Ball shear strength and fracture mode of lead-free solder joints prepared using nickel nanoparticle doped flux

Ball shear strength and fracture mode of lead-free solder joints prepared using nickel nanoparticle doped flux G. K. Sujan a, A. S. M. A. Haseeb a, *, Chong Hoe Jian b, Amalina Afifi a a Department of Mechanical Engineering, Faculty of Engineering, University of Malaya 50603 kuala Lumpur, Malaysia. b ON Semiconductor, SCG Industries (M) Sdn. Bhd, Lot 122, Senawang Industrial Estate, 70450 Seremban, Negeri Sembilan, Malaysia. hoejian.chong@onsemi.com *Corresponding author e-mail address: haseeb@um.edu.my Abstract The trend towards the miniaturization and the replacement of lead-based solders in microelectronic devices raise concerns over reliability in the recent year. Particularly, the rapid growth of interfacial intermetallic compound layers can lead to brittle fracture in solder joints. In the present study, a novel nanoparticle doped fluxing method was used to prepare ball grid array solder joints in between lead-free Sn-3.0Ag-0.5 solder balls and organic solderability preservative- pads. In this method, nickel nanoparticles were mixed with a water soluble flux prior to its use. The shear strength and fracture modes of resulting solder joints were determined as a function of aging time The results showed that the average shear strength of solder joints prepared by using 0.1 wt.% Ni doped flux was only marginally higher than that of solder joints prepared with undoped flux. In case of undoped condition, the percentage of catastrophic brittle fracture mode increased with increasing the aging time. On the other hand, 0.1wt.% Ni doped flux solder joints showed excellent resistant against brittle fracture up to 30 days of aging. 1. Introduction Now a days, the use of tin-based lead-free solders have been widely used and firmly established in microelectronic consumables. However, some reliability issues have to be overcome such as, growth of excessive intermetallic compounds (IMCs) layer. This can lead to catastrophic failure of the solder joints [1] etc. There are two ways to control the growth of IMC layer in the solder joints. One is metallurgical alloying of elements in the bulk solder [2]. Another method is particle reinforcement of solder [3,4]. Recently it has been reported that the addition of Ni nanoparticles [5] and Co [6] nanoparticles into lead-free SAC solder suppresses the growth of IMC layer during long time isothermal aging. This is expected to lead better solder joint reliability since IMC is considered to be more brittle and involved with kirkendal void formation [5]. Despite of the advantages of nanoparticles addition to lead-free solder alloys, research on the reinforcement of lead-free solder alloys with nanoparticles is still in academic level. So far no practical use of particle reinforced solders has been reported. The reason is that the reinforcement of nanoparticles to lead-free solders is solely committed to enhance the overall mechanical strength of the solder joint which is the opposite way to go. Due to the miniaturization and higher packaging density in microelectronic devices, the diameters of solder joints in ball grid array (BGA) and flip chip (FP) packages become much smaller. This will increase the proportion of brittle IMCs in the solder joint enhancing brittleness [7]. Thus, at present, a tougher and low mechanical strength joints are more preferable rather than hard and high mechanical strength joints in BGA and FC packages. In BGA and FC packages, a popular method for solder joint preparation is flux dipping method [8,9]. Recent studies suggest that this method has improved drop impact reliability over the conventional paste printing method [9]. Thus, at present, the most suitable approach should be the flux dipping method to prepare BGA and FC assemblies utilizing nanoparticle in such a way that it mainly modify the interfacial IMC without affecting the solder bulk. In this study, the microstructural evaluation and fracture modes of BGA solder joints have been investigated in between SAC305 solder balls and OSP- substrate using 0.1 wt.% Ni nanoparticles doped flux and compared with undoped condition. 2. Experimental procedures Lead-free Sn-3.0Ag-0.5 (SAC305) solder balls of diameter 450 µm were used in this study. 0.1wt.% of Nickel (Ni) nanoparticles (Accumet Materials, Co., USA) with the average size of 44 nm were mixed with a commercially available water soluble flux (Sparkle Flux WF-6317, Japan) to prepare the 0.1wt.% Ni nanoparticle doped flux. The BGA printed circuit board (PCB) used in this study as the substrate had double sided polyimide solder mask with opening diameter of 300 µm. The underlying pads were 30 µm in thickness with OSP surface finishing at the opening end. Before reflow, the PCB substrates were dipped in 2- Propanol solution (R & M Marketing, Essex, U.K.) for 120s to clean the opening pads of OSP surface finished substrates. After that, the substrates were rinsed with deionized water and dried in acetone. Solder joint preparation steps are schematically illustrated in Fig. 1 [10]. 0.1wt% Ni nanoparticle doped flux was placed on the clean OSP- substrate. Solder balls were then placed on top of the flux and were reflowed in a reflow oven (Forced Convection, FT02) at 240 C for 45s. Isothermal aging test was conducted at 150 C for 10, 20, 30 and 42 days respectively. Microstructural characterization and elemental compositions of reflowed and aged BGA solder joints were examined by using field emission scanning electron microscopy (FESEM) equipped with energy dispersive X-ray spectroscopy (EDX). The ball shear test was performed using a global solder bond tester (DAGE BT 4000) with the shear height and shear speed of 20 µm and 650 µm/s respectively. Twenty solder bumps were

considered for each experimental condition. A detailed quantitative fracture surface analysis was carried out to identify various types of fracture modes by using FESEM fractographs and EDX analyses. Solder mask pad PCB Solder mask Ni doped flux Solder ball Intermetallic compound (IMC) Fig. 1. Schematic diagram illustrating the experimental steps for solder joint preparation. 3. Results and discussion 3.1 Effects of Ni nanoparticle doped flux on interfacial IMCs: Fig. 2 shows the cross-sectional FESEM microstructures of 0 and 0.1 wt.% Ni doped flux solder joints after reflow and reflow followed by 42 days of aging respectively. In Fig. 2a, scalloped and elongated morphology of IMC (confirmed by point EDX analysis) is observed for undoped condition just after reflow. A relatively planar and less irregular morphology of thinner (,Ni) 6 Sn 5 IMC (confirmed by EDX spot analysis) layer is formed in case of doping 0.1wt.% Ni into flux (Fig. 2b) after reflow comparing with undoped condition (Fig. 2a). Moreover, a very thin IMC (confirmed by EDX analysis) layer is also formed having darker contrast than visible or (,Ni) 6 Sn 5 IMC for both undoped and 0.1wt.% Ni doped flux samples. Since the growth of IMC is diffusion and reaction controlled, longer contact time is required for its growth in between liquid solder and copper substrate [11]. Upon 42 days of aging, IMC layer is clearly visible in both 0 and 0wt.% Ni doped flux solder joints (Fig. 2c-d). Comparing Fig. 2c and d, it is seen that IMC thickness is reduced in case of 0.1wt.% Ni doped flux samples. On the other hand, the thickness of (,Ni) 6 Sn 5 IMC (Fig. 2d) is higher than that of IMC (Fig. 2c) after isothermal 42 days of aging. Fig. 3a shows the effect of doping 0.1wt.% Ni into flux on IMCs after isothermal aging. It is seen that IMC thickness is suppressed for the solder joints with 0.1wt.% Ni doped flux comparing with undoped condition in all aging conditions. On the other hand, (,Ni) 6 Sn 5 IMC thickness is lower than that of IMC just after reflow as shown in Fig. 3b. However, (,Ni) 6 Sn 5 IMC thickness becomes higher than that of IMC in all aging conditions. In Fig. 3c, the total IMC thickness (summation of /(,Ni) 6 Sn 5 and IMCs) is plotted against the aging time. Undoped flux solder joints always gives higher total IMC thickness than that of solder joints prepared with 0.1wt.% Ni doped flux after reflow and isothermal aging respectively. Reduction in and an increase in IMCs thickness were also reported in previous studies when Ni was used as alloying element [12] and nanoparticle reinforcement [5]. It has been suggested that (,Ni) 6 Sn 5 intermetallic compound has the same crystal structure of wherein Ni atoms substitute the space sites in the monoclinic crystal structure of [13,14]. It is reported in the literature that the heat of formation of the complex compound (,Ni) 6 Sn 5 will become more negative in the presence of Ni in the [13]. Thus, doping Ni nanoparticles into flux will eventually thermodynamically stabilize the layer. Moreover, Gao, F., et al. [15] have shown on the basis of thermodynamic calculations that the affinity between and Ni was stronger than that for and Sn. As a result, (,Ni) 6 Sn 5 IMC growth is more favorable and faster in the presence of Ni atoms. Gao et al. [15] has also suggested the growth of through the reaction + 9 = 5. Since Ni increases the stability of, the above reaction is suppressed. This effectively reduces the growth kinetics of layer as has been found in the present study. 3.2 Ball shear test and fracture surface analysis: Fig. 4 shows the effect of doping 0.1wt.% Ni into the flux on the average shear strength as a function of aging time. It is seen that the average shear strength of 0.1wt.% Ni doped flux solder joints is marginally higher than that of undoped solder joints. In order to determine the fracture surface modes and study the crack initiation and propagation mechanisms, the elemental compositions of the fracture surfaces were determined by the means of elemental

(a) 0% Ni_0 day (b) 0.1% Ni_0 day (,Ni) 6 Sn 5 (c) 0% Ni_42 days (d) 0.1% Ni_42 days (,Ni) 6 Sn 5 Fig. 2. High resolution FESEM cross-sectional images: for 0wt.% Ni doped flux at (a) 0 and (c) 42 days of aging; for 0.1wt.% Ni doped flux at (b) 0 and (d) 42 days of aging. (a) (b) Fig. 4. Effect of Ni nanoparticle doped flux on average shear strength with respect to aging time. (c) Fig. 3. Effect of Ni nanoparticle doped flux on (a) IMC thickness, (b) IMC thickness and (c) total IMC thickness with respect to aging time. mapping analysis. According to the previous studies done by other researchers, there were five types of fracture modes found after the ball shear test in BGA solder joints: ductile (Mode I), quasi-ductile (Mode II), quasi-brittle (Mode III), brittle (Mode IV) and pad-lift (Mode V) [16,17]. In the present study, there are only four types of failure modes observed in both undoped and 0.1wt.% Ni doped flux samples after the ball shear test of aged BGA solder joints. As for example, Fig. 5 shows the fractographs of different failure modes and the elemental mapping of different elements on fracture surfaces for undoped condition after 42 days of aging. Cross-sectional schematic diagrams of different failure modes are also given to visualize the initiation and propagation of cracks in the fracture surface. Mode I is a ductile failure mode as the crack is propagating along the ductile solder matrix without exposing brittle IMCs. Mode II (quasi-ductile) is one of the mixed mode

Fractograph Sn Overlapped image Shear Direction Mode I: Ductile Sn-3.0Ag-0.5 Mode II: Quasi-Ductile Sn-3.0Ag-0.5 Mode III: Quasi-Brittle Sn-3.0Ag-0.5 Mode IV: Brittle Sn-3.0Ag-0.5 200 µm Fig. 5. Elemental maps and distribution of Sn (green) and (red) elements in the resulted fracture surfaces after 42 days of aging for 0wt.% Ni doped flux. Schematic diagrams illustrating various fracture modes in BGA solder joints: Mode I (ductile); Mode II (quasi-ductile); Mode III (quasi-brittle) and Mode IV (brittle). failures where less than 50% IMC has been exposed. Mode III (Quasi-brittle) is another type of mixed mode failures where the IMC has been exposed to more than 50%. In both types of mixed failure modes, the crack propagation has been attributed through the combination of solder matrix and brittle IMC. Mode IV is a brittle failure mode where the crack propagates along the brittle IMC layer leaving no or minor amount of ductile solder on the fracture surface. However, when it comes to brittle IMC fracture, the crack propagation has become more complicated since it is involved with two IMC layers. As a result, high magnification FESEM images of brittle IMC fracture surfaces were studied in Fig. 6. It is seen from Fig. 6a and b that the IMCs (confirmed by EDX analysis) were exposed on the fracture surface for undoped condition. The fracture surface was mostly covered with IMCs with some fraction of particles. Similar results have also been observed in case of 0.1wt% Ni doped flux solder joints (Fig. 6 c and d). Thus, during brittle IMC fracture in Mode II, III or IV, the crack propagation occurs along the or (,Ni) 6 Sn 5 and IMCs interface. (a) 0% Ni_42 days (c) 0.1% Ni_42 days (,Ni) 6 Sn 5 (b) 0% Ni_42 days (d) 0.1% Ni_42 days (,Ni) 6 Sn 5 Fig. 6. Brittle fracture images after 42 days of aging for 0wt% Ni doped flux (a) at low (686 ) magnification and (b) at high (2574 ) magnification; for 0.1wt% Ni doped flux (c) at low (686 ) magnification and (d) at high (3432 ) magnification.

Fig. 7 summarizes the percentage of fracture modes observed in 0 and 0.1wt.% Ni doped flux solder joints as a function of aging time. It is seen from Fig. 7a, the percentage of brittle IMC fracture mode increased with increasing the aging time in case of undoped condition. For 0.1wt.% Ni doped flux solder joints, excellent resistant against brittle fracture observed up to 30 days of aging. The presence of Ni stabilizes the hexagonal (,Ni) 6 Sn 5 structure and hinders its transformation to monoclinic structure. As a result, the volume change required for phase transformation has been suppressed which will eventually prevent IMC cracking [18]. From Fig. 3a and Fig. 7, it is seen that there is a critical value of IMC thickness which leads to interfacial fracture giving either Mode II, III or IV. When the thickness was 1.12 ± 0.05 µm for undoped condition after 10 days of aging (Fig. 3a), 25% quasi-ductile (5 out of 20 solder bumps) and 5% brittle (1 out of 20) failures were observed (Fig. 7a). On the other hand, since 0.1wt% Ni doped flux suppressed the IMC thickness (Fig. 3a), there was no interfacial failures observed even up to 20 days of aging. However, the IMC thickness reached to a value of 1.2 ± 0.2 µm after 30 days of aging for 0.1wt.% Ni doped flux (a) (b) Fig. 7. Summary of fracture modes with respect to aging time for (a) 0wt.% Ni and (b) 0.1wt.% Ni doped flux. solder joints (Fig. 3a). Then, 5% quasi-ductile (1 out of 20), 5% quasi-brittle (1 out of 20) and 10% brittle (2 out of 20) failures were observed (Fig. 7b). Thus, when the IMC thickness reaches to an optimum value of 1.1-1.2 µm, the catastrophic interfacial failures can not be avoided and cracks begins to initiate and propagate along the (,Ni) 6 Sn 5 / interface. 4. Conclusions The following conclusions can be drawn: 1. The total IMC thickness was suppressed for 0.1wt.% Ni doped flux comparing with undoped condition after reflow. After long time aging, thickness of was increased and was decreased by doping 0.1wt.% Ni into flux comparing with undoped flux. 2. 0.1wt.% Ni doped flux solder joints showed marginally higher average shear strength values than that of undoped flux solder joints. 3. Fractography suggests the percentage of brittle fracture mode increased with increasing the aging time in case of undoped condition. On the other hand, 0.1wt.% Ni doped flux solder joints showed excellent resistant against brittle fracture up to 30 days of aging. 4. The initiation and propagation of crack was changed from solder (ductile mode) to solder/( /) (quasi-ductile or quasi-brittle mode) and / (brittle mode) after isothermal aging test. 5. It is suggested that the suppression of IMC thickness is responsible for the better resistant against catastrophic brittle fracture mode after isothermal aging in case of 0.1wt.% Ni doped flux solder joints ensuring long term solder joint reliability. Acknowledgments The authors acknowledge the financial support of High Impact Research grant (UM.C/HIR/MOHE/ENG/26, Grant No. D000026-16001) and University of Malaya Research grant (UMRG, Grant No. RP021-2012D). The authors are grateful to M. M. Arafat for his sincere cooperation and valuable advice. References 1. Frear, D. R., et al, Intermetallic growth and mechanical behavior of low and high melting temperature solder alloys, Metall Mater Trans A, Vol. 25 (1994), pp. 1509-1523. 2. Yang, C., et al, Impact of Ni concentration on the intermetallic compound formation and brittle fracture strength of Sn--Ni (SCN) lead-free solder joints, Microelectron Reliab, Vol. 54 (2014), pp. 435-446. 3. Tang, Y., et al., Influence of TiO 2 nanoparticles on IMC growth in Sn-3.0Ag-0.5-xTiO 2 solder joints in reflow process, J Alloy Compd, Vol. 554 (2013), pp. 195-203. 4. Haseeb, A. S. M. A., et al, Stability of Molybdenum nanoparticles in Sn-3.8Ag-0.7 solder during multiple reflow and their influence on interfacial intermetallic compounds, Mater Charact, Vol. 64 (2012), pp. 27-35. 5. Tay, S. L., et al, Influence of Ni nanoparticles on the morphology and growth of interfacial compounds

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