A D V A N C E D M A T E R I A L S & P R O C E S S E S | A P R I L 2 0 2 1 3 8 iTSSe TSS iTSSe TSS The cold spray (CS) technique is an innovative solution to join copper and aluminum and overcome the issues associated with soldering and brazing. The CS process is known to deposit the powder particles in solid-state far below the material’s melting point; thus, it can avoid common temperature-induced problems such as high-temperature oxidation, thermal stresses, and phase-transformation. Cold spray is a powder-based technology in which micron-size powder particles are accelerated in the supersonic flow of a compressed working gas through a de Laval nozzle. These powder particles impact the substrate, plastically deform, and create bonding with the substrates. CS offers short production times, virtually unlimited component size capability, and flexibility for localized deposition. THERMAL RESISTANCE EXPERIMENTS To demonstrate the process, the Impact Innovations ISS 5/11 cold spray system and Impact’s cold spray grade copper powder (iMatP_Cu01) were used to produce hybrid-heat sinks. A copper layer was deposited on a base plate of a commercially available extruded aluminum heat sink, as shown in Fig. 1. The thickness of such a copper layer can be adjusted to the electronic device’s design and operational temperature. When discussing a heat sink’s performance, its cooling capability is typically quantified in terms of the thermal resistance, a measure of the temperature rise above ambient on the top of the device per dissipated unit of power. The lower the value of thermal resistance, the higher the cooling ability of the heat sink. To demonstrate the performance of hybrid heat sinks, Impact Innovations conducted experiments to compare the performance of identically structured copper, aluminum, and hybrid heat sinks. The experiment was performed three times, each time with a different heat sink design. Thermal impedance and thermal resistance were measured. The thermal impedance of heat sinks was evaluated by running power cycles at specific load currents heating the device until reaching the thermal equilibrium. Then the load current was switched off, and the voltage drop was recorded. When an aluminum heat sink was tested, a maximum temperature of 438 K was registered. This value corresponds to a thermal resistance of 0.7 K/W. For the copper heat sink, the maximum temperature was just 348 K, and the corresponding thermal resistance was 0.33 K/W. Testing the hybrid-heat sink, themaximum temperature was just slightly higher at 349 K, and the thermal resistance was 0.36 K/W. These results show that the copper and hybrid-heat sinks have almost identical thermal results and outperformed the aluminum heat sink in a substantial fashion, thus showing the importance of quick heat spreading along the base. At the same time, the hybrid heat sinkweighed and cost less than the copper heat sink. Indeed, hybrid heat sinks manufactured by cold spraying have slightly higher production cost than commercially available aluminum heat sinks. However, adding a layer of copper on an aluminum heat sink decreases its thermal resistance by 48%. This has a direct affect on the production costs because the semiconductor area can be decreased by 94%. In addition, the deposition efficiency and deposition rates of copper powder by the cold spray process are 95% (including overspray) and 10 kg/h, respectively, indicating the potential of the CS process to realize a cost-effective large-scale industrial production. ~iTSSe Formore information: Dr. Reeti Singh, principal scientist, Impact Innovations GmbH, Bürgermeister-Steinberger-Ring 1, 84431 Rattenkirchen, Germany, info@impact-innovations. com, www.impact-innovations.com. Fig. 2 — Thermal resistance and maximum temperature obtained at the device using aluminum, hybrid, and copper heat sinks. 6 FEATURE 6
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