Reliability investigations of fluxless flip-chip interconnections on green tape ceramic substrates

The use of flip chip bonding technology has a growing importance in the construction of novel hybrid microelectronic devices and is of increasing interest for the application in consumer oriented products. Fluxless processes, especially, are demanded for the compatibility with underfill materials and for an improved reliability performance. This paper describes the development of a fluxless flip chip mounting process by use of Au/Sn solder bumps on different thick film metallizations of green tape ceramic substrates. The results of the investigations show that fluxless flip chip bonding is possible directly on Au as well as Ag and Pd/Ag thick film pattern and via metallizations. The hip chip assembly process is performed by single chip bonding and requires substrates with narrow planarity tolerances. For the different substrate metallizations, the range of optimal bonding parameters are determined. Best mechanical and electrical results are achieved with Au/Sn bumps on Pd/Ag thick film metallizations. For this system, the investigations are performed to show the influence of the chip size and bump height on reliability. The fatigue life of solder joints, which is limited by the thermal expansion mismatch between chip and substrate, could be significantly increased by an adequate encapsulation process. The reliability results of the fluxless flip chip joints after thermal cycling, temperature storage, temperature-humidity, and pressure cooker tests are presented.