key: cord-1020903-eshaseis
authors: Rajendran, Sri Harini; Kang, Hyejun; Jung, Jae Pil
title: Ultrasonic-Assisted Dispersion of ZnO Nanoparticles to Sn-Bi Solder: A Study on Microstructure, Spreading, and Mechanical Properties
date: 2021-02-10
journal: J Mater Eng Perform
DOI: 10.1007/s11665-021-05518-5
sha: 8c4d6de096f9cdcade1485544d791c6314e2d7ef
doc_id: 1020903
cord_uid: eshaseis

Nanocomposite Sn-Bi solders received noticeable attention for flexible electronics due to their improved mechanical properties. The main limitation is the dispersion of nanoparticles in the solder alloy. Accordingly, in this work, varying additions of ZnO nanoparticles were successfully dispersed into Sn57Bi solder via the liquid-state ultrasonic treatment. Nanocomposite solders were prepared using the melting and casting route. The solder alloys were then characterized for microstructure, spreading and mechanical properties. With increasing ZnO addition, the microstructure revealed significant refinement of Bi- and Sn-rich phases. Consequently, the eutectic lamellar spacing also decreases. The spreading improved up to 0.1 wt.% ZnO addition. For higher additions, nanocomposite solders experienced deterioration in spreading characteristics. The tensile strength of the solder increases with an increase in the amount of ZnO nanoparticles. High ductility is achieved for nanocomposite solder containing 0.05 wt.% ZnO. An attempt was made, to explain the effect of increasing ZnO nanoparticle addition on microstructural, spreading, and mechanical properties of Sn57Bi solder.

Flexible and wearable devices have become a forefront topic in the research and development of the electronic industry. However, integration of functional components such as semiconductors and inorganic chips remains a barrier in achieving the robustness of the flexible electronic device (FET) (Ref 1). Presently, conductive adhesives are utilized as an interconnect material for flexible electronics. Conductive adhesives have poor electrical conductivity and impact strength at high operating temperatures ( Ref 1, 2) . Research toward establishing solder materials for high power FETÕs are highly focused, in particular, eutectic Sn-Bi solders capture the growing interest due to their cost effectiveness and melting temperature (139°C) (Ref 2-7) . Meanwhile, brittleness of Bi phase intermetallic compounds (IMC) accounts to a reliability concern since FETÕs may experience mechanical shock upon bending reported the addition of 1 wt.% ZnO NPs, substantially reduced the Cu-Ni-Sn thickness at the interface and increased the shear strength of Sn-57.6Bi-0.4Ag solder joints on Ni metallized Cu pads. However, the result of ZnO NPs addition on the wetting characteristics and the tensile property of Sn-Bi solder alloy has not be studied. Besides, dispersion of ceramic NPs in the molten solder is a challenging task due to the poor wetting between ceramic NPs and the molten metal. NPs are generally expelled from the molten solder during the fabrication (Ref 9) . Further, nanoparticles tend to agglomerate due to their high surface energy and hence may retard the dispersion during conventional mixing techniques such as stir casting and mechanical solder paste mixing. Hence, advanced mixing techniques must be adapted to successfully disperse the nanoparticles in the solder alloy.

Ultrasonic treatment (UT) of molten metal has been reported by many researchers as one of the effective methods to disperse the NPs in Al and Mg alloys (Ref 10). The shock waves generated from the acoustic bubble collapse can de-agglomerate the NPs during dispersion ( Ref 9, 11) . Additionally, it is reported that sono-capillary effect by acoustic cavitation can enhance the wettability of molten metals on ceramic surface ( . Tzanakis et al. (Ref 13) confirmed the sonocapillary effect through the penetration of molten aluminum on the pre-existing oxide groove in the presence of UT. The benefits of UT are utilized in various metal processing techniques such as dispersion of NPs (Ref 9), infiltration (Ref 11) and alloy refinement ( Ref 12) . However, dispersion of nanoparticles in the molten solder by UT is yet to be achieved. Therefore, in the present work, ZnO NPs are dispersed in Sn-Bi solder using UT and the influence of ZnO on the microstruc-This article is an invited paper selected from abstracts submitted to the 5th International Conference on Nanojoining and Microjoining (NMJ 2020), that was to be held from September 20-23, 2020, in Leipzig, Germany. The conference was postponed due to the coronavirus (COVID-19) pandemic. The paper has been expanded from the planned presentation. ture, spreading and mechanical properties of Sn57Bi solder are discussed.

Sn57Bi solder ingot of purity 99.9% acquired from BNF, South Korea and ZnO nanoparticles (NPs) of purity 99.5% and an average individual particle size of 15-25 nm purchased from Ditto Technology, South Korea were used as starting materials. Figure 1 (a) shows the field emission scanning electron microscope image and EDS analysis of as purchased ZnO nanopowders. The average aggregate (sintered individual particles) size of as-purchased ZnO powders were found to be 53 ± 10 nm. Series of Sn57Bi-xZnO (x = 0.05, 0.1, 0.2 and 0.4 wt.%) nanocomposite solders were prepared by melting and casting method. The Sn-Bi solder was heated to 400°C in a melting furnace and accurately weighted ZnO NPs in pellet form was incorporated in the molten solder in the presence of UT. Piezoelectric ultrasonic transducer (Hangzhou Dowell Ultrasonic Technology, Co. Ltd, China) coupled with SS304 sonotrode with diameter of 3 cm, frequency of 20 kHz and 70% amplitude was used for dispersing the NPs. The molten solder was subjected to an UT for 3 min and cast on a stainless steel mold. During melting and casting, Ar gas was flowed continuously on the surface of the molten solder to prevent the oxidation. Microstructure samples were mounted in an epoxy resin, polished and etched with a solution containing 3 vol.% HCl, 5 vol.% HNO 3 and 92 vol.% CH 3 OH. The microstructure of the nanocomposite solder alloys was analyzed using analytical scanning electron microscope (SEM) (JEOL JSM-6010PLUS, Tokyo, Japan) attached with energy dispersive spectrometer (EDS). The eutectic spacing was measured using Image-Pro Plus 6.0 program.

The spreading ratio (S) of solder bump is calculated in accordance with Japanese Industrial Standard (JIS-Z-3197) (Ref 12):

D is the diameter of the solder spread (assumed to be a sphere); D = 1.24V 1/3 ; V is the mass/specific gravity of the solder alloy; H is the height of the solder after spreading. 99.99% pure copper test substrate (30 mm  30 mm  0.3 mm) and 0.3 g of near-spherical as prepared solder were used for the spreading test in the presence of rosin mildly activated (RMA) flux. Spreading test was conducted at a peak temperature of 165°C for 60 s. After cooling, the samples were cross sectioned to measure the spreading height (H). Mechanical property of ZnO added Sn-58Bi solders were characterized through AUTO-TENFORCE M/C (Korea-tech) tensile testing machine. Tensile tests were performed in room temperature at a fixed rate of 3.0 mm/min. Tensile tests were conducted in accordance with ASTM: E8 standard. The dimensions of tensile test sample was given in Figure 1 (c). For each condition, five samples were tested and the average values were reported. The alternating Bi and Sn-rich eutectic morphology changes from coarse to a fine lamellar structure with the increasing addition of ZnO NPs up to 0.2 wt.%. The refinement has resulted in the significant reduction in the average eutectic spacing between Sn-rich and Bi phase. However, as seen from Fig. 2(f) , increasing the NPs addition to 0.4 wt.%, tends to coarsen the eutectic lamellar structure. Figure 2 (g) shows the average eutectic lamellar spacing in monolithic and ZnO added Sn57Bi solder. Eutectic lamellar spacing was measured using the linear intercept method from ten high magnification microstructures. The average eutectic lamellar spacing reduces with the addition of ZnO, reaches a minimum value of 1.4 ± 0.5 lm for 0.2 wt.% addition and thereafter increases for the higher addition amount. However, the extent of coarsening is insignificant compared with the monolithic alloy.

Microstructural refinement can be attributed to ZnO NPs acting as active nucleation sites in the solder melt during solidification. Nucleation sites in the melt reduce the free energy barrier and promote the nucleation events during solidification Figure 2(g) shows the ZnO aggregates and individual particles dispersed in the Sn57Bi alloy matrix. At higher addition amount, the increased NPs interactions can lead to clustering of ZnO NPs. These clusters can be pushed during the dendritic growth presumably reducing the number of ZnO nucleation sites resulting in the coarsening of Bi and Sn-rich phases.

The higher spreading ratio reflects a good wettability of the solder and is considered as one of the most desirable properties for soldering. Figure 3(a) shows the spreading ratio for monolithic and ZnO NPs added Sn57Bi solders on the Cu substrate. The average spreading rate of monolithic Sn-57Bi solder on the Cu substrate is 70.7%. With the addition of 0.05 and 0.1 wt.% of ZnO NPs, the average spreading ratio increases to 72.6% and 74.4%, respectively. However, upon increasing the NP addition amount to 0.2 wt.%, spreading ratio starts to decrease. Addition of 0.4 wt.% of ZnO NPs leads to a spreading ratio of 70.1%.

Addition of ZnO NPs up to 0.1 wt.% has improved the wettability of the Sn57Bi by reducing the surface tension between the molten solder and the Cu substrate. Generally, capillary forces and the contact angle are considered as the main driving force for the spreading of molten metal towards a curvature shape ( Ref 17) . The contact angle (h) is the balance of the surface energy forces existing between the solid, liquid and the vapor phase as given by the classical Young-Dupre Eq 2: (Ref 18) .

where c SV is the solid-vapor surface energy; c SL is the solidliquid interface surface energy and c LV is the liquid-vapor surface tension. For a good spreading Cos h should be maximum. In reactive spreading, the spread rate is dependent on thermodynamics and kinetics of metallurgical and chemical reaction at the interface. Formation of Cu 6 Sn 5 IMC during wetting reduces the c SL term as given by Eq 3: (Ref 18)

where c 0 SL is the solid-liquid interface surface energy before the Cu 6 Sn 5 formation and DG Cu6Sn5 is the Gibbs free energy per unit area for the Cu 6 Sn 5 formation (DG Cu6Sn5 is a negative number). As per the theory of adsorption, NPs in the molten solder gets adsorbed at the interface and increase the number of Cu 6 Sn 5 grains per unit area ( Ref 19) . In other words, NPs increases the driving force for the nucleation of Cu 6 Sn 5 phases which eventually lead to the decrease in the surface tension ( Ref 20) . Besides, upon spreading, ZnO NPs redistributes within the molten solder. At higher addition amount, accumulation of NPs at the leading edge can slow down the capillary flow, thus reducing the spreading ratio as shown schematically in Fig. 3(b) . 

In this work, ZnO nanoparticles were successfully dispersed in the molten Sn-Bi solder using a liquid-state ultrasonic treatment. The influence of ZnO NPs on the microstructure, wettability and mechanical properties of Sn57Bi solder are analyzed. The results are as follows: 

Low-Temperature-Solderable Intermetallic Nanoparticles for 3D Printable Flexible Electronics

Mechanical Properties of Sn-Bi Bumps on Flexible Substrsate, 2013 IEEE 63rd Electronic Components and Technology Conference

Figure 4 Tensile results of monolithic and ZnO nanocomposite Sn57Bi solder, (a) stress-strain curves, (b) tensile properties, (c) and (d) fractured surface of nanocomposite Sn57Bi solder with 0.05 wt.% and 0.4 wt.% ZnO addition, respectively 3

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Effects of nanoscale Cu 6 Sn 5 Particles Addition on Microstructure and Properties of SnBis Solder Alloys

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