Printed Circuit Design & Fab - July 2008 - (Page 34)

EMERGING tEchnoloGiES FiGurE 4. Water drop on hydrophobic 80 nm silver coating. FiGurE 5. Self-assembled silver nanoparticles monolayer on a polymer surface through hydrophobic surface modification of a normally hydrophilic system. The iNEMI Nano-Solder Project is working to put these materials into a printable solder paste. The project team is characterizing the metals and working to develop a proof of concept demonstration, using a model similar to the SnAgCu system developed for Pb-free solders (inemi.org/cms/projects/ ba/Pb-free_nano-solder.html). Nano-solders could potentially be available within the next three years. A method has been developed to synthesize lead-free solders such as Sn-3.5Ag-xCu (x=0.2, 0.5, 1.0) by chemical reduction methods.8 Sinterable silver systems are currently available from such companies as Cabot, Cima NanoTech and NanoDynamics. enhanced adhesives. Nanomaterials can enhance mechanical properties of adhesives (even a 0.1% addition of multi-wall carbon nanotubes can raise the flexural strength of an unfilled epoxy by 30%), as well as the electrical properties because of the large number of potential contact or tunneling events when nano-sized particles are present. A reduction of 10x in particle size (for example, from 1 µm to 100 nm) with the same weight content yields a 10 x 10 x 10 (1000-fold) increase in the number of particles present. Several iNEMI members are participating in a University of Binghamton SPIR project (Strategic Partnership for Industrial Resurgence, http://watson.binghamton.edu/level2/industry.html#SPIR) to quantify effects of nanoparticles – metal and carbon-based – in resin-based systems in order to get a consistent dataset to characterize performance. Information from this project will be published. nano-attach technologies. Hook-and-loop fasteners (e.g., Velcro) used the ideas generated by plant burrs to create a new fastening paradigm. An extension would be to use carbon nanotubes, as suggested by researchers at Michigan State University in 2003.9 Simulation of entangled carbon nanotubes demonstrated nanotubes made in curved shapes could be used to develop very high-strength room temperature interconnects. Another innovative application of nanotechnology is biomimetic nano-attach. Textured dry adhesives, based on the gecko foot approach (bio-inspired) where nano-sized hairs attach to surface roughness using van der Waals forces, have been the subject of a great deal of research.10,11 Bio-inspired materials systems 34 using carbon nanotubes or polymer nano-filaments have been demonstrated at a number of research locations with strengths far higher than those that comfortably attach a gecko to a ceiling! The iNEMI Nano-Attach research project is identifying and attempting to demonstrate these types of nano-adhesion techniques that have the potential to replace traditional solder or conductive adhesive assembly processes currently employed in electronics manufacturing. Implementation Challenges Percolation and particles. The addition of fine particles to improve polymer system conductivity has been well documented in the literature. However, duplicating the performance in real-life systems tends to be much more complex. Nano-product implementation can only be achieved if the following factors are addressed: ■ Protective coating. Virtually all nano-metals need a protective coating that permits dispersion in liquids and prevents oxidation or self-sintering. Uncoated silver will sinter to itself at room temperature, and uncoated copper will quickly turn black. Uncoated aluminum and many other metals are pyrophoric. Coatings may be applied in vapor phase by precipitation or chemical reaction. ■ Agglomeration. Nanomaterials have high surface energy and tend to stick to each other. Because the number of particles per unit volume is high, the inter-particle distance is lowered as the particle size decreases for a given content. At the sub 5 µm level, many colloids agglomerate at concentrations of only 3 to 5%. ■ Dispersion. Nanoparticles supplied as dry powders are notoriously difficult to disperse in viscous liquids. The tendency now is to supply them as dispersions in compatible liquids or polymer master batches. ■ Segregation. By varying the hydrophobic/hydrophilic nature of the surface coating, it is possible to encourage the material to disperse in a similar medium or to preferentially congregate on a second phase or at the surface (FiGurES 4 and 5). ■ reactivity. Reaction with catalysts, fillers or other JULY 2008 printEd circuit dESign & fAB http://inemi.org/cms/projects/ba/Pb-free_nano-solder.html http://inemi.org/cms/projects/ba/Pb-free_nano-solder.html http://watson.binghamton.edu/level2/industry.html#SPIR http://watson.binghamton.edu/level2/industry.html#SPIR

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