Lead-Free Solder Films Via Novel Solution Synthesis Routes

We report two novel routes, sol-gel and electroless plating, for the synthesis of lead-free solders. Novel processes with these routes were developed and demonstrated for Sn-Ag-Cu, Sn-Ag systems to achieve thin bonding layers for assembly of fine pitch integrated circuits onto substrates. Sol-gel route can be used to accurately control the final alloy composition and incorporate additives leading to the designed thermomechanical properties. In this process, the inorganic polymer solutions were spin coated and then heat-treated in a reducing atmosphere to form thin films of lead-free solders. The presence of Ag and Cu enabled easy reduction of tin oxide to tin at 400degC that was not possible with Sn precursor. With the alternate solution reduction (electroless plating) approach, bonding layers can be deposited at almost room temperatures directly on organic substrates. With this approach, the deposition selectively occurs on the metal bonding pads, which eliminates the need for any lithography. Using this approach, electroless Sn-Ag films were demonstrated on organic laminates. These thin film synthesis routes can enable short interconnections that are critical for high density, high frequency, and embedded active component packaging.