Reliability of nano-structured nickel interconnections replacing FlipChip solder assembly without underfill

This paper reports the reliability of fine pitch interconnections using nano-structured nickel as the primary interconnection material. Assembly was accomplished with different bonding methods to provide organic compatible low-temperature fabrication. Au-Sn and Sn-Cu were used for solder- based assembly of nanonickel interconnections. Low modulus conductive adhesives impart lower stresses in the interconnections and enhance reliability though they add electrical parasitics. These were used as an alternate bonding route and compared to solders. Test vehicles were fabricated at 200 micron pitch to evaluate the reliability with different bonding routes. Different CTE substrates - FR4 with 18 ppm/C, advanced organic boards with 10 ppm/C, novel low CTE (3 ppm/C) substrates based on Carbon-Silicon Carbide (C-SiC) were evaluated. No underfilling was used in all the test vehicles evaluated in this study. High frequency electrical characterization was performed to compare the electrical parasitics from different bonding routes. Nanometal bumps bonded with conductive adhesives showed the highest reliability withstanding 1500 cycles. This technology can be easily downscaled to submicron and nanoscale unlike the current solder technologies leading to true nanointerconnections.