Advanced Metallurgical Reliability Issues in Flip Chip Technology

University Of California-Los Angeles, Los Angeles CA

Investigators

Abstract

In this grant, interactions among the electrical force, thermo-mechanical force, and chemical force on flip chip solder joint are examined. A previous NSF project established the basis reasons why electromigration in flip chip solder joints becomes a weak link in the interconnect system on silicon devices. When electromigration is combined with thermal stress and solid-state aging (Kirkendall void formation), many new and unexpected reliability problems of flip chip solder joints are encountered by electronic companies. These unique metallurgical reliability issues in flip chip solder joints are studied in this grant. Scientific understanding from the study not only enhances basic knowledge of reliability of solder joints, but also enables optimization of advanced manufacturing processes and materials. It is not because atomic diffusion in the low melting point solder alloy is faster than that in Al or Cu. Actually, grain boundary diffusion in Al and surface diffusion on Cu is close to that of bulk diffusion in solder alloys at the device operation temperature. Rather it is because the.critical product of electromigration of solder alloys is two to three orders of magnitude smaller than that in Al and Cu. So electromigration can fail a solder joint by a current density that is two to three orders of magnitude less than that needed to fail Al or Cu lines. In addition, there are several other factors that enhance electromigration in solder joints. The unique configuration of line-to-bump of the flip chip solder joint produces a serious current crowding phenomenon at the contact between the line and the bump. The joints at the corners of a Si chip are under a very large thermal stress, and the stress concentration overlaps the current crowding. The solder reacts very fast with under-bump metallization and the reaction rate and products can be affected by electromigration due to the polarity effect; the reaction at the cathode is different from that the anode. The demand for flip chip technology in high-density system-in-packaging for advanced electronic consumer products is growing rapidly. Flip chip technology with an area array of solder bumps is the only existing technology that can meet the requirement of a very large number of input/output interconnects for direct chip attachment. Today, little basic materials research is conducted in industry, although materials processing and reliability problems in electronic devices are quite subtle and complicated. In the past, electronic companies were able to beat the timetable projected in the International Semiconductor Technology Roadmap (published by the Semiconductor Industry Association) and gained market lead-time. Today, the schedule for integrating low-k materials into Si devices is pulling back! Similarly, due to the lack of knowledge on reliability of Pb-free solders, mainframe computer makers have obtained exemptions for allowing the use of high-Pb solder until 2010. These two cases reveal the important role of materials research in universities related to electronics manufacturing. In the near future, there is no other technology that can deliver the large number of input/outputs on-chip interconnections as flip chip technology. This study of the basic behavior of solder joints under a combination of electrical, mechanical and chemical forces will benefit mainframe computers as well as consumer electronic products in the US.