William E. Bernier

Flip-Chip Interconnections: Past, Present, and Future

Flip-chip interconnection technologies have been extensively used in many microelectronic applications for high performance systems as well as consumer electronics in recent years. There are several types of flip-chip interconnects being used today in the industry, which include high-Pb solder bumps joined to a ceramic substrate, high-Pb bumps on chip joined to eutectic PbSn on a laminate substrate, all eutectic PbSn bumps, Pb-free bumps, Cu pillar bumps, and Au-stud bumps. Steady improvements have been made in high performance packages, such as having interconnects greater than 10,000 I/O’s with a pitch less than 200 μm, migrating from ceramic to low-cost organic substrates, replacing high-Pb with Pb-free interconnects, and others. Fundamental reliability issues, especially with the Pb-free solder bumps, remain to be solved. As the expiration of European Union (EU) Reduction of Hazardous Substance (RoHS) exemption approaches soon, Pb-containing solder bumps for flip-chip interconnects are expected to be phased out in a few years. However, the recent introduction of fragile low-k, or very fragile ultra low-k interlayer dielectrics (ILD) into the back-end interconnect architectures in the advanced semiconductor devices has imposed serious technical challenges in integrating Pb-free technologies for high-performance systems. How to control the cracking of low-k ILD layers during chip joining has become an urgent issue to be resolved together by the semiconductor and packaging industries. Another reliability challenge associated with the implementation of Pb-free solder in flip-chip packages is the poor electromigration (EM) performance of Sn-rich solders. This is mainly due to the highly anisotropic crystal structure of Sn, causing the fast solute diffusion along its c-axis, combined with the aggressive interfacial reactions with under-bump metallurgy (UBM) and laminate pads.

Reactive ion-exchange precipitation procedure for the determination of trace amounts of oxalate

The feasibility of using reactive ion-exchange for the determination of oxalate in aqueous solution has been demonstrated. The procedure consists of pH adjustment of the analyte solution, concentration of oxalate on a lead(II)-loaded cation exchange mini-column in the form of lead oxalate precipitate, reactive elution of oxalate by sulphuric acid dissolution and analysis of the concentrated eluate by permanganate titration. Almost quantitative yields of oxalate are obtained for concentrations typically found in human urine. The procedure represents a simple and reliable technique to concentrate oxalate for determination by classical methods.