Development of Chip-on-Dot flip chip technique utilizing Gold Dot/sup TM/ flexible circuitry

Abstract

A novel flip chip assembly technique, called Chip-on-Dot, has been developed for assembling chips which does not mandate either under-bump metallization (UBM) treatment to the aluminum input/output pads or die bumping. Instead, the die are flipped upon flexible Gold Dot/sup TM/ substrates possessing raised contact features integral to the circuitry. The copper conductors possess small and exactingly shaped raised contact bumps (Gold Dots). When the Al die pads are aligned and pressed upon the mating dots of the substrate in conjunction with temperature, diffusion between aluminum and gold establishes an inter-metallic bond layer at the contact interfaces. Electrical connection is thus established. A non-electrically conductive adhesive, which is dispensed prior to bonding and simultaneously snap cured during the bonding process enhances the mechanical integrity of these interconnections. Finite element contact analysis was conducted to study the pressure distribution in the contact area of a dot. By simulating the Chip-on-Dot assembly process, the optimal dot geometry was designed which provided a uniform contact pressure across the contact area. This additionally promoted diffusion bonding across the whole contact area and produced little damage to the Al pads. Using the PCMCIA card format, an electrically functional prototype was constructed and populated with 12 die using the Chip-on-Dot technique. Subsequently, the test vehicle was subjected to accelerated environmental testing. The following test results show that this assembly process is robust for commercial packaging applications.