STTR Phase I: Low Temperature, Lead-Free Nanosolder for Microelectronics

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This Small Business Technology Transfer (STTR) Phase I Project is to develop a nanomaterials approach for a low temperature lead-free solder technology for heat-sensitive microelectronic, nanoelectronic and MEMS device that is based on the depression of the bulk melting temperature exhibited by sub-10nm particles. Preliminary results have shown a depression of up to 30ºC. With these alloy nanoparticles combined into a paste using organic liquids and a flux that suppresses nanoparticle oxidation as the paste is heated, the paste can be applied to form an interconnect with existing microelectronics assembly processes.The elimination of Sn-Pb solder in electronics enacted by the European Union has led to widespread industry adoption of Pb-free solders with significantly higher melting temperatures than the Sn-Pb eutectic alloy they replaced. Heat sensitive components, such as sensors, system-in-package, and MEMS devices, were barely surviving the 183°C eutectic temperature of Sn-Pb. With the introduction of Pb-free alloys that melt more than 30°C higher, significant damage can be done to critical electronic and MEMS components under standard assembly conditions. The key feature of the new technology is the use of solder alloy nanoparticles of approximately 5-10nm in diameter to create a material with a melting point of approximately 185°C, some 30° lower than the bulk melting point. With these alloy nanoparticles combined into a paste using organic liquids and a flux that suppresses nanoparticle oxidation as the paste is heated, the paste can be applied to form an interconnect with existing microelectronics assembly processes. The solder alloy nanoparticles melt at a lower temperature than their bulk powder counterparts. The nanoparticles will then coalesce and, as they are cooled, will solidify. An important feature of the technology is that once the solder joints are solidified, because of their large size, their melting temperature will be the bulk melting temperature. This new technology therefore allows for step soldering in which heat-sensitive components may be attached sequentially without damaging components with subsequent soldering steps.