Feature: Batteries


robust method of making electrical connections in hybrids, switches and regulators. Also known as ‘friction welding’, there is minimal localised heating to the wire or battery, and the process is far better with tolerances in cell heights (relative to the busbar). In addition, there are several industry specifications relating to wirebonding and bond quality that are being adopted within the automotive sector. For example, MIL-STD-883E, Notice 4, Method 2011.7, is a test to measure bond strengths; see Figure 2. Since durability and safety are key here, ultrasonic


wirebonding has the edge over welding. Depending on the vehicle’s intended operating environment, the battery pack may be subjected to significant vibration and mechanical shock. Any interconnect technology used at the cell level must withstand the external forces expected, to ensure a good operational lifetime. The bond wire tends to be high-purity aluminium, with a


diameter between 0.2 and 0.5mm, with a degree of softness and flexibility (annealing). It’s worth noting that multiple bonds can be made side by side to accommodate high currents. As for safety, with a suitable diameter a bond wire can act


as a fuse, so a failing/shorting cell will effectively self-isolate, reducing the risk of fire or explosion.


In practice In the automotive sector in particular, keeping manufacturing costs down is crucial, and this applies to its suppliers, too. For example, Steatite’s Power Business Unit has recently taken delivery of an Asterion EV wire bonder; see Figure 3. Steatite makes custom battery packs, which need to be of a particular size and shape. The new bonder helps the company establish electrical connections using wirebonding much quicker than spot welding. In addition, as a result of the very low resistance of the bonding wire, battery packs can offer higher discharge capabilities and better performance. Steatite’s engineers have been successfully spot and arc


welding battery pack components for several years. Spot welding in particular is suitable for most of the company’s products, in which some parts are up to 3mm thick. Wirebonding is therefore a complement to welding in making most of the battery packs. On a general note, for any high-value manufacturing process,


the ability to rework process steps to improve assembly yield is important, especially in the initial prototyping and pilot production stages. In this respect, wirebonding has the edge, since weak or failing bonds can easily be reworked. Moreover, wire bonders like the Asterion EV can automatically perform wire pull tests to verify that the bond has taken. Reworking a failed or imperfect weld is more problematic,


since there will be more surface material to remove and the cell will be exposed to another temperature process as it is rewelded. Also, depending on its design, and once in place, a busbar might not allow access to individual joints for rework purposes.


Figure 3: The Asterion EV wedge bonder at work within Steatite’s Power Business Unit


Both, welding and wirebonding both have key roles in the


construction of EV battery packs. As for which process to use, this depends on the pack architecture, ease of access to the parts to be connected, whether or not fuses are required, the ability to accommodate reworks, the volumes being manufactured, production costs (including time) and the end application.


www.electronicsworld.co.uk April 2023 25


Figure 2: MIL-STD-883E, Notice 4, Method 2011.7, specifies the minimum forces to be applied during a bond wire pull test, depending on the material and its diameter. This is a non-destructive test; the bond wire is pulled midway along its length. Some wire bonders can perform the test using a hook as part of the bonding process, so the quality of every bond can be checked during production


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