sn intermetallic phase within the solder ball and appears as needle or platelet like in the solder, as shown in Fig. 4a. A little bit of Pb (<0.02 wt.%) was found in the Sn0.7Cu solder as impurities and has appeared as white region in the structure. Some Pb-rich phase has been entrapped into the IMC due to the rapid reaction and formation of IMC at the interface, as shown in Fig. 4b. This commercially available Sn Cu solder ball might not be produced with necessary measures to make it 100% lead-free. To produce lead-free Sn base solder ball, Sn should be treated with electrolytic refining before alloying with other elements. The Au Sn phase is closely associated with Pb-rich phase in the solder. Cu Sn intermetallics are also found at different locations in the bulk solder, as shown in Fig. 4a. The thickness of intermetallics is m. According to EDX analysis, Sn Ni Cu IMC form on the electrolytic Ni layer after the solder ball bonding process, Fig. 4b, similar result has reported by other investigators [11,14,15]. Sn content is found more in the solder-side IMC than the substrate-side IMC and Ni decreases from the substrate-side to the solderside IMC. The interface roughness is very high, which seems to be related to fast kinetics of the formation and growth of the IMC layer at the beginning of the solder reaction. The initial shear strength of the solder joints is good Reaction kinetics and cross-sectional studies of the interface To investigate the shear strength and reaction kinetics of the solder joints, detailed cross-sectional studies were carried out by SEM. During reflow, molten solder absorbs the entire Au layer into solution, allowing Sn and Cu from the solder to react with the Ni layer and to form different types of IMCs at Fig. 4. After as-reflowed: (a) bulk solder; (b) interfacial structure of solder joint.

4 M.N. Islam, Y.C. Chan / Materials Science and Engineering B 117 (2005) Fig. 5. After 30 min reflow: (a) bulk solder; (b) interfacial structure of solder joint. Fig. 6. Different type of hexagonal IMCs, after 120 min reflow: (a) bulk solder; (b) interfacial structure of solder joint Interface after long time molten reaction A reflow time for industrial applications above 10 min may not 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 is than helpful for prediction of life and for designing new components. After 10 min reaction in molten condition, it is seen that the IMCs thicknesses of samples grow gradually with time. The shear strength of the solder joints has increased slightly. The little increase of shear load of the solder joints may be related to the homogenization of solder alloy with the time of reflow. After 30 min reaction in molten condition, it is seen that IMCs thickness of solder joint increases with molten time whereas original Ni layer thickness decreases. The IMCs thickness is m. According to the EDX analysis, the IMCs are composed of Sn Ni Cu (Table 1). The atomic percentage of Ni gradually increases in the solder-side IMC with reflow time. Au has higher affinity to form more stable compounds within the solder and interfaces. Au Sn, Cu Sn Au and more stable Sn Cu Ni Au compounds are found within the solder ball, so significant amount of Au cannot diffuse back to the interface (Fig. 5a). It is seen that some Ni atoms have diffused through the IMCs and form quaternary compounds in the solder, similar result has reported by other investigators [16]. Small amount of Au has diffused from the solder to the interface and reacts with IMC to form quaternary Sn Ni Cu Au compounds on the top of the ternary Sn Ni Cu IMCs (Fig. 5b), similar result has reported by other investigators [16,17]. These quaternary compounds are also formed in several places on the IMCs. Quater- nary compounds do not form during initial stage of soldering since the high solubility of Au in the molten solder causes the Au to dissolve, separating it from Ni. The shear strength of solder joints has decreased slightly. It is due to the formation of some quaternary Sn Cu Ni Au compounds at the interface which creates weak interface with the solder [17]. Different forms of ternary IMCs are found in the interface due to long time (120 min) molten reactions. Some Sn Cu Ni IMCs grows as hexagonal shape and hexagonal rods like shape that protrude into the molten solder. Pb-rich phase with solder are found in the center of hexagonal ternary IMC, (Fig. 6a). Au reacts with the ternary IMC from all side and form quaternary Sn Cu Ni Au compounds. From the result of EDX, the brighter region in the IMC is a quaternary phase composed of Sn 0.44 Cu 0.32 Ni 0.16 Au 0.08 (Fig. 6b). Quaternary compounds and Au Sn compounds are also found within the solder. The slightly brighter IMCs have formed on the top of the electrolytic Ni layer. The bright IMCs are composed of mainly Ni and Sn i.e. Ni Sn IMC, but there is also small amount of Cu present. This slightly bright low-cu containing IMCs layer is clearly identifiable from the Sn Cu Ni IMCs (Fig. 6b). Due to long time molten reaction, this bright IMCs grow beneath the Sn Cu Ni IMCs layer because most of the Cu have been consumed and incorporated in the Sn Cu Ni IMCs and the only metal left to react with Ni is Sn on the top of electrolytic Ni layer. At this stage the average shear strength of the solder joints has decreased slightly (Fig. 2a). After 180 min molten reaction of solder, it is seen that the quaternary Sn Cu Ni Au compounds are found in the solder ball and in several places on the solder-side ternary IMCs. The maximum percentage of Au is about 11 at.% and Ni is around 19 at.% in the quaternary compounds. Au Sn

5 250 M.N. Islam, Y.C. Chan / Materials Science and Engineering B 117 (2005) Table 1 Atomic percentages of Sn, Cu, and Ni in the Sn Cu Ni IMCs (by EDX analysis) of the solder joints at different reflow time Position in the Sn Cu Ni IMCs IMC near Ni layer IMC middle IMC upper (solder-side) As reflow (at.%) 24 27Ni, 32 36Cu, 36 40Sn 17 21Ni, 31 35Cu, 44 49Sn 16 18Ni, 34 38Cu, 43 50Sn After 30 min reflow (at.%) 20 25Ni, 26 34Cu, 38 43Sn 20 23Ni, 30 35Cu, 41 44Sn 17 20Ni, 34 37Cu, 42 45Sn After 120 min reflow (at.%) Ni Sn IMCs with low-cu (on Ni layer). On the top of it: 23 26Ni, 21 27Cu, 41 49Sn 21 24Ni, 31 35Cu, 42 45Sn 18 23Ni, 33 35Cu, 42 46Sn, negligible amount of Au After 180 min reflow (at.%) Ni Sn IMCs with low-cu (on Ni layer). On the top of it: 22 27Ni, 18 26Cu, 39 49Sn 19 25Ni, 32 35Cu, 38 44Sn 19 24Ni, 30 34Cu, 43 46Sn, negligible amount of Au compounds also still exist in the solder. Some ternary IMCs protrude into the solder from top of IMC and Au from solder has attacked around the IMCs to form Sn Cu Ni Au compounds. It is interesting that this attack is from both inside and out side of the IMCs. This protruded IMCs structure may not be so dense and may have slight variation in Ni content, so Au has preferentially diffused into the IMCs. The Au-containing quaternary compounds are brighter and easily distinguishable from the darker ternary IMCs (Fig. 7a). Some IMCs in the bulk solder are converted more or less to round shape from rod shape. The interfacial energy between these IMCs and the liquid solder may be high. For that reason, these IMCs become round to possess less free energy thermodynamically. The thickness of IMCs increases with an increase of reflow time. But the shear strength of the solder joints does not change significantly with reflow time. It is concluded that shear strength does not depend on the thickness of IMCs during reflow. Different types of ternary Cu Sn Ni IMCs have formed on the top of Ni layer. The atomic percentage of ternary IMCs in different positions (Fig. 7b) are given Fig. 7. After 180 min reflow: (a) broken IMCs and quaternary compounds within the solder; (b) interfacial structure of solder joint. in Table 1. The atomic percentage of Ni in the top of solderside ternary IMC increases with an increase of reflow time. The percentage of Cu towards the substrate-side decreases significantly with an increase of reflow time. There is no appreciable difference of Sn content in the through thickness of the IMCs for longer time reflow. Park [18] also has proven that Sn can easily diffuse through the Sn Ni Cu IMCs. Negligible amount of Au is found in the upper solder-side IMCs. Au is not found in the middle ternary IMCs and low-cu containing IMCs. It may be stated that Au cannot diffuse into the high Ni-containing IMCs. The Ni layer after 180 min reflow is still intact (about 2.93 m). The average consumption rate of Ni layer is m/min Interface after long time aging The IMCs thickness gradually increases with an increase of aging time, as shown in Fig. 8b. After 4 days aging Au Sn and Cu Sn compounds with small amount of Au are found within the solder (Fig. 9a). The intermetallics have changed from a rough morphology to a slightly planar type. Two types of IMCs are easily identifiable in the backscattered electron micrograph as shown in Fig. 9b. The main differences of these IMCs are that the slightly darker IMCs contain more Ni and low Au than the slightly brighter IMCs (Table 2). It is due to the preferential penetration of Au into the low Nicontaining ternary Sn Ni Cu IMCs from the solder. During ball shear test ductile fracture is observed mainly within the solder (Fig. 3a). In this stage the highest shear strength is found, it may be due to the removal of residual stress and strength hardening effects of the solder alloys. During long time aging, the shear strength of solder joints turn to decrease gradually after 4 12 days of aging. According to the EDX analysis, Au Sn, Cu Sn compounds with small amount of Au and Ni are found within the solder (Fig. 10a). It is seen that Ni atoms have diffused into the solder through the IMCs, similar result has reported by other investigators [19]. The percentage of Au in the solder-side IMCs increases with an increase of aging time and become 3 4 at.% and make weaker interface, thus slightly brittle failure occur within the IMCs and/or solder IMCs interface (Fig. 3b). The minimum shear strength is found after 12 days aging; it may be due to the increases of Au in the ternary IMCs and also may be due to the increases of IMCs thickness. Au-containing compound seems to be more brittle and creates weak inter-

6 Fig. 9. After 4 days aging at 175 C: (a) bulk solder; (b) interfacial structure of solder joint. Fig. 8. IMCs thickness: (a) during reflow; (b) during aging of the solder joints. Table 2 Atomic percentages of Sn, Cu and Ni in the Sn Cu Ni IMCs (by EDX analysis) of the solder joints at different aging time Position in the Sn Cu Ni IMCs As reflow (at.%) 4 days aging (at.%) 8 days aging (at.%) 12 days aging (at.%) 16 days aging (at.%) IMC near Ni layer IMC middle IMC upper (solder-side) 24 27Ni, 32 36Cu, 36 40Sn 17 21Ni, 31 35Cu, 44 49Sn 16 18Ni, 34 38Cu, 43 50Sn 25 28Ni, 30 32Cu, 38 41Sn, negligible amount of Au Dark IMC, 14 18Ni, 34 38Cu, 43 46Sn, 1 2Au Dark IMC, 13 17Ni, 35 38Cu, 44 46Sn, 1 2Au Bright IMC, 8 11Ni, 38 40Cu, 43 48Sn3 4Au Bright IMC, 7 9Ni, 30 36Cu, 49 60Sn, 3 4Au 24 28Ni, 19 27Cu, 43 48Sn, 0 1Au 22 25Ni, 28 30Cu, 43 46Sn, 1 2Au 18 20Ni, 30 33Cu, 42 45Sn, 3 4Au Ni Sn IMC with low-cu. On the top of it: 25 30Ni, 19 24Cu, 42 46Sn, 0 1Au 22 24Ni, 25 29Cu, 43 45Sn, 1 3Au 15 19Ni, 31 35Cu, 43 45Sn, 3 4Au Ni Sn IMC with low-cu. On the top of it: 23 28Ni, 18 23Cu, 42 49Sn, 0 1Au 22 24Ni, 28 30Cu, 41 46Sn, 1 3Au 15 18Ni, 33 36Cu, 42 44Sn, 3 5Au M.N. Islam, Y.C. Chan / Materials Science and Engineering B 117 (2005)

7 252 M.N. Islam, Y.C. Chan / Materials Science and Engineering B 117 (2005) Fig. 10. After 12 days aging at 175 C: (a) bulk solder; (b) interfacial structure of solder joint. face with the solder, similar results are also reported by other investigators [17]. A very thin layer of Ni Sn IMC with small amount of Cu has formed on the top of electrolytic Ni layer, which is slightly brighter in the backscattered electron micrograph, (Fig. 10b). Fig. 11 shows backscattered electron micrograph of sample after 16 days aging. Slightly brighter Sn Cu Ni Au quaternary compounds are found within the solder and in some places on the top of solder-side IMCs. The Fig. 11. After 16 days aging at 175 C: (a) bulk solder; (b) interfacial structure of solder joint. atomic percentages of different types of compounds are Sn 0.37 Cu 0.35 Ni 0.16 Au 0.12,Sn 0.36 Cu 0.39 Ni 0.18 Au 0.07, and Sn 0.34 Cu 0.42 Ni 0.21 Au 0.03 (Fig. 11a). It is found that if the percentage of Ni is higher, the percentage of Au is lower in the compounds. Au Sn compounds still exist within the solder. The thickness of IMCs is about m. The intermetallics have changed from a rough morphology to a slightly planar type and in some places IMCs are still quite rough. The interfacial intermetallics are well adhered to the electrolytic Ni layer. The Pb-rich phases in the bulk solder are converted more or less to round shape. The interfacial energy between the solder and the Pb-rich phase may be high. For that reason Pb-rich phase becomes round to possess less free energy thermodynamically. The electrolytic Ni layer of about 2.18 m was still intact in the interface. Different types of ternary IMCs with small amount of Au have formed in the interface (Fig. 11b). The atomic percentage of ternary IMCs are given in Table 2. The atomic percentages of Au and Cu decrease from the solder-side to the substrate-side IMCs. The percentage of Ni decreases towards the solder-side IMCs. The percentage of Sn in the Sn Ni Cu IMCs is almost the same. The maximum 3 5 at.% Au is found in the solder-side ternary IMCs. It may be due to the lower percentage of Ni within the ternary IMCs, so Au has diffused into the IMCs during long time aging. No significant amount of Au is found within the low-cu containing Ni Sn IMCs, similar results are also reported by other investigators [20]. It is due to higher percentage of Ni that is present in the IMCs. At this stage Ni Sn IMCs with small amount of Cu are easily identifiable from the ternary Sn Ni Cu IMCs (Fig. 11b) Comparison of reflow and aging The interfacial intermetallics adhered better to the electrolytic Ni layer during solid-state aging than during the reflow. More than 2 m of the electrolytic Ni layer was still intact in both cases during the observed time period. The growth rate of IMCs thickness during long time reflow is more than 0.05 m/min and during aging is m/min. The intermetallics have changed from rough morphology to slightly planar morphology during aging. Ni Sn IMCs with small amount of Cu has formed due to the lower supply of Cu from the solder, after 120 min reflow and for aging it is found after 12 days. No significant amount of Au is found within these IMCs. No significant change is observed in the percentage of Sn in the interfacial IMCs throughout reflow and aging. The percentage of Ni in the solder-side IMCs is higher in the reflow than the solid-state aging. It may be due to the increases of diffusion of Ni at high temperature reflow. No significant amount of Au is found in the reflow IMCs. But 3 5% Au is found within the aged low Ni-containing IMCs during aging. During reflow solder joints give relatively stable ball shear strength than aging. It may be due to the presence of negligible amount of Au within the ternary IMCs after reflow. Fracture occurs within the solder and/or the solder IMCs interface for

8 M.N. Islam, Y.C. Chan / Materials Science and Engineering B 117 (2005) reflow and for long time aging within the IMCs and/or the solder IMCs interface. It may be concluded that high Ni and negligible Au-containing ternary IMCs give more stable ball shear strength. After 30 min reflow and after 12 days aging, different types of quaternary compounds are found for both cases within the solder. As Au has been consumed by the Cu-containing compounds within the solder, Au cannot resettle at the interface. So, it can be stated that Cu prevents the resettlement of Au and helps the formation of more stable quaternary compounds. Au Sn compounds still existed in the solder over the whole duration of aging and reflow. 4. Conclusions The interfacial reactions at the solder joints, formation and growth of the intermetallic compounds and shear strength of the interface are investigated during extended molten reaction and solid-state aging of Sn Cu solder with electrolytic Ni. The resistance of electrolytic Ni layer as a barrier of Sn is lower for reflow than aging. The IMCs at the interface of solder joint have lower growth rate and are more stable and adhere better on the electrolytic Ni layer during aging than reflow. Au Sn and Sn Cu Ni Au compounds are more stable within the solder rather than Cu Sn and Cu Sn Au compounds during long time reflow and aging. Preferential diffusion of Au is observed into the IMCs. Less amount of Au is found in the high Ni-containing ternary and quaternary compounds in both reflow and aging. High Ni and negligible Au-containing IMCs have relatively stable shear strength during extended reflow. Shear strength is not stable during aging due to creation of weak interface between the higher amount of Au and low Ni-containing ternary IMCs and solder. Cu prevents the resettlement of Au and high Ni atoms inhibit the diffusion of Au atoms into the IMCs. So, Cu and Ni play a significant role for interfacial reactions and formation of different type of IMCs which affect the shear strength of solder joint during extended reflow and aging. It can be concluded that 5 m electrolytic Ni layer can protect the Cu layer from the higher Sn content of Sn0.7Cu solder for more than 180 min reflow at 250 C and 16 days aging at about 175 C. Acknowledgements The authors would like to acknowledge the financial support provided by Innovation and Technology Fund (Teaching Company Scheme) Ref. UIT/31 and CityU Ref and Compass Technology Co. Ltd. in Hong Kong. References [1] D.R. Frear, D. Grivas, J.W. Morris, J. Met. 40 (18) (1988) [2] A.C.K. So, Y.C. Chan, IEEE Trans. Comp. Pack. Manuf. Technol.: Part B 19 (3) (1996) [3] Y.C. Chan, A.C.K. So, J.K.L. Lai, Mater. Sci. Eng. B 55 (1998) [4] A.C.K. So, Y.C. Chan, J.K.L. Lai, IEEE Trans. CMPT: Part B 20 (1997) [5] P.L. Tu, Y.C. Chan, K.C. Hung, J.K.L. Lai, Scripta Mater. 44 (2) (2001) [6] P. Kay, C.A. McKay, Trans. Inst. Met. Fin. 57 (1997) 169. [7] S.K. Kang, W.K. Choi, D.Y. Shih, P. Lauro, D.W. Henderson, T. Gosselin, D.N. S Leonard, Electronic Components and Technology Conference, 2002, pp [8] P.G. Kim, J.W. Jang, T.Y. Lee, K.N. Tu, J. Appl. Phys. 86 (12) (1999) [9] K.N. Tu, K. Zeng, Mater. Sci. Eng. R 34 (2001) [10] C. Zhang, J.-K. Lin, L. Li, Electronic Components and Technology Conference, 2001, pp [11] W.T. Chen, C.E. Ho, C.R. Kao, J. Mater. Res. 17 (2) (2002) [12] M. Abtew, G. Selvaduray, Mater. Sci. Eng. 27 (2000) [13] P.G. Harris, et al., Soldering and Surface Mount Technology, No. 30, MCB University Press Ltd., 1998, pp [14] J.K. Lin, et al., Electronic Components and Technology Conference, 2001, pp [15] J.W. Jang, D.R. Frear, T.Y. Lee, K.N. Tu, J. Appl. Phys. 88 (11) (2000) [16] R.K. Kinyanjui, A. Zribi, E.J. Cotts, Electronic Components and Technology Conference, 2002, pp [17] K. Zeng, K.N. Tu, Mater. Sci. Eng.: R Rep. 38 (2) (2002) [18] J.Y. Park, et al., Investigation of interfacial reaction between the Sn Ag eutectic solder and Au/Ni/Cu/Ti thin film metallization, J. Electron. Mater. Warrendale 30 (9) (2001) [19] F. Zhang, C.C. Chum, M. Li, Electronic Components and Technology Conference, 2002, pp [20] C.E. Ho, R. Zheng, G.L. Luo, A.H. Lin, C.R. Kao, J. Electron. Mater. 29 (10) (2000)