Several noteworthy events occurred on June 18, 1996, including: suspected Unabomber Ted Kaczynski was indicted on 10 criminal counts; New Zealand’s Mt. Ruapehu volcano erupted with an eight-mile high plume of ash; and MSE Adjunct Professor Iver Anderson and MSE Professor Emeritus John F. Smith and their colleagues published their first U.S. patent on an invention they called “Pb-free Sn-Ag-Cu Ternary Eutectic Solder” [1]. Eighteen years later, the only item of lasting importance in that list has proven to be the lead-free solder.
That first “motherhood” patent on Sn-Ag-Cu alloy, U.S. Patent 5,527,628, is the top-earning patent in the history of both Ames Laboratory and Iowa State University with royalty income that totaled $58,628,649 before the patent expired in 2013. More than 50 companies based in 13 countries licensed the invention.
In retrospect the invention’s success is not surprising. Several billion computers, mobile devices, and other electronic systems are in use today. The dangers of lead in the work place and in land-fills have caused rising concern about human health and environmental safety, leading to adoption of lead-free requirements for all but a small number of special electronic devices (e.g., medical implants and satellite circuitry).
Despite their environmental drawbacks, Pb-based solders provide a combination of properties (low melting point, low cost, good wettability, high tensile and fatigue strengths, and good corrosion resistance) that are challenging to match with Pb-free solders. Anderson’s team’s invention exploits an error he discovered in the Sn-Ag-Cu ternary diagrams then in use. The actual melting temperatures of “SAC” ternary alloys in this system are lower than the older diagrams indicate, but are still about 30°C higher than the Pb-Sn eutectic temperature. Fortunately, the typical electronic assembly equipment and circuit components have sufficient elevated-temperature tolerance to accommodate the SAC eutectic melting temperature of 217°C. Also, by combining additional elements such as Ni, Zn, or Al, the mechanical properties and temperature stability of the new solder can surpass those of Pb-Sn solders, although the wettability of Pb-Sn solder is still unequaled.
Anderson and his group continue to study and improve the lead-free solders. Resistance to thermal-mechanical fatigue and to drop impact shocks are the next targets for major property improvements for SAC+X Pb-free solders, where Zn and Al additions seem to have a major advantage.
1.) Anderson, I. E., Yost, F. H., Smith, J. F., Miller, C. M., and Terpstra, R. L., “Pb-Free Sn-Ag-Cu Ternary Eutectic Solder,” U. S. Patent No. 5,527,628, issued June 18, 1996.