This paper aims to investigate the characteristics of ultra-fine lead-free solder joints reinforced with TiO2 nanoparticles in an electronic assembly.
This study focused on the microstructure and quality of solder joints. Various percentages of TiO2 nanoparticles were mixed with a lead-free Sn-3.5Ag-0.7Cu solder paste. This new form of nano-reinforced lead-free solder paste was used to assemble a miniature package consisting of an ultra-fine capacitor on a printed circuit board by means of a reflow soldering process. The microstructure and the fillet height were investigated using a focused ion beam, a high-resolution transmission electron microscope system equipped with an energy dispersive X-ray spectrometer (EDS), and a field emission scanning electron microscope coupled with an EDS and X-ray diffraction machine.
The experimental results revealed that the intermetallic compound with the lowest thickness was produced by the nano-reinforced solder with a TiO2 content of 0.05 Wt.%. Increasing the TiO2 content to 0.15 Wt.% led to an improvement in the fillet height. The characteristics of the solder joint fulfilled the reliability requirements of the IPC standards.
This study provides engineers with a profound understanding of the characteristics of ultra-fine nano-reinforced solder joint packages in the microelectronics industry.
The findings are expected to provide proper guidelines and references with regard to the manufacture of miniaturized electronic packages. This study also explored the effects of TiO2 on the microstructure and the fillet height of ultra-fine capacitors.
Che Ani, F., Jalar, A., Saad, A., Khor, C., Ismail, R., Bachok, Z., Abas, M. and Othman, N. (2018), "SAC–xTiO2 nano-reinforced lead-free solder joint characterizations in ultra-fine package assembly", Soldering & Surface Mount Technology, Vol. 30 No. 1, pp. 1-13. https://doi.org/10.1108/SSMT-04-2017-0011Download as .RIS
Emerald Publishing Limited
Copyright © 2018, Emerald Publishing Limited
Miniaturization and diversification are current trends in integrated circuit (IC) packaging. Miniature IC packages and passive components (e.g. capacitors and transistors) require reliable solder joints to sustain the performance and overall reliability of the products. To sustain the reliability of solder joints, multifarious researches have been carried out to improve the material, mechanical and thermal properties of the lead-free solders. Various types of ceramic or metal-based nanoparticles are often added to lead-free solders to reinforce the solder joints during the electronic assembly process (Noor and Singh, 2014). Several lead-free solder materials, such as Sn-3.5Ag-0.7Cu (SAC), SnAg, SnCu and SnZn, are commonly blended with various types of nanoparticles.
The presence of nanoparticles, such as aluminium oxide (Al2O3) (Tsao et al., 2010), zirconia (ZrO3) (Chang et al., 2011) and rhodium (Fouzder et al., 2011), in lead-free solders slightly alter the melting temperature. In addition, NiO (Chellvarajoo and Abdullah, 2016), Fe2NiO4 (Chellvarajoo et al., 2015a, 2015b), diamond (Chellvarajoo et al., 2015a, 2015b) and TiO2 (Tang et al., 2013) nanoparticles are also used to reinforce the microstructure and mechanical properties of lead-free solders. The addition of TiO2 nanoparticles into SAC solder improves the hardness value by up to 34 per cent compared to plain SAC305 (Tang et al., 2014a, 2014b), and slightly improves the tensile properties. Moreover, the average size and spacing of the intermetallic compound (IMC) layer are reduced by the presence of TiO2 nanoparticles in the SAC solder (Tsao, 2011). The addition of TiO2 also suppresses the Cu6Sn5 layer of the solder. The wettability of the solder material is improved by the addition of TiO2 nanoparticles at 0.05-0.1 wt. per cent (Tang et al., 2013). An excessive amount of TiO2 will decrease the beneficial effects of the nanoparticles on the solder materials, and might create unexpected features in the solder. Besides, other factors such as aging temperature, aging time (Tang et al., 2014a, 2014b) and reflowing time (Tang et al., 2015) also influence the thickness of IMC layer. The IMC layer of SAC-TiO2 increases with the increasing of aging temperature and time. However, the short reflowing time does not affect the overall thickness of the IMC layer of SAC-TiO2. Furthermore, the real application of nano-reinforced solder pastes into ultra-fine package assemblies via the reflow soldering process is still limited in the literature. There still remains a wide research gap in investigations into the effects of nanoparticle-reinforced solder pastes on the actual electronic components.
The innovation of this study was on the real application of nano-reinforced solder pastes in the assembly of an ultra-fine package in the industrial reflow soldering process. Thus, a real electronic component (i.e. the capacitor) was considered in this study using a new form TiO2 nano-reinforced lead-free solder paste for the assembly process. Different weighted percentages of the TiO2 nanoparticles were reinforced into a lead-free solder paste (SAC305; 96.5Sn –3.0Ag–0.5Cu) for the assembly of ultra-fine solder joints during a reflow soldering process. The characterization of the ultra-fine solder joints was conducted through microscopic investigations to study the effects of the TiO2 nanoparticles on the microstructure, nanoparticle distribution and fillet height of the solder joints, and the results were also compared with plain lead-free solder paste, SAC305. The correlation between the weighted percentage and the fillet height was also established from the experimental results using a box plot analysis.
2. Materials and methods
2.1 Preparation of the surface-mounted packages using a Nano-reinforced lead-free solder assembly
The ultra-fine package used in this study was a 01005 capacitor (as illustrated in Figure 1). The dimensions of this ultra-fine package were 0.4 × 0.2 mm. Then, TiO2 nanoparticles (Aldrich, ≥ 98 per cent trace metal basis, particle size: <25 nm) were mixed with the 96.5Sn–3.0Ag–0.5Cu solder paste (SAC305; Alpha OM-353) at a nominal percentage of 0.01, 0.05 and 0.15 Wt.% using a mechanical stirrer (Fritsch Planetary Mill PULVERISETTE 5) for approximately 10 min (300 rpm) to achieve homogeneity prior to the assembly. Smaller amounts of TiO2 were dispersed in the solvent onto a lacey carbon-coated copper grid prior to the high-resolution transmission electron microscope (HRTEM) image analysis. The average particle size of the as-received nanoparticles was measured using an FEI Tecnai G2 F20 HRTEM, and the HRTEM micrograph in Figure 2 shows that the TiO2 nanoparticles had an average particle size of 15 nm, which almost tallied with the particle size value specified by the supplier (<25 nm). The average particle size of the as-received 96.5Sn–3.0Ag–0.5Cu was 15-25 μm (Type 5, −500/+635 mesh designation as per ASTM B214). The ultra-fine package was then mounted on a printed circuit board (PCB) with a thickness of 2.0 mm (PCB organic solderability preservative surface finish) using a nano-reinforced lead-free solder (Figure 3). The nano-reinforced lead-free solder paste was printed (DEK Horizon) with a thickness of 0.127 mm using a laser cut stainless steel stencil (nano-coated with 1:1 aperture – 0.2 × 0.2 mm) and a steel squeegee. The ultra-fine package was mounted onto the printed nano-reinforced lead-free solder paste using robotic Fuji NXT equipment (customized nozzle). The soldering process with a lead-free reflow profile was performed under a nitrogen atmosphere in a full convection reflow oven (Vitronics Soltec XPM2). The reflowed PCBs were washed using an aqueous cleaning machine (Electrovert Aquastorm 200).
2.2 Characterization of nano-reinforced lead-free solder joint
An X-ray inspection system (Nikon XT V 160) was used to examine the formation of voids in the ultra-fine solder joint. The presence of TiO2 nanoparticles in the ultra-fine solder joint was investigated using a FEI Tecnai G2 F20 HRTEM system equipped with an energy dispersive X-ray spectrometer (EDS). Because of the ultra-fine solder joint, the typical mechanical technique using a diamond blade had its limitations. Therefore, the HRTEM lamella of the ultra-fine joints was prepared using an advanced technique known as the FEI Helios NanoLab 650 Dual Beam system (Figure 4). In this technique, an ion beam was used to perform the cross-sectioning so as to create an HRTEM lamella sample. The system was incorporated with both a high-resolution electron beam and a finely focused ion beam. The HRTEM lamella was lifted out from the bulk sample and attached to a molybdenum (Mo) grid finger using an in-situ lift out technique by means of an Omni probe needle. It was then thinned down to electron transparency with a thickness of less than 100 nm.
The height of the soldered fillet in the ultra-fine solder joint was inspected to determine its effect on the reliability of the joint in the ultra-fine package. It was quantified using scanning electron microscopy. The minimum fillet height should be above the solder thickness plus 25 per cent termination height (IPC, 2010). Figure 5 illustrates the measurement of the fillet height for the 01005 capacitor after the reflow soldering process.
The relationship between the solder land dimensions, component termination, solder volume and solder paste flux wetting behaviour is given as below (Wassink and Verguld, 1995):
For an ultra-fine package (01005 capacitor), a 0.2-mm PCB solder pad was used, with a stencil width of 0.2 mm and stencil thickness of 0.127 mm, while the volume fraction of solder in the paste was 0.5.
The calculation is described below as:
There were four columns, labelled as i, ii, iii and iv, in the focused area for the fillet height measurements. Each column had ten rows, labelled as a, b, c, d, e, f, g, h, i and j (Figure 6). Each capacitor had two solder fillet heights, left and right. The readings were only taken from the capacitors in rows a and j.
In addition, the assembled ultra-fine packages (01005 capacitors) on the PCB were chosen for the investigation (Figure 3). As noticed, the SAC 305-XTiO2 nano-reinforced lead-free solder was taken into consideration in the investigation into the thickness of the IMC as well as the microstructure. The four reflowed samples, as depicted in Figure 3, were then cut to obtain the cross-sectional view. They were mounted in cold epoxy and ground down to 240, 320, 600, 800, 1000, and 2000-grit sizes by using a silicon carbide paper cooled with flowing water. The surfaces were polished with 1-µm Al2O3 suspension, followed by 0.3 and 0.05 µm of alumina. The polished, cold-mounted samples were placed in an ultrasonic cleaner to remove any existing polishing agent prior to etching the surface in a mixture of 2 per cent HCl, 5 per cent HNO3 and 93 per cent methanol for a few seconds. Subsequently, the IMC thickness and microstructure of the SAC305 nano-reinforced lead-free soldered samples were measured and observed using an ultra-high-resolution field emission scanning electron microscope equipped with an energy-dispersive EDS (FEI Nova NanoSEM 450).
Four cross-sections of the IMC layered samples were measured using field emission scanning electron microscopy (FESEM) with the following parameters: variable pressure mode, extra-high tension of 10 kV, working distance of 5.4 to 5.9 mm, and a concentric backscatter detector (CBS). Figure 7 presents a detailed view of the selected region, where the IMC layer is indicated by the dotted line.
3. Results and discussion
3.1 Solder voids
The presence of voids in the solder joint can affect the reliability of the joint in terms of the industry standard. The voids in the ultra-fine joints were examined. No voids were found in the ultra-fine joints, as depicted in the X-ray images [Figure 8 (a) – (d)]. The requirement for an acceptable level and dimensions of the non-BGA solder voids depends on the conformity of the customer, according to the IPC standards (IPC, 2010). The dependability of a product can decrease by 25 to 50 per cent if the voids in the joints make up more than 50 per cent of the solder joint area, as reported by M. Yunus et al. (2003). The observation indicated that the addition of TiO2 nanoparticles to the lead-free solder paste did not lead to the formation of voids in the solder joint.
3.2 Investigation into TiO2 nanoparticles in ultra-fine solder joint
The characterization and analysis of the ultra-fine solder joint package presented new challenges for the engineer and researcher due to the miniature size of the package. The dimensions of the copper pad and the volume of the solder paste used in the experiment corresponded to the package size. Therefore, the formation of the ultra-fine solder joint was restricted only to the copper pad area. To analyse the ultra-fine package, HRTEM was used to examine the presence of TiO2 nanoparticles in the solder joint at different weighted percentages. The HRTEM micrographs depicted the presence of a single TiO2 nanoparticle on the ultra-fine solder joint [Figure 9(a)-(c)]. This was caused by a small increase in the weight percentage and the small volume of the printed solder paste in the assembly process. The titanium (Ti) element in the EDS results indicated the presence of the nanoparticle in the solder joint. The increase in the TiO2 nanoparticles to 0.05 Wt.% provided a clear view of the distribution of the nanoparticles in the solder joint, as illustrated in Figure 10. The TiO2 nanoparticles were evenly distributed in the ultra-fine solder joint (SAC 305-0.05 Wt.% TiO2 nano-reinforced lead-free solder), as shown in Figure 10(a). This was due to the homogeneous mixing of the SAC 305-0.05 Wt.% of TiO2 nano-reinforced lead-free solder during the preparation phase. Thus, the EDS results confirmed that titanium and oxygen were present in the ultra-fine solder joint [Figures 10(b) and (c)]. An interesting phenomenon was observed when the TiO2 nanoparticles was increased to 0.15 Wt.%, where the HRTEM results revealed an accumulation of TiO2 nanoparticles at the top of the solder fillet, as shown in Figures 11(a) and (b). This phenomenon was attributed to the movement of the molten solder in the constrained copper pad area during the reflow soldering process. Besides, the density of the molten solder (∼7.38 g/cm3) was greater than the density of TiO2 nanoparticles (∼3.9 g/cm3). Thus, the buoyancy of the molten solder contributed to an upward force on the immersed TiO2 nanoparticles. The upward force was able to keep the TiO2 nanoparticles afloat in the molten solder. In addition, the gravity effect caused the capacitor to move downward when the solder melted, thus providing an external force to the movement of the molten solder. Both situations encouraged the accumulation of TiO2 nanoparticles in the solder joint, as was obviously observed in the HRTEM analysis. The mechanism of the solder joint, as shown in Figure 12, clearly explains the accumulation of the nanoparticles (at 0.15 Wt.%) during the formation of the solder joint. Furthermore, this accumulation of nanoparticles was not part of the metallurgical formation and did not form an IMC layer. Besides, the HRTEM analysis also clearly revealed that the TiO2 accumulation did not occur around the lower bulk of the solder, as depicted in Figures 11(c) and (d).
3.3 Inspection of solder fillet height in the ultra-fine solder joint
One of the important requirements in the industry standard after the reflow soldering process is that the solder height of the capacitor must be inspected because an insufficient solder height may lead to failure in the interconnection between the capacitor and the PCB. The formation of an adequate solder height, which fulfils the J-STD-001E-2010 Industry Standard Requirements for Soldered Electrical and Electronic Assemblies may be influenced by several factors such as the wettability and thickness of the printed solder paste. Thus, the analysis of the solder fillet height was crucial for the newly-formed TiO2 nanoparticle-reinforced solder paste. By means of a statistical approach, a box plot analysis (Figure 13) indicated that by combining all the levels, it appeared that Level 4 had the highest mean (0.180250) and the lowest standard deviation (0.023926) compared to Level 2 (0.144244; 0.047307) and Level 3 (0.175513; 0.030296). Level 4 had a higher mean than Level 1 (0.166187) in terms of the fillet height measurements. In other words, Level 4 was the best candidate in terms of the fillet height formation. However, it had a downside in that it had a thicker IMC layer compared to the other levels, which was not feasible. Therefore, a p-value of 0.016 for the one-way ANOVA test showed that there was a significant difference in the fillet heights when different compositions of nanoparticles were used as well as when there were no nanoparticles in the solder paste. Figure 13 shows the average fillet height versus the weighted percentage of TiO2 in the SAC 305 solder. From the experimental results, the SAC solders with 0.05 and 0.15 Wt.% of TiO2 doping yielded the highest fillet height.
3.4 Thickness of intermetallic compound and microstructure
The effect of the TiO2 doping on the thickness of the IMC and the microstructure of the solder joint were investigated. A box plot analysis (Figure 14) combining all the levels showed that Level 3 had the lowest mean (1.9135) and the highest standard deviation (0.654901) compared to the other levels. This was desirable because the thinner the IMC layer, the better the bonding interface (copper pad/solder). The correlation between the average IMC thickness and the weighted percentage of TiO2 was plotted in Figure 15. The average IMC thickness decreased polynomially from 0 to 0.05 Wt.%. However, the IMC thickness increased drastically when applied with 0.15 Wt.%. of TiO2. This phenomenon was investigated and discussed based on an FESEM analysis.
base metal dissolution; and
formation of an intermetallic layer.
During the reflow soldering of the SAC 305-xTiO2 nano-reinforced lead-free solder on the OSP substrate, the dissolved Cu atoms reacted with Sn to form a Cu6Sn5 intermetallic phase as a metallurgical bond at the interface between the solder and copper pad. The reactions of the intermetallic formation are described in three consecutive stages (Laurila et al., 2005), namely, dissolution, chemical reaction and solidification.
The results of the EDS analysis, as shown in Figure 17(e), revealed that the intermetallic phase was Cu6Sn5 (η-phase). Figure 17(b) and (c) shows that a rounded type of Cu6Sn5 intermetallic layer was formed at the SAC 305-xTiO2 nano-reinforced lead-free solder/Cu interface. Thus, with an increase in the proportion of nanoparticles, the morphology of the Cu6Sn5 intermetallic layer changed slightly. As the proportion of nanoparticles increased from 0.01 to 0.15 Wt.%, the morphology of the Cu6Sn5 intermetallic layer was gradually transformed to become scallop-like, as shown by the arrows in Figure 17(d).
The nano-reinforced lead-free solder containing 0.01 and 0.05 Wt.% of TiO2 nanoparticles had almost identical intermetallic layers with a thickness of 2.554 and 1.914 μm (on average), respectively [Figure 17(b) and (c)]. This could be explained by the agglomeration and segregation of the TiO2 nanoparticles in the bulk solder (Liu et al., 2008; Seo et al., 2009b; Seo et al., 2009a) and could also have been due to the van der Waals forces that caused the TiO2 nanoparticles to become entangled with each other, thereby resulting in no further significant increase in the intermetallic layer of Cu6Sn5.
The FESEM micrographs in Figure 17(d) revealed that the IMC layer thickness (5.390 μm on average) increased with increasing amounts of TiO2 nanoparticles in the SAC 305 solder. This caused the accumulation of TiO2 nanoparticles on the upper side (solder fillet) during the soldering process without retarding the growth of the IMC layer at the composite/Cu interface (Figure 18). The TiO2 nanoparticles were deposited on the larger η-Cu6Sn5 IMC grains due to the Gibbs–Thomson effect, which caused the highly soluble nanoparticles to adhere to the IMC grains. The increased amount of TiO2 affected the ductility of the solder and also encouraged the agglomeration and segregation of the TiO2 nanoparticles, which created micropores in the bulk solder. The solder material incorporating 0.15 Wt.% TiO2 caused the TiO2 to accumulate at the upper side during the soldering process. The inhomogeneous dispersion of TiO2 in the solder matrix and the insufficient bonding between the TiO2 and the matrix elements were the factors that limited the suppression efficiency. Furthermore, the addition of a considerably high amount of TiO2 (in this case, it was 0.15 Wt.%) to the solder joint provided a lower electro-migration resistance as the atomic diffusion induced by the electro-migration in the solder could not be retarded by the TiO2 particles. The agglomeration of TiO2 particles resulted in the formation of micropores in the matrix. The micropores increased the interfacial reaction due to the diffusion of the Sn atoms to Cu, thus resulting in increased IMC growth. This was due to the agglomeration and uneven distribution of the excessive nanoparticles, which reduced the suppression of the IMC growth. This phenomenon occurred in the region of the solder fillet, whereby in previous studies with nano-reinforced lead-free solder printing of the copper plate (without component attachments) the agglomeration of nanoparticles was discovered near the surface of the solid semi-circular solder (Chellvarajoo et al., 2015a). The hypothesis was confirmed through the HRTEM analysis, as mentioned earlier. A thin IMC layer is preferable, as it produces strong bonding at the interface (Laurila et al., 2005; Tay et al., 2013). The intermetallic layer is crucial in enhancing the strength of the joint during its service (Wiese and Wolter, 2004; Tang et al., 2016).
In addition, the selection of the solder paste composition was based on a survey of the literature, which had been a common practice in the previous work. Furthermore, in an initial experiment using 1.0 Wt.% of TiO2 nanoparticles (Figure 19), it was discovered that there was a severe solder residue across the ultra-fine solder joint, and the joint failed to meet the industry standards with respect to cosmetics criteria (IPC, 2010). The solder residue was analysed using an X-ray diffraction (XRD) machine. A Bruker AXS D9 with monochromatized Cu Kα radiation (λ = 1.5406 Å) over the range of 10° < 2ϴ < 90° was used to identify the structural properties of the solder residue. The structural analysis of the solder residue clearly showed the dominant peaks, which were attributed to the TiO2 nanoparticles. This phenomenon was attributed by the flux out gassing from the solder boundary to the surface. A weak electrostatic force between TiO2 nanoparticle and metallic SAC alloys that encourages the TiO2 nanoparticle expelled from the solder matrix during the soaking stage of the reflow process. Thermal loads above 150°C imposed on the solder paste during the soaking phase (Lee, 2002; Mokhtari et al., 2012) activate the flux. Hence, the TiO2 nanoparticles were upwardly displaced because of the weak intermolecular forces. It was concluded that the residue that was formed consisted of TiO2 nanoparticles that had exited the system by means of the flux activation during the reflow.
A new form of TiO2 nanoparticle-reinforced lead-free solder paste was successfully applied to an ultra-fine package assembly using the reflow soldering technique. The addition of TiO2 nanoparticles crucially improved the fillet height and suppressed the IMC layer. An HRTEM analysis indicated that the TiO2 nanoparticles were evenly mixed into the lead-free solder paste. The ultra-fine solder joints were formed successfully with different weight percentages of TiO2 nanoparticles and fulfilled the IPC standards. The minimum IMC layer and the maximum fillet height were obtained when the solder paste was reinforced by 0.05 Wt.% of TiO2 nanoparticles. The increase in the weight percentage of TiO2 to 0.15 Wt.% improved the wetting, thereby yielding the highest fillet height for the ultra-fine solder joint. However, the severe accumulation of TiO2 nanoparticles caused an increase in the thickness of the IMC layer. This phenomenon indicated that there was an accumulation of TiO2 nanoparticles during the reflow soldering process when the solder paste was reinforced with 0.15 Wt.% of TiO2 nanoparticles. The optimum weighted percentage of the TiO2 nanoparticles is crucial to improving the microstructure, IMC layer and the fillet height of the ultra-fine solder joint.
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The authors would like to thank the Universiti Kebangsaan Malaysia (Research grant–DIP-2014-012), Universiti Sains Malaysia (304/PMEKANIK/60312038) and Jabil Circuit Sdn Bhd for their financial support.