Advances in composite materials for thermal management in electronic packaging

Improved hollow cathode magnetron deposition for producing high thermal conductivity graphite-copper composite

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 [1–3]. 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 K7W) [1]. 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 [4–6]. 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 various 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.

Polymer Matrix Composite Thermal Materials

With increased performance requirements for smaller, more capable, and more efficient electronic systems, thermal challenges have become critical issues in electronic packaging design. Breakthroughs are needed in advanced cooling solutions and pragmatic design at chip, board, and system package levels. In response to these critical needs, advanced materials and process improvements in packaging and cooling technology are required to provide high thermal transfer efficiency, well-controlled thermal transient behavior, environmental compatibility, low weight, and acceptable material and fabrication cost. The lowest device temperature and expected reliability will be achieved when the lowest package thermal resistance is achieved. This chapter provides an outline of thermal challenges in advanced electronic packaging, heat transfer theory, and its application in thermal design, development of thermal management solutions, advanced thermal management materials selection and component design guideline, as well as environmental compatibility requirements in thermal management of electronic packaging.

Metal Matrix Composite Thermal Management Materials

This article discusses use of metal matrix composite materials in electronic packaging. Composites can have higher thermal conductivity than traditional materials, low thermal stress, and tailorable coefficients of thermal expansion. They can reduce weight by as much as 80% and size by as much as 65%. They can also be used in low-cost, net-shape fabrication processes. The silicon carbide content can be adjusted to differing percentages to vary the composite's coefficient of thermal expansion. Traditional microelectronic packaging materials used to achieve low coefficients of thermal expansion include blends of copper and tungsten or of copper and molybdenum, and a nickel–cobalt–iron alloy called Kovar, a trademark owned by CRS Holdings Inc., a subsidiary of Carpenter Technology Corp. of Wyomissing, Pennsylvania. A key firm involved in promoting pyrolytic graphite in packaging is Advanced Ceramics Corp. of Cleveland. An expert predicts that, in the future, many of these materials will also be used in the optoelectronics industry. This will be a new market for the materials, but one with enormous potential.

The dissociation of NZP (Ca 0.5Sr 0.5Zr 4P 6O 24) during plasma spraying

A fin heat sink (FHS) is a thermal heat transfer device employed to dissipate heat from a high temperature heat source to a lower temperature surrounding. A typical FHS consists of a flat metal base with an array of cooling fins on top. A problem normally encountered in thermal management of electronic packages is thermal heat spreading resistance which occurs as heat flows by conduction from a high temperature heat source to a low temperature heat sink with different cross-sectional areas. As high powered semiconductor chips are made more compact and requiring greater heat dissipation, more effective cooling systems have to be devised. There are various methods employed to minimize this heat spreading resistance. These include increasing the thickness of the base of the FHS or height of the fins. Another method is to use more expensive highly conductive materials like aluminum, copper and diamond which would increase cost. A more economical alternative would be to combine a flat heat pipe (HP) sometimes termed a vapor chamber (VC) with a conventional FHS to increase effective thermal conductivity at the base. Thermoelectric (TE) is the direct conversion of temperature difference between the junctions of two dissimilar materials (thermocouple) to electricity. The converse is true. A voltage applied between the junctions of the thermocouple creates a temperature difference between them. This effect could be utilized as a heat pump to transfer heat from the cold junction to the hot junction. A dc voltage imposed across a thermoelectric (TE) module causes a temperature difference to be imposed across the surfaces of the resulting in one face to be at a temperature higher than the other face. Heat is absorbed from a heat source in contact with the cold surface and dissipated to a heat sink in contact with the hot surface. This paper presents the results of an investigation conducted to evaluate the performance of VCs and TEs for the thermal management of LEDs.

The materials selection for thermal management of electronic packaging is influenced by thermal, electrical, physical and thermomechanical requirements of the device and its surrounding electrical system and by the environment to which the device will be exposed. The reliability of the finished device and electric system will depend not only on the characteristics of the individual materials but also on the interaction of package and thermal management materials at interfaces during exposure to such stresses as thermal gradients, temperature cycling, moisture, and contamination. This chapter will introduce general characterization methodologies of thermal management materials for assessing performance and reliability of electronic packaging, including thermal properties, electrical properties, thermomechanical analysis, as well as material microstructure and interface characterization, surface finish and contact interface compatibility, and reliability analysis and environmental evaluation.

The substantial heat generation due to miniaturization and high-degree integration of electronic devices is one of the major issues to facilitate efficient thermal management in power electronics. Though epoxy-based composites have shown great interest in different applications such as laminated circuit board, electronic component encapsulations, and potting, they have low application temperature (up to 150 °C) and higher mismatch of coefficient of thermal expansion (CTE) between the heat source and heat sink. Here, poly(ether ether ketone) (PEEK) composites reinforced with hexagonal boron nitride (hBN) nanoplatelets have been developed by liquid mixing and re-melting method for a step change in composite materials with lower CTE and significantly improved thermal dissipation capability. The lowest achieved CTE is 2.1 µm m−1 K−1, and the highest thermal conductivity is 1.04 W m−1 K−1 in PEEK/hBN composites at 30 wt% hybrid hBN content (hBN platelets with two different sizes, i.e. 70 nm and 500 nm, taken as 1:1 weight ratio), due to the formation of thermally conductive inter-filler networks. The composites show negligible variation in K with the working temperature up to 250 °C. The developed composites also exhibit excellent electrical insulation properties; thus, they will have good potential in thermal management for power electronic applications.Graphical abstract

Composite materials are engineered materials made from two or more constituent materials. They have significantly different physical or chemical properties which remain separate and distinct on a macroscopic level within the finished structure. The advantage of composite material is that they exhibit the best qualities of their components or constituents and often some qualities that neither constituent possesses. The properties that can be improved by forming a composite material are strength, stiffness, corrosion resistance, wear resistance, weight, thermal insulation, thermal conductivity, etc. Composite materials can be classified and characterized into four commonly accepted types; (1) fibrous composite materials that consist of fibers in a matrix, (2) laminated composite materials that consist of layers of various materials, (3) particulate composite materials that are composed of particles in a matrix and (4) the combination of some or all the first three types (Jones, 1999). According to the matrix phase the composites are divided into three groups; (1) metal matrix composites (MMCs), (2) polymer matrix composites (PMCs) and (3) ceramic matrix composites (CMCs). Ceramic materials in general have very attractive properties e.g.: high strength and high stiffness at very high temperatures, chemical inertness and low density. In the presence of flaws (surface or internal) they are prone to catastrophic failures. Ceramic materials can be toughened by incorporating fibers and thus exploit the attractive high-temperature strength and environmental resistance of ceramic materials without risking a catastrophic failure (Chawla, 1987). According to the basics written above, a monolithic silicone carbide (SiC) was used as a matrix phase, which has been recognized as one of the most promising structural materials for many thermo-mechanical applications because of its excellent hightemperature strength and modulus, good oxidation resistance, high hardness, low specific weight and low density (Xin-Bo & Hui, 2005; Xin-Bo et al., 2000; She et al., 1999). The problem with monolithic SiC is its low thermal shock resistance, which leads to cracking and catastrophic failure of the material. Thermal shock resistance and crack propagation can 6

In this study, we present a newly developed carbon aluminum thermal management composite material that has a CTE less than 7.5 ppm/K, thermal conductivity over 400 W/mK, and the thermal diffusivity is 2.5 - 3 times higher than that of Cu and Al. In addition, the composite material is lightweight, and can be easily machined. We also show anisotropy in thermal performance aligning along carbon matrix extrusion direction during manufacturing. Even though this composite material is anisotropic, its thermal conductivity normal to the extrusion direction is comparable with that of Al or Al alloys. Moreover, the electrically insulating composite plates can be obtained by bonding ceramic sheets on composite surface during the composite fabrication process without the use of foreign adhesives.

At present, the growing needs and design requirements with significant weight savings as well as high strength-to-weight ratio as compared to conventional materials raised a growing interest towards composite materials. Metal matrix composites (MMCs) are extensively used in fields like aerospace, automobiles and army-defence applications due to their high specific strength, low coefficient of thermal expansion, wear resistance, high durability, excellent physical and mechanical properties. The filler reinforcements like SiC, TiC, Al2O3 improve the stiffness, specific strength, creep properties of MMCs, but the presence of such hard reinforcements makes MMCs difficult to machine through conventional machining processes. Wirecut electrical discharge machining (WEDM) is highly precise thermoelectrical process and is considered ideal for machining very hard materials, intricate geometries and tooling. This paper reviews the researches in WEDM of metal matrix composites and the relation between different process parameters like pulse on time, pulse off time, wire feed rate,voltage, wire tension, dielectric flow rate on the machining performances such as material removal rate (MRR), Kerf widthand surface roughness (SR). The future trends in WEDM researches were also recommended in this paper.

Advanced composites for high performance marine craft

Purpose This research outlines the development and characterization of advanced composite materials and their potential applications in the aerospace industry for interior applications. Advanced composites, such as carbon-fiber-reinforced polymers and ceramic matrix composites, offer significant advantages over traditional metallic materials in terms of weight reduction, stiffness and strength. These materials have been used in various aerospace applications, including aircraft, engines and thermal protection systems. Design/methodology/approach The development of design of experiment–based hybrid aluminum composites using the stir-casting technique has further enhanced the performance and cost-effectiveness of these materials. The design of the experiment was followed to fabricate hybrid composites with nano cerium oxide (nCeO2) and graphene nanoplatelets (GNPs) as reinforcements in the Al-6061 matrix. Findings The Al6061 + 3% nCeO2 + 3% GNPs exhibited a high hardness of 119.6 VHN. The ultimate tensile strength and yield strength are 113.666 MPa and 73.08 MPa, respectively. A uniform distribution of reinforcement particulates was achieved with 3 Wt.% of each reinforcement in the matrix material, which is analyzed using scanning electron microscopy. Fractography revealed that brittle and ductile fractures caused the failure of the fractured specimens in the tensile test. Practical implications The manufactured aluminum composite can be applied in a range of exterior and interior structural parts like wings, wing boxes, motors, gears, engines, antennas, floor beams, etc. The fan case material of the GEnx engine (currently using carbon-fiber reinforcement plastic) for the Boeing 7E7 can be another replacement with manufactured hybrid aluminum composite, which predicts weight savings per engine of close to 120 kg. Originality/value The development of hybrid reinforcements, where two or more types of reinforcements are used in combination, is also a novel approach to improving the properties of these composites. Advanced composite materials are known for their high strength-to-weight ratio. If the newly developed composite material demonstrates superior properties, it can potentially be used to replace traditional materials in aircraft manufacturing. By reducing the weight of aircraft structures, fuel efficiency can be improved, leading to reduced operating costs and environmental impact. This allows for a more customized solution for specific application requirements and can lead to further advancements in materials science and technology.