Elsevier

Applied Surface Science

Volume 258, Issue 19, 15 July 2012, Pages 7507-7514
Applied Surface Science

Effect of surface oxide on the melting behavior of lead-free solder nanowires and nanorods

https://doi.org/10.1016/j.apsusc.2012.04.074Get rights and content

Abstract

Lead-free nanosolders have shown promise in nanowire and nanoelectronics assembly. Among various important parameters, melting is the most fundamental property affecting the assembly process. Here we report that the melting behavior of tin and tin/silver nanowires and nanorods can be significantly affected by the surface oxide of nanosolders. By controlling the nanosolder reflow atmosphere using a flux, the surface oxide of the nanowires/nanorods can be effectively removed and complete nanosolder melting can be achieved. The complete melting of the nanosolders leads to the formation of nanoscale to microscale spherical solder balls, followed by Ostwald ripening phenomenon. The contact angle of the microscale solder balls formed on Si substrate was measured by direct electron microscopic imaging. These results provide new insights into micro- and nanoscale phase transition and liquid droplet coalescence from nanowires/nanorods to spheroids, and are relevant to nanoscale assembly and smaller ball grid array formation.

Highlights

Surface oxide of nanosolders plays a critical role in the melting of solder nanowires and nanorods. ► Flux can effectively remove the surface oxide layer and ensure complete nanosolder reflow. ► An interesting Ostwald ripening phenomenon was observed during the melting of solder nanowires and nanorods, which led to the formation of nano- and micro-sized solder balls. ► The smallest solder balls that we have observed are in the range of tens of nanometer. ► A significant shape change (quasi-melting) was observed for these nano-solders at a temperature well below the apparent melting point temperature, which underlines the importance of the solder reflow conditions.

Introduction

One-dimensional nanostructures, especially nanowires, have received substantial interest in recent years due to their outstanding electrical, optical, magnetic and biological properties. However, there are still several technical obstacles against fully use of the unique properties of nanowires, which hinder the fast growth and adoption of nanowire applications. Among the important issues, a common problem in nanowire assembly and integration is unreliable interconnection between nanowires and nanowires or between assembled nanowires and electrodes/contact pads; thus, the joining of nanowires has become a critical issue for nanoelectronics assembly and packaging. Various joining processes such as welding, soldering and mechanical bonding have been proposed for the formation of nanowire interconnects [1], [2]. Among various techniques proposed in the past several years, nanosoldering is a unique technique gaining increasing interest. Solder materials have been widely used in electronics assembly and board level packaging. Due to the environmental and health concern of lead, the classical tin/lead (Sn/Pb) solders are being phased out and lead-free solders in the form of binary, ternary or quaternary alloys are being extensively studied as replacements. Nanowires that contain nanosolders can be used to bond various nanocomponents or integrated surfaces. Besides the intensive research on the synthesis and fabrication of nanostructured solders, including tin/silver, tin/copper and tin/silver/copper (SAC), the thermal and electrical properties of Pb-free nanosolders in both nanoparticles and nanowires are being studied and their applications are being explored [3], [4], [5].

Reflow soldering is the most common method of chip level packaging and joining electrical components to circuit board by heating the solders and adjoining surface. There are many parameters to evaluate the property and quality of solders during the reflow. Wettability is one of those important parameters which can be experimentally assessed by measuring the contact angle of wetting [6]. A good wetting result can only be achieved if the oxides of solders are completely removed. A flux, normally an inorganic or organic acid, is often used to remove the surface oxides and therefore enhance wetting by solder in the molten state. For most soldering process, the effect of fluxes on the wettability of Sn/Pb alloy over various substrates has been extensively reported; however, the melting property of lead-free solders with one-dimensional nanostructures have not been reported. Also, due to the higher surface to volume ratio of nanostructures, oxidation effect may become prevail in the reflow soldering and this issue needs to be addressed.

In this paper, we present the melting behavior of nanosolder systems (tin and tin/silver alloy nanowires/nanorods) under flux influence and the micron/nano-structures formed on Si substrate. We found that the flux vapor, rather than the liquid flux, can effectively remove the nanosolder surface oxide and facilitate the phase transformation and shape change of nano-solders at temperatures even below the melting point of the bulk solder materials. Under the influence of the flux vapor, an Ostwald-ripening assisted spheroid formation was observed. The micron-scale contact angle of reflowed solders was measured by direct scanning electron microscopy imaging. Finally, the effect of temperature and type of fluxes on the melting, especially the phase transition/shape change from nanowire/nanorod to spheroids was investigated.

Section snippets

Materials and methods

Sn and Sn/Ag alloy nanowires were fabricated using polycarbonate (PC) porous membranes (Whatman) with pore size of 30 nm and 50 nm in diameter through the electrodeposition method. First, a thin layer of Ag was evaporated on one side of a commercial PC membrane by a Nano-Master NTE-3000 thermal evaporator (Nano-master, Inc.). The silver coated side of the membrane contacted with a copper plate and restrained by a glass joint with O-ring seal. After that, the membrane was filled with tin or

Structure and composition characterization

The structure and morphology of the solder nanowires and nanorods were characterized by FESEM as shown in Fig. 1(a)–(c). The size of the nanowires can be controlled by using the template with different pore sizes. The pure Sn nanowires with 30–50 nm in diameter and 5–6 μm in length and the Sn/Ag alloy nanowires with 50–80 nm in diameter and 5–6 μm in length were shown in Fig. 1(a) and (b), respectively. Fig. 1(c) shows the SEM image of the Sn nanorods with 70–100 nm in diameter and about 300 nm in

Conclusions

In summary, it is found that the thin oxide layer on both solder nanowires and nanorods can dramatically affect the melting behavior of nanosolders. The reflow environment, especially the flux, can effectively clean the surface oxide and influence Sn and Sn/Ag alloy solder phase transition from nanowires/nanorods to spheroids. After oxide removing, micro- and nano-sized solder balls have been successfully formed from lead-free solder nanowires and nanorods with different lengths and aspect

Acknowledgements

Financial support from the NSF Center for High-rate Nanomanufacturing (CHN) is acknowledged. We also thank partial funding support from 3M Company (3M Non-tenured Faculty Grant).

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