iTSSe TSS ADVANCED MATERIALS & PROCESSES | JULY/AUGUST 2024 53 iTSSe TSS 9 The Center for Thermal Spray Research (CTSR) team at Stony Brook University received a major grant during 1999-2000 from the Defense Advanced Research Projects Agency (DARPA). The goal was to develop a new technology for digital printing of mesoscale circuits directly from a CAD file onto plates, cylinders, and complex 3D components and structures. The so-called “direct writing” technique is in some ways similar to the layer by layer process of additive manufacturing, but the goal here was to print traces of thick film electronic circuits conformally to achieve functionality. The DARPA program was known as MICE, which stands for Mesoscopic Integrated Conformal Electronics. Several teams competed with different types of direct writing technologies including inks, inkjet, and laser-induced forward transfer. The key requirement was maskless deposition directly from design to circuit. The technology was geared toward the prototyping of both novel circuits and sensors for manufacturing. EARLY DEVELOPMENT The Stony Brook thermal spray team was among the recipients of the DARPA award. The team worked on multiple pathways to both miniaturize thermal and cold spray processes and to develop innovative products and applications based on the approach. In the main embodiment, a plasma spray-based printing technology along with accessories was developed to print submillimeter traces of metals, alloys, and ceramics onto 3D components. The goal was to develop mesoscale circuits (50 microns to millimeter dimensions) that could serve applications in thick film devices for radio frequency electronics and thick film sensors such as thermocouples, strain gauges, magnetic sensors, and chemical sensors. Over the course of the four-year project, a number of concepts and application ideas were developed. The technology was then commercialized through licensing from Stony Brook via the spin-off company MesoScribe Technologies. MesoScribe developed and deployed products that are still in commercial use today. COLD SPRAY EXPLORATION Concurrent to the plasma spray approach, the team also explored cold spray as this technology was being introduced in the U.S. during this timeframe. The team realized that cold spray of soft metals—particularly silver particles— was feasible even in subsonic mode, providing significant opportunity to print very small traces in 2D and 3D. The process featured a fine nozzle and a fluidized bed powder delivery system to transport micron-sized silver particles onto the ceramic and thin polymeric substrates. Because silver has relatively low critical velocities for deposition and is soft, it was not only feasible to deposit these traces but also to obtain high quality performance. Typical silver traces were 100-200 micrometers in width with thickness from 25-100+ microns. The as-deposited silver conductor featured an electrical resistivity that was two times that of bulk silver, which is more than adequate from an applications point of view. Figure 1 shows a cross section of the deposited silver along with a computed microtomography picture obtained from synchrotron imaging. Traces in the submillimeter dimension can be placed side by side with similar gap spacing to produce circuits. DIRECT WRITING OF MESOSCALE CIRCUITS USING SUBSONIC COLD SPRAY With the evolution of cold spray technology and new powder materials, as well as advancements in control and robotics, innovative thermal spray applications are on the horizon. Sanjay Sampath, FASM, TSS-HoF,* Center for Thermal Spray Research Stony Brook University, New York *Member of ASM International Fig. 1 — From left: cross section of silver conductor; computed microtomography image; and 3D white light scanning interferometer graph of coil circuits. Coating thickness is roughly 75 µm. FEATURE
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