Copper foil windings are used in high-current transformers and inductors because they offer controlled layer geometry and efficient window use. Connecting thin foil to a terminal, flexible braid, or thicker tab is challenging. Solder can add heat and wick along the foil; brazing can damage insulation. Ultrasonic welding copper foil creates a solid-state bond through pressure and high-frequency mechanical motion, but it needs a validated material and tooling window.
Joint stack definition
Specify copper alloy, temper, foil thickness, plating, tab material, number of layers, overlap length, surface condition, and orientation. Small changes in hardness or thickness alter energy transfer and deformation. A recipe for annealed copper may crack a harder foil or under-weld a thicker stack.
The joint should sit where tooling can support it without crushing winding insulation. Bending loads after welding should not concentrate at the weld edge.
Surface preparation
Oil, oxide, varnish, adhesive, fingerprints, and particles interfere with bonding. Cleaning must be controlled and compatible with nearby enamel, tape, and bobbin material. Abrasive cleaning can thin foil or leave particles; chemical cleaning can leave residue.
Define maximum time from cleaning to welding and handling method. Surface condition can change during storage even when parts look bright.
Process parameters
Horn amplitude, force, weld energy or time, trigger force, hold time, and collapse determine bond formation. Too little energy leaves partial contact. Too much can tear foil, extrude material, thin the stack, create edge cracks, or mark adjacent insulation.
Tool knurl pattern and anvil support control slip and local strain. Horn wear changes energy coupling. Tooling should have inspection limits and maintenance history.
Fixture and alignment
Overlap, centering, stack flatness, and support must remain repeatable. A tilted stack concentrates pressure. Flexible foil can wrinkle before the horn descends. Fixtures should position the joint without contaminating it or constraining necessary motion.
Electrical and mechanical verification
A weld can look textured and still have incomplete bonded area. Four-wire resistance measurement checks electrical performance when fixture and temperature are controlled. Peel, pull, or shear tests should load the joint in a direction relevant to assembly and service.
Metallographic cross sections reveal bonded regions, voids, cracks, thinning, and interface deformation. Destructive teardown during qualification and periodic audits complements production monitoring.
Environmental qualification
Thermal cycling tests differential expansion and residual stress. Current cycling heats the joint. Vibration and shock challenge the foil-to-rigid-tab transition. Humidity or corrosive exposure may reveal surface contamination or dissimilar-metal issues. Resistance should be compared before and after aging.
Production controls
- Lock material, temper, thickness, overlap, and surface preparation.
- Control machine recipe access and tooling identity.
- Monitor energy, force, displacement, time, and equipment alarms.
- Use first-off and periodic resistance and mechanical tests.
- Inspect horn and anvil wear and keep reference samples.
- Apply change control to foil, tab, plating, cleaner, tool, fixture, and software.
BaoHui Tech links weld records to transformer production lots and verifies the joint within the complete insulation and thermal construction.
Frequently asked questions
Does a low initial resistance prove the weld is reliable?
No. Mechanical integrity, bonded area, cracks, contamination, and resistance stability after aging also matter.
Can aluminum foil use the same recipe?
No. Material properties, oxide, tooling, energy, and dissimilar-metal behavior require a separate qualified process.