Adhesive and Conductive - Inkjettable nano-filled inks for use in Microelectronics and Microsystems Technology

Adhesive and Conductive – Inkjettable nano-filled inks for use in microelectronics and microsystems technology Edouard Marc Meyer**, Andreas Arp***, Francesco Calderone**, Jana Kolbe*, Wilhelm Meyer***, Helmut Schaefer*, Manuela Stuve* * IFAM - Fraunhofer Institut fuer Fertigungstechnik und Angewandte Materialforschung, Klebtechnik und Oberflaechen, Wiener Strasse 12, D-28359 Bremen, Germany ** Metalor Technologies SA, Avenue du Vignoble, CH-2009 Neuchâtel, Switzerland *** Microdrop Gesellschaft für Mikrodosiersysteme GmbH, Muehlenweg 143, D-22844 Norderstedt, Germany ABSTRACT Current technology, Inkjet is an accepted technology for dispensing small volumes of material (50 – 500 picolitres). Currently traditional metal-filled conductive adhesives cannot be processed by inkjetting (owing to their relatively high viscosity and the size of filler material particles). Smallest droplet size achievable by traditional dispensing techniques is in the range of 150 µm, yielding proportionally larger adhesive dots on the substrate. Electrically conductive inks are available on the market with metal particles (gold or silver) 99.997%. The technical approach retained to match the specificities of ink-jetting is based on a 2-step curing system: Dispensing into a final form, pre-cure to a material that can be easily handled (the jettable material would not allow easy processing outside of the inkjet itself), followed by the component assembly step and final curing of the conductive adhesive. cannot penetrate deeply into the formulation to initiate the first radical curing reaction. Again, not every silver powder can be used. Surface morphology plays an important role and some degree of UV reflectivity could be achieved with the selected Ag material. Special attention had to be paid to include various UV initiators. They are essential for propagation of the reaction into the resin matrix shadowed by the filler particles. With the present adhesive, curing of layers up to 30 µm thick was achieved successfully. (This fact is noteworthy, as to the best of our knowledge, there are no commercially available conductive adhesives based on radical UV curing.) The second, thermal curing step is accelerated by imidazoles. A specific electrical resistance of 10-4 Ωcm and bond strength of 10-15 N were achieved with SMDresistors (case size 1206) on copper. The resin system was designed with a first UV-cure with radical mechanism. After inkjet deposition on the substrate, the adhesive is partially cured by UV (Prepreg) and can be processed immediately or stored at 10 g/ cm3 for silver! 8 15 N were achieved with SMD-resistors (case size 1206) on copper substrate. This joint development of resin system, silver particles and inkjet device has opened the door to apply the technology for electrically conductive joining in the microsystem and microelectronic fields. 10 ACKNOWLEDGEMENT The present work was supported by the European Commission (Growth-Program) under G1RD-CT-200200656 and by the Swiss Confederation under OFES 01.0575. Thanks also to Dr. C. Dullaghan and K. McNeilly (both Metalor) for their valuable contributions. OUTLOOK Future work will be done with a 50 µm nozzle. This will reduce the dot diameter. The current objective is to generate 80 µm dots. Additionally a new dispenser system (Dispenser Patent n° DE 10153708) is under development. It is characterized by a reduced dead volume and integrates a fluid circulation device. With this system the sedimentation of particles in the fluid can be prevented. First tests with ethylene glycol and fluids with low particle content have been performed already. They show a similar performance as the system described above. The optimization is still ongoing. Figure 8: Single ICA dot deposited by inkjet 9 CONCLUSIONS Arrays of ICA dots of 130 µm diameter with a pitch of 200 µm have been produced reproducibly. Specific electrical resistance of 10-4 Ωcm and bond strengths of 10- 444 NSTI-Nanotech 2005, www.nsti.org, ISBN 0-9767985-1-4 Vol. 2, 2005