Active Ultrasonic

Compared to other technologies and other ultrasonics solutions this new and innovative ultrasonic system offers significant yield improvements. To meet the demands of modern solder powder applications this technology also offers tightly controlled powder properties including particle size, shape, and surface chemistry.

Principle of capillary-wave atomization:

A thin layer of a liquid, wetting the surface of a solid resonator, which vibrates vertically to its surface plan, forms a pattern of standing capillary waves. This occurs when the vibration amplitude A exceeds a threshold value. On further increase of the amplitude, ligament breakup of the liquid follows and droplets are hurled from the crests of the capillary waves.

Drop size is correlated to sonotrode frequency, amplitude, liquid physical properties, and liquid-film thickness.

Capillary-wave atomization

The acoustic activity of a conventional “Langevin” transducer is attenuated by the attached mechanical load. Material selection and dimensions for both an acoustical wave-guide and the sonotrode are limited. As a result, the sonotrode has to be located as close to the transducer as possible. Such a configuration is problematic for ultrasonic transducers that are inherently heat sensitive. In molten metal atomization the interconnected sonotrode is heated at melt temperature and care must be taken not to damage the ultrasonic transducer element.

Attenuation also decreases the vibration amplitude of the sonotrode thereby reducing the amount of wave crests where ultrasonic atomization occurs. To maintain a viable atomizing efficiency, the flow rate has to be increased and that will cause larger drop sizes.

High temperatures combined with high amplitude standing waves, ultrasonic cavitation, and chemical activity between liquid metal and the sonotrode are known to quickly degrade and severely limit the life of atomizing sonotrodes used in conventional ultrasonic metal atomization.

Our new ultrasonic atomizing system allows the transducer to be kept outside the heating and powder making vessel. Consequently, the melting point of the material to be atomized is no longer a limiting factor for the process. Provided that there exist a resonator material with appropriate acoustic properties and mechanical strength, any molten metal can be atomized.

For production of Type 3 (25 to 45 µm) and Type 2 (45 to 75 µm) solder powder using metal alloys with a melt point below 500° C we have developed new and proprietary rotating sonotrodes. Subsequent high frequency system development efforts are expected to result in high yield production of Type 4 (15-30 µm) solder powder. All sonotrode designs are capable of an operational life exceeding 6 months. Some other conventional ultrasonic atomizing methods have a short sonotrode operating life of less than one week. Our new sonotrode technology also offers a much improved yield of spherical powder with a focused distribution of the target particle size in the range of 50% to 70% depending on the solder alloy, flow rate and operational conditions. Target production volumes are 40 to 80 kg per hour depending on material processed.

The atomization vessel comprises a hemispheric upper cap and an asymmetric cone. Since ejected droplets have much lower velocities than gas atomized droplets, the vessel dimensions can be reduced to approximately 1.2 meters diameter and 2 meters height. Fast cooling rates are achieved under circulating nitrogen atmosphere. Inert gas consumption is low in ultrasonic atomization, since gas is not the atomizing medium. This allows the unit to be hermetically tight to improve environmental security. The new design offers a continuous supply of melt and increases the energy efficiency of the process to a few Watts per liter of melt.

• Active Ultrasonics has constructed prototypes of the system’s ultrasonics elements.
• A new production vessel and control system has been design and can be constructed within 3 to 6 months depending on resources provided.
• We seek a client/partner interested in final stage co-development and production.