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Open Access Publications from the University of California

Silver-Indium Solid Solution on Copper as Passivation and Study of its Reaction with Tin Solder

  • Author(s): Wu, Yipin
  • Advisor(s): Lee, Chin C
  • et al.

As the most commonly used electrode, leadframe, and package material in electronic packaging, copper (Cu) has severe oxidation problem, preventing wetting during soldering processes and prohibiting successful wire bonding. Thus leads and bond pads of leadframes and packages are often coated with a passivation layers such as palladium, silver, or tin, which suppresses the growth of copper oxides. In this research, I investigated the use of silver-indium ((Ag)In) solid solution as a new alternative coating material for passivation. First, a reaction study of bulk (Ag)-9.5In solid solution disks with Sn was conducted to evaluate possible intermetallic compounds in the Ag-In-Sn system. Next, (Ag)-9.5In solid solution layer was coated on copper substrates by E-beam evaporation. The (Ag)9.5In-passivated Cu substrates and bare Cu substrates were aged at 150°C for up to 1000 hours in air. No oxide was detectable on (Ag)9.5In-passsivated Cu substrates while thick Cu2O3 was grown on bare Cu. This result illustrates that (Ag)9.5In solid solution is an excellent passivation coating material. Then, (Ag)9.5In-passivated Cu substrates were electroplated with tin. As a reference, bare Cu substrates were also plated with tin. These two different types of samples were reflowed and aged for different amount of time. No flux was used. The reaction results were examined by scanning electron microscope/energy dispersive X-ray spectroscopy (SEM/EDX) and compared. The experimental results show that tin wetted and reacted well with (Ag)9.5In-passivated Cu substrates. The growth of intermetallic compound (Cu6Sn5, Cu3Sn) on (Ag)9.5In-passivated Cu substrates is also suppressed compared to the growth on bare Cu substrates. The design of (Ag)In solid solution passivation is an invention and its superior performance is a discovery. It provides an economical, environmentally friendly, and valuable passivation alternative on copper electrodes, leadframes, and packages.

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