Full Length ArticleThermo-mechanical evolution of ternary Bi–Sn–In solder micropowders and nanoparticles reflowed on a flexible PET substrate
Graphical abstract
Bi–Sn–In composite solder, consisting of micropowders (MPs) and nanoparticles (NPs), was reflowed on a flexible PET substrate at 110 °C because the plastic film would not be damaged at such a low temperature
Introduction
Near-future electronics will be flexible, bendable, and wearable [1], [2]. Following such trends, the microelectronic packaging industry requires more advanced solders, suitable for the junction between organic microchips and plastic substrates [3]. However, the future use of flexible, polymer-based components requires the development of a novel solder that can be reflowed at a temperature below 110 °C due to damage to the organic microchips and/or plastic substrates at higher temperatures [3]. In this context, Bi-based alloys are an effective solder material for flexible microelectronics because of their high solderability and low melting temperature [3], [4], [5], [6]. Nonetheless, their easy thermal cracking and high brittleness, which result from the formation of intermetallic compounds (IMCs), are predominant demerit factors, particularly regarding the solder reliability and durability [3], [7], [8]. Hence, these characteristics must be resolved before the Bi-based alloys can attain practical use as flexible interconnections. Two approaches are frequently used to improve the thermal behavior and/or mechanical strength [9], [10]. First, alloying elements can be used to control the intrinsic mechanical properties of the solder [9]. For example, the use of In within the binary Bi–Sn alloy system improves the ductility of the solder through the formation of oxidation resistant phases, despite the deteriorating electromigration behavior [3], [9]. In the second approach, fine additives, such as FeCo and TiO2 nanoparticles are used as reinforcements to enhance the fracture strength of the solder without substantial sacrifice to the other mechanical strengths, providing the amount of the secondary phases do not exceed their limits [9], [11], [12].
In contrast to a rigid Cu pad, the introduction of a flexible plastic substrate requires the development of more advanced solder bumps that are engineered to withstand high strain [3], [13]. Nanoparticles are a useful reinforcement candidate to improve the adhesion stability of solder bumps. In particular, nanoparticles have high diffusivity due to their small size and are thus ideal for liquid-phase sintering among micropowders, even at a low reflow temperature [9], [14]. Likewise, the capillary action of nanoparticles allows them to neck readily between the micropowders and the substrates without any defects [15]. However, the intrinsic oxide layers that form on the surface of the nanoparticles are the primary issue when producing ductile joints because the typical metal oxides are fragile, degrading easily in flexible devices [16]. Furthermore, metal oxides have low electrical conductivity, which retards the electrical/electronic signal transmission between microchips and substrates [17].
In the present study, the thermo-mechanical evolution of the composite solder material, consisting of as-fabricated Bi–Sn–In micropowders and as-synthesized nanoparticles of the same composition, is evaluated via microscopy, spectroscopy and mechanical analyses. Their electrical resistivity is also investigated, regarding the IMC formation and oxidation degree according to the amount of the solder nanoparticles added into the solder micropowders. In particular, it highlights the ability of the composite solder bumps to strongly adhere on the PET substrate, even after the reflow process at a low temperature of 110 °C and the reinforcement effect of the solder nanoparticles, which is associated with their shear strength resistance during the scratch test.
Section snippets
Experimental
Bi pieces (99.99%), Sn granules (99.9%), and In shots (99.9%) were obtained from Alfa Aesar (Ward Hill, MA, USA). Two types of Bi-based micropowders, composed of binary Bi–Sn and ternary Bi–Sn–In, were fabricated using a gas atomizer (Hot Gas Atomization System, PSI Ltd., Hailsham, East Sussex, UK) under identical conditions. Then, the gas-atomized powders were classified using a series of standard sieves to obtain powders of a specific size range. Bi (III) nitrate pentahydrate (4.2 × 10−4 mol),
Results and discussion
The gas-atomized Bi–Sn and Bi–Sn–In micropowders had a diameter less than 10 μm, as shown in Fig. 1a and b, respectively. The as-synthesized Bi–Sn–In nanoparticles that were used as a reinforcement material are shown in Fig. 1c. The composite solder micropowders and nanoparticles, showing the presence of nanoparticles on the surface of the micropowders, can be seen in Fig. 1d.
Fig. 2a shows a low magnification TEM image of the as-synthesized Bi–Sn–In nanoparticles. The nanoparticles were
Conclusions
Composite Bi–Sn–In solder bumps, consisting of micropowders and nanoparticles, were successfully patterned without cross-linking using a roll-to-plate printing method and reflowed on a flexible PET substrate at 110 °C. A scratch test demonstrated that the addition of 5.0 wt.% nanoparticles into the micropowders, effectively improved the adhesion strength (0.43 N average shear force) of the composite solder bumps by improving their viscosity and diffusivity. However, when the nanoparticles amount
Acknowledgements
This work was supported by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIP) (No. CAP-12-6-KIMS).
References (29)
- et al.
Controllable growth of polyaniline nanowire arrays on hierarchical macro/mesoporous graphene foams for high-performance flexible supercapacitors
Appl. Surf. Sci.
(2017) - et al.
Performance of flexible capacitors based on polypyrrole/carbon fiber electrochemically prepared from various phosphate electrolytes
Appl. Surf. Sci.
(2016) - et al.
Preparation of property-controlled Bi-based solder powders by a ball-milling process
Metals
(2016) - et al.
Solder wetting behavior enhancement via laser-textured surface microcosmic topography
Appl. Surf. Sci.
(2016) - et al.
In situ study on growth behavior of interfacial bubbles and its effect on interfacial reaction during a soldering process
Appl. Surf. Sci.
(2014) - et al.
Thermal property, wettability and interfacial characterization of novel Sn–Zn–Bi–In alloys as low-temperature lead-free solders
Mater. Des.
(2015) - et al.
Interfacial segregation of bismuth in copper/tin–bismuth solder interconnect
Scr. Mater.
(2001) - et al.
Structure and properties of lead-free solders bearing micro and nano particles
Mater. Sci. Eng. R: Rep.
(2014) - et al.
Effects of TiO2 nanoparticles addition on microstructure, microhardness and tensile properties of Sn–3.0Ag–0. 5Cu–xTiO2 composite solder
Mater. Des.
(2014) - et al.
Magnetic nanoparticle-based solder composites for electronic packaging applications
Prog. Mater. Sci.
(2015)
Activation of electroplated-Cu surface via plasma pretreatment for low temperature Cu-Sn bonding in 3D interconnection
Appl. Surf. Sci.
Effect of silver (Ag) nanoparticle size on the microstructure and mechanical properties of Sn58Bi–Ag composite solders
J. Alloys Comp.
Oxidation resistant effects of Ag2S in Sn–Ag–Al solder: a mechanism for higher electrical conductivity and less whisker growth
Corros. Sci.
Densification and microstructural development during sintering of powder injection molded Fe micro–nanopowder
Powder Technol.
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