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A Fundamentally New Approach to Solving the Lead-Free Solder Challenge

Nanotechnology Copper Solder Developed at Lockheed Martin Advanced Technology Center

PALO ALTO, Calif.—A worldwide effort to phase out hazardous materials in electronics has created an urgent need for lead-free electrical interconnect material, or solder. At the Lockheed Martin Space Systems Advanced Technology Center (ATC) in Palo Alto, Calif., scientists in the Advanced Materials and Nanosystems directorate are excited about their development of such a material, which, in this case, is a nanotechnology copper-based electrical interconnect material (solder) that can be processed at temperatures around 200°C. Although a number of technical hurdles remain, the CuantumFuse^TM solder material is expected to eventually produce joints with up to 10 times the electrical and thermal conductivity of tin-based materials currently in use.

Another big reason for the excitement, however, is the material's potential to solve the reliability concerns that have plagued the use of traditional lead-free solders in tough military and commercial applications, where conditions such as vibration, shock, and extreme temperatures require the use of robust components with long service lives. Applications in military and commercial systems are reportedly already under consideration.

The principal lead-free replacement—a combination of tin, silver, and copper (Sn/Ag/Cu)—has proven acceptable to the consumer electronics industry, in which short product life cycles and relatively benign operating environments are common. However, multiple issues have arisen that make it difficult to quantify reliability. High processing temperatures drive higher cost, the high tin content can lead to tin whiskers that can cause short circuits, and fractures are common in challenging environments. Such reliability concerns are particularly acute in systems for the military, aerospace, medical, oil and gas, and automotive industries. Long service life and robustness of components are critical in these applications, where vibration, shock, thermal cycling, humidity, and extreme temperature use can be common.

"To address these concerns, we realized a fundamentally new approach was needed to solve the lead-free solder challenge," said Dr. Alfred Zinn, materials scientist at the ATC and inventor of CuantumFuse^TM solder, in a statement. "Rather than finding another multi-component alloy, our team devised a solution based on the well-known melting point depression of materials in nanoparticle form. Given this nanoscale phenomenon, we've produced a solder paste based on pure copper."

A number of requirements were addressed in the development of the CuantumFuse^TM solder paste, including sufficiently small nanoparticle size; a reasonable size distribution; reaction scalability; low cost synthesis; oxidation and growth resistance at ambient conditions; and robust particle fusion when subjected to elevated temperature. Copper was chosen because it is already used throughout the electronics industry as a trace, interconnect, and pad material, minimizing compatibility issues. It is cheap (1/4th the cost of tin; 1/100th the cost of silver, and 1/10,000th that of gold), abundant, and has 10 times the electrical and thermal conductivity compared to commercial tin-based solder.

"We are enormously excited about our CuantumFuse^TM breakthrough, and are very pleased with the progress we're making to bring it to full maturity," said Dr. Kenneth Washington, vice president of the ATC. "We pride ourselves on providing innovations like CuantumFuse^TM for space and defense applications, but in this case, we are excited about the enormous potential of CuantumFuse^TM in defense and commercial manufacturing applications."

The ATC has demonstrated CuantumFuse^TM with the assembly of a small test camera board. "These accomplishments are extremely exciting and promising, but we still have to solve a number of technical challenges before CuantumFuse^TM will be ready for routine use in military and commercial applications," said Mike Beck, director of the Advanced Materials and Nanosystems group at the ATC. "Solving these challenges, such as improving bond strength, is the focus on the group's ongoing research and development."