Ultrasonic welding is a connection technique with which one or more copper wires can be joined together. This is done by means of high pressure and high friction frequency (in the ultrasonic range) in an ultrasonic welding machine. This causes the wires to become so hot that they fuse together and this creates a welding plug. You can evaluate this welding plug with the IPC/WHMA-A-620 standard. You can also weld wires to contacts with this technique (they often do this in the automotive industry).