Nanoparticle synthesis and formation of composite ...

Nanoparticle synthesis and formation of composite solder for harsh environments R. Ashayer1, A. Cobley2, O. Mokhtari1, S. H. Mannan1, S. Sajjadi1, T. Mason2 1 Department of Mechanical Engineering, King’s College London Strand, London WC2R 2LS, UK 2 The Sonochemistry Centre at Coventry University Priory Street, Coventry, CV1 5FB, UK obtained from Aldrich Chemical. All chemicals were used as received. HPLC grade water was purchased from BDH and used in all experiments unless otherwise stated.

Abstract The demand for electronics capable of operating at high ambient temperatures above 150°C is increasing in the oil/gas drilling and automotive industries in particular. These demands have accelerated the progress of materials development and processing technology. Nanoparticle enhanced solders have been reported to have superior creep and reliability properties compared to simple alloyed materials. The nanoparticles, typically added at 12 vol% concentrations into the solder serve to harden the solder, stabilize the microstructure and improve reliability in high temperature environments. The nanoparticles may be added to the solder before production of solder particles, or added as a separate ingredient of the solder paste. This paper explores the latter approach.

2.2 Core Syntheses. The steps taken in production of gold nanoshell are outlined as follows: 2.2.1 Preparation of Silica Nanoparticles. A modified Stöber method [8], updated by Pham et. al. [9] is being used to manufacture the silica particles. In general, 3 mL of ammonium hydroxide solution was added to 50 mL of absolute ethanol. The mixture was stirred vigorously, and 1.5 mL of Si (OC2H5)4 (tetraethyl orthosilicate, TEOS) was added drop wise. The initial reaction mixture was clear. After 45 minutes, the reaction mixture began to turn cloudy as nanosilica particles were grown and eventually turned white. The solution was stirred overnight. Analysis by TEM indicated that the silica nanoparticles were spherical in shape with an average diameter of approximately 100 nm.

1 Introduction Nanoparticles can be introduced into solder by various methods in order to enhance mechanical properties [1-7]. However, in this investigation, the nanoparticles are mixed into the solder paste, and only incorporated into the solder during reflow soldering. The nanoparticles were composed of a silica core and Au metallic shell to ensure solder wettability. Two different synthesis approaches, including magnetic stirring and ultrasonic irradiation have been applied to generate the Au nanoshell formation. Transmission electron microscopy (TEM) photographs showed that the formation of the nanoshell depends on the concentration of the metal and the speed of the mixing. However, even with solder wettable shells, it was found that a large proportion of the particles were expelled from the SAC solder during reflow in air, and the causes were examined with the aid of Computational Fluid Dynamics (CFD) to model the reflow process and nanoparticle expulsion. CFD simulations indicate that while many nanoparticles dispersed into the flux may not come into contact with the solder / flux boundary during reflow, there should still be significant uptake into the solder.

2.2.2 Functionalization of Silica Nanoparticles. The amount of Poly diallyldimethyl ammonium Chloride (PDADMAC) needed for surface functionalization is to be in excess of (~50µL) for a 50 mL quantity of silica nanoparticle solution. Therefore, ~50µL of PDADMAC was added to 50 mL of the vigorously stirred silica nanoparticle solution and allowed to react for 2 hour. In order to enhance bonding of the PDADMAC groups to the silica nanoparticle surface, the solution was then gently boiled for 1 hour. The volume of the solution was kept constant by adding ethanol during the process of heating. The PDADMAC coated silica nanoparticles were purified by centrifuging at 2000 rpm and redispersed in ethanol. Analysis of the purified nanoparticles by TEM showed no difference between pre- and post functionalization with PDADMAC. 2.3 Assembly of Colloidal Gold Nanoparticles. Aqueous solutions of small gold nanoclusters were prepared by reduction of chloroauric acid with tetrakishydroxymethylphosphonium chloride (THPC) as described by Duff et al. [10]. These colloidal Au particles are highly monodispersed. First, 0.5 mL of 1 M NaOH, 1 mL of THPC solution (prepared by adding 12 µL of 80% THPC in water to 1 mL of water) was added to 45 mL of water. The solution was vigorously stirred for 5 min. After which, 2.0 mL of 1% wt HAuCl4 in water was added to the stirred solution. The THPC gold solution preparation produced a clear cola brown colour solution within 2-3 seconds of chloroauric acid addition. Analysis by TEM indicated that the gold nanoparticles were

2 Experimental Method and Procedure 2.1 Materials Hydrogen TetraChloroAurate (HAuCl4) (99.9%), Tetraethylorthosilicate (TEOS) (99.9%), Tetrakiss (hydroxymethyl) phosphonim (THPC) 80% solution in water, Poly (diallyldimethyl ammonium chloride) (PDADMAC) (Low molecular weight), Potassium carbonate (99%), Formaldehyde, Ammonium hydroxide solution (33% NH3) and Ethanol (99%) were

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2.0%HAuCl4 (Fig.1). From this Fig., it can be seen that at the nucleation stage for the concentration of 2%HAuCl4 , there is no gold cluster formation when ultrasound has not been applied (Fig.1 c). Therefore the gold nanoparticles preferably attach to the surface of the functionalized silica. However, using ultrasound at this concentration favours agglomeration of the gold particles resulting in formation of gold clusters, preventing particle attachment to the silica surface. On the other hand at a lower gold concentration (i.e. 1%) the process seems to favour the usage of sonifier. Using the sonifier also led to better coverage at the shell formation stage (data not shown).

spherical in shape with the size of 2-3 nm in diameter. Freshly prepared gold was used since changes in pH with time have been observed previously [11]. 2.4 Attachment of colloidal gold to silica. 4 mL of functionalized silica nanoparticle solution was added to 40 mL of gold colloid in a tube. After shaking the tube for 3 minutes using a magnetic stirrer, It was left to stand for further 2 hours. To remove non attached small gold nanoclusters, the solution was centrifuged (2000rpm), the supernatant was removed, and the remaining pellet was redispersed in water. 2.5 Nanoshell growth. In order to grow the gold overlayer on the Au/PDADMAC/silica nanoparticles, 25 mg of potassium carbonate (K2CO3) was dissolved in 100 mL of water. The mixture was stirred for 10 minutes prior to adding 1.5 mL of 1%wt HAuCl4 solution. The solution initially appeared transparent yellow and slowly became colourless over the course of 20-30 min. 20 mL of this solution was vigorously stirred and 1 mL of the solution containing the Au/PDADMAC/silica nanoparticles was added. The colour in both cases changed to purple/pink. After addition of 40 µL of formaldehyde the colour changed to blue which is a characteristic of nanoshell formation. 3 Effect of ultrasound on gold nanoshell formation The equipment used for this part of the experiment was the Meinhardt 850kHz Ultrasonic Power Generator which works at a high frequency (850kHz). In this model transducers are placed at the bottom of the sonifier which enabled the generator to be used as a water bath. Two types of setting were used giving an estimated power of 0.8 and 1 W respectively. In order to see the effect of a higher speed on the formation of nanogold shell, a digital sonifier was also used. This equipment was composed of a 20 kHz ultrasonic horn with a 13mm diameter tip attached to an ultrasonic generator. The generator was a Sonic and Materials Inc VibraCell Model VCX600 supplied by Jencons. The powers used in this experiment were at 14 and 30 W. It should be noted that ultrasound was applied at the attachment and shell growth stages of the nanogold shell processes using same method that has been described. Characterization. All the prepared samples were characterized by transmission electron microscopy (TEM). TEM was performed with a FEI TecnaiT20 electron microscope operating at a bias voltage of 200 kV. Sample preparation involved deposition of the nanoparticles in their original solution onto a carboncoated copper grid. The excess solvent was absorbed with the means of filter paper. The grid was then set aside to allow drying before analysis.

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Fig. 1. TEM images of silica particles at nucleation stage: using a)1% and b) 2% HAuCl4 concentration without ultrasound and c)1% and d)2% HAuCl4 concentration with ultrasound at 1 W. In order to prevent the agglomeration of the gold particles at 2% gold concentration, the nucleation and the shell growth stage were conducted at a lower intensity (i.e.0.8). However, the results showed no improvement as such (Fig.2.)

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4 Results and Discussion When ultrasonic irradiation was applied at power 1W (frequency of 850kHz), results showed that by using the normal magnetic stirrer, the best coverage at the nucleation stage occurred at the concentration of

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Fig.2. TEM micrograph of nanoshell formation using2%gold for both stages with ultrasound at 0.8 W

5. Nanoparticle retention in the solder Despite the improvement in nanoshell coating consistency achieved, during reflow the majority of nanoparticles are still expelled, leaving around ~0.1vol% nanoparticles in the solder (see fig. 5). Nanoparticles are suspended in a solvent compatible with the flux, and then mixed with the flux, which is finally mixed with the Sn0.7Cu solder powder (type 3) to form a solder paste. The solder paste is reflowed in a bench top oven and cross sectioned using a Jeol SM-090010 Cross Section Polisher for 24 hours at 5.5kEV ion accelerating voltage. The advantage of this ion beam milling technique is that no hard polishing particles (e.g. diamond or Silicon Carbide) are embedded in the solder during preparation. For figure 5, slightly larger (250 nm diameter) silica particles have been used to ease particle identification; 100nm silica particles are less easy to spot (see fig. 5b)

Since the results obtained for 1% gold concentration (Fig.1) showed a significant improvement in the number of nucleation sites when power of 1W (850 kHz frequency) was applied, the effect of the speed was further investigated. Fig.3 shows the TEM images taken at the nucleation stage when 1% gold concentration was used at 14 and 30 W (20 kHz frequency). The attachment of the nanogold particles seems to be influenced by the power applied to the solution. However further increase of the power at nucleation stage (i.e. 14-25 W) showed the opposite effect (Fig. 3b-c). It can be seen that the number of nucleation sites decreased rapidly as the power increased. This effect could suggest that the deposition of gold particles on nanosilica particles are dependent on the speed of mixing in a way that limits the attachment of negatively charged gold particles. Formation of the shell also showed the same trend (Fig.4).

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a b Fig.5. Cross section of reflowed solder showing silica nanoparticles embedded within solder a) 250nm diameter particles b) 100nm diameter particles a

b CFD simulations indicate that a high proportion of particles approach the solder surface before being expelled from the paste with the flux. Figure 6-7 show a sequence of snapshots of nanoparticles moving in the flux as the solder particles coalesce. The simulation was developed using the Volume of Fluid method (VoF) and implemented in Fluent 6.3. The 100 nm diameter nanoparticles were assumed to flow with the flux and no nanoparticle – solder interaction forces were included in the simulation. The solder spheres were modelled as 1820 micron diameter spheres at a volume fraction of 45% in the paste. The surface tension of solder in the presence of flux was given as 0.04N/m and the viscosity of flux taken as 100Pa.s. Gravity was not included in the simulation because the timescale for solder particle coalescence was much smaller than that for gravity driven processes. The simulation captures only particles that were originally on a symmetry plane as plotting the nanoparticle trajectories relative to solder surfaces in 3-d was considered too complicated. Nevertheless, the simulations show that even for this worst case scenario, large numbers of nanoparticles do approach a solder boundary during their expulsion from the solder.

Fig.3. TEM images of nucleation stage using 1% gold at, a) without ultrasound, b-c) with ultrasound at 14 and 30 W respectively.

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Fig.4. TEM images of gold nanoshell formation with 1% gold at nucleation stage using ultrasound a: a) 14W for both stages and b) 30W for both stages

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Solder 6. Conclusions Overall, the best results for gold attachment on the surface of nanosilica particles were obtained at 1% HAuCl4 concentration at the nucleation and the shell growth stages by means of a sonifier at a low power setting (i.e. power of 1W and frequency of 850kHz). Further increase in the gold concentration using ultrasonic irradiation caused the formation of large gold clusters, thus resulting in a poor surface coverage. Since the ultrasound seems to favour a lower gold concentration, it can be concluded that by using a sonifier, the concentration of gold can be reduced which in turn makes the gold nanoshell procedure more cost effective. Further work is in progress on attempting to maximize particle capture within the paste, but it has been determined that the solder volume fraction within the paste and reflow profile have a vital role in determining whether the nanoparticles are captured or expelled.

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Fig. 6. Initial CFD simulation setup. Solder particles are hexagonally close packed in three layers. On the left the arrangement of particles in layers is shown while on the right the particle intersections with the x-z and y-z symmetry planes are shown. The nanoparticles are originally placed in 4 lines, with darker particles in the solder and light particles in the flux. The dashed vertical line denotes the z axis.

Acknowledgments This work was funded by IeMRC under grant code SP/06/03/01 in collaboration with Henkel Loctite Adhesives Ltd., Sondex plc, and NPL. References 1. Lin, D. C., Liu, S, Guo, T. M., Wang, G., -X., Srivatsan, T. S., Petraroli, M., “An investigation of nanoparticles addition on solidification kinetics and microstructure development of tin-lead solder,” Materials Science and Engineering, Vol. A360 (2003), pp. 285-292. 2. Marshall J. L., Calderon, J., Sees, J., Lucey, G., Hwang, J. S., “Composite Solders”, IEEE Trans. on Comp. Hybrids and Manf. Tech., Vol. 14, No. 4 (1991), pp.698-702. 3. Mavoori, H., Jin, S., “New Creep-Resistant, Low Melting Point Solders with Ultrafine Oxide Dispersions,” J. Electr. Mat., Vol. 27, No. 11 (1998), pp. 1216-1222. 4. Guo, F., Lee, J., Lucas, J. P., Subramanian, K. N., Bieler, T. R., “Creep Properties of Eutectic Sn-3.5Ag Solder Joints Reinforced with Mechanically Incorporated Ni Particles,” J. Electr. Mat., Vol. 30, No. 9 (2001), pp.1222-1227. 5. Liu, J. P., Guo, F., Yan, Y. F., Wang, W. B., Shi, Y. W., “Development of Creep-Resistant, Nanosized Ag Particle-Reinforced Sn-Pb Composite Solders”, J. Mat. Sci., Vol. 33, No. 9 (2004), pp. 958-963. 6. Lee, A., Subramanian, K. N., “Development of NanoComposite Lead-Free Electronic Solders,” J. Electr. Mat., Vol. 34, No. 11 (2005), pp. 1399-1407. 7. Guo, F., “Composite lead-free electronic solders,” J. Mat. Sci. Mat. In Electr., Vol. 18 (2007), pp.129-145. 8. Stober, W., Fink, A. J., “Controlled growth of monodisperse silica spheres in the micron size range,” Colloid Interface Sci., Vol. 26 (1968), pp. 62-69.

Fig. 7. Simulation after 7,21 and 70 ms

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