iTSSe TSS 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 7 iTSSe TSS Electronic devices, such as those used in telecommunications and high-power systems, generate heat during normal operation. Yet this heat must be dissipated to avoid junction temperatures exceeding tolerable limits, as it can lead to performance inhibition and deterioration of reliability. It has been shown that every 10 K reduction in the junction temperature will increase the 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 a preferred choice for heat sinks due to its cooling capacity superiority over 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 devices 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 the aluminum heat sink with electronic devices, 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 lowthermal conductivity, affecting theoverall aluminumheat sink’s performance. HYBRID HEAT SINKS 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. HYBRID HEAT SINK MANUFACTURING BY COLD SPRAY Hybrid heat sinks produced via cold spray technology can be a cost effective and lightweight alternative for cooling electronics. Reeti Singh Impact Innovations GmbH, Rattenkirchen, Germany However, there are many issues associated with soldering and brazing, such as corrosion at the interfaces and solder materials with different electrical resistance, as well as thermal expansionmismatch. Fig. 1 — Cold sprayed hybrid heat sinks: (a) application of copper powder coating and (b) resulting sprayed plates. 5 (a) (b) FEATURE 5
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