Electronic devices, including those in telecommunications and high-power systems, generate heat during normal operation. That heat must be dissipated to avoid junction temperatures exceeding tolerable limits, which can lead to performance inhibition and deterioration of reliability.
It has been shown that every 10K (Kelvin) reduction in the junction temperature will increase a device’s life and performance. Thus, maintaining the junction temperature below the maximum allowable limit is a primary issue.
The most common way to cool devices has been air/liquid cooling using a heat sink. Conventionally, copper and aluminum heat sinks are used in combination with such cooling systems.
Copper is always a preferred choice for heat sinks due to its cooling capacity being superior to aluminum; however, copper’s weight and cost limit the size, especially for large electronics systems. Whereas, due to lower thermal conductivity, aluminum heat sinks do not spread the heat quickly enough; thus, a large surface area or taller fins are required, which is not a plausible option in many cases.
Moreover, a problem arises if a heat sink is substantially larger than the integrated circuit device it resides on. If the electronic device generates heat faster than the heat sink spreads, portions of the heat sink far away from the device do not contribute much to heat dissipation. In other words, if the base is a poor heat spreader, much of its surface area is wasted.
Furthermore, to connect an aluminum heat sink with an electronic device, a thermal interface material is generally used because soldering of aluminum with direct-bond copper of the electronic device is difficult. Typically, this material has a very low thermal conductivity, affecting the overall aluminum heat sink’s performance.
A hybrid heat sink, combining the thermal benefits of copper with lightweight aluminum presents an exciting alternative to overcome the issues associated with conventionally available copper and aluminum heat sinks. In such a concept, the portion of the heat sink that comes in contact with the electronic device is made of copper, while the other portion is made of cheaper and lighter aluminum.
However, joining aluminum and copper is a difficult challenge. Soldering and brazing is commonly used to join aluminum with copper in industrial refrigeration, air conditioning, and heat exchangers. However, there are many issues associated with soldering and brazing, such as corrosion at the interfaces, and solder materials with different electrical resistance and thermal expansion mismatch. 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.
1. Cold spray is a power-based technology. It deposits powder particles in solid-state far below the material’s melting point. This image shows the cold spraying process, along with some cold-sprayed hybrid heat sinks. Courtesy: Impact Innovations
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 (Figure 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 rise in temperature above ambient on the top of the device per dissipated unit of power. The lower the value of thermal resistance, the higher is the cooling ability of the heat sink.
To demonstrate the performance of hybrid heat sinks, Impact Innovations conducted experiments on 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 the 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.
2. This graph shows thermal resistance and maximum temperature obtained at the device using aluminum, hybrid, and copper heat sinks. Source: Impact Innovations
When an aluminum heat sink was tested, a maximum temperature of 438K was registered. This value corresponds to a thermal resistance of 0.7 K/W. For the copper heat sink, the maximum temperature was just 348K, and the corresponding thermal resistance was 0.33 K/W. Testing the hybrid heat sink, the maximum temperature was just slightly higher at 349K, and the thermal resistance was 0.36 K/W (Figure 2).
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 sink weighed 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 effect on the production costs since the semiconductor area can be decreased by 94%. Besides, the deposition efficiency and deposition rates of copper powder by the cold spray process are 95% (including overspray) and 10 kilograms/hour, respectively, indicating the potential of the CS process to realize cost-effective large-scale industrial production. ■
—Reeti Singh, PhD is principal scientist at Impact Innovations GmbH.