ting capillary rise. However, the acoustic propagation characteristics (i.e., acoustic attenuation) in the capillary should also be considered because both of these aspects contribute to the capillary rise of the liquid. To calculate the height and speed of the capillary rise of the liquid, we should resolve a hydrodynamic equation of the liquid movement with the integral acoustic pressure from the cutoff point to the capillary surface. On the basis of the above analysis, ultrasonically induced capillarity is not wetting based but rather significantly dependent on the acoustic pressures at the cutoff of the capillary P0 and in the capillary P. P0 is generally a function of the input acoustic power, the acoustic velocity in the liquid, and the location of the capillary end. P is illustrated in Equations 1 and 2. Considering these conditions, we can easily understand the natures of the abnormal capillary phenomena occurring in ultrasonically agitated water or liquid metal systems mentioned in Refs. 13, 20–23. Effect of Soldering Variables on the Solder Rise A high ultrasonic amplitude markedly improves the solder rise. The mentioned ultrasonic amplitudes were those on the top surface of the sonotrode. The vibration of the sonotrode was transmitted through the titanium plate to the bottom of the vessel and finally injected into the solder bath. That is, the vibrating bottom surface of the vessel served as the agitator for the solder bath. Our previous study (Ref. 26) revealed that the surface vibration of a solid plate agitated by a sonotrode follows a linear relationship with the input amplitude. Thus, the input energy to the solder bath increases with increasing sonotrode vibration, resulting in a significant elevation of the acoustic pressure level of the solder. Given that the ultrasound incident into the capillary would be dissipated, the difference in acoustic pressure between the outside and the inside of the capillary is enlarged when the acoustic pressure in the solder pool is increased. Thus, solder rise improvement is associated with an increase in applied ultrasonic amplitude. Solder rise is highly sensitive to the variation in joint clearance. Decreasing the joint clearance not only decreases the volume of the ultrasonic energy transmitting into the capillary but also accelerates the dissipation of the ultrasonic waves inside. As depicted in Equation 2, the acoustic attenuation coefficient is mainly dependent on the diameter of the capillary for a given ultrasonic system. Figure 15 shows the variation in acoustic attenuation coefficient with capillary diameter d for the Sn-9Zn solder. increases significantly when d is reduced from 500 μm to 0. By contrast, presents an extremely small value and changes minimally with the variation in d when d exceeds 500 μm. Therefore, the acoustic pressure difference in the liquid outside and inside the capillary, as well as the acoustic gradient along the capillary, is substantially augmented when the capillary is sufficiently small. In other words, the driving force of solder rise is increased in the capillary with a diameter under a critical value. Combining Figs. 11 and 15, we conclude that the critical capillary diameter in the current study is 500 μm. In addition, smaller clearance contains less liquid to be driven, resulting in a decrease in driving resistance, which also contributes to the enhancement of solder rise when the capillary dimension is reduced. The effect of heating temperature on solder rise is apparently complex. The physical properties of the solder, the base material, and the vessel vary with the temperature. These variations consequently influence the propagation characteristics of the ultrasonic waves in the system. For example, temperature elevation normally reduces the solder viscosity and density, which strengthens the cavitation effect in the solder bath by lowering the cavitation threshold. However, this condition also declines the elastic modulus of the vessel and thus reduces the ultrasonic energy transmitted to the solder bath. To date, clarifying which factor behaves dominantly during ultrasonication remains challenging, and further investigation is necessary. Conclusions The rise and wetting of liquid filler in a vertical joint clearance under ultrasonication WELDING RESEARCH was investigated, and the following conclusions could be drawn. 1. Filling of liquid solder in a vertical joint above the normal solder level was realized under ultrasonic agitation regardless of the presence of oxide layers on the surface of the base material. Surface oxides were gradually removed from the clearance bottom to the solder head by prolonging ultrasonic agitation time, which resulted in metallurgical bonding between the solder and the base material. 2. The mechanism for the ultrasonically induced capillary rise of liquid was proposed. The propagation characteristics of acoustics in the capillary significantly decreased the acoustic pressure at the entrance of the capillary and the acoustic pressure gradient along the capillary. This phenomenon was basically responsible for the nontraditional capillary rise of the liquid under ultrasonic agitation. 3. Solder rise increased roughly linearly with the applied ultrasonic amplitude and decreased with increased joint clearance and heating temperature. The acoustic pressure at the entrance of the capillary and the acoustic pressure gradient along the capillary depended on ultrasonic intensity and joint clearance. The process by which heating temperature influences the propagation characteristics of ultrasonic waves in the solder warrants further investigation. Acknowledgments This project is supported by the National Natural Science Foundation of China (Grant No. 50905044), Postdoctoral Science-Research Developmental Foundation of Heilongjiang Province (Grant No. LBH-Q12075), and Fundamental Research Funds for the Central Universities (Grant No. HIT. NSRIF. 201129). References 1. Graff, K. 1977. Macrosonics in industry: Ultrasonic soldering. Ultrasonics 15(2): 75–81. 2. Faridi, H. R., Devletian, J. H., and Le, H. P. 2000. A new look at flux-free ultrasonic soldering. Welding Journal 79(9): JULY 2016 / WELDING JOURNAL 271-s
Welding Journal | July 2016
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