Sandia National Laboratories researcher Tom Crenshaw, co-author on the paper, sets up a specimen in a test frame that will pull a solder joint apart to determine its tensile strength. Credit: Sandia National LaboratoriesTechnology used in downhole applications—such as geothermal or oil-well monitoring—must endure punishing conditions, from very high temperatures to tremendous pressures. In developing such technology, researchers have run into various materials snags, not the least of which is finding a solder material that can perform under these harsh environments. But researchers at Sandia National Laboratories in New Mexico have repurposed a solder alloy once intended for defense applications to meet these challenges.

Originally developed for neutron tube components, a gold-silver-germanium alloy has all the right properties for downhole applications, says Paul Vianco, an electronics manufacturing and reliability researcher at Sandia Labs, who specializes in soldering technologies. The components for high temperatures electronics, such as microprocessors and capacitors, are ready for testing, he says. Unfortunately, electronic packaging, including soldering technology, has lagged behind, because of the unique service conditions.

“Having a higher strength material, starting with the solder, was a driving force to begin looking at alternative materials,” says Vianco.

When he began to investigate high temperature solder options, the gold-silver-germanium alloy looked like a perfect fit. When he and fellow researchers were first investigating the material 15 years ago, they created a full profile of the alloy’s properties, including its ability to hold up to vibrations based on initial strength performance.

The alloy was also able to clear the biggest solder hurdle: melting temperature. Most solders melt around 350 C, which is problematic for downhole applications that can easily reach much higher temperatures. The other option is to use a brazing material, a technique that is not ideal for most electronics. Most brazing materials melt around 700 C, temperatures that would damage even electronics designed for downhole use.

Though there are solders available that are usable around 450 C, they are almost universally lead-based, says Vianco. Though functional, lead-based products work counter to the current environmental movement to use materials that are as nontoxic as possible.

“A lot of the high temperature, lead-based solders were usable, so people were making high temperature electronics with them, but such lead-containing solders were obviously not the best choice,” says Vianco. It became a hunt, then, for other, less toxic materials that would enable electronics to be used in high temperature applications.

The gold-silver-germanium alloy fits the bill perfectly, says Vianco. It has a melting range of 420–440°C and is lead-free. And since it was nearly fully characterized during the previous study over a decade ago, the alloy is ready for prototyping, says Vianco. The research, which was presented at Surface Mount Technology Association International where it won best of proceedings category, has generated some buzz in the industry. All that’s missing now is a substrate to solder the circuits onto, says Vianco. Once that piece of the puzzle is in place, a new wave of high temperature electronics can be developed for the downhole oil, gas, and geothermal industries.

Source: University of Cambridge, Materials 360 Online