What Is Fully Insulated Wire (FIW)? How It Compares with Triple Insulated Wire (TIW)

Power products keep getting smaller, and transformers have to shrink with them. But a transformer must keep enough safety insulation between its primary and secondary windings. With conventional enameled wire, that usually means adding insulating tape or margin barriers, which take up a lot of space.

Triple insulated wire (TIW) and fully insulated wire (FIW) both solve this problem: the wire itself provides enough insulation, so no separate barrier is needed. The goal is the same, but the structure, size, soldering method and cost are different.

What is triple insulated wire (TIW)?

As the name suggests, triple insulated wire has three insulation layers around the conductor. Compared with conventional enameled wire, it increases the insulation strength on the secondary side, so the safety margin can be met without a barrier layer or insulating tape. This greatly reduces transformer size.

TIW is particularly well suited to high-frequency transformer windings in compact, high-efficiency switching power supplies, and is a mature, widely used solution.

What is fully insulated wire (FIW)?

Fully insulated wire was developed around 2010. Instead of three extruded layers, it uses a multi-layer coating process that applies special insulating varnish to the conductor. Yoshikura’s fully insulated wire KFIW(3~9)-B, for example, has 20 to 40 layers of enamel film around the conductor, reaches reinforced insulation and has a pinhole-free surface.

Because the insulation is thinner, the overall diameter of FIW is about 30% smaller than TIW. That brings several direct benefits:

• Smaller winding volume → smaller transformers
• Higher output in the same volume → better performance
• Strong, abrasion-resistant film that works with most automated winding equipment
• Lower overall cost

FIW vs TIW specifications

The key differences are:

1. Size: FIW insulation is roughly half the thickness of TIW, so the same winding window holds more turns, or the transformer can simply be made smaller.

2. Soldering: FIW can be soldered directly at a lower temperature and in less time, and the heat-damage distance on the pins is only about 2 mm. Lower soldering temperatures also reduce energy use and improve safety on the line.

3. Mechanical strength: the FIW coating is tough with good adhesion, so it handles friction and tension during winding well and suits automated production.

4. Cost: TIW costs more; in volume production FIW can come down to a medium cost level.

When to choose FIW, and when to choose TIW

Fully insulated wire (FIW) is usually the better choice when:

• The product is being miniaturized and transformer space is tight
• The line uses automated winding equipment and needs a tough, abrasion-resistant wire
• You want lower soldering temperatures and less heat damage to the pins
• It is a new design, so safety certification can be planned around FIW from the start
• The application is automotive electronics or high-frequency transformers

Triple insulated wire (TIW) is usually the better choice when:

• The product has specific safety requirements, such as medical equipment. Rubadue’s triple insulated wire TCA3, for example, meets IEC 60601-1, IEC 61558 Annex K, IEC 60950-1 Annex U and UL & CSA requirements
• You need a 155 ℃ thermal class and a highly durable outer layer. The Type F version of Yoshikura’s triple insulated wire KTW-B/F has a Teflon outer layer
• An existing product is already certified with TIW and you do not plan to re-certify soon

Whichever you choose, changing the wire usually affects safety certification. Confirm the certifications your end product needs at the design stage, then test samples on your own winding process.

Insulated wires from Altra Tech

• Fully insulated wire KFIW(3~9)-B (Yoshikura, Japan): diameter 0.08–1.2 mm; the first EIS certification internationally, with VDE and component certification; UL Type B (130 ℃) and Type F (155 ℃).
• Triple insulated wire KTW-B/F (Yoshikura, Japan): diameter 0.15–1.00 mm; Type B (130 ℃) and Type F (155 ℃); solderable without stripping.
• Triple insulated wire TCA3 (Rubadue, USA): 18–40 AWG (0.079–1.024 mm); UL Type F (155 ℃); breakdown voltage 10 kV; for medical and appliance applications.

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FAQ

Q: Can fully insulated wire directly replace triple insulated wire?
Technically it often can, and it lets you make the transformer smaller. Because it affects safety certification and the winding process, test samples first and check whether your end product needs to be re-certified.

Q: Does fully insulated wire need stripping before soldering?
No. Fully insulated wire can be soldered directly, and KFIW(3~9)-B can be tinned directly at a low temperature.

Q: What diameters are available?
KFIW(3~9)-B covers 0.08–1.2 mm, one of the widest ranges in the industry.

Q: What is fully insulated wire used for?
Mainly high-frequency transformers, inductors and automotive electronics, especially products that need miniaturization or automated production.

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