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Ultrasonic metal fusion-Joining is finding application in the fabrication of electrical and electronic assemblies. Ultrasonic soldering and brazing without the use of flux are particularly effective in Joining connections to silicon or germanium transistors, in the Joining of insulated wires without pre-stripping, and in the type of multiple Joining necessary for printed circuitry and the like. In the fabrication of silicon transistor assemblies, ohmic contacts must be obtained, and conventional Joining methods are relatively difficult. Ohmic contacts can readily be achieved with ultrasonic fluxless soldering techniques. Equipment has been devised for evaluation of the resistance of such Joints, and comprehensive test data have been assembled by outside sources, indicative of the work currently being undertaken in the nation's laboratories. Mlcrographic evaluation of typical Joints shows minimum penetration of the solder into the base metal. Reliable electrical wire Joints can be achieved without the use of flux by ultrasonic techniques. Conductivity and burn-off tests have been used to evaluate the quality of ultrasonically-brazed wire Joints. The effect of solder or braze-alloy composition, as well as the effect of ultrasonic exposure time on the electrical properties of the Joints, were considered. Of particular significance, sound Joints can bo obtained on Formvar-coated wires without the necessity for pre-stripping. With ultrasonic fusion-Joining methods, fine-coated aluminum wires can be bonded together or aluminum wire can be bonded to copper magnet wire without the costly production procedure of first removing the insulation. Double-wire, triple-wire, and even quadruple-wire Joining is possible in a single operation. Techniques for accomplishing such Joints were described. Test data indicate that effective bonding is dependent on Joining time and temperature. Other limiting mechanical and metallurgical factors were discussed. A primary consideration in the selection of a solder or braze alloy for aluminum is the intended end use of the Joint, particularly its corrosion environment. Data have been obtained showing the typical strength behavior of a variety of solder alloys in several corrosion environments, and metallographic evaluation has indica- ted that different corrosion media may attack different solders in different ways; in some instances failure may occur at the solder-base metal interface, while others show solder-matrix failure. The adaptability of ultrasonic soldering techniques was illustrated by their use in simultaneous multiple soldering operations. Multiple-head ultrasonic units have been devised for soldering several points at one time, as on printed circuitry. The operation of such a unit was described with regard to operational sequence, effective coverage, limitations in its use, and potential applications.