Assembly Planbook - April 2008 - (Page 34) ASSEMBLY Ultrasonics Soldering the Unsolderable With ultrasonics, assemblers can solder aluminum, titanium and other difficult materials without flux. U In ultrasonic soldering, the vibrational energy induces cavitation in the molten solder. This enhanced image shows cavitation in water. Cavitation is the sequential formation and collapse of vapor bubbles and voids in a liquid subjected to acoustic energy of high frequency and intensity. Photo courtesy NASA By John Sprovieri Editor ltrasonic energy can be used to weld metal and plastic; clean and degrease parts; and spray thin coatings of flux, conductive ink and other liquids. Now, assemblers can add one more application to that list— soldering. In ultrasonic soldering, high-frequency vibrational energy is applied to a molten filler metal to solder parts without flux. The technology is different from ultrasonic plastic welding, in which the vibrations generate heat to melt the plastic. In ultrasonic soldering, heat from a separate energy source melts the filler metal before vibrational energy is applied. Ultrasonic soldering is similar to ultrasonic metal welding, in that both processes rely on high-frequency vibrations to break up and remove oxides on the surface of each part. In ultrasonic metal welding, however, the metal does not melt. Actually, ultrasonic soldering is more akin to ultrasonic cleaning than either welding process. In ultrasonic cleaning, the vibrational energy induces cavitation in the water bath. Cavitation is the sequential formation and collapse of vapor bubbles and voids in a liquid subjected to acoustic energy of high frequency and intensity. The parts are scrubbed clean by the erosive effect of the bubbles. Similarly, in ultrasonic soldering, the vibrational energy induces cavitation in the molten solder. The cavitation breaks up and disperses surface oxides, allowing the liquid solder to wet and bond pure metal. It also ensures that the solder joint is free of voids. “With ultrasonic plastic welding, the ultrasonic energy is applied up and down. You’re creating friction to melt the plastic,” explains Tom Dunn, president of Phase 4 Inc. (Gilbert, AZ). “With ultrasonic metal welding, the ultrasonic energy is applied side to side. You’re using friction to disrupt oxides at the “Ultrasonic soldering lets you solder materials that ordinarily couldn’t be soldered.” —Tom Dunn, Phase 4 Inc. interface. With ultrasonic soldering, the ultrasonic energy doesn’t melt the solder. It helps the solder to wet the parts. The solder is the ultrasonic bath.” Besides direct metal-to-metal bonding, ultrasonic soldering creates strong attachments through two other mechanisms. The first is mechanical. The vibrational energy forces the liquid solder into tiny crevices and pores in the substrates. This helps to seal the parts and greatly increases the surface area to which the solder can bond. The other mechanism is chemical. The filler metal is specially formulated 34 ASSEMBLY / April 2008 www.assemblymag.com http://www.assemblymag.com
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