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24 SMT Magazine • October 2014 nanOcOPPer-baseD sOLDer-Free eLectrOnic assembLY materiaL continues FEATurE plastic substrates used for flexible electronics such as polyethylene terephthalate and poly- imide. conclusions We have developed an all-copper electron- ic assembly material that can be processed at 200°C. Through improvements in nanocopper synthesis, paste formulation, and processing techniques, a fully functional camera board was successfully assembled using only nanocopper paste. Coupled with the proven scalability of our unique nanocopper synthesis process, this dem - onstration illustrates the utility and potential of nanocopper as a replacement for SnPb and Pb- free solders. The electrical conductivity of nano- copper is already 2–3 times higher than standard solder currently in use. Also, the tensile strength is approaching that of the best solders available. Nanocopper is still in the early development stages, yet improvements in strength, thermal conductivity, and electrical conductivity have been rapid. There is plenty of room for further improvement as the full materials properties po - tential of nanocopper have yet to be realized. We are aggressively pursuing further improvements in all areas of performance. The fact that this ma- terial already performs similar to or better than existing materials shows the exciting potential of nanocopper to be a robust alternative to the cur- rent library of lead-free solders. smt references 1. "Directive 2011/65/EU on the restriction of the use of certain hazardous substances in electrical and electronic equipment," Official Journal of the European Union, 2011. 2. Shapiro, A., "Implications of Pb-Free Mi- croelectronics Assembly in Aerospace Applica- tions," IEEE Transactions on Components and Packaging Technologies, Vol. 29, No. 1, March 2006. 3. NASA: Tin Whisker Literature References 4. "The Lead Free Electronics Manhattan Project—Phase I," Department of the Navy, Sci- ence & Technology, 2009. 5. "Lead Free and Green Electronics Forum," Center for Advanced Life Cycle Engineering, University of Maryland, 2011. 6. Borgesen P., Bieler T., Lehman L. P., Cotts E. J., "Pb-free solder: New materials consider- ations for microelectronics processing," MRS Bulletin, 32, pp. 360–365, 2007. 7. This information, tin whisker images, and much more can be found at the following NASA website NASA: Whisker Failures Data 8. Frederickson, M., Morris, E., "Pb-free Elec- tronics Research Manhattan Project Overview," LMC Defense Manufacturing Conference, Nov 30–Dec 3, 2009. 9. Buffat, Ph., Borel, J-P., "Size effect on the melting temperature of gold particles," Physical Review A, Vol 13, No. 6, June 1976. Figure 7: photograph of flexible LEd circuits with stencil printed nanocopper traces.

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