Long Term Interconnect Creep and Weld Fatigue under Cyclic Thermal Load in Heavy Formats
Thermal expansion and cell swell induce cyclic shear stresses that trigger weld fatigue and interconnect creep, requiring compliance flexure loops.
Ultrasonic Wire Bonding
Meaning
A micro-welding process uses high-frequency ultrasonic energy and pressure to connect thin aluminum or copper wires between battery terminals and busbars. This ultrasonic wire bonding is an alternative to laser or resistance welding, especially in high-density cylindrical cell packs. In cell sourcing, the presence of this joint technology indicates a pack designed for high vibration resistance and serviceability.
The boundary applies to thin-wire connections that connect individual cells to a common conductor plate. It ceases to apply to heavy busbar welding and direct mechanical clamping.
Bonding Process
The process starts by feeding a thin wire through a bonding tool called a wedge. The tool presses the wire onto the terminal while applying ultrasonic vibrations at several dozen kilohertz. This action creates friction that breaks down the surface oxide layer and fuses the metal molecules.
The joint is formed without applying external heat, which prevents damage to the cell’s internal seal. Once the first bond is complete, the tool moves to the busbar to create the second bond. The wire is then cut, leaving a flexible electrical bridge.
This entire process is completed in a fraction of a second per cell.
Production Strategy
Sourcing teams select this connection method for its high manufacturing yield and diagnostic capabilities. This ultrasonic wire bonding technique operates as an integrated fuse that protects individual cells. If a single cell experiences an internal short circuit, the thin wire bond melts and isolates it.
This action prevents the short circuit from draining the rest of the battery pack. Sourcing managers verify that the bonding equipment includes pull-testing mechanisms that check the strength of each joint. This real-time quality control ensures that no weak bonds leave the factory.
Vibration Resistance
Flexible wire bridges are highly effective at absorbing the mechanical stresses of automotive applications. When ultrasonic wire bonding is used, the connections do not crack under continuous road vibration. This flexibility prevents the electrical contacts from separating over the vehicle’s lifetime.
Failed connections lead to disconnected cells and reduced module capacity. The process requires high-purity aluminum or copper wire to ensure low resistance and consistent bond strength. Regular calibration of the ultrasonic transducer is necessary to maintain uniform weld quality across all production batches.
Buying Cells against Buying Packs and Where Responsibility Sits
Buying cells requires owning BMS development, weld quality, thermal isolation, and pack safety files; buying packs trades unit margin for transferred liability.