Thermomechanical Reliability of Nickel Pillar Interconnections Replacing Flip-Chip Solder Without Underfill

Abstract

Interconnect technologies between ICs and packages or boards have a significant impact on the IC performance and packaging density. Today, the interconnections are typically accomplished with either wire bonding or flip-chip solders. While both of these technologies are incremental, they also run into either electrical or mechanical barriers as they are extended to higher density of interconnections. Downscaling traditional solder bump interconnect might not satisfy the thermomechanical reliability requirements at very fine-pitches. Alternate interconnection approaches such as compliant interconnects typically require lengthy connections and are therefore limited in terms of electrical properties, although expected to meet the mechanical requirements. This paper reports fine-pitch interconnection technologies using nano-structured nickel as primary interconnection material. The nano-grained nickels are produced by electroplating process. The primary nano-structured interconnects are assembled with different bonding methods to provide organic compatible low-temperature fabrication. Au-Sn and Sn-Cu are used for solder-based assembly of nano-nickel interconnections. Low modulus anisotropic conductive films (ACFs) are also used as an alternate bonding route of the solders. No underfilling is used in all the interconnect structures evaluated in this paper. Assembly are accomplished on different coefficient of thermal expansion (CTE) substrates including FR-4 with 18 ppm/degC, advanced organic substrates with 10 ppm/degC, novel low CTE (3 ppm/degC) substrates based on carbon-silicon carbide (C-SiC). The thermomechanical reliability of all the nano-interconnects assembled on different CTE substrates with different bonding approaches is evaluated by thermal shock testing and finite-element analysis. Nano-nickel interconnects bonded with the ACF showed the highest reliability withstanding 1500 cycles. In all cases, no apparent failure was observed in the primary nano-nickel metal interconnects. This technology is expected to be easily downscaled to submicrometer and nano-scale unlike the current solder technologies leading to true nano-interconnections.