Influence of process parameters on SAC305 lead-free solder powder produced by centrifugal atomization
Graphical abstract
This present work is aimed to investigate the influence of operating parameters of centrifugal atomization process on the production of SAC305 lead-free solder powder to propose a more attractive choice for the industry that demands a high productivity and quality product of SAC305 powder.
Highlights
► Median particle size of produced SAC305 solder powder could be decreased by increasing rotating speed, decreasing melt feed rate and using a larger size atomizer. ► Cup shaped atomizer was able to give about 11% finer powder compared to flat disc shaped one. ► Median particle size appeared to be smaller and shape of the particles tended to be rounder with decreasing oxygen content in the chamber. ► Fine particles of SAC305 powder (-45 microns) containing oxygen less than 100 ppm could be synthesized by purging nitrogen gas into the atomizing chamber. ► Production yield of SAC305 powder could be increased with increasing rotating speed of atomizer, reducing melt feed rate, and with the use of larger atomizer.
Introduction
Centrifugal atomization has been widely practiced in mass production of high quality metal powders in the size range of 50–400 μm. Additionally, it can be applied for cylindrical part preparations as well [1], [2]. Advantages of this process are: narrow particle size distribution, high production yield, less impurities, no need for an atomizing fluid medium, and modest energy consumption [2]. Prior works reported that this process is quite flexible for producing several types of materials such as organic matter, chemical substance and metals [1], [2], [3], [4], [5], [6], [7], [8]. Although its benefits were obviously propagated, it is still overlooked and is little used by plant owners owing to lacking of scientific knowledge in this area [9].
Theoretically in the centrifugal atomization process, after a melt jet is impinged onto a high speed rotating disk, it will be radially spread out by the centrifugal force. The moving melt then develops a thin film covering the disk surface. During its flow on the disk, the melt exhibits 3 regions: potential region, jet boundary layer region, and outer boundary layer region [10], [11]. The melt film will ideally begin disintegrating at the edge of the atomizer and forming small droplets. The mean size of these droplets can be predicted using Eq. (1) derived by Champagne and Angers [12], [13]where γ is the surface tension of the melt (N/m), ρ is the density of the melt (kg/m3), ω is the angular speed of the disk atomizer (s−1), and R is the atomizer radius (m).
The three regimes of melt film disintegration, namely: (1) direct droplet formation (DDF), (2) ligament formation (LF), and (3) film disintegration (FD) had been proposed by Champagne and Angers [12], [13]. Transition between each regime could be by the ratio of the process parameters and melt properties. Melt disintegration could sometimes prematurely take place before it reaches the edge of the atomizer disk. This occurrence can cause an inefficient atomization of the molten spray. Its magnitude depends on the melt feed rate and the rotating speed [15]. Actual observation of melt flow on the disk is difficult to conduct unless expensive high speed camera is available. Thus, modeling and simulation are considered as cost-effective means to predict the behavior of melt flow on a rotating disk [16]. During melt spreading on the atomizer in the jet boundary layer, hydraulic jump, peripheral discontinuous of melt, and solidification of melt on the rotating disk — known as skull formation, are frequently observed. These incidents have been perused and their damages have been reduced by using modeling and simulation [17], [18].
In 2006, Zhao [19] proposed a number of equations for atomizer design. Meanwhile, a number of works had been developing on new techniques in centrifugal atomization [20], [21]. New hybrid processes were developed in order to improve the performance of conventional processes.
SAC305 solder alloy is one type among the Sn–Ag–Cu solder family which has been used instead of conventional Pb–Sn solder alloy due to its superior strength, better ductility, good creep, and fatigue resistance [22]. This alloy has also become more of interest because of legislations and regulations in many countries, for example, RoSH, WEEE [23]. This present work aims to investigate the influence of operating parameters on the production of the SAC305 lead-free solder powder to propose a more attractive choice for the industry that demands high productivity and quality product of the powder. This work reports the effects of melt feed rate, angular speed, atomizer size and shape, and oxygen content on the median particle size and the production yield of the SAC305 powder.
Section snippets
Experimental
Commercial grade SAC305 lead-free solder alloy used in this research was obtained from the Thailand Smelting and Refining Company (THAISARCO). Chemical composition of this alloy is given in Table 1. Its melting point is approx. 217–219 °C. Density and surface tension of the alloy at 317 °C, reported by Moser et al. [24], are 7050 kg/m3, and 0.54 N/m, respectively.
A centrifugal atomizer has been constructed for this study. Depicted in Fig. 1, the centrifugal atomization unit consists of 4 main
Results and discussion
In the early stage of this study, poor wetting between the SAC305 melt and the surface of the atomizer disk was found to be a pronounced problem since most melt poured onto the rotary disk had slipped and dropped nearby the atomizer, resulting in an inefficient atomization as shown in Fig. 3. This problem can be solved by coating the disk surface with the atomized material [3]. In the present study, the atomizer surface was coated with the SAC305 alloy. After coating, the atomizing process was
Conclusions
From our experimental results, it could be concluded that:
- 1)
Median size of SAC305 powder decreases with increasing rotating speed, decreasing melt feed rate and with the use of larger atomizer.
- 2)
At same processing conditions, SAC305 powder atomized by using a cup shaped atomizer is approximately 11% smaller in size than that atomized by using a flat disk shape atomizer.
- 3)
Reduction of oxygen content in the atomizer chamber only slightly affects the median size of SAC305 powder.
- 4)
SAC305 powders produced
Acknowledgments
The authors would like to gratefully thank: the Center of Excellence in Nanotechnology at the Prince of Songkla University and the Prince of Songkla University Research Grant (ENG540012S) for financial supports; the Department of Mining and Materials Engineering, Faculty of Engineering, Prince of Songkla University for research facilities; the Department of Mechanical Engineering, Faculty of Engineering, Prince of Songkla University for apparatus creation and assimilation; plus many of our dear
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