Gallium based interconnects for flip-chip assembly

Abstract

A feasibility study into alternative methods of producing interconnection between a PCB and flip chip has been undertaken. A number of initial ideas were investigated, the least promising being discarded at an early stage, whilst the ideas showing the greatest chance of success were subject to a more rigorous examination. Of the initial ideas, the most promising were amalgam materials and magnetic alignment of ferromagnetic particles. These two ideas were combined to produce a new type of anisotropic conducting adhesive (ACA) which may have the potential to overcome problems due to z-axis irregularities and have the ability to form fine pitch metallurgical bonding. In order to promote bonding, amalgam compositions that enhance surface wetting while retaining good mechanical properties have been investigated. The possibility of incorporating liquid/semi-solid metallic interconnects within the ACA which retain contact during the thermal expansion of the polymeric materials was also explored. During the course of the study, various techniques such as DSC and SEM have been used to characterise the thermal stability of Ga based alloys and discrepancies with current phase diagrams have been found.