WELDING RESEARCH A B contact between the solder and the base metal. The solder rise reached a maximum height when the ultrasonic exposure time was prolonged to 16 s. Comparative results between Figs. 7 and 8 show that solder wetting to the base metal was markedly improved not only at the clearance bottom but also at the solder head under prolonged ultrasonic exposure time. The amount of cavity and oxide inclusion was markedly reduced at the wetting interfaces of the solder head with prolonged ultrasonic exposure time. Further extension of the ultrasonic exposure time did not enhance solder rise but improved the wetting between the solder and the base metal — Fig. 9. On the one hand, most oxide inclusions were removed from the wetting interfaces, and the solder presented a close contact with the base metal at the solder head — Fig. 9A. On the other hand, the interaction between the solder and the base metal became more intense either at the bottom or in the middle of the clearance, as evidenced by the increased number of aluminum dendrites being transferred into the solder — Fig. 9B. However, a focus on the filling front revealed that an approximately 500-μmlong solder exhibited poor wetting to the base metal — Fig. 9C. No obvious oxidation occurred at the wetting interfaces, and most of the solder shared an intimate contact with the base metal in this region. Nevertheless, the several microcracks and limited aluminum dendrites 268-s WELDING JOURNAL / JULY 2016, VOL. 95 that grew from the Fig. 9 — Microstructure of the wetting interface after 24 s of ultrasonic exposure: A, C — solder head; B — the middle. solder/base metal interfaces indicated that no alloying or metallurgical bonding occurred between the solder and the base metal. The above observations illustrate several distinctive aspects of solder rise under ultrasonic agitation compared with those found in previous ultrasonic soldering applications (Refs. 13–15) or in the conventional soldering process (Ref. 19). First, liquid solder filling can exceed the normal solder level and reach a stable height after 16 s of ultrasonication. Second, filling is not dependent on the removal of the surface oxides of the base metal; that is, the liquid solder rises between the two layers of the surface oxide regardless of the absence of simultaneous wetting. In addition, the surface oxides of the base metal are gradually removed from the clearance bottom to the solder head with prolonged ultrasonic agitation time, and metallurgical bonding is formed between the solder and the base metal. Influence Factor of UltrasonicInduced Solder Rise Figure 10 shows the effective capillary rise of the solder as a function of ultrasonic intensity. The filling height of the solder increased, roughly linearly, with the ultrasonic amplitude applied. The solder rise was 4 mm when the input ultrasonic was approximately 11 μm, which is only 2 mm over the solder pool level. The level reached 8 mm when the ultrasonic input was increased to 20 μm. Figure 11 shows the variation in solder filling height with the joint clearance value. Solder rise was favored when the joint clearance was less than 500 μm and became negligible when the joint clearance exceeded 700 μm. Therefore, the solder filling height decreased with increasing joint clearance. This decrease was more significant for capillaries with diameters lower than 500 μm. Figure 12 illustrates the variation in solder filling height with heating temperature. In the range of 250°– 350°C, the solder increased to a lower level at an elevated temperature. Discussion Driving Force of Sonocapillarity In addition to the ultrasonic soldering system, a similar abnormal ultrasonic induced capillary filling has also been observed in several other fields, and several hypotheses in terms of Fig. 8 — Microstructure of the wetting interface after 16 s of ultrasonic exposure: A — clearance bottom; B — solder head. A B C
Welding Journal | July 2016
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