Litz wire is made from many individually insulated strands that are transposed so each strand occupies different positions within the bundle. At the correct strand diameter and frequency, this construction reduces skin and proximity loss in a high frequency transformer. The benefit can be lost at the connection if only part of the bundle conducts, if solder wicks into a flexing section, or if the stripping process damages fine copper.
A reliable litz wire termination method must make electrical contact to the intended strands, withstand production handling and thermal cycling, fit the insulation system, and remain repeatable across bundle tolerance. There is no universal best method. Soldering, crimping, resistance welding, ultrasonic welding, hot-bar processes, and purpose-designed terminals each suit different wire constructions and production volumes.
Start with the actual litz construction
Important inputs include strand diameter, strand count, bunching and cabling pattern, serving or textile wrap, outer insulation, conductor material, enamel chemistry, overall bundle dimensions, and required current. A bundle of several large strands behaves differently from hundreds of very fine strands. Some enamels are solderable at controlled temperature; others require mechanical, thermal, chemical, or laser removal.
The drawing should identify the approved wire construction and supplier. Nominal equivalent cross-sectional area is not enough because strand insulation and bundle geometry affect stripping, wetting, compression, and terminal fit.
Soldered terminations
Solder can join many strands simultaneously when enamel is fully removed or designed to dissolve in the process. Dip soldering is common, but bath alloy, temperature, dwell time, immersion depth, flux, preheat, and post-cleaning need control. Low temperature or short dwell can leave insulated strands. Excessive heat can oxidize copper, damage serving, recede insulation, or embrittle the lead.
Capillary action can draw solder beyond the intended termination and create a rigid section. If that rigid-to-flexible transition sits at a vibration or bend point, strand fatigue can develop. Define maximum wick length and support the lead so movement does not concentrate at the solder boundary.
Crimping and terminal compression
A crimp avoids prolonged heat, but it must penetrate or remove strand insulation as intended. Some terminals use serrations or controlled deformation to break through enamel. Others require pre-stripped and consolidated strands. Barrel size, conductor fill, crimp height, tool geometry, and wire insertion determine whether inner strands join the contact.
Pull force alone is insufficient. An overcompressed bundle may show high short-term retention while cutting strands. A visually acceptable crimp can also trap insulated inner strands and have excessive resistance. Cross sections, four-wire resistance, current cycling, and environmental aging complement mechanical tests.
Welding methods
Resistance and ultrasonic welding can create low-resistance joints without solder, often after consolidating the bundle into a compact end. Weld energy, force, time, electrode or horn condition, material stack, and cleanliness define the process window. Too little energy leaves partial bonding; too much thins or fractures fine strands.
Welding is sensitive to conductor mass and placement. Recipe control should prevent a setting developed for one bundle from being used on a different strand count or terminal thickness. Electrode wear and surface contamination need scheduled monitoring.
How to verify strand participation
A bright exterior joint does not prove inner-strand contact. Qualification can combine metallographic cross sections, resistance measurement, thermal imaging under current, pull and bend tests, and teardown. The joint resistance should be low and stable relative to the winding resistance, with measurement fixtures and temperature controlled.
High-frequency current distribution at the terminal can differ from DC behavior. A long solidified section or large terminal can introduce local proximity and eddy-current loss. For high-current designs, BaoHui Tech reviews termination geometry as part of the winding-loss model and confirms temperature on assembled prototypes.
Insulation and safety considerations
The termination may sit near another winding, core, clamp, or PCB. Stray strands, sharp terminal edges, solder spikes, insulation recession, and conductive residue can reduce creepage or puncture tape and sleeving. The required basic or reinforced insulation must be maintained after the complete assembly process, not only on the bare bobbin.
Production control checklist
- Lock the litz construction, supplier, terminal, alloy, flux, and tooling.
- Define strip length, exposed-strand coverage, wick limit, crimp geometry, or weld nugget criteria.
- Use first-off and periodic resistance, pull, appearance, and dimensional checks.
- Schedule bath analysis, tip or electrode maintenance, calibration, and reference samples.
- Link wire, terminal, machine, recipe, and test data to the transformer lot.
- Requalify changes to enamel, serving, strand count, terminal plating, process chemistry, or tooling.
Frequently asked questions
Can litz wire be soldered without stripping every strand?
Only when the enamel and controlled solder process are specifically designed and validated to expose all required strands. A successful-looking outer surface is not enough evidence.
What should an OEM send to BaoHui Tech?
Provide current and frequency spectrum, litz construction, allowable terminal size and temperature, insulation requirements, connection technology preferences, vibration duty, resistance limit, and production volume.