Fluxless soldering process technology

This paper describes a reflow soldering process not requiring the use of flux. There are basically two fluxless soldering processes. Initially, we developed a process in which an argon-atom sputter-etching technique is employed to clean the surface, and reoxidation is prevented by carrying out aligning, mounting, and reflowing in an inert gas in a vacuum chamber. Then, for purposes of simplification, we developed a more convenient process in which, following the surface cleaning, the aligning and mounting are done in an atmosphere of ordinary air and then the reflowing is done in a belt furnace. This causes the reoxidation on surfaces activated by sputter-etching to be approximately the same both in a vacuum and in an air at room temperature. The thin oxide film is easy to break by volume expansion when it is melted. This allows the solder to wet. Applications to F.C.B. (Flip Chip Bonding) are possible between Au-metallized Ni surfaces and two types of solder (Pb3.6Sn and Sn3Ag). Excellent soldering results have been obtained using this new process. Basic characterization of a soldering process is determined by wettability tests. After sputter-etching, samples can be stored in air for one week. Oxygen density in a reflow atmosphere is allowed to reach 20 ppm. Based on our evaluations, it is concluded that this process is simple and convenient for manufacturing circuits.<
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