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Historical alloy suited for high-temperature applications

Sandia National Laboratories in the US say a gold-silver-germanium alloy developed 15 years ago can be used to improve high-temperature electronics used in oil and geothermal wells.

Researchers at the lab first investigated the gold-silver-germanium alloy about 15 years ago. It was based on the gold-germanium system, which has traditionally been a die attachment material used in microelectronics packaging.

With the application at that time (a neutron tube) requiring a higher melting temperature, the scientists added silver and adjusted the concentrations to reach a near-uniform melting point for the alloy.

A later design change saw the material shelved, before it was resurrected a few years ago, when researchers working on projects with applications inside a downhole asked Sandia’s geothermal researchers to develop electronics to monitor well conditions in field operations.

Circuit boards placed downhole in oil and geothermal wells must withstand high temperatures and pressures, excessive vibrations and other extreme environments.

The gold-silver-germanium alloy, the researchers found, is suited as a solder in these extreme applications, providing a good material fitting into the temperature gap between normal solder and brazing materials.

Conventionally, most soldering materials melt as temperatures approach 350°C, while aluminium-based brazing alloys are the next tier up, melting at about 600°C. However, aluminium-based alloys are difficult to process for electronics.

There was thus a gap in suitable soldering material for downhole applications where temperatures are as high as 350°C.

The researchers found the gold-silver-germanium alloy melts at 431°C, and used it to join a Kovar alloy (iron-nickel-cobalt) and alumina with nickel-over molybdenum/manganese metallisation.

The joint was then pulled apart. At room temperature, it displayed tensile strengths of 111±7 MPa when using a peak reflow temperature (Tp) as low as 455°C.

At 300°C, the pull strength dropped to 74±12 MPa, and the researchers determined that solders using the gold-silver-germanium alloy have a service temperature ceiling of at least 350°C.

In addition, the gold-silver-germanium alloy is lead-free, making it compliant with current lead-free initiatives and regulations. Based on the latest research as well as data from 15 years ago, the alloy’s fundamental mechanical and processing properties are nearly fully characterised, which will help with speed to market.

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