Flip-Chip Interconnection Using Anisotropic Conductive Adhesive with Lead Free Nano-Solder Particles

The flip-chip interconnections obtained using anisotropic conductive adhesives (ACA) containing nanoparticles of Sn-4.0Ag-0.5Cu and Sn-0.4Co-0.7Cu (wt% composition) lead free solder alloys as fillers were studied in order to make use of the nanosize effect in flip-chip interconnection applications. The ACAs were formulated using epoxy resin, curing agent, coupling agent and lead free solder nanoparticles. The filler content used in this study was 1wt%. The silicon chips with electroless nickel and immersion gold platted bumps, and FR-4 test boards with copper, nickel and gold platted pads were used in this work. The average contact resistances of the flip chip joints, measured by means of four probe measurement method were 7.08 mOmega and 6.69 mOmega for joints with Sn-4.0Ag-0.5Cu & Sn-0.4Co-0.7Cu nanoparticles respectively. As epoxy resins are hygroscopic in nature and they absorb water, pressure cooker test was carried out in order to study the effect of severe humidity and temperature on the integrity and reliability of nanointerconnects formed in flip chip packages. It was observed that after 24 hours of pressure cooker testing, there was a slight increase in contact resistance due to moisture absorption causing hygroscopic swelling which induced residual stresses in the package. The measured contact resistance values after the pressure cooker test were 10mOmega~32mOmega