Mechanical Fracturing of Core-Shell Undercooled Metal Particles for Heat-Free Soldering

Abstract

Phase-change materials, such as meta-stable undercooled (supercooled) liquids, have been widely recognized as a suitable route for complex fabrication and engineering. Despite comprehensive studies on the undercooling phenomenon, little progress has been made in the use of undercooled metals, primarily due to low yields and poor stability. This paper reports the use of an extension of droplet emulsion technique (SLICE) to produce undercooled core-shell particles of structure; metal/oxide shell-acetate (‘/’ = physisorbed, ‘-’ = chemisorbed), from molten Field’s metal (Bi-In-Sn) and Bi-Sn alloys. These particles exhibit stability against solidification at ambient conditions. Besides synthesis, we report the use of these undercooled metal, liquid core-shell, particles for heat free joining and manufacturing at ambient conditions. Our approach incorporates gentle etching and/or fracturing of outer oxide-acetate layers through mechanical stressing or shearing, thus initiating a cascade entailing fluid flow with concomitant deformation, combination/alloying, shaping, and solidification. This simple and low cost technique for soldering and fabrication enables formation of complex shapes and joining at the meso- and micro-scale at ambient conditions without heat or electricity.

Heat‐Free Fabrication of Metallic Interconnects for Flexible/Wearable Devices

“…The spherical liquid particles deform into uniform films that are solid, continuous, and conductive (resistivity = 15 µΩ‐cm). As expected, all solidification occurs with concomitant spinodal decomposition (Figure c,h,m) 9b…”

“…Undercooled metal particles were prepared using the SLICE method 9b. Field‘s metal—a low‐melting, fusible eutectic alloy of bismuth, indium, and tin (Bi: 32.5/In: 51/Sn: 16.5 wt%, T m = 62 °C)—was significantly undercooled (degree of undercooling, Δ T/T m = 0.34) at high yields (>97%, see Figure S1, Supporting Information).…”

“…Undercooled Particles Synthesis : Undercooled metal particles made from Field's metal (eutectic bismuth, indium, tin alloy, Bi:In:Sn = 32.5:51.0:16.5 wt%) were synthesized following the SLICE method 9b. 1 mL glacial acetic acid was mixed into 200 mL of Diethylene glycol in a graduated cylinder.…”

“…Utilizing material surface properties, such as interfacial excess ( Γ i ), surface tension (γ), and shear modulus ( G ), has inspired quantitative synthesis of undercooled liquid metal core–shell (ULMCS) particles through the so‐called SLICE technique . The synthesized core‐shell architecture frustrates heterogeneous nucleation through a thin (≈<2 nm) oxide shell that is polished in situ to form a smooth continuous surface.…”

Simplicity as a Route to Impact in Materials Research

“…It is useful to find a basic phenomenon to leverage, starting from those that are either highly reversible or highly irreversible (it narrows the search and facilitates achieving reliability and predictability in the final solution), and that are out of left field (if the problem is longstanding, out of left field solutions are more likely to work). In a recent example, the metastable nature of undercooled metal particles was exploited to prepare heat‐free ‘solder’ which circumbents many drawbacks of conventional joining techniques and is especially valuable for flexible electronic device fabrication …”