What joining methods optimize EV battery production efficiency? part 2

Resistance spot welding and wire bonding are popular choices for creating joints during EV battery production. However, every joining technology comes with a trade-off, giving packaging engineers room to select the appropriate one for their battery design. This is the second part of the multipart FAQ on the joining methods for EV battery production and how efficient these joints are.

Resistance spot welding — for low-cost, good-quality control

Resistance spot welding uses pressure and electrical current to fuse materials in specific areas. Through controlled melting at the contact point, this process joins tabs up to 0.4 mm thick made of steel, nickel, copper, and aluminum, among other materials. The schematic working of this phenomenon is shown in Figure 1.

Figure 1. A schematic working principle of resistance spot welding. (Image: World Electric Vehicle Journal, MDPI)

Advantages of resistance spot welding

The method has several important benefits for making batteries. It is cost-effective because it does not need replacement materials and can be done automatically. Compared to laser and ultrasonic welding, his method uses a lot less heat, which protects battery parts that are sensitive to heat.

In addition, resistance spot welding makes connections that are strong enough to resist vibration, which protects the integrity of the battery pack. There are well-known quality control methods that make it easier to keep an eye on and confirm the quality of welds.

Disadvantages of resistance spot welding

Resistance spot welding has trouble making big weld joints, working with thick materials, and joining multiple layers simultaneously, which limits the types of applications that can be used. Material issues also create problems, especially when working with highly conductive materials like copper and aluminum tabs or when joining different materials that melt at different temperatures.

Figure 2. Wire bonding EV battery cells can happen at ambient temperature, which makes the process safer and easier. (Image: Elsevier)

During the resistance spot welding, the sonotrode and anvil approach each other to complete the welding process, as shown in Figure 2. Electrode sticking, when material from the object sticks to the electrode tips, is a recurring problem. Also, because resistance spot weld nuggets are smaller, there is more current density when the battery runs, which could affect its long-term performance.

Wire bonding — for fuse functionality, low energy consumption

Figure 3. Wire bonding EV battery cells can happen at ambient temperature, which makes the process safer and easier. (Image: Inseto)

Wire bonding utilizes ultrasonic energy and pressure to create a metallurgical bond between a thin wire and a substrate. This technology has been adapted for EV battery production, connecting individual cells to form battery modules (Figure 3). The process involves laser cleaning of terminals, placement of wire bonds, and testing of wire bonds.

Advantages of wire bonding

Wire bonding occurs at room temperature, eliminating the need for external heat sources and minimizing the risk of heat damage to the battery cells. This is a significant advantage over welding processes like laser or resistance spot welding that involve melting the materials, especially considering the heat sensitivity of battery chemicals.

Figure 4. Experimental results showing the thermal analysis comparing wire and spot bonding for a moving vehicle condition. (Image: Nexcharge)

The flexibility of wires allows wire bonding to accommodate height differences between cell terminals and busbars, simplifying module design. Wire bonding also doesn’t require the strict tolerances in joint alignment that laser welding demands. Wire bonds can also act as fuses, isolating failing cells and preventing a potential chain reaction that could lead to a battery pack meltdown.

Because wire bonding allows for single-sided connections, battery cells can be placed directly against the enclosure with cooling fins, improving heat dissipation. This phenomenon is evident in Figure 4, which shows a thermal analysis of the battery packs using wire bonding and spot welding technologies for a moving EV at 5 Km/hr. The average temperature of the battery pack using wire bonding technology is down by 19.4°C compared to the spot welded design.

Disadvantages of wire bonding

Wire bonds are generally weaker than welds created by fusion processes like laser welding, especially for high-current applications. Wire bonding is primarily suitable for joining thin wires to battery terminals, but is less effective for joining larger components or thicker materials.

Wire bonding equipment can be complex and require specialized expertise to operate and maintain. The build-up of oxides on bond wires can compromise bond quality, necessitating careful handling and storage.

Summary

Resistance spot welding is advantageous because it lowers costs while maintaining reasonable quality control over EV battery production. The process is also easy to automate. However, it is less suitable for dissimilar materials and making bigger welds than other joining methods.

The wire bonding is good for two-wheeler EVs that operate at lower power but face more vibrations and shocks, as the wire bonds are flexible and result in lower thermal issues. However, they are not as strong as other joining technologies and are difficult to use for high current flow because they act as a fuse and result in power discontinuity.

The next FAQ will focus on lesser-used joining technologies, such as micro-TIG, micro-clinching, and magnetic pulse welding, to name a few.