Nanofiber based composites for thermal management

Since the invention of the integrated circuit increasing transistor integration density has been the main path to increased performance of microelectronic devices. With traditional transistor scaling approaching fundamental limitations, the field of electronic packaging has seen increasing effort towards increased integration density, e.g. three-dimensional packaging. Overall, this has led to tremendous power densities in the order of 100 W/cm². Consequently, thermal management of electronic packages has attracted increasing research interest and has been identified as a research priority by industry consortia iNEMI and ITRS. There is a wide consensus within the microelectronics industry that current thermal interface materials constitute a bottleneck hindering a reduction of junction to ambient thermal resistances to future required levels (<; 0.2 K/W). Hence, there has been a significant increase in research towards improved thermal interface materials. A clear trend observed in thermal interface materials research is the shift from traditional particulate composites, i.e. randomly dispersed high thermal conductivity particles in polymer matrices relying on percolation mechanisms, towards structured composites providing thermal conductivity through a continuous high thermal conductivity phase. In combination with the application of nanotechnology a number of interesting composite materials have been realized, based on e.g. vertically aligned silver and nickel nanowires and carbon nanotubes. In this paper we introduce and discuss a new type of composite based on nanotechnology for thermal management applications. The core technology behind the composite is electrospun nanofibers. Electrospinning is a method for fabrication of polymeric fibers with diameters in the nano- and micrometer range, offering high flexibility in terms of choice of materials. A huge number of polymeric materials have been electrospun and documented in literature, including var- - ious polymeric precursors (e.g. to carbon fibers). In this work we present films of electrospun fibers based on a flexible thermoplastic polyurethane elastomer (Desmopan 9370A, Bayer Material Science AG) and high temperature resistant polyimide (Matrimid 5218, Huntsman Advanced Materials). A typical electrospun polyimide film is exhibited in figure 1. As seen in the image, the film is composed of randomly oriented fibers (mean diameter 780 run) forming a three-dimensional porous polymer structure. The porous polymer film is liquid phase infiltrated with a metal (typically In, Sn, InSnBi and similar alloy systems) under high pressure (30 MPa), leading to the formation of a topologically connected metal network within the polymeric structure (see figure 2). High thermal conductivity is achieved via the continuous metal phase, while the polymeric phase defines geometry (bondline thickness) and composition. In contrast to pure solder interfaces, the inclusion of a polymeric phase lowers stiffness, indicating a potential for lower bondline-thicknesses while still absorbing CTE-mismatch in assemblies. Results from xenon flash thermal characterization of a representative composite; polyimide infiltrated with pure indium, sandwiched between two copper plates is exhibited in figure 3. As seen, the composite exhibits a total contact resistance (2 interfaces) of 8 Kmm²/W. The thermal conductivity is found to be 28 W/mK, approximately 1/3 of the inherent thermal conductivity of the indium metal phase (83.7 W/mK). This clearly indicates the potential of the material in thermal management applications.