In either case the process can be automated for large-scale production or can be done by hand for prototyping or repair work.
In these devices, piezoelectric crystals are used to generate high frequency (20—60 kHz) acoustic waves in molten solder layers or batch, to mechanically disrupt oxides that form on the molten solder surfaces.
In the cases of soldering direct to glasses and ceramics, ultrasonic soldering filler metals need to be modified with active elements such as In, Ti, Hf, Zr and rare earth elements (Ce, La, and Lu).
The use of ultrasonic soldering is expanding, since it is clean and flux-less in combination with active solders being specified for joining assemblies where either corrosive flux can be trapped or otherwise disrupt operation or contaminate clean production environments or there are dissimilar materials / metals / ceramic / glasses being joined.
To be effective in adhering to surfaces, active solder's own nascent oxide on melting need to be disrupted and ultrasonic agitation is well suited.