CNTs - A comparable study of CNT-filled adhesives with common materials

Electronics packaging must be designed to meet the increasing requirements of the microelectronics industry. Future packages will have an even higher number of I/O's and pitches down to 20 microns resulting in high dissipation losses and extreme current densities. When using conventional materials, design engineers will face physical barriers and limitations in performance and new material solutions have to be found. Reliable interconnects are a major concern of packaging technologies. The steadily increasing mechanical, thermal and electrical loadings open up new areas research. One way to solve the problems of future electronics is the use of nanotechnologies and nanomaterials. Among the world's most researched materials are carbon nanotubes (CNTs) [1-5]. CNTs have excellent thermal, electrical and mechanical properties. They can be used in various ways. One researched field of application are CNT-polymer composites which combine common technologies with advanced materials. This paper focuses on the latest results obtained for CNT-filled adhesives for electronics packaging and compares the new materials with conventional, electrical Ag-filled conductive adhesives. Based on previous investigations on CNT-epoxy composites [1-4], a comprehensive study of the mechanical, electrical and thermal properties of selected promising candidates was conducted. Additionally, the performance of the novel composites as conductive adhesives in electronics packages was included in the evaluation. In preliminary investigations, the bulk properties were studied for the cured and uncured states of the materials. The rheological properties (viscosity vs. rate, amplitude) were critically compared for the screen printing performance for the electronics manufacturing process. To determine the effect of the differently modified fillers in the adhesive, an amplitude sweep and a frequency sweep were performed using the rheometer "Rheostress 600" (Haake). From the results it is obvious that- the different filler materials and volume fraction are very influential parameters. The viscosity shown was changed by the filler fraction and the surface modification of the CNTs. Another factor are the thermo-mechanical properties of the bulk materials in the cured state. Polymers are known to exhibit viscoelastic behaviour which is much more complex than that of elastic materials. As for viscoelastic materials, deformation time or frequency influences the reaction of the material. Internal stresses can be reduced by relaxation. The Dynamic Mechanical Analysis (DMA) was applied to determine the temperature and time dependent modulus for the composites. Additional tension and torsion tests were performed to check the consistency of the results. An alternative ultrasonic method was critically benchmarked against mechanical methods. The combination of different test procedures helps gather advanced information on the thermal mechanical behaviour of the adhesive, which is important for future applications in the automotive and aviation industry, for example. Dissipating thermal loss in miniaturized electronic packages is a major challenge for performance issues. The conductivity of the filled adhesives is determined by a laser flash system to get information about their improved thermal properties. It allows measuring the thermal diffusivity of the composite which can be used to calculate the thermal conductivity. Concerning the electrical properties, a test board was used to measure thin film resistance as well as contact resistance of electrical components. The test board structures were manufactured in thick film and printed wiring board (PWB) technologies and cover a wide application range. The PWBs had a chemical tin surface finish on standard FR4 substrate. The thick film application was done on gold metallization. The components were zero Ohm resistors in the sizes 0805- 0402 with tin finish. Different environmental conditions are critical for the technical reliability of adhesive conductive elements. For investigating the moisture and thermal cycle stability of the adhesive joints, a relative humidity test (85% r.h./ 85°C) and a temperature cycle test (-40°C/125°C) were performed. The evaluation criteria were shear strength and the electrical contact resistances of the electrical components. This study investigates the potential and problems of implementing nanomaterials - especially CNTs - in modern electronics packaging applications. The use of nano composites is just one way of introducing CNTs to electronics packaging. Alternative ways of using CNTs are described in different publications.