Carbon nanotube (CNT) filled adhesives for microelectronic packaging

Abstract

This project evaluated the use of carbon nanotubes as a filler in electrically conducting adhesives in order to enhance the electrical, mechanical and thermal performance. As the carbon nanotubes caused a marked increase in the viscosity of the adhesive, a low viscosity polymer matrix (< 100 mPas) was chosen. This allowed a high CNT content. Multi-wall carbon nanotubes (MWNTs) were chosen for the experiments because these are available in favorable quantities and at reasonable prices. In order to enhance the dispersion properties, the MWNTs were also treated chemically via ozone/UV and low pressure plasma. For mixing the MWNTs into the polymer matrix different methodologies were tested. Ultrasound was found to be a very effective method, but the dispersion energy drops dramatically when viscosities above 10,000 mPas are reached. Hence high viscosity adhesives were treated using a calander as described in [1]. In general, good dispersion could be achieved by applying combinations of these two dispersion techniques. Adhesives were produced with varying CNT content, different CNT types and using different dispersion techniques. The influence of these parameters on the electrical resistance, thermal conductivity, impedance behavior and mechanical strength was investigated. In parallel, many samples were analyzed by TEM in order to get detailed knowledge of the adhesive nanostructure and learn how this structure is influenced by the above- mentioned parameters. Furthermore, the absorption of microwave radiation by the samples was investigated in order to further our understanding of microwave absorption by CNT-filled composites. Based on these results, the two standard formulations showing the best performance were chosen for bonding SMD components on a test assembly. For the curing of the adhesives two methods were used: Curing by microwave radiation and curing in a conventional oven at 130degC. Whilst the conventional curing took 30 minutes, it was possible to completely cure the adhesives using microwave radiation in only 3 minutes. Finally, the bonded assemblies were subjected to a system integrity test. The mechanical stability of the adhesive bonds showed very high resistance to ageing after exposure to humidity and after 1000 thermal cycles.