Low cost flip chip (LCFC): An innovative approach for breakthrough reduction in flip chip package cost

Abstract

A new low cost flip chip (LCFC) packaging solution is developed that dramatically reduces flip chip package cost. The solution entails innovations and improvements in the bump, interconnect structure, substrate design and underfilling process. Cu column bumps with solder caps are used to form a “Bump on Lead (BOL)” interconnection with narrow substrate pads with no solder mask confinement (“open solder mask”, or “open SR”) and no solder-on-pad finish (“no SOP”). This results in a substrate design with relaxed design rules despite high I/O escape densities, enabling a majority of IC designs in the sub-1000 pin range to fit in 2-lyr laminate substrates. The novel BOL/open SR/no-SOP interconnect structure is combined with a mold underfilling (MUF) process using high density 1-up matrix substrates to achieve high substrate utilization and low process cost. Furthermore, it is shown through empirical data and FEA modeling that the unique mechanical structure of the BOL interconnection remarkably reduces the stress on Si subsurface layers resulting in elimination of the low K damage phenomenon commonly observed in sub-45 nm Si nodes. The LCFC technology is Pb-free, scalable to very fine pitch in the sub-100 um range and highly electromigration-resistant, providing a natural migration path to 3D/TSV micro bump / micro bond and green solutions for the future. In the present paper, we present in detail, the structure, assembly process and reliability of this packaging solution. We also present data on the application of the solution to advanced sub-45 nm Si nodes with ELK (Extra Low K) dielectrics.