54 September/October 2023 | E-Mobility Engineering Sound joints Ultrasonic welding (UW) uses the combination of pressure and ultrasonic vibration that, in metals, causes highpressure dispersion of surface oxides. In turn, this brings clean surfaces into very close contact under high pressure and at localised temperatures, which while high, are below the melting point of the metals. Under these conditions, metal surfaces diffuse into each other to form diffusion bonds. As the metals don’t melt, UW does not make fusion joints. According to Holtkamper, UW principles are well-known, standardised production equipment is available, the process is well-suited to materials with high thermal conductivity and offers good traceability. But although UW needs no elaborate support tooling, it does require a rigid underlying structure. Wiegel recently showed that UW is also very effective for creating busbars of composite thickness (meaning the thickness varies along and across the bar), having been awarded a patent for it earlier this year. The process involves cutting out a base component and a thinner foil sheet, loading both into a fixture according to how they are to be joined, then passing the fixture holding them through a UW machine and then trimming away excess material to produce the final part. Compared with alternative methods such as laser welding and brazing, using UW to produce composite thickness busbars for battery current collectors produces wider and stronger welds of greater cross-sectional area, the company says. Mark Boyle notes that UW handles dissimilar metals well, but the vibration can damage nearby components. The process also produces fine dust that has to be removed from the battery, or prevented from entering it. Carr points out that, in battery applications, the process is only used on non-ferrous metals and is limited to joining interconnects and busbars. Holtkamper adds that ‘smart welding’ combines high welding power of conventional ultrasonic welding equipment with the flexibility, precision, speed and advanced process control of wire bonding machines. Wire bonding Wire bonding can be described as an ultrasonic and compression process in which a combination of time, ultrasonic energy and pressure is used to create an intermetallic bond between the wire and the contacts. The wire is fed through the machine and the welding head to provide the connector material; the tool on the head is called a wedge, hence the term ‘ultrasonic wedge bonding’. The technology has been used in the semiconductor industry for decades, and is also familiar to the automotive industry which conforms to its standards, Holtkamper says. He adds that the looped form of the connection allows movement that absorbs stresses created by thermal expansion, that the process provides good control of electrical resistance, and the wires can also act like fuses, parting to isolate faults. Further, because the welding tool moves from one contact point to another, it is easy to reprogram it within its motion limits to adapt it to different products and contact configurations. Finally, any faulty wire bonds can easily be sheared off and in some cases remade, he says. The main limitation of the process is in the dimensions of the wire it can handle. For round wire, the upper limit is between 500 and 600 µm in diameter, while for ribbon wire it is 2000 x 400 µm, a consequence of which is that contact areas are limited. Also, applications that use multiple wires to connect each cell to a current collector need relatively long welding times. Micro-TIG welding is a version of the tungsten inert gas (TIG) electric process used for small parts made from nonferrous metals. Enabled by advances in high-frequency power supplies that improve arc stability and control of small currents, it is very good for joining copper while offering what Amada Weld Tech describes as a fairly relaxed process window with respect to part fit-up and positioning tolerances of the electrode to the parts. But although relatively cheap to implement, it is slow and consumes electrodes. In batteries, it offers a good solution for welding busbars that would otherwise need a brazing material for Ultrasonic smart welding is designed for high speeds with precise control in battery module and pack production and to handle cells, flexible busbars and tabs that connect BMS and voltage sensing contacts (Courtesy of Hesse Mechatronics)
RkJQdWJsaXNoZXIy MjI2Mzk4