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There are times when it is desirable to solder without the use of any flux. To meet this need, soldering devices utilizing ultrasonics were developed. Although ultrasonic soldering has been in relatively common use for many years, the availability of ultrasonic equipment powerful enough to be practical for industry is new. Most equipment available up to a couple years ago, was usable for laboratory purposes and process evaluation as well as some pilot run situations. This WRC report describes the basic principles, the equipment, current materials joined, solders used in joining, applications and future production potential of ultrasonic soldering.