What is Wire Bonding and how it helps prolong the life of Li-ion battery packs

Nexcharge, a brand of the JV between Exide Industries Limited and Leclanché SA, is using wire bonding technology to connect cells in its Li-ion battery packs. We caught up with CEO, Stefan Louis to understand more about the technology and its advantages.

This interview was first published as a part of EVreporter e-magazine for April 2021. Check out our magazines here.

What is wire bonding?

Wire-bonding is an ultrasonic, metal- metal friction welding process that is used to connect cells into a battery pack. The process takes place at room temperature and no external heat is necessary for welding.

The technology is not new per se and has been widely utilized in microelectronics and power electronics industries since the 1970. In fact, all battery packs in Tesla cars use this technology. We are the first ones in India to use wire bonding for battery pack manufacturing.

How does wire bonding work?

It is a combination of three parameters that forms the bond: vertical force, ultrasonic power and time. An ultrasonic transducer generates vibrations in the range of about 60kHz. Vibrations are transferred to the welding area for a period in the order of 100ms.

The 3 step production process includes:

• Laser cleaning of terminals
• Placing of wire bonds
• Testing of wire bonds

What is the more commonly used process that wire bonding replaces?

It replaces spot welding that is commonly used to connect cells in the battery pack. The battery packs using spot welding are suitable for low power applications only. Though spot welding provides a low capex entry to manufacturing, it introduces multiple issues with respect to cell connections and thermal design.

Manual spot welding may show variation in quality. There is also a risk of short circuit due to loose wires to BMS and it is difficult to extract heat from the battery pack. Spot welding design has much higher cell temperature gradient that leads to greater battery imbalance.

What are the pros of wire bonding in making cell connections?

• Makes fast and fully automated connections
• All wire links are tested automatically
• Makes battery packs safer by the virtue of fuse link effect per cell
• Enables lower resistance and better battery performance
• Lighter batteries as intercell busbars are optional
• Allows better thermal relief as cells can be placed against the enclosure
• Less production scrap as any faulty wire bonds can be replaced

What are the pros of wire bonding in terms of thermal design?

Wire bonding design allows cells to be connected from one side, which allows the cells to be pressed against the enclosure with fins. This results in much better heat dissipation than spot welded designs. Without air movement, our tests have shown up to ~8°C average temperature difference under real operating conditions and ~11.4°C max temperature difference.

Whereas for slowly moving vehicle conditions, our tests show ~19.4°C max temperature difference under heavier operating conditions. Wire bonded batteries run cooler and can offer double the life as compared to spot welded batteries.