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Three-dimensional (3D) integration using through-silicon vias (TSVs) and low-volume lead-free solder interconnects allows the formation of high signal bandwidth, fine pitch, and short-distance interconnections in stacked dies. There are several approaches for 3D chip stacking including chip to chip, chip to wafer, and wafer to wafer. Chip-to-chip integration and chip-to-wafer integration offer the ability to stack known good dies, which can lead to higher yields without integrated redundancy. In the future, with structure and process optimization, wafer-to-wafer integration may provide an ultimate solution for the highest manufacturing throughput assuming a high yield and minimal loss of good dies and wafers. In the near term, chip-to-chip and chip-to-wafer integration may offer high yield, high flexibility, and high performance with added time-to-market advantages. In this work, results are reported for 3D integration aider using a chip-to-wafer assembly process using 3D chip-stacking technology and fine-pitch interconnects with lead-free solder. Stacks of up to six dies were assembled and characterized using lead-free solder interconnections that were less than 6 [micro]m in height. The average resistance of the TSV including the lead-free solder interconnect was as low as 21 m[OMEGA].

The replacement of lead (Pb)-bearing solders used in the electronic industry with Pb-free solders will become a reality in the near future. Several promising Pb-free solders have recently been identified, including Sn--0.7Cu, Sn--3.5Ag, Sn--3.8Ag--0.7Cu, and Sn--3.5Ag--4.8Bi (in wt. % with slight variations in composition). These are all Sn-rich solders with melting temperatures between 210[degrees]C and 227[degrees]C, and are recommended for various soldering applications, including surface mount technology (SMT), plated-through-hole (PTH), ball grid array (BGA), flip-chip bumping, and others. Although a considerable amount of information on Pb-free solders has been published in the last few years, the database on these new materials is still at an infant stage compared with that for Pb-containing solders. This paper addresses several aspects of the current fundamental materials understanding associated with Pb-free solders and various issues regarding their imminent use in electronic interconnect applications, including microstructure-processing-property relations, mechanical properties, intefacial reactions, and the thermal-fatigue life and failure mechanisms of Pb-free solder joints.