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.