Issue



Thermal Management Goes Nano with CNT Fabrics


07/01/2007







BY PETER ANTOINETTE, N

Concepts from the textile industry may soon infiltrate thermal management in advanced packaging, in the form of manufactured spun threads and felted mats composed of single-wall carbon nanotubes (CNTs). These CNTs are grown in lengths of hundreds of microns for fabrics that are highly conductive, thin, light weight, and as strong as carbon steel. Initially developed as advanced fabric for military applications, the nonwoven sheets and yarns have the potential to become heat pipes, thermal interface materials (TIMs), integrated antennae, and even inter-die thermal material in 3D stacked-die packages.

Click here to enlarge image

CNTs are grown in tube furnaces in a continuous-flow process, using custom chemistries and metal-based catalysts. Then, manipulated with the aid of magnetic fields, the CNTs form a spun yarn with electrical and thermal conductivity, which is essentially a conductive “wire.” Nonwoven sheets are produced similarly but use a moving belt to capture CNTs into flat mats. Thinner than paper, the resulting fabricated CNT textile mat is shiny, black, flat, and flexible in thicknesses from 40 μm down to less than 10 μm. Several square feet of the textile are produced every few hours. The figure shows nanotube fibers in a CNT yarn under scanning electron microscopy (SEM) spun as CNT thread; the product is flexible enough to be tied into a knot. Manufacturing techniques have been developed on semi-automated batch systems, which will become continuous automated production, shifting CNTs from laboratory applications to volume processes, in a similar manner as early semiconductor chip fabrication.

The composition of CNTs for these textiles can be varied with additives and process-condition changes to produce CNTs that match specific properties. CNTs are generally grown as 100% pure single-walled structures for the majority of applications. However, the process can dial in double- or multi-wall CNTs for high yields of clean, pure CNTs. Adding different metals to the material can adjust conductivity, coefficient of thermal expansion (CTE), mass, and other properties.

The felted CNT sheets suit conventional pick-and-place systems, inserted as an interface between die and package lid, between a package and heatsink, or integrated into molded caps. CNT conductive threads offer the potential of a woven thermally and electrically conductive fabric, and may replace metal wires, antennae, or heat pipes. Integrating this technology could generate a new format of thermal dissipation in which CNT fabrics pipe heat laterally to a dislocated heatsink attached in a less-dense or cooler region of a module.

Laminating CNT wires and sheeting into other structures could add thermal, electrical, and EMI-shielding capacity in new areas around electronic assemblies. For example, using CNT fabric around the die would dissipate heat, then wrapping the package with CNT sheeting would provide enough EMI shielding to move packages closer together on a module, with CNT wires acting as heat pipes to carry excess heat away to a distant heatsink. Replacing heavy-metal conductors, transformers, antennae for wireless devices, and electro-storage components with CNT materials facilitates the construction of lighter-weight, lower-power packages.

However, CNT fabrics and wires face several challenges in the advanced packaging space. While the material is thin enough for insertion between dice in a high-power/high-heat 3D package, no information is currently available about whether this would offer enough mass to move heat in channels away from the dice. While the fabric is thin and flexible, it does not stretch, which may limit applications within or outside the package, such as wrapping a mobile-phone casing with a CNT sheet. Scale is everything; manufacturing CNT fabric and threads in high yields - with a cost-effective process and few or no post-processing purification steps - will open viable end markets. The materials potentially will serve applications with high heat and limited space, entering end products in gaming, laptop, cell-phone, and related markets. With EMI shielding, thermal conductivity, light weight, ease-of-integration, and beneficial electrical properties, CNT wires and sheets could infuse thermal management into many stages of the packaging process, enabling denser, higher-power, smaller-form-factor assemblies on a high-volume production level using existing equipment.

Peter Antoinette, president and CEO, may be contacted at Nanocomp Technologies, Inc., 162 Pembroke Rd. Concord, NH; 603/442-8992; E-mail: plantoinette@nanocomptech.com.