Self-aligning flip-chip assembly using eutectic gold/tin solder in different atmospheres

New applications in the field of optoelectronic packaging as well as environmental concerns lead to a growing interest in fluxless assembly processes. Eutectic gold/tin solder offers the possibility of fluxless flip chip soldering on various substrate materials and substrate metallizations. It has been shown that gold/tin solder is applicable for flip chip assembly by different techniques. Using the self-alignment mechanism of the solder, cost effective flip chip assembly with high accuracy can be realized, as the process requires only coarse positioning prior to the soldering. The scope of this paper is to investigate the self-alignment capability of gold/tin solder for fluxless soldering processes in different atmospheres. During the soldering, the atmosphere determines the thickness of the oxide layer which inhibits the wetting of the pads and therefore the self-alignment. In this study, the impact of inert and reducing atmospheres on the self-alignment capability is investigated. The experiments are performed in nitrogen, active atmosphere and hydrogen. Vernier patterns on chips and substrates allow the quantification of the positioning accuracy. Comparing the results, a conclusion about the optimum atmosphere for self-aligning flip chip assembly can be drawn. As an example, the application of the choice process is shown for optoelectronic devices.