Most of the compliant interconnects that are currently being developed have inductance and resistance higher than desirable. There is definitive evidence that nano-structured interconnects can provide better resistance to crack growth and fatigue resistance and hence improve mechanical reliability without sacrificing the electrical properties. Current approaches to interconnects such as reflowed solder paste and electroplated interconnects cannot easily achieve nano-grained structures and also impose restrictions on the processibility. For example, screen-printing solder pastes cannot achieve very fine pitches, while electroplated interconnects are restricted to a few material systems. The current wafer level packages are at a pitch of 250-400 microns. We propose solution derived reworkable nano-interconnects as a viable technology to meet the needs of reducing pitch in the die package. This paper proposes solution derived (sol-gel based) nano-grained copper and lead free solders for fine-pitch high strength nano-structured interconnects. In this process, metal-based organic polymer solutions are heat-treated in reducing atmosphere to form metallic copper and lead-free solders (Sn-Ag-Cu). The key is to achieve ultra homogeneous mixing at atomic to molecular level. The precursors were mixed in solvent and refluxed under inert atmosphere at 125°C to form metallic Cu/Sn-Ag-Cu complex solutions. The homogenous gel obtained from the hydrolysis of sol-gel precursor solution is heat treated under inert atmosphere at temperatures below 350°C to achieve the desired compositions. Among a large number of methods for metal deposition, sol-gel technology, based on thermal decomposition of metal-organic compounds, can prove to be the most convenient and inexpensive for industrial applications.
