Key takeaways
- The termination has to carry the current, survive the assembly and resist the vibration of the application.
- Enamel must be removed from the joint area, or the weld or solder will sit on insulation instead of copper.
- Strain relief belongs on the lead or the bobbin, not on the fine wire at the joint.
- A pull test, a resistance check and a section are more useful than a visual check alone.
Why the termination decides coil life
A coil can be wound perfectly and still fail at the point where the wire leaves the bobbin. The joint carries the full current in a small area, it is often the hottest part of the winding and it is the place where movement concentrates.
The termination also has to survive the assembly. A lead that is pulled during handling, a pin that is bent during insertion or a tab that is twisted during soldering can damage the joint before the part ever reaches the customer.
Three routes are common: a pin or post moulded into the bobbin, a flexible lead wire, and a welded tab. Each has a different failure mode, and each needs a different inspection.
Pins and posts
A pin is a rigid terminal moulded into or pressed into the bobbin. The winding wire is terminated to the pin, usually by solder, welding or a wrapped connection. The pin then plugs into a connector or receives a spade terminal.
Pins suit automated assembly and a defined connector interface. The pin material and plating have to match the mating terminal, and the pin has to withstand the insertion force without moving in the bobbin.
The weak point is the joint between the fine wire and the pin. A solder joint can wick along the wire and create a stiff point that cracks under vibration. A weld avoids the wicking but needs the enamel removed and the wire in close contact with the pin.
The pin also conducts heat away from the joint. That can help a solder process, but it can make a resistance weld harder to start. The process has to be set for the actual pin and wire combination.
Lead wires and flying leads
A lead wire is a flexible conductor that carries the connection away from the coil. It is spliced or welded to the winding and usually protected by sleeving, tape or a moulded strain relief.
Leads are common where the coil has to connect to a terminal block or a harness. They allow tolerance in the assembly and they isolate the fine winding wire from the movement of the harness.
The lead insulation has to match the thermal class of the coil. A class H winding with a class B lead is a class B weak point. The lead gauge also has to carry the current without adding significant resistance.
The splice is the critical joint. It should be supported so that the load is carried by the lead insulation or a tie, not by the fine wire. A splice that hangs on the wire will fatigue at the edge of the tape.
Welded tabs
A welded tab is a metal flag or a terminal that is resistance welded or ultrasonically welded to the wire. The weld forms a metallurgical bond without adding solder, which suits high temperature and high reliability applications.
Resistance welding passes a current through the joint and the resistance at the interface generates the heat. The process needs clean metal, the right electrode force and a controlled energy. Enamel left on the wire acts as an insulator and produces a weak or no weld.
Ultrasonic welding uses high frequency vibration to break the oxide and bond the metals. It works well on copper and aluminium and avoids the heat of resistance welding, but the tooling and the parameter window are specific to the joint.
A welded tab is strong and compact. The risk is that the weld looks good from the outside and has little bond area. A pull test and a section are the usual way to confirm the process.
Enamel removal is part of the process
The joint area has to be bare copper before welding or soldering. Abrasion, chemical stripping or a hot solder pot can remove the enamel, but each method needs its own control so the wire is not damaged.
Choosing the joint method
The table below compares the common routes. The right choice depends on the wire size, the current, the temperature, the volume and the equipment already in the winding house.
| Method | Best for | Main risk |
|---|---|---|
| Solder to pin | Small wire and low volume | Wicking, a stiff joint and flux residue |
| Resistance weld | Copper wire to a tab or pin | Weak weld if enamel or oxide remains |
| Ultrasonic weld | Copper and aluminium, high current | Narrow process window and tool wear |
| Crimp terminal | Leads and harness connections | Wrong die or wire range and a loose barrel |
| Welded lead splice | Flexible leads on a coil | Strain on the fine wire at the splice |
Many coils use more than one method, such as a welded winding tab and a crimped lead. Each joint needs its own process control.
Strain relief and routing
The fine wire that leaves the winding has almost no strength. Any load on the joint should be carried by the bobbin, a tie, a tape wrap or the lead insulation. A loop of slack between the winding and the anchor is a simple and effective relief.
The lead route should avoid sharp edges, heat sources and moving parts. A lead that rubs on a housing will fail long before the winding does. A lead that runs close to a hot component needs a higher temperature insulation or a heat shield.
Pins need support too. A pin that is bent during mating puts a load on the internal joint. A moulded boss or a connector housing that takes the insertion force protects the winding.
The drawing should show the route, the anchor point and the minimum bend radius. A note that says route the lead neatly is not a specification.
Inspecting the joint
Visual inspection catches the obvious faults: loose strands, burnt enamel, a cold solder joint, a weld that has expelled too much material and a lead that is pinched. It does not prove the bond area.
A pull test measures the strength of the joint. The sample size, the direction of pull and the acceptance value should be agreed. A pull test that pulls the lead out of the splice without loading the weld proves very little.
A resistance measurement across the joint detects a high resistance connection. A four wire measurement is better on a low resistance joint, because the lead and contact resistance can swamp the result.
A section through the joint shows the bond area, the voids and the enamel remnants. It is the most useful check for a new process, and a photograph from the first article should be kept with the records.
What belongs on the drawing
State the termination type, the pin or tab material and plating, the wire gauge and the joint method. Add the pull test value, the resistance limit and the inspection level.
For a lead, state the length, the insulation material and class, the colour if it matters and the stripping length. For a pin, state the mating interface and the insertion force if the connector is part of the design.
Add the strain relief and the route. A drawing that leaves the lead route to the winding house will get a different route from each supplier, and the assembly may not fit.
Finally, state the test after assembly. A high potential test between the winding and the terminal, a resistance check and a pull test on a sample are the minimum. The records should travel with the first shipment.
Frequently asked questions
Why does a coil joint fail after a few months?
Common causes are a weak weld or a cold solder joint, enamel left in the joint, a stiff joint that cracks under vibration and a lead that loads the fine wire. A section and a pull test on the first article usually reveal which one applies.
Can I solder directly to enamelled wire?
Only if the enamel is removed or is a solderable grade designed to melt at soldering temperature. For most grades, the enamel has to be stripped mechanically or chemically before the joint is made.
Which is better, a welded tab or a lead wire?
A welded tab is compact and suits a direct connection or a high temperature joint. A lead wire is flexible and suits a harness or a terminal block. The choice depends on the assembly, the current and the movement in service.
How do I test a coil termination?
Use a pull test for strength, a four wire resistance measurement for electrical quality and a section for bond area. Add a high potential test to confirm the termination did not damage the insulation.
Sources and further reading
- IEC 60317 series, Specifications for particular types of winding wires
- IPC-A-610, Acceptability of electronic assemblies
- Qumek coil termination and inspection notes
Qumek Engineering Desk
Engineering team
The engineering team behind Qumek drawings, DFM notes, tolerances and inspection standards. Every guide is reviewed against the drawings and processes used on live programmes.
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