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Soldering to Gold Using Indium-Based Alloys

Category:
  • Soldering
  • Indium Alloy
  • Gold-plated surfaces are widely used in electronics assembly. But, using common tin-based solder alloys against these surfaces can scavenge the gold, destroying the gold conduction patterns and creating crack-inducing platelets in the solder joint when the plating is greater than 1.0 micron.

    The use of InPb solders against gold metallization layers dramatically reduces the scavenging.  Since the gold is essentially insoluble in indium this slows the dissolution rates.

    Another advantage is that the InPb family of solder alloys is their good wettability.  Some alloys can directly replace SnPb solder to solve the gold scavenging challenge.

    Reflow temperatures of InPb solder alloys range from 149°C to 300°C, although alloys with greater than 80% Pb will have poorer wettability.  The most commonly used InPb solder alloy is Indalloy #7 (50In 50Pb) which has a solidus temperature of 184°C and a liquidus of 210°C.

    Using InPb solders requires a couple of considerations:

    • InPb should only be used in applications where the final device operating temperature (continuous use) will be lower than 125°C.  Solid state diffusion can occur above this temperature, creating a AuIn intermetallic.
    • Avoid halides, which can corrode the indium.  This is both in the operating environment (such as a marine environment) and in the flux. 

    If you'd like to test a variety of InPb alloys to find the one best suited for your application, check out our Soldering to Gold Research Kit; it contains six alloys and two fluxes.  You can also get more information on our web site at our Soldering to Gold page, including an InPb alloy Product Data Sheet.  Contact us at askus@indium.com, or me directly, cgowans@indium.com, for more information.