Gallium alloy interconnects for flip-chip assembly applications

For miniature interconnection applications, innovative material systems based on gallium alloys offer potentially attractive alternatives over commonly used bonding materials, such as solders and conductive adhesives, without the reliability and environmental drawbacks. Gallium alloys are mechanically alloyed mixtures of a liquid metal and metallic powders, formed at room temperature which cure to form solid intermetallic interconnects. Through the course of this work, gallium alloys have been investigated for flip-chip interconnect applications. Specifically, this paper presents the results of a preliminary feasibility study demonstrating the applicability of gallium alloys as flip-chip on laminate interconnect materials. The topics covered include the test vehicle assembly process, reliability screening results, preliminary failure mode analysis, and interconnect microstructure analysis. To demonstrate preliminary feasibility and application, gallium alloyed with copper and nickel was used as micro-miniature interconnects between bare silicon chips and printed circuit boards. This initial study shows feasibility of such interconnects and the reliability tests demonstrate sufficient cyclic fatigue reliability in the presence of underfill material. Moreover, through the course of this work a new micro-dispensing technology for gallium alloys was developed which leverages existing industry infrastructure. This initial study represents a significant advancement in microelectronic interconnect materials unveiling the potential for an innovative lead-free interconnect alternative.