Fabrication And Characterization Of Mono-sized Spherical Micro Particles

Mono-sized spherical micro particles are a special kind of spherical powder particles, which have a wide application. As for various applications, not only the particles applied directly in BGA electronic packaging, spherical semiconductor and so on, but also droplets used in net-form manufacture, rapid deposition and so on, all the particles or droplets prepared by different methods are required to be with characteristics of narrow size distribution, high sphericity, particle size controllable and the identical cooling history. While, the traditional preparation methods of micro particles, such as Cutting Punching Remelting Method, Atomization, Uniform Droplet Spray Method (UDS) have more or less faults, and cannot meet the requirements of the above application fields. Therefore, this work aims to innovate in principle, and consequently develop a new kind of equipment used for preparing mono-sized spherical particles with the desired size.The main parts of independently developed low temperature POEM, including uniform droplets generating system, heating and cooling system, pressure difference controlling system, piezoelectric actuator driving system, particles collecting system, vacuum system and etc., have been introduced in this paper. Spherical and mono-sized micro particles of63Sn-37Pb eutectic composition have been prepared by above low temperature equipment. The effects of technological parameters on the particle size and size distribution are discussed. Furthermore, characterization for Fe-Co based metallic glass alloy particles prepared by high temperature POEM is carried out, and critical cooling rate to realize fully crystalline phase is also estimated. Based on results and discussion, some conclusions can be drawn in the following.63Sn-37Pb micro particles with the characterization of narrow size distribution, high sphericity, good surface quality and fine grains can be stably prepared by independently developed low temperature POEM. The obtained micro particles meet requirements of the BGA packaging. Technological parameters, such as the pulse signal waveform applied to piezoelectric actuator, applied voltage, frequency and pressure difference between the crucible and chamber, have important effects on particle size and size distribution. With the orifice size of200μm, the optimal parameters applied in preparation of63Sn-37Pb particles is as followed:trapezoidal wave is selected as driving signal for piezoelectric actuator; the applied voltage ranges from90V to120V; the time to reach the pulse voltage from zero (tup) is set as250μs, and the holding time (thold) and the time to return back from pulse voltage to zero (tfall) are both1000μs; frequency of piezoelectric actuator motion ranges from10Hz to200Hz; the applied pressure, which is the difference between the pressure in the crucible and chamber is adjusted within the range of2-3torr.[(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb4metallic glass alloy particles of different particle size with narrow size distribution and high sphericity were separately prepared by high temperature POEM in He or50%Ar+50%He atmosphere, respectively. The particles obtained in the fully glassy phase have a diameter of less than645μm in He atmosphere and less than515μm in50%Ar+50%He atmosphere. The phase transitions of particles from the fully crystalline phase to the mixed phase which was mixture of the glassy phase and the crystalline phase to the fully glassy phase occurred as the particle diameter is decreased regardless of any atmosphere. Critical cooling rate for fully glassy phase of [(Fe0.5Co0.5)0.75B0.2Si0.05]96Nb4metallic glass alloy particles prepared by POEM is estimated to be within the range of800-1100K.sec-1. No changes of the critical cooling rate occur in any atmosphere.