Gold Soldering with Indium Alloy

Surface gold plating is widely used in the field of electronic assembly. But using common tin-based solder alloys on such gold-plated surfaces removes the gold layer and disrupts the gold's conduction mode. Also, if the thickness of the coating exceeds 1.0 microns, a thin layer that induces cracks will form on the solder joints.

The use of indium lead (InPb) solder on the gold plating can significantly mitigate this removal effect. Since gold is substantially insoluble in indium, the rate of dissolution is slowed.

Another great advantage of the indium lead solder alloy family is their good wetting properties. Some alloys can also directly replace tin-lead (SnPb) alloys to solve the problem of gold removal.

Indium-lead solder alloys have reflow temperatures between 149°C and 300°C, although alloys with more than 80% lead have poor wetting properties. Indalloy #7 (50In 50Pb) is the most commonly used indium lead solder alloy with a solidus temperature of 184°C and a liquidus temperature of 210°C.

Two factors should be considered when using indium lead solder:

1. Indium lead should only be used in applications where the final device operating temperature (continuous use) is below 125°C. Above this temperature, solid-phase diffusion occurs, resulting in gold-indium intermetallic compounds.

2. Avoid contact with halides, which corrode indium. The requirements for working environment (such as marine environment, etc.) and flux are the same.