SBIR-STTR Award

We are proposing the development of adhesives that will be capable of conducting heat and electricity at levels significantly above currently available systems while offering improved processability. A novel thermoplastic/epoxy hybrid paste adhesive will use an advanced formulation approach to achieve unusually high conductive filler and thermoplastic loadings while still maintaining a low viscosity. It will have a thermal conductivity of more than 15 W/mK combined with an electrical resistivity of less than 40 microhm-cm. This is a combined performance better than the separate film adhesives currently used for the F-22 sub-array assembly. Further improvement of the electrical or thermal properties will be possible with additional development. A high thermoplastic content will allow rework and will provide unusual toughness to resist thermal cycling failure. The advanced formulation approach to both high filler and thermoplastic content will still allow a material that flows like current pastes, thus allowing the productivity gains of automatic dispensing. In order to provide a technology backup and process alternative, we will also develop an epoxy film with both a thermal conductivity (>40 W/mK) and an electrical resistivity (<20 microhm-cm) better than solder. This z-axis film is the next generation of technology of our z-axis thermally conductive adhesive that has been developed for the BMDO/USAF. This will eliminate the need for separate adhesives for the F-22 sub-array assembly in areas of very high thermal or electrical transport across an adhesive bond line and will open many applications for solder replacement. The best thermally conductive adhesives are diamond filled and have a thermal conductivity of 10-12 W/mK with very high electrical resistance. The highest electrically conductive adhesives have a resistance of 100 to 500 microhm-cm and a thermal conductivity of 2-5 W/mK. Our proposed automatic dispensable paste adhesive will initially provide up to 20 W/mK thermal conductivity together with less than 40 microhm-cm electrical resistivity. A flowable paste adhesive with these much higher conductivities will open up many high performance electronic assembly applications due to the productivity improvements created by automated processing. The proposed z-axis film adhesive will be able to out perform solder in both thermal and electrical conductivity while requiring a sharply lower processing temperature. Such significant performance enhancement will find application in a variety of electronic packaging products and processes to reduce the yield loss caused by the high processing temperature of solder

Following successful initial testing during Phase I, we are proposing continuing the development of both paste and film adhesive candidates during the first half of the Phase II program, then qualifying one of the adhesives for the F-22 radar sub-array production program. The prototype paste adhesive has already exceeded F-22 bonded joint thermal and electrical conductivity requirements with a metallic filler that will preclude galvanic corrosion of the aluminum substrate. The key Phase II challenge will be developing this paste to meet the unique F-22 assembly requirements necessary for maximizing production labor and cycle time savings. Significant improvements in current needle dispensed paste processing will be essential in making this adhesive a viable candidate: rapid large area dispense coverage (up to 1 square foot in a few minutes); maintaining tight tolerance component location and moderate adhesive strength of large bond areas during a fast pre-cure stage; and maintaining an easy rework capability (< 5 lbs. force@ < 130 C for > 10 cm2 bond area). Using metallic coated fibers oriented in the z-axis, the prototype film adhesive came close to meeting the bonded joint conductivity requirements. Phase II development will use a heavier metallic coating to ensure sufficient conductivity margin. The key Phase II challenge will be to modify the polymeric adhesive formulation so that a low pressure (<15 psi) applied during component placement will replace the current process that uses constant autoclave pressure during the temperature cure cycle. Also, film adhesive tackiness at low temperatures ( >60 degrees C) needs to be increased to maintain tight tolerance component location while the substrate is flipped upside down for component placement on the opposite side of the substrate.

Keywords:
Adhesive, Thermally Conductive, Z-Axis, Anisotropic, Electrically Conductive, Film, Paste