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Development of an adhesive chip assembly technology for dedicated electronic applications


Objectives and content

The future production of electronic systems combines a clear trend of an increasing complexity with a size reduction of the subcircuits. This results in an increasing chip functionality density and I/O pin count. The needs for chip packaging and interconnection dictated by this trend, cannot be met by the traditional assembly technologies. All viable solutions to replace the traditional technologies are flip-chip (FC) solutions. Although this technology has been existing for some decennia now, there has never been a major breakthrough because of the required expensive IC back-end processing and dedicated assembly technology. Today the most widely used FC technology is the IBM "C4" process, using solder bumps. This process has some important drawbacks. It requires expensive additional wafer processing and cannot be used on non-solderable substrates like flex and glass.

A very promising alternative is offered by the recent developments of adhesive technologies. Flip-chip with isotropic conductive adhesives (ICAs) and anisotropic conductive adhesives (ACAs) have been used for several years now for mounting driver IC's on liquid crystal displays (chip-on-glass, COG). However ICAs are presently not suitable for pitches below 200 m and need an underfill; ACAs do yet not achieve low contact resistance. A third category of adhesives, nonconductive adhesives (NCAs) are used only marginally until now.
It is obvious that there is a need for further development of adhesive joining technologies to solve the shortcomings listed above. It is believed that a conductive adhesive joining technology will lead to higher packaging densities at lower costs. Furthermore adhesive joining is considered as one of the candidates for replacing Pb-based solders and make electronic assembly technology more environmental friendly.

In the proposed DACTEL project, new adhesives (ICAs, ACAs and NCAs) and assembly technologies will be developed for various kinds of flip-chip assemblies: chip-on-glass (COG), chip-on-flex (COF), chip-on-board (COB) and Ball Grid Array (BGA) assemblies. The joining technology to be developed in this project will have the following innovations:
- Low cost back-end processing of standard available wafers with aluminium bondpads to enable flip chip assembly (with or without bumps). - Development of adhesives and adhesive joining technology (isotropic, anisotropic and non-conductive) with improved electrical, processing, repairability and reliability properties.
- Reduction of process steps by combining the interconnect and the underfill function within one adhesive (anisotropic or non conductive). - Introduction of flip chip bonding onto low cost substrates like polyester flex and laminate. Improvement of interconnection density down to 100 m pitch. Improvement of contact resistance and current density - Development of mathematical tools for calculation of stress distribution and probability of short and open circuits.

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Beteiligte (12)