Fluxless soldering

The hot stage has not only been applied to optical and atomic force microscopes, but also to scanning electron microscopes. Hot-stage accessories are available on environmental SEMs that can collect ESEM images at elevated temperatures. Applications to the electronics industry include fluxless soldering, intermetallic growth studies, and copper thick-film sintering studies (92-94). 

Because flux is such a concern, one contractor is exploring ultrasonic bonding (10) using prepunched aluminum interconnects that are attached to electroplated copper cell metallization with a seam welder. Others are examining fluxless bonding concepts, such as vapor-phase solder reflow. 

Hosking, F. M., Frear, D. R., Vianco, P. T, and Keicher, D. M., Sandia National Labs Initiatives in Electronic Fluxless Soldering, Proceedings of the 1st International Congress on Environmentally Conscious Manufacturing, Santa Fe, September 18,1991. 

Frear Keicher, D. Fluxless soldering using activated acid vapors. Proceedings of NEPCON West, Des Planes, IL, Cahners Exhibition Group Des Plaines, IL, 1992 1704—1715 pp. 

Kuhmann, J.F. Preuss, A. Adolphi, B. Maly, K. Wirth, T. Oesterle, W. Pittrof, W. Weyer, G. Fanciulli, M. Oxidation and reduction kinetics of eutectic SnPb, InSn, and AuSn a knowledge base for fluxless solder bonding applications. IEEE Trans. Compon. Packag. Manuf. Technol., Part C April 1998, 21 (2). 

Hilhnan, D. Frommelt, D. Tench, M. Anderson, D. Jambazian, P. White, J. Lucy, G. Piekarski, B. Reduced oxide soldering activation process a fluxless soldering alternative. Proceedings of the Surface Mount International Conference and Exposition, San Jose, CA Aug/Sept 1994 347-362. Tench, D.M. Anderson, D.P. Jambazian, P. Kim, P. Warren, L.F. Hillman, D. Lucey, G.K. Reduction of metalhc surface oxides via an electrochemically-generated redox species. J. Appl. Electrochem. 1995, 25, 947-952. 

Fluxless methods rely on oxide removal prior to assembly, typically achieved via a plasma cleaning system. In plasma systems, a plasma gas is made to impinge upon the metal pads to be wet by solder. The plasma radicals bond with the oxygen molecules of the pad surface oxides, which removes them and leaves a clean exposed metal surface. 

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