Nanotech and the Semiconductor Industry: Will Packaging Be Early Adopter?
The semiconductor industry’s position on nanotechnology has been interesting to observe over the last decade. The industry has not only embraced it as a concept, but claimed ownership — rightfully so I think — as the original inventors of nanotechnology. It’s hard to argue that point, since work at dimensions well beyond the nano level have been a matter of course for quite some time.
However, aside from routinely working at incredibly small dimensions, the semiconductor industry has not really implemented what many would consider the true value of nanotechnology, where materials behave in a unique and useful way at very small dimensions. The point could be argued that the transistor does exactly that, but there’s been no widescale implementation of shall we say “modern” nanotechnology. Carbon nanotubes in vias, for example, are not in production, although that’s been fairly heavily researched and shows a high level of promise. Ditto for self-directed assembly, which could get around the need for expensive and challenging lithography steps. The same is true for carbon nanowires.
For the most part, ICs are still fabricated with the same deposition and patterning techniques as they have for the last decade, albeit with an increasing number of new materials. Where nanotechnology is showing great promise, and could by quickly implemented, is in the assembly and packaging area.
At the recent Nanotech Conference and Expo, researchers from the Singapore Institute of Manufacturing Technology described how nanoparticles and carbon nanotubes could be used to enhance the properties of lead-free solders. They reported that an attractive and viable method of enhancing the performance of a solder is to introduce nano-size, non-coarsening second phases to a conventional solder alloy, so as to form a nanocomposite solder.
Solder materials containing nano-sized metals exploit the high surface area and high surface energy of nano-sized particles to lower the apparent melting point below the conventional melting point. Thus the melting points of tin, silver and copper, the ingredients of most popular lead-free solders, all can be depressed below 200??C, well below the eutectic melting point of 217??C.
The lead-free nanocomposite solders (Sn-Ag-Cu) solders investigated by the Singapore Institute were fabricated using powder metallurgy technique followed by extrusion. The researchers said that characterization results “convincingly established” that composite technology coupled with nanotechnology in electronic solders can lead to simultaneous improvement in thermal performance — in terms of better thermal stability with increasing amount of nano-size reinforcements — and mechanical performance — in terms of better 0.2% yield strength (13???23% increase) and better ultimate tensile strength (17???34% increase). There was also no compromise on the melting temperature and electrical performance. With the addition of nano-size reinforcements, better wettability (in terms of lower contact angle) of the nanocomposite solders was also observed.
In other work, also presented at the Nanotech Conference, researchers from the Norwegian University of Science and Technology reported that metalized and monodispersed polymer particles with diameters ranging from 0.5 to 30 ??m are increasingly used in developing new electrical packaging technology, such as anisotropic conductive adhesives in flat panel displays. In these applications, the large deformation of the particles is required to achieve a reliable and low resistance connection.
Nanotechnology may also have potential in solders at the board level. Traditional SMT assembly processes rely on mass solder reflow, which introduces a potential product reliability risk with its high thermal excursion. The risk is particularly high for assemblies with an uneven thermal mass, thermally sensitive components, or critical field reliability requirements, according to industry group iNEMI. Lead-free processing exacerbates the issue, with reflow peak temperatures 20?????40??C higher than eutectic reflow. Often post-reflow processing is required to attach sensitive components, increasing process complexity, cost and cycle time. A project at iNEMI called Nano-Attach identified electronic assembly applications that would benefit from nano-attach adhesion techniques and the necessary requirements for their adaptation. Nano-attach technology gaps were identified from the team’s technology benchmarking work. Currently, the team is preparing for the second phase of the project, which will undertake an experimental evaluation of key parameters of available nano-attach materials.