Microelectronics Reliability

Microelectronics Reliability 51 (2011) 975 984 Contents lists available at ScienceDirect Microelectronics Reliability journal homepage: www.elsevier.com/locate/microrel Microstructure, thermal analysis and hardness of a Sn Ag Cu 1 wt% nano-tio 2 composite solder on flexible ball grid array substrates Asit Kumar Gain a, Y.C. Chan a,, Winco K.C. Yung b a Department of Electronic Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong b Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong article info abstract Article history: Received 29 August 2010 Received in revised form 9 January 2011 Accepted 10 January 2011 Sn Ag Cu composite solders reinforced with nano-sized, nonreacting, noncoarsening 1 wt% TiO 2 particles were prepared by mechanically dispersing TiO 2 nano-particles into Sn Ag Cu solder powder and the interfacial morphology of the solder and flexible BGA substrates were characterized metallographically. At their interfaces, different types of scallop-shaped intermetallic compound layers such as Cu 6 Sn 5 for a Ag metallized Cu pad and Sn Cu Ni for a Au/Ni and Ni metallized Cu pad, were found in plain Sn Ag Cu solder joints and solder joints containing 1 wt% TiO 2 nano-particles. In addition, the intermetallic compound layer thicknesses increased substantially with the number of reflow cycles. In the solder ball region, Ag 3 Sn, Cu 6 Sn 5 and AuSn 4 IMC particles were found to be uniformly distributed in the b-sn matrix. However, after the addition of TiO 2 nano-particles, Ag 3 Sn, AuSn 4 and Cu 6 Sn 5 IMC particles appeared with a fine microstructure and retarded the growth rate of IMC layers at their interfaces. The Sn Ag Cu solder joints containing 1 wt% TiO 2 nano-particles consistently displayed a higher hardness than that of the plain Sn Ag Cu solder joints as a function of the number of reflow cycles due to the well-controlled fine microstructure and homogeneous distribution of TiO 2 nano-particles which gave a second phase dispersion strengthening mechanism. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Over the past several decades, there has been significant interest in eliminating lead in solders because of its hazardous nature to health and the environment [1 3]. This has prompted a search for lead-free solders and more attention in the research activities in this field. In addition to solders, printed circuit boards (PCBs) surface finishes also have to be lead-free. The flexible ball grid array (BGA) substrates with different surface finished have been widely used in high-volume microelectronic packaging industries last few years. BGA joints provide both the mechanical strength and electrical conductivity, which play a significant role in the connection between electronic component and printed circuit boards [4]. Recently, many Sn-based lead-free solders have been studied such as Sn Ag, Sn Cu, Sn Au, Sn Ag Cu and Sn Zn as replacements for the traditional Sn Pb solder [5 7]. Among the numerous substitutes for Sn Pb solders, Sn Ag Cu lead-free solder has been regarded as a promising candidate to replace the conventional Sn Pb solder due to its modest melting temperature, reasonable solderability, comparable electrical performance and good mechanical properties [8,9]. However, many problems with the ternary Sn Ag Cu lead-free solders still exist such as an excessive Corresponding author. Tel.: +852 2788 7130; fax: +852 2788 7579. E-mail address: eeycchan@cityu.edu.hk (Y.C. Chan). reaction between Sn Ag Cu solder and substrates, the formation of large brittle intermetallic compounds (IMCs) and the short creep-rupture life time in service [10]. In addition, the formation of large primary Sn dendrite reduces the resistance to thermal mechanical fatigue [11]. Moreover, through the years, the advancement of micro-/nanosystem packaging technologies have prompted a rapid increase in the density of solder joints in microelectronic products. Therefore, conventional solder technology can no longer guarantee the solder joint reliability of electronic components due to the higher diffusivity and the softening nature of the solder [12]. To solve this problem, a potentially viable and economically affordable innovative approach to improve the mechanical properties of a conventional solder is to add micro-/nano-sized metallic, intermetallic or ceramic particles to a solder matrix so as to form a composite [13 15]. In the existing literature, composite solders have improved the reliability of solder joints because the reinforcement particles suppress grain boundary sliding, grain growth, IMC formation and redistribute stress uniformly [16]. As reported earlier, lead-free composite solders prepared by the addition of micro/ nano-sized alloying elements i.e. silver, copper, nickel, antimony and bismuth significantly improved the mechanical properties while simultaneously reducing the melting point [17,18]. Tai et al. [19] prepared a 20 vol.% Cu 6 Sn 5 reinforced Sn 3.5Ag composite solder by an in situ method and the composite solder joint 0026-2714/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2011.01.006

976 A.K. Gain et al. / Microelectronics Reliability 51 (2011) 975 984 exhibited a better steady-state creep strain rate, less thermomechanical fatigue damage and a higher shear strength after different numbers of thermo-mechanical fatigue cycles as compared to a plain Sn 3.5Ag solder joint. Li and Gupta [20] noted that a 91.4Sn 4In 4.1Ag 0.5Cu solder alloy reinforced with Al 2 O 3 nanoparticles had significantly enhanced hardness, yield strength and ultimate tensile strength. Recently, Tsao and Chang [21] developed a series of Sn 3.5Ag 0.25Cu composite solders reinforced with different weight percentages (0, 0.25, 0.5 and 1 wt%) of TiO 2 nanoparticles and measured their mechanical properties. Among them, Sn 3.5Ag 0.25Cu composite solders containing 1 wt% TiO 2 nanoparticles indicated significant improvements in the yield strength, microhardness and ultimate tensile strength. Microhardness measurement is a very sensitive technique to detect structure changes of different solders at different temperatures. Usually microhardness testing is a non-destructive testing but it leaves a small pit in the structure. Microhardness testing can be the easiest way to determine the mechanical properties of the different phases of the structure [22]. The rule of thumb is the higher the hardness the higher is the mechanical strength. For the soldering technology it is very important to study the microhardness of the structure because in soldering many soft and hard phases form, which are hard and brittle, and induce some hardness in the structure [23]. However, the result of a literature search revealed that no studies have been reported so far on lead-free Sn Ag Cu solder joints containing TiO 2 nano-particles on flexible BGA substrates with different surface finishes i.e. Ag metallized Cu pad, Au/Ni metallized Cu pad and Ni metallized Cu pad. Much concern has been made in this study on the IMC layer due to interaction between lead-free Sn Ag Cu and Sn Ag Cu 1TiO 2 composite solders with flexible BGA substrates i.e. Ag metallized Cu pad, Au/Ni metallized Cu pad and Ni metallized Cu pad. In addition, microhardness of Sn Ag Cu and Sn Ag Cu 1TiO 2 composite solders on flexible BGA substrates with different surface finishes was measured as a function of the number of reflow cycles. 2. Experimental procedure Composite solders were prepared by mechanically dispersing 1 wt% TiO 2 nano-particles (Inframat Advanced Materials LLC, USA) into Sn 3.0 wt%ag 0.5 wt%cu (AMTECH, USA) solder powder. The mixture was blended manually for at least 30 min to achieve a uniform distribution of TiO 2 nano-particles with a water-soluble flux (Qualitek Singapore (PTE) Ltd.). Then, the paste mixture was printed onto alumina substrates using a stainless steel stencil with a thickness of 0.15 mm and reflowed in a reflow oven (BTU International, Pyramax-100N) at 250 C to prepare approximately 0.76 mm diameter solder balls. These lead-free solder balls with a diameter of 0.76 mm, were placed on the prefluxed BGA substrates with different surface finishes i.e. Ag metallized Cu pad, Au/Ni metallized Cu pad and Ni metallized Cu pad as shown in Fig. 1 and reflowed at 250 C in a convection reflow oven. The melting characteristics of plain Sn Ag Cu solder and the Sn Ag Cu composite solder containing 1 wt% TiO 2 were investigated using a differential scanning calorimeter (DSC Q 10). For the DSC analysis, 10 mg of each solder was placed into an aluminum pan and scanned from 100 C to 270 C at a rate of 10 C min 1 under a nitrogen atmosphere. X-ray diffraction (XRD, Philips PW 3040 X Pert PRO) was used to determine the crystalline phases of the TiO 2 nano-particles and the composite solder containing TiO 2 nano-particles. Fig. 1. SEM micrographs of ball grid array substrates with different surface finishes: (a) Au/Ni metallized Cu pad, (b) Ag metallized Cu pad and (c) Ni metallized Cu pad.

A.K. Gain et al. / Microelectronics Reliability 51 (2011) 975 984 977 Fig. 4. XRD profiles of (a) TiO 2 nano-particles and (b) the Sn Ag Cu 1TiO 2 composite solder alloy. Fig. 2. Bright field TEM (a) and (b) HRTEM micrographs of TiO 2 nano-particles. To find out the formula composition of the IMC particles, the chemical analyses of the EDX spectra were corrected by standard atomic number, absorption, fluorescence (ZAF) software. Before SEM observations, the samples were sputter coated with Au to avoid the effects due to charging. Transmission electron microscopy (TEM, CM 20, Philips) was used for the observation of the TiO 2 nano-particles. The average thickness of the IMC layer was calculated by using the following equation: T =(t 1 + t 2 + t 3 +...+ t n )/n where t 1, t 2, t 3,... t n are the thickness of the IMC layer at various position of the interface. The average IMC layer thickness at ten positions was taken for each sample. The cross-sectioned polished samples were placed in a Vickers hardness tester (FV-700) to measure the microhardness in the solder ball region. The applied load was 0.3 kgf for 10 s. The average hardness at 15 points was taken for each condition. 3. Results and discussion Fig. 3. DSC curves of (a) Sn Ag Cu and (b) Sn Ag Cu 1TiO 2 composite solder alloys. To characterize the microstructures, the reflowed samples were cross-sectioned and mounted in resin, then ground by different grit sized emery papers and polished with a 0.5 lm Al 2 O 3 suspension and etched with 5% dilute HCl. Finally, the interfacial morphology at the solder alloy/bga substrate interface was observed using a scanning electron microscope (SEM, Philips XL 40 FEG) with the back-scattered electron (BSE) imaging mode and an energy dispersive X-ray spectrometer (EDX, International, model No. DX-4) was utilized to determine the chemical composition of the IMCs. The accuracy of the compositional measurements was typically ±5%. Fig. 2 shows bright field TEM (a) and high resolution transmission electron microscope (HRTEM) (b) micrographs of TiO 2 nanoparticles. In the TEM image (a), TiO 2 nano-particles appeared with a spherical shape and with an average particle size of about 60 nm in diameter, but some agglomeration was also clearly observed. In the HRTEM micrograph (b), some lattice distortion can be seen in the inner region as marked with circles. However, internal defects such as twins or dislocations were not found in the TiO 2 nanoparticles. Fig. 3 shows the differential scanning calorimetry (DSC) results of: (a) the plain Sn Ag Cu solder and (b) the solder containing 1 wt% TiO 2 nano-particles. The DSC results showed that the melting point of the plain Sn Ag Cu solder and the solder containing TiO 2 nano-particles ranged from 217 C to 217.64 C with only a eutectic peak. There was an elevation in the melting temperature of less than 1 C for the Sn Ag Cu composite solder doped with TiO 2 nano-particles. The plausible explanation for changing the melting point of the composite solders is that the reinforcing TiO 2 nano-particles may change the surface instability and the physical properties of the grain boundary/interfacial characteristics. From DSC analysis, Tsao et al. also reported [24] that the addition of Al 2 O 3 nano-particles did not significantly affect the melting point of Sn 3.5Ag 0.5Cu solder. From this DSC profiles, it was confirmed that it was not necessary to change the existing solder process parameters such as the reflow temperature when applying this Sn Ag Cu composite solder doped with TiO 2 nano-particles.

978 A.K. Gain et al. / Microelectronics Reliability 51 (2011) 975 984 Fig. 4 shows XRD profiles of: (a) TiO 2 nano-particles and (b) the Sn Ag Cu solder doped with 1 wt% TiO 2 nano-particles. From XRD profile (a), it was clear that the TiO 2 nano-particles appeared as a crystalline phase with sharp peaks. In the composite solder alloy in XRD profile (b), Ag 3 Sn, Cu 6 Sn 5 IMCs and the TiO 2 phase were easily detected as well as b-sn phase. It is well known that dissolved Ag and Cu precipitate by forming Ag 3 Sn and Cu 6 Sn 5 IMCs in the solder matrix during solidification. Fig. 5 shows the backscattered scanning electron micrographs of (a and b) plain Sn Ag Cu solder joints and (c and d) solder joints containing 1 wt% TiO 2 nano-particles after one reflow cycle on Ag metallized Cu pad on BGA substrates. At their interfaces, a scallop-shaped Cu 6 Sn 5 IMC layer was clearly observed in both solder joints. However, in the plain Sn Ag Cu solder joint, some rod like-shaped Ag 3 Sn IMCs as marked in Fig. 5a were clearly observed. In general, the top most Ag layer was dissolved completely in the molten solder, leaving the Cu layer exposed to the molten solder. Therefore, the Ag layer disappeared completely at the interface. Such complete consumption of the top Ag layer was also found in other studies on the rapid dissolution of Ag in liquid solder and has been explained based on thermodynamics and kinetics [25]. SEM micrographs (b and d) were taken from solder ball regions. In these solder ball regions, Ag 3 Sn and Cu 6 Sn 5 IMC particles were found in an elongated-shaped b-sn matrix in plain Sn Ag Cu solder joints. However, after the addition of TiO 2 nano-particles, these IMC particles size as well as the b-sn grain size was substantially decreased and appeared with a network type microstructure as shown in Fig. 5d. Fig. 6 shows the backscattered scanning electron micrographs of (a and b) plain Sn Ag Cu solder joints and (c and d) solder joints containing 1 wt% TiO 2 nano-particles after (a and c) eight and (b and d) 16 reflow cycles on Ag metallized Cu pad on BGA substrates. By increasing the number of reflow cycles, the Cu 6 Sn 5 IMC layer thickness was substantially increased as shown in Fig. 5a and c and Fig. 6. In addition, after 16 reflow cycles, the upper part of the IMC shows a loose flake-shaped layer and the bottom side is a very thin, about 1 lm in thickness, Cu 3 Sn IMC layer and found with dark contrast as marked in Fig. 6b and d. Fig. 7 shows the backscattered scanning electron micrographs of (a, c and e) plain Sn Ag Cu solder joints and (b, d, and f) solder joints containing 1 wt% TiO 2 nano-particles after (a and b) one, (c and d) eight and (e and f) 16 reflow cycles at 250 C which were taken at the interface between the solder and Au/Ni metallized Cu pad on BGA substrates. At their interfaces, a ternary scallopshaped Sn Ni Cu IMC layer was clearly observed with a dark contrast in both the plain and composite solder joints. Yoon et al. [26] noted that there is a small atomic size difference between Cu and Ni, and since both have the same FCC lattice structure, the substitution of Ni into Cu 6 Sn 5, without causing a lattice distortion or the formation of a new phase, was reasonable. It can be seen that the outer very thin Au layer in the Au/Ni metallized Cu pad on BGA dissolved into the molten solder. Therefore, no Au-containing IMC or Au layer was found at the interface. In addition, by increasing the number of reflow cycles the scallop-shaped Sn Ni Cu IMC layer thickness was substantially increased in both types of solder joints. However, after the addition of TiO 2 nano-particles, the scallopshaped Sn Ni Cu IMC layer growth rate was relatively lower than that in plain Sn Ag Cu solder joints. The reason may be that the second phase TiO 2 nano-particles change the driving force and diffusivity of the IMC layer growth. Li et al. [27] reported that rare earth elements reduce the rate of IMC layer growth in two ways i.e. by altering the diffusion coefficient and the thermodynamic Fig. 5. SEM micrographs of (a and b) Sn Ag Cu and (c and d) Sn Ag Cu 1TiO 2 solder joints on Ag metallized Cu pads after one reflow cycle.

A.K. Gain et al. / Microelectronics Reliability 51 (2011) 975 984 979 Fig. 6. SEM micrographs of (a and b) Sn Ag Cu and (c and d) Sn Ag Cu 1TiO 2 solder joints on Ag metallized Cu pads after (a and c) eight and (b and d) 16 reflow cycles. parameters of elemental affinity. The Sn Ni Cu IMC layer thicknesses of plain Sn Ag Cu solder joints after one and 16 reflow cycles were about 2.5 and 6.8 lm, while the thicknesses of this layer in the composite solder were about 1.9 and 5.6 lm, respectively. From the interfacial microstructure, it is clear that the TiO 2 nanoparticles contained in solder joints inhibited the formation of the IMC layers. Fig. 8 shows the backscattered scanning electron micrographs of (a and b) plain Sn Ag Cu solder joints and (c and d) solder joints containing 1 wt% TiO 2 nano-particles after (a and c) one and (b and d) eight reflow cycles on Au/Ni metallized Cu pads on BGA substrates which were taken from the solder ball region. After one reflow cycle, in the solder ball region needle-shaped Ag 3 Sn, spherical-shaped Cu 6 Sn 5 and AuSn 4 IMC particles were clearly observed in a b-sn matrix. The formation of the AuSn 4 IMC particles in the solder ball region is due to the dissolution of the top thin Au layer from the Au/Ni metallized Cu pads on the BGA substrates. The AuSn 4 IMC particles was found only as scattered particles in the solder ball region and no AuSn 4 layer segregation next to the intermetallic layers was observed. However, after the addition of TiO 2 nano-particles, needle-shaped Ag 3 Sn, spherical-shaped Cu 6 Sn 5 and AuSn 4 IMC particles as well as the b-sn grain size was substantially decreased and appeared with a fine microstructure as shown in Fig. 8c. However, by increasing the number of reflow cycles, the needle-shaped Ag 3 Sn, spherical-shaped Cu 6 Sn 5 and AuSn 4 IMC particles as well as the b-sn grain size were significantly increased in plain Sn Ag Cu solder joints as compared with composite solder joints as shown in Fig. 8b and d. The elongatedshaped b-sn grain size after one reflow cycle of a plain Sn Ag Cu solder joint was about 1015 lm while the grain size in a Sn Ag Cu 1TiO 2 solder joint was about 710 lm. From this result, it is also clear that the second phase TiO 2 nano-particles retarded the growth rate of IMCs. The reason may be that the second phase reinforcement TiO 2 nano-particles promote a high nucleation density during solidification. Usually, the plane with maximum surface tension grows fastest, while its adsorption amount of surfaceactive material is maximized. However, as increasing in adsorption elements decreases its surface energy and therefore decreases the growth velocity of this plane. The addition of TiO 2 nano-particles may be adsorbed at the grain boundary and changes relative relationship of the growth velocities between crystalline directions of the IMC particles, which reduces the IMC particles size. Fig. 9 shows the backscattered scanning electron micrographs of (a and b) plain Sn Ag Cu solder joints and (c and d) solder joints containing 1 wt% TiO 2 nano-particles after (a and c) one and (b and d) 16 reflow cycles at 250 C which were taken at the interface between the solder and the Ni metallized Cu pad on BGA substrates. At their interfaces, a scallop-shaped ternary Sn Ni Cu IMC layer was found in both solder joints. From the EDX profile, the Sn, Ni and Cu atom percentages in these phases were detected to be around 48.9, 27.4 and 23.7, respectively. In addition, some needle-shaped Cu 6 Sn 5 IMC particles were clearly observed at the top surface of the Sn Ni Cu IMC layer as marked in Fig. 9a and c. By increasing the number of reflow cycles, the IMC layer thickness was substantially increased in both solder joints. However, in the composite solder joints, the Sn Ni Cu IMC layer growth rate was relatively lower than that in the plain Sn Ag Cu solder joints. Fig. 10 shows the backscattered scanning electron micrographs of (a and b) plain Sn Ag Cu solder joints and (c and d) solder joints containing 1 wt% TiO 2 nano-particles after (a and c) one and (b and

980 A.K. Gain et al. / Microelectronics Reliability 51 (2011) 975 984 Fig. 7. SEM micrographs of (a, c, and e) Sn Ag Cu and (b, d, and f) Sn Ag Cu 1TiO 2 solder joints on Au/Ni metallized Cu pads after (a and b) one, (c and d) eight and (e and f) 16 reflow cycles. d) 16 reflow cycles on Ni metallized Cu pad on BGA substrates which were taken from the solder ball region. In a plain Sn Ag Cu solder joint after one reflow cycle, needle-shaped Ag 3 Sn, spherical-shaped Cu 6 Sn 5 IMC particles and elongated-shaped b Sn grains were clearly observed. The elongated-shaped b Sn grain size was about 12 15 lm in diameter. However, after the addition of TiO 2 nano-particles, needle-shaped Ag 3 Sn, spherical-shaped Cu 6 Sn 5 IMC particles and the elongated-shaped b Sn grain size was substantially decreased and appeared with a fine microstructure. In addition, by increasing the number of reflow cycles from one to 16 reflow cycles, the elongated-shaped b-sn grains, needle-shaped Ag 3 Sn and spherical-shaped Cu 6 Sn 5 IMC particles were dramatically increased in plain Sn Ag Cu solder joints (Fig. 10b). On the other hand, the growth behavior of the elongated-shaped b-sn grains and these IMC particles in composite solder joints was lower than that in the plain Sn Ag Cu solder joints. The average elongated-shaped b-sn grain sizes after one and 16 reflow cycles in composite solder joints were about 7 and 10 lm in diameter, respectively. Anderson et al. [28] reported that the formation of large primary dendrite-shaped b-sn grains in a Sn Ag Cu solder reduced the resistance to thermal mechanical fatigue. From this result, it is clear that the solder joint containing TiO 2 nano-particles retarded the growth behavior of the elongatedshaped b-sn grains. The measurement of hardness, especially microhardness, is the usual method to characterize the mechanical properties of solid-state interfaces. The hardness of a material is often equated with its resistance to abrasion or wear and is a characteristic of

A.K. Gain et al. / Microelectronics Reliability 51 (2011) 975 984 981 Fig. 8. SEM micrographs of (a and b) Sn Ag Cu and (c and d) Sn Ag Cu 1TiO 2 solder joints on Au/Ni metallized Cu pads after (a and c) one, (b and d) eight reflow cycles. Fig. 9. SEM micrographs of (a and b) Sn Ag Cu and (c and d) Sn Ag Cu 1TiO 2 solder joints on Ni/Cu pads after (a and c) one and (b and d) 16 reflow cycles.

982 A.K. Gain et al. / Microelectronics Reliability 51 (2011) 975 984 Fig. 10. SEM micrographs of (a and b) Sn Ag Cu and (c and d) Sn Ag Cu 1TiO 2 solder joints on Ni/Cu pads after (a and c) one and (b and d) 16 reflow cycles. Fig. 11. Hardness of Sn Ag Cu and Sn Ag Cu 1TiO 2 solder joints as a function of the number of reflow cycles on BGA substrates with different surface finishes: (a) Ag metallized Cu, (b) Au/Ni metallized Cu and (c) Ni metallized Cu pad.

A.K. Gain et al. / Microelectronics Reliability 51 (2011) 975 984 983 practical interest since it determines the durability of a material during use and it also decides the suitability of the material for particular applications. Microhardness tests have been performed to determine the hardness of total grains, phases and structural components of alloys. The microhardness of a solder alloy depends on the motion of dislocations and growth and configuration of grains. So the mechanical properties such as the microhardness depend particularly on the microstructure, processing temperature, and the composition. Fig. 11 shows microhardness values of the plain Sn Ag Cu solder joints and solder joints containing 1 wt% TiO 2 nano-particles as a function of the number of reflow cycles between the molten solders and BGA substrates with different surface finishes i.e. Ag metallized Cu pad (a), Au/Ni metallized Cu pad (b) and Ni metallized Cu pad (c). The hardness of solder joints containing TiO 2 nano-particles displayed consistently higher values than that of plain solder joints due to the homogeneous distribution of TiO 2 nanoparticles and refinement of the IMC particles i.e. Ag 3 Sn, Cu 5 Sn 6 and AuSn 4 which can act as reinforcement. According to the dispersion strengthening mechanism, the fine IMC particles and TiO 2 nano-particles can improve the mechanical properties of a solder joint. This can be attributed to: (1) pinning grain boundaries, (2) obstacles to the movement of dislocations and increased dislocation densities and (3) a strengthening mechanism of the matrix by finely dispersed IMC particles and TiO 2 nano-particles [29]. Among the three BGA substrates with different surface finishes, the solder joints on Ag metallized Cu pad BGA substrates exhibited a higher hardness as compared with other BGA substrate solder joints due to a well controlled the microstructure (Fig. 5b and d) and a change in the composition i.e. dissolution of Ag layer. The hardness values of the plain Sn Ag Cu solder joints on Ag metallized Cu pad, Au/Ni metallized Cu pad and Ni metallized Cu pad after one reflow cycles were about 17.3, 16.1 and 15.5 Hv, respectively and their hardness values in composite solder joints were about 19.3, 17.4 and 17.1 Hv, respectively. On the whole, the hardness of composite solder joints changed very little as a function of the number of reflow cycles, perhaps because of the uniform distribution of TiO 2 nano-particles and the well-controlled fine microstructure. 4. Conclusions The solid-state interfacial reactions between plain Sn Ag Cu solder joints and Sn Ag Cu solder joints containing TiO 2 nano-particles with flexible BGA substrates with different surface finishes i.e. Ag metallized Cu pad, Au/Ni metallized Cu pad and Ni metallized Cu pad were investigated as a function of the number of reflow cycles. In Ni metallized Cu pad and Au/Ni metalized Cu pad BGA substrates after the reflow process, a Sn Ni Cu IMC layer was formed at their interfaces and the IMC layer thickness was substantially increased with an increase in the number of reflow cycles. On the other hand, in the Ag metallized Cu pad on BGA substrate, the topmost Ag layer dissolved very quickly into the molten solder and at the interface Cu 6 Sn 5 and Cu 3 Sn IMC layers were observed. In addition in the solder ball region, needle-shaped Ag 3 Sn, spherical-shaped Cu 6 Sn 5 and AuSn 4 IMC particles were found to be uniformly distributed in the b-sn matrix. However, after the addition of TiO 2 nano-particles, Ag 3 Sn, AuSn 4 and Cu 6 Sn 5 IMC particles as well as the b-sn grains appeared with a finer microstructure. The solder joints containing TiO 2 nano-particles showed a consistently higher hardness than that of plain Sn Ag Cu solder joints at all numbers of reflow cycles. It is reasonable to suggest that the hardness of the bulk solder was enhanced by the addition of TiO 2 nano-particles due to the homogeneously dispersion of TiO 2 nano-particles and well controlled finer IMC particles. Acknowledgements The authors acknowledge the financial support provided by City University of Hong Kong for the project 9041222 CERG grant of Hong Kong Research Grants Council and RGC Ref. No. 111307 (Development of a nano-activator doped surface modifier for Sn Zn based lead-free soldering). Professor Brian Ralph is thanked for proof reading the manuscript. References [1] Kim DG, Jung SB. 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