Evolution and Adaptation in Battery Connectivity for E-Mobility

Ultrasonic and laser welding

With the rapid advancement of e-mobility, the requirements for battery connectivity are constantly evolving, creating an environment conducive to innovation. This article, while recognizing the importance of mass wiring in Giga factories, focuses on solutions more suited to mixed and medium-scale productions. 

We will explore three methods for connecting battery cells: ultrasonic wire bonding, ultrasonic welding (Smart Welding), and laser welding. Each of these techniques, distinct in its approach, aims to achieve precise process control and reliable traceability, indispensable aspects in today’s technological world. They stand out for their flexibility and ability to meet the diversified requirements of modern productions. 

This analysis aims to evaluate these technologies by placing them in a decision-making framework suited to specific situations, such as those encountered by design teams or within a corporate strategy. This approach offers an enlightened perspective for determining the most suitable solution, moving away from the usual constraints of large uniform productions. 

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Key Takeaways: 

  • Diversity of Connection Techniques – Explore ultrasonic wire bonding, ultrasonic welding, and laser welding for varied production needs. 
  • Adaptability and Precision of Wire Bonding – Ideal for prototypes, small and medium series, offering flexibility and precision. 
  • Efficiency of Ultrasonic Welding – Secure and efficient for high-capacity connections, with a cold process reducing risks. 
  • Advances in Laser Welding – Exceptional precision for fine welds, suitable for serial productions, with continuous innovations. 

 

Ultrasonic Wire Bonding, Mastering the Process 

Wire bonding process

Wire bonding is a frequently used technique in the manufacturing of semiconductor components and sensors. This process typically uses gold, copper, or aluminum wires, with diameters ranging from 17 to 50 µm. For connections requiring high current capacity, such as in power modules and components, aluminum wires of 300 to 500 µm are used. The process can also include the use of aluminum ribbons up to 2000 µm x 400 µm. The proven reliability and efficiency of this method make it a preferred option for battery cell connections, where the requirements for reliability, efficiency, and repeatability are extremely high. 

For Prototyping and Small Series 

In the context of prototypes and small productions, the flexibility and precision provided by ultrasonic wire bonding, whether manual or semi-automatic, are fundamental. Equipment like the HB series from TPT stands out for its versatility and precision, proving indispensable for both research and development and small production series. These machines are characterized by a user-friendly interface, with the ability to handle a variety of conductive materials, ensuring precise assembly adapted to each operator’s skills. 

For Medium-Scale Production 

As production increases, the adoption of automatic wire bonding machines becomes essential. Machines such as the Bondjet BJ985 from Hesse Mechatronics are ideal for handling large volumes, thanks to their spacious work areas that facilitate automatic handling. These equipments are constantly improved, particularly in terms of the user interface, with features designed to make operation easier. These improvements include automatic calibration and tools designed to minimize handling errors. In line with Industry 4.0 principles, these automated machines offer online programming capabilities and process data collection, contributing to rigorous quality control. They are also equipped with advanced networking features, facilitating interaction with production management systems (MES) for efficient cross-checking, data download and traceability, and remote troubleshooting for errors.  

Technical Advantages and Innovations of Ultrasonic Wire Bonding 

Ultrasonic welding is a high-precision, proven, and reliable technique, ideal for battery assembly. This process creates robust and durable connections with the help of proven industrial machines. From a design perspective, the use of wires as fuses in case of overcurrent improves the safety of battery systems. Moreover, the possibility of repairing these wires is a major asset for managing end-of-cycle batteries, their recycling, or replacement. 

The automatic navigation and high precision of the machines bring notable flexibility in creating battery cell connections. This method reduces the time required to integrate new product designs and adapts easily to production flexibility needs due to the absence of the need for specific tooling. However, it is important to emphasize that this process can be slower than other welding techniques, especially in situations where managing high currents is necessary and the use of multiple wires or ribbons is required. 

Ultrasonic Welding or “Smart Welding” – Waste-Free Welding 

Smart welding process

Ultrasonic Welding: Safety and Precision in High-Capacity Connections 

Ultrasonic welding is widely recognized for its safety and low risk, making it an attractive option. This type of welding excels in creating high-capacity current connections, especially for directly connecting cells to copper or aluminum bars. Compared to laser welding, ultrasonic welding offers several notable benefits. It is a cold method, without fusion or mixing of materials, which reduces the risk of creating intermetallic phases that could weaken the joints. Moreover, this process takes place at the material interface, thus avoiding any risk of compromising the cell’s seal. In addition, this technique ensures a safer production environment, without pollution or particle emissions that could cause short circuits in the battery pack. 

The significant advancement of intelligent ultrasonic welders lies in their ability to deliver high ultrasonic energy and force, necessary for assembling robust connections, while preserving the finesse and precision essential for power circuits. Their high precision, up to 10 µm, ensures that cell balancing remains unchanged despite resistance variations due to differences in weld positioning. In the production phase, the precision of movements, controlled by camera and axes, allows optimal adjustment of part positioning. The absence of the need for special tools to position or hold the parts facilitates the use of these machines, especially during design changes. Finally, the ultrasonic force guarantees high repeatability and reliability in assembly. 

Monitoring and Quality Control 

Rigorous control of the welding process produces essential data for evaluating the quality of each welded joint. During the process, several physical parameters are monitored, including force profile, deformation, ultrasonic power, and frequency. Careful observation of these parameters, especially oscillations, provides an in-depth analysis of the joint’s evolution and factors such as friction and vibration. This method not only ensures the quality of each weld; it also allows for comparing welds with each other to ensure uniformity throughout the production. The data collected continuously and in real-time allow for a complete history for the traceability of the welding process. 

Technical Innovations of the Smart Welder SW1085 

The Smart Welder SW1085 exemplifies innovation in ultrasonic welding. This machine stands out with its large work area of 374 mm x 883 mm, offering great flexibility for automation and handling of large battery packs. Equipped with the latest technologies, it has a wide range of ultrasonic forces up to 1000N and a maximum power of 1.5kW, making it capable of efficiently welding copper connection surfaces up to 9 mm2. The SW1085 is designed for a variety of welding applications and is distinguished by its long and thin sonotrodes, allowing deep access and tight spacing between welding points. 

Challenges and Limitations 

As with wire bonding, ultrasonic welding presents certain constraints related to material choice and assembly configuration. These restrictions are well known and documented. The significant force required for ultrasonic welding can limit the size of connections, particularly regarding material thickness and welding surface. 

Laser Welding – Challenges and Innovative Solutions 

Laser welding stands out for its remarkable precision, capable of performing fine and complex welds without physical contact, thus avoiding damage to sensitive components. Fast and efficient, it is perfectly suited for serial production where welding speed is crucial. Moreover, laser welding is versatile, capable of assembling a wide variety of materials and sizes. This characteristic is particularly advantageous for applications requiring very high currents, which would demand excessive force with ultrasonic methods. 

Smart laser welding process

Issues and Complexities of Laser Welding 

Even though laser technology is currently preferred for large-scale productions, like in gigafactories, it is not always the ideal solution, especially for medium-sized productions. In the field of batteries, the risk of fire due to particle ejection is a major concern. Implementing safety measures to reduce these risks is costly but necessary to avoid disastrous consequences. The efficiency of laser energy absorption depends on the surface reflectivity and can vary during melting, where inadequate control could damage the battery cell. 

Moreover, laser welding requires perfect contact between the parts, implying the use of clamping tools in production. Although welding speed is optimized by scanning and correct beam orientation, it is important to note that the beam shape can change if the angle of incidence is not vertical. Adapting to these variations is possible, but it requires additional time and costs for setup. 

Optimization and Innovation: Hesse’s Strategy 

Hesse aims to simplify the use of laser welding, making it more accessible, especially for SMEs. The LW1089 model from Hesse is characterized by a mobile welding head that travels to each welding point and applies controlled force for high-quality laser welding. This innovation eliminates the need for specialized tools, thus reducing the costs and complexity associated with traditional laser welding methods. Additionally, the machine is equipped with a shape recognition system and a tactile sensor to adjust welds according to the required dimensions. Laser welds, performed in a closed nozzle, allow for efficient smoke extraction and particle capture, reducing setup time for new applications. The flexible programming of weld shapes increases adaptability to different welding processes. 

Real-time monitoring of the integrity of the welding process is ensured by measuring the emission spectrum during welding, providing key information on the melting temperature and material composition. Improved smoke extraction enhances the accuracy of these measurements, and the collected data can be used for traceability in production. 

Finally, the LW1089 from Hesse integrates the same control software and axis systems used in their wire bonding machines, allowing for the reuse of proven automation solutions. This complete integration with factory software facilitates the management of the machine, program, and production line, ensuring seamless integration into a production environment. 

Challenges and Technical Considerations 

In recent developments, the focus has been on flexibility, control, and safety, which can affect the speed of transition between welding positions. It is important to emphasize that risks should not be overlooked: despite the presence of safety measures, a visual inspection of particles is always necessary. Moreover, acquiring a laser welder represents a significant investment, with costs varying according to the power of the laser. 

It is also necessary to consider the materials and intermetallic alloys formed during the fusion process. The crystalline structure of these materials can be fragile, leading to the creation of joints that may break. Thus, laser welding technology, particularly the choice of materials, is a constantly evolving field that requires dedicated attention. 

Conclusion 

Each method examined here – ultrasonic wire bonding, ultrasonic welding (Smart Welding), and laser welding – offers specific advantages, lending themselves well to various scales of production, including medium volumes. The choice of connection technique closely depends on the specific needs of each project, taking into account factors such as budget, production size, the need for repeatability, and the ability to adapt to evolving requirements. Each of these techniques plays a significant role in the field of battery production. To succeed, a careful and precise evaluation of each application’s needs is necessary to choose the most appropriate method. Thus, success depends on a judicious and flexible selection among these technologies.

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Ultrasonic and laser welding

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