The influence of the drop-size effect on the wetting kinetics in SAC305/Ni system

The studies of the drop-size effect on the wetting kinetics of the nickel substrate by lead-free solder were performed by the sessile drop method in a vacuum at a temperature of 260°C.
In this work, Sn-based alloy SAC305 (Ag - 3.0 wt. %, Cu - 0.5 wt. %) and polished nickel substrate (99.8%) were used. During the wettability tests, a separate heating of the couple of materials to a test temperature was performed by placing a sample of SAC305 alloy in a graphite capillary above the nickel substrate. After reaching the test temperature, the SAC305 drop was squeezed from the capillary and deposited on the Ni substrate. After 30 seconds, the second drop was squeezed and deposited on the first one, thereby increasing the volume of the drop twice. The process was repeated 9 times every 30 seconds reaching a 10 times larger drop volume, compared to the first drop.
After wettability tests, the solidified drop/substrate couple was cooled, cut and subjected to a shear test and structural analysis by optical microscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy analysis.
Wettability studies have not shown a significant difference between the values of the contact angle obtained for each drop produced by subsequent adding of nine drops despite their different size of ten drops produced on the substrate, their contact angle values formed after 30 second contact time were similar showing θ = 35 ±2°. High repeatability of the measured contact angle value θ demonstrates not only physical homogeneity of the substrate surface but it presents a lack of size effect over a range of SAC305 drop volume used in this study.
The obtained results demonstrate the suitability of the sessile drop method for surface quality evaluation, particularly for quality tests of thin technological layers or coatings used in joining dissimilar materials.

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