Fluxless flip chip bonding on flexible substrates: A comparison between adhesive bonding and soldering

During the last few years an increasing number of flip chip (FC) interconnection technologies have emerged. While flip chip assembly offers many advantages compared to conventional packaging techniques, several aspects hinder this technology from entering the high volume market. Among these are the availability of bumped chips and the costs for the substrates, i.e. ceramic substrates with closely matching coefficient of thermal expansion (CTE) to the chip, in order to maintain a high reliability. Only recently, with the possibility of filling the gap between chip and organic substrate with an encapsulant, was the reliability of flip chips mounted on organic substrates significantly enhanced. This paper presents two approaches to a fluxless process, the one based on soldering techniques using the Au-Sn metallurgy and the other on adhesive joining techniques. Soldering is performed with a thermode and with a laser based system. For both of these FC-joining processes, alternative bump metallurgies based on electroplated gold, electroplated gold-tin, mechanical gold and electroless nickel gold bumps are applied