Nano nickel-tin interconnects and electrodes for next generation 15 micron pitch embedded bio fluidic sensors in FR4 substrates

Lead free solder bumping requirements have challenged researchers to develop technologies to achieve fine pitch interconnects. ITRS has predicts that by 2017 the industry require 70 micron pitch area array lead free interconnects for flip chips. This paper describes bumping, assembly and reliability evaluation of a nano composite 15 micron pitch interconnect technology. Nanoparticles are the most common building blocks in several applications of nanotechnology. Beside, the enormous increase of their surface area, the high surface to volume ratio of the nano particles results in extraordinary high reactivity, and unusual physical properties (optical, magnetic, etc.). Several types of nanostructures (nano particles, nano-wires and nano-rods) can be fabricated. Dispersed nano particles are used in several important applications, e.g., catalysis, biomedical applications, nano coatings and nanocomposites. Nanostructured materials, prepared by consolidating nanoparticles with a very high density of grain boundary, have been shown to lead to dramatically improved mechanical and physical properties. Lead free interconnect reliability has been a concern due tin pest (whiskers), intermetallics and high temperature reflow problems. This problem becomes even more pronounced when the pitch of the flip chip reduces to 100 microns or lesser. This paper proposes a nano metal composite for fine pitch lead free interconnect as an alternative to solder for fine pitch flip chip interconnect as presented in R. R. Tummala (2001) and W. D. Brown (1999). This paper describes design, process, fabrication and reliability test for composite interconnects fabricated using Ni nano particles distributed in a tin core. A wafer level process is developed for flip chip having 15 micron pitch composite nano structured interconnect. The composition and intermetallics formed at the chip-to- interconnect-interface are evaluated when sent through 4 reflow cycles. Composition of the intermetallics is eva- - luated to study the distribution Ni3Sn4, Cu6Sn5 and Cu3Sn. Shear test on interconnects provide bump adhesion strength and fatigue resistance information. The failure mode associated with shear test is also presented. The chip is thermally cycled in a oven from -55degC to 125degC and after every 100 cycles is evaluated for defects using x-ray and Sonoscan techniques. A cross section is made and the composition at interface is also evaluated for intermetalic diffusion. The paper provides optimized data for next generation nano composite interconnect for fine pitch flip chip to achieve high reliability. The paper also describes a process to fabricate substrates with nano lines and embedded fluidic channels