COMPLIANT MICROSTRUCTURES FOR ENHANCED THERMAL CONDUCTANCE ACROSS INTERFACES
With the extreme increases in power density of electronic devices, the contact thermal resistance imposed at interfaces between mating solids becomes a major challenge in thermal management. This contact thermal resistance is mainly caused by micro-scale surface asperities (roughness) and wavy profile of surface (nonflatness) which severely reduce the contact area available for heat conduction. High contact pressures (1~100 MPa) can be used to deform the surface asperities to increase contact area. Besides, a variety of conventional thermal interface materials (TIM), such as greases and pastes, are used to improve the contact thermal conductance by filling the remaining air gaps. However, there are still some applications where such TIMs are disallowed for reworkability concerns. For example, heat must be transferred across dry interfaces to a heat sink in pluggable opto-electronic transceivers which needs to repeatedly slide into / out of contact with the heat sink. Dry contact and low contact pressures are required for this sliding application.
This dissertation presents a metallized micro-spring array as a surface coating to enhance dry contact thermal conductance under ultra-low interfacial contact pressure. The shape of the micro-springs is designed to be mechanically compliant to achieve conformal contact between nonflat surfaces. The polymer scaffolds of the micro-structured TIMs are fabricated by using a custom projection micro-stereolithography (μSL) system. By applying the projection scheme, this method is more cost-effective and high-throughput than other 3D micro-fabrication methods using a scanning scheme. The thermal conductance of polymer micro-springs is further enhanced by metallization using plating and surface polishing on their top surfaces. The measured mechanical compliance of TIMs indicates that they can deform ~10s μm under ~10s kPa contact pressures over their footprint area, which is large enough to accommodate most of surface nonflatness of electronic packages. The measured thermal resistances of the TIM at different fabrication stages confirms the enhanced thermal conductance by applying metallization and surface polishing. Thermal resistances of the TIMs are compared to direct metal-to-metal contact thermal resistance for flat and nonflat mating surfaces, which confirms that the TIM outperforms direct contact. A thin layer of soft polymer is coated on the top surfaces of the TIMs to accommodate surface roughness that has a smaller spatial period than the micro-springs. For rough surfaces, the polymer-coated TIM has reduced thermal resistance which is comparable to a benchmark case where the top surfaces of the TIM are glued to the mating surface. A polymer base is designed under the micro-spring array which can provide the advantages for handling as a standalone material or integration convenience, at the toll of an increased insertion resistance. Through-holes are designed in the base layer and coated with thermally conductive metal after metallization to enhance thermal conductance of the base layer; a thin layer of epoxy is applied between the base layer and the working surface to reduce contact thermal resistance exposed on the base layer. Cycling tests are conducted on the TIMs; the results show good early-stage reliability of the TIM under normal pressure, sliding contact, and temperature cycles. The TIM is thermally demonstrated on a pluggable application, namely, a CFP4 module, which shows enhanced thermal conductance by applying the TIM.To further enhance the potential mechanical compliance of microstructured surfaces, a stable double curved beam structure with near-zero stiffness composed of intrinsic negative and positive stiffness elastic elements is designed and fabricated by introducing residual stresses. Stiffness measurements shows that the positive-stiffness single curved beam, which is the same as the top beam in the double curved beam, is stiffer than the double curved beam, which confirms the negative stiffness of the bottom beam in the double curved beam. Layered near zero-stiffness materials made of these structures are built to demonstrate the scalability of the zero-stiffness zone.