Other Methods

Ultrasonic (Vibrational) Atomization. This very specialized technique was, until recently, merely a laboratory curiosity, despite being reported as early as the 1960s. The principle is that a liquid film, if vibrated sufficiently strongly, will form waves that will in turn break off to form droplets. This effect is observable at low frequencies and has been demonstrated on metals such as zinc using low kHz frequencies in the Ukraine. However, it has recently been developed by several leading solder producers to make electronic solder powder for paste production. As mentioned above, this is needed with very narrow particle size and perfectly spherical shape.

The major drawback of the process is that, similar to the spinning-cup process, it places high demands on materials of construction for the solid surface. It must resist melt attack, be wettable (attack and wetting are two sides of the same coin, so this is always a compromise), and have suitable acoustic properties. Therefore, it is not surprising that it has not found application on higher-melting materials. However, it can be made to work on solders, although with a low output, typically 20 to 40 kg/hour (for one horn). Frequencies used range from 20 to 80 kHz, and the particle size distribution is practically free of ultrafine particles. Standard deviation (Te of 1.4 or better is achievable, giving yields similar to centrifugal techniques and far better than gas atomization. Equipment is compact (typical vessel diameter less than 1 m) and energy use small ( 1 kW). Production by this method is probably around 1000 tons/year (gross).

Other Methods of Atomization. Besides the industrial types of atomization processes, there are also many atomization methods of potential scientific interest that have been reported in the literature. These methods include:

• Roller atomization: A mechanical process that feeds a stream of molten metal between rapidly rotating rolls and discharging it into flake, acicular, or irregular spherical particles. This has been found to be generally impractical due to accretions on the rolls and very variable heat transfer to them.

• Vibrating electrode: This mechanical approach produces high-purity powder by the vibration of a consumable electrode. The electrode, forming a resonant rod with a fixed and free end, is continuously moved between rollers toward a slowly rotating, water-cooled copper disk. Atomization occurs in an arc struck between the disk and vibrating electrode end. The size of spherical particles formed is controlled by changing the length of the resonant rod. Productivity is low, cost high, and particles coarse.

• Melt drop (vibrating orifice) technique: In this method, molten metal contained within a closed, pressurized crucible is subjected to vibratory oscillations. Forcing the metal through a nozzle at the bottom of the crucible and into a vacuum or inert gas chamber causes it to form a jet and undergo Rayleigh breakup into very uniform droplets. Values of <J„ of less than 1.05 have been reported. The technique has been demonstrated with aluminum, beryllium, copper, lead, mild steel, and some superalloys (Ref 45) and has recently been proposed for solder (Ref 46). The technique is in limited industrial use in the 1990s, making precision solder balls for electronics with sizes in the range 500 to 1000/Jm. It is limited as production is related to the square of particle size and nozzle blockage is obviously a worry for holes in the 200 to 20/'m range.

• Variants of centrifugal atomization: Besides the more common methods of the rotating electrode and rotating disk or cup processes, other methods include centrifugal impact atomization, shot casting, laserspin atomization, and the Durarc process (Ref 1). Few of these are commercially exploited, but new variants on old ideas are frequently reported.

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