Copper vs Aluminium Windings in Custom Transformers: When Cheaper Metal Costs More

The Question Behind the Cost Request

When a buyer asks a magnetics supplier to “look at aluminium to take some cost out”, the honest answer is that aluminium is not a cheaper version of copper — it is a different design with different failure modes. It works well in some applications and is genuinely poor in others, and the difference is decided by the thermal budget and the current density, not by the metal price on the day.

The Physical Comparison

Copper vs aluminium winding material
Property	Copper	Aluminium	Consequence
Resistivity	1.72 × 10^-8 Ωm	2.82 × 10^-8 Ωm	About 64 % higher loss for the same cross-section
Density	8.96 g/cm³	2.70 g/cm³	About 70 % less weight for the same volume
Cost per kg (typical)	Index 1.0	0.45–0.6	Material cost falls, but more volume is needed
Thermal conductivity	400 W/mK	237 W/mK	Hot spots are less well spread along the conductor
Solderability and termination	Direct	Needs flux, special solder or welded terminations	Additional process step and a real reliability risk
Oxide behaviour	Stable	Tenacious oxide forms immediately	Termination quality depends on process control

The naive calculation is that aluminium costs half as much per kilogram, so a 50 % saving follows. In practice the winding must be redesigned: roughly 1.6 times the cross-section for the same loss, which means a bigger window, a bigger core, or a higher temperature rise. Once the core grows to fit, 20–40 % of the expected saving disappears.

Where Aluminium Is a Reasonable Choice

Weight-critical applications. If the product is air-freighted or mounted on a moving platform, an aluminium winding can cut transformer mass by 40–50 %, and the freight or payload saving may exceed the material delta.
Large, low-frequency designs. At 50/60 Hz the window is usually generous and the current density is low, so the extra cross-section fits without growing the core.
Foiled windings on wide windows. Aluminium foil in a wide, shallow window keeps the loss penalty small and the termination can be welded rather than soldered.
Non-critical, cost-driven consumer products where the thermal margin is large and the lifetime expectation modest.

Where It Should Not Be Used

High frequency. Above roughly 20–50 kHz the AC resistance factor rises faster for aluminium because the skin depth is larger relative to the conductor, and the proximity effect in multilayer windings is worse.
Dense, sealed enclosures. With no thermal headroom, a 64 % higher conductor loss cannot be absorbed by a bigger rise.
Vibration environments. Aluminium terminations are more sensitive to process variation; a cold or oxidised joint becomes an open circuit under thermal cycling.
Applications with a hard efficiency target. The loss shows up in the efficiency figure the customer measures, and no amount of cost saving compensates.

How to Evaluate an Aluminium Proposal Fairly

Ask for the loss budget at rated load, copper and aluminium versions side by side, not just the price.
Ask whether the core changed. If the core grew, the comparison must include the new core cost and the new footprint.
Ask for the termination method and its validation: pull test, thermal cycling, and a cross-section of the joint.
Ask for the temperature rise at the worst-case ambient, not at 25 °C.
Ask what the material declaration says — RoHS and REACH declarations must name the alloy, not just “aluminium”.