A comprehensive analysis of copper foil diffusion bonding technology for Multilayer Copper Foils Flexible BusBar

The Multilayer Copper Foils Flexible BusBar (hereinafter referred to as "flexible busbar") is made by stacking multiple layers of 0.05–0.3 mm high-purity ductile copper foil (T2 or TU1). It is then subjected to pressure (5–15 MPa) and vacuum (≤ 5×10⁻² Pa) at 750–850 ℃ for 20–40 min, allowing copper atoms to diffuse and form a metallurgical bond. Both ends are welded to create rigid terminals for electrical connections, while the central section retains unwelded areas between the foils, forming flexible "Ω", "U", or "S" shaped bends. This design combines a large cross-sectional area for high conductivity at the terminals with flexibility for bending, allowing it to absorb vibrations and thermal expansion in three dimensions.

1. Current Carrying Capacity: Hard terminal contact surface ≥ 3 A/mm² (based on 40 ℃ temperature rise), flexible section dynamic current ≥ 2 A/mm².
2. Contact Resistance: Same cross-section hard terminal ≤ 5 μΩ, flexible section increases by ≤ 2 μΩ per 100 mm.
3. Bending Life: R=5×thickness, ±30° bending angle, 1 Hz, >10⁵ cycles without interlayer cracking.
4. Thermal Shock Resistance: -40 ℃↔+150 ℃, 1000 cycles, 0/10 cracking rate in the welded area.
5. Flame Resistance: UL94 V-0 rating after welding, oxygen index ≥ 32%.

1. Copper Foil Pretreatment: Degreasing → pickling → inert gas purging, residual carbon ≤ 0.3 mg/cm², ensuring a pure diffusion layer.
2. Layer Positioning: Using 0.02 mm laser-cut positioning holes, layer misalignment ≤ 0.05 mm, preventing "steps" on the welded surface that could increase local resistance.

3. Vacuum Diffusion Welding: The temperature gradient during heating should be ≤ 10 ℃/min. Maintain a temperature of 600 ℃ for 10 min to degas, then increase the pressure to the process temperature. Maintain pressure during cooling to prevent thermal shrinkage voids.
4. Post-Weld Shaping: Use a 2000 t four-post hydraulic press for cold-pressing and flattening. The flatness of the hard end should be ≤ 0.1 mm/100 mm, meeting the IP65 sealing requirements.
5. Surface Treatment: Tin-plate the hard end with 8–12 μm or silver-plate with 3–5 μm; no substrate corrosion after 48 h salt spray test. Cover the flexible section with 0.1 mm flame-retardant silicone rubber or heat-shrink tubing; withstand voltage ≥ 2 kV AC/1 min.

1. The length L of the flexible section can be estimated by ΔL=α·ΔT·L₀, where α (copper) = 17×10⁻⁶ /℃. Ensure thermal expansion compensation ≥ 1.5×ΔL.
2. Bend radius R ≥ 5×total thickness. If space is limited, multiple "hard-soft-hard" units can be welded in sections to reduce bending stress.
3. The current direction should be parallel to the foil layers to avoid eddy currents. When the operating frequency > 1 kHz, increase the soft section cross-sectional area by 20% to reduce skin effect heating.
4. The weld zone width ≥ 1.2×soft section width for smooth current transition; the distance from the hard end hole to the weld zone edge ≥ 1.5×hole diameter to prevent delamination.

1. New Energy Battery Packs: Copper Flexible BusBar Replace traditional copper busbar and cable solutions, improving space utilization by 25% and absorbing 2 mm thermal expansion displacement.
2. Wind Power Converter Cabinet: Connect the IGBT and busbar capacitor, reducing terminal stress by 30% under 50 Hz vibration, and extending module lifespan.

3. Railway Traction Inverter: Multilayer Copper Foils Flexible BusBars Meets EN 45545 HL3 flame retardancy requirements; 18% lighter than rigid copper busbars, improving vehicle weight reduction.
4. Energy Storage Container: Integrates flexible busbars and liquid cooling plates, allowing operation between -30°C cold start and +55°C full load without additional maintenance.

1. Interlayer Microcracks: Often caused by rapid heating, leading to trapped gas between layers; extend the 600°C degassing stage and maintain a vacuum <5×10⁻² Pa.
2. Terminal Warping: Pressure release during cooling occurs too early; release pressure only after ≤ 300°C, and use a stainless steel pressure plate for even cooling.
3. Surface Oxidation: Exposed to air at >60°C after cooling; control cooling temperature ≤ 40°C or purge with nitrogen.
4. High Contact Resistance: Insufficient welding temperature or insufficient holding time; re-weld at 850°C for 30 min, but re-test the bending life.