Authored By:
Gustaf Mortensson
Chalmers University of Technology, Goteborg, Sweden
Kalland, Erik, Redford, Keith
Mosaic Solutions AS, Skjetten, Norway
Oppland, Ottar
FFI, Kjeller, Norway
Summary
The development of novel interconnection materials for production of electronics is of considerable interest to fulfill increasing demands on interconnect reliability in increasingly demanding environments with respect to temperature extremes, mechanical stresses and/or production limitations. Adhesives are playing an increasingly significant role in the continuously evolving electronics industry.
Electrically conductive adhesives have developed to the stage that they offer a viable alternative to traditional solders for applications that demand high reliability in structurally challenging environments. Conductive adhesives are often divided into groups based on conductive directions; Isotropic Conductive Adhesives (ICA) normally provide almost equal electrical properties in all spatial directions, and Anisotropic Conductive Adhesives (ACA) which are insulating in an unstressed state, but provide directional electrical conduction through connections between filler particles and the local connection points.
Both ICA and ACA have traditionally demanded a high filler content to ensure adequate electrical connectivity. Epoxy based adhesives are often selected due to the vast selection of combinations availability, and traditionally silver fillers are used for obtaining electrical conductivity in ICA. Silver is advantageous since even its oxide maintains high conductivity. Unfortunately, the high cost of silver prevents many applications from using it. An ICA was developed at a much lower cost where solid silver is replaced with metal coated polymer spheres. The polymer spheres are essentially mono disperse in size and can be specifically chosen for different applications. The silver coating of the spheres is approximately 100 nm thick.
Specific applications will be presented that highlight the feasibility of the technology with respect to conductivity, structural reliability and lifetime standards. The deposition of the novel ICA has been performed using a jet printing technology to ensure both precise and accurate positioning, size and volume delivery.
Conclusions
From the tests that were performed in this study, it has been demonstrated that a conductive adhesive utilising spherical, polymer silver particles can be used for no-contact jet printing. The electrical properties of such an adhesive can be controlled by changing the particle size, suspension concentration and/or silver thickness. The electrical resistivity measured in this study is low enough to ensure no measurable resistive losses on the test PCBs. Further work is needed for optimising the properties of the adhesive matrix to improve reliability and deposition quality and reliability.
Initially Published in the IPC Proceedings
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