CUPAL vs. Copper–Aluminium Transition Methods: Which One Lasts?

CUPAL is a copper–aluminium bimetal made by cold roll-bonding (Kaltwalzplattieren) and diffusion welding, in which a Cu-ETP copper layer and an EN AW-1050A aluminium core are joined inseparably and hermetically. In the field, though, several methods compete to connect copper and aluminium: antioxidant paste, copper-clad aluminium (CCA) conductor, the welded or brazed bimetal cable lug, and the riskiest of all — the bare bolted Cu–Al joint. This article works through the engineering decision of how to build a copper–aluminium transition when the joint has to run reliably for decades with no maintenance.

Comparison at a glance

Antioxidant paste (contact grease)

Antioxidant paste (contact grease) has been used on aluminium joints for decades: the non-conductive filler displaces air and moisture from the contact face, slowing the onset of oxidation and galvanic corrosion. On an aluminium-to-aluminium joint, that is a sensible supplementary measure.

On a copper–aluminium transition, however, paste alone is not a solution — only a delay. It does not remove the electrochemical potential difference between copper and aluminium; the grease dries out, runs, or is squeezed out of a joint that heats up and works under thermal expansion, and the moment the electrolyte returns, galvanic corrosion begins. That is why a pasted joint typically demands periodic re-torquing and paste renewal — which turns into a lifetime cost and a maintenance liability. The mechanism of galvanic corrosion is covered in detail in the galvanic corrosion prevention article.

Why CUPAL is the durable choice: CUPAL seals the critical Cu/Al interface inside the composite, in a moisture-protected zone where no oxide layer can form. There is no filler to dry out and no re-torque cycle — the joint stays stable for the life of the equipment.

Copper-clad aluminium (CCA) conductor

In a copper-clad aluminium (CCA) conductor, a thin copper layer covers an aluminium core. The conductor surface behaves like copper, while its weight and cost stay close to aluminium — which is why CCA has spread mainly in weight- and cost-sensitive conductor applications.

As a copper–aluminium transition element, however, CCA is limited. The copper cladding is thin and does not amount to a full bimetal transition: wherever the coating is breached by mechanical damage, cutting, a drilled hole or corrosion, the aluminium core is directly exposed to the electrolyte and the copper surface, and galvanic corrosion begins locally. Cut ends and hole edges expose the aluminium core by definition. CCA can make a good conductor, but it offers no permanent guarantee as a transition connector.

Why CUPAL is the durable choice: CUPAL is not a galvanic coating but a bimetal bonded through the full cross-section by cold roll-bonding and diffusion welding — the copper and aluminium layers are thick and workable, and the interface stays metallurgically sealed even at cut or drilled edges. There is more on the CUPAL sheet product page.

Welded or brazed bimetal cable lug

A bimetal cable lug offers a genuine metallurgical Cu–Al bond: the copper and aluminium halves of the lug are joined by friction welding or brazing, so the installer connects copper to copper and aluminium to aluminium. This works on the same principle as CUPAL and is a good solution for cable terminations — the CUPAL range itself includes a bimetal cable lug.

The difference is not in the principle but in the form and the flexibility. A cable lug has a fixed geometry, with set hole and cross-section dimensions; it is made for a cable end. For flat, surface-to-surface transitions — busbar connections, distribution-board rails, large-area terminations — the lug does not fit. There, a CUPAL washer, a flat CUPAL sheet or a CUPAL strip is the right form, one that can be cut to size from a DXF drawing to match the actual bolt pattern. You get the same permanent bimetal bond without the rigid geometry of a lug, fitted to the real layout of the cabinet. One caution on names: toothed copper-alloy washers are sold as transition washers too, and they are not a bimetal — there the transition is made as the bolt is tightened, not by the material. Bimetallic or toothed transition washer sets the two side by side.

Why CUPAL is the durable choice: the bimetal cable lug and CUPAL both deliver a metallurgical bond — and as a flat element, CUPAL also covers the surface-mount and custom-geometry transitions that fixed lug sizes cannot.

Bare bolted Cu–Al joint (no transition)

Bolting a copper lug straight onto an aluminium busbar with no transition element is the classic bad joint. In the presence of moisture — condensate or airborne humidity — copper and aluminium form a galvanic couple: the less noble aluminium starts to dissolve as the anode, an insulating aluminium-oxide layer forms, contact resistance rises, the joint heats, and it loosens and corrodes further. It is a self-reinforcing process that, on a high-current joint, can escalate to fire-hazard temperatures. This is not an alternative — it is the baseline condition you are trying to prevent.

Why CUPAL is the durable choice: insert a CUPAL transition element and like metal meets like metal on both sides, the galvanic couple disappears, and the joint's contact resistance stays low and stable.

Documented compliance

Several standards set requirements for copper–aluminium joints on contact resistance, temperature rise and long-term stability: IEC 61238 for cable connectors, IEC 61439 for low-voltage switchgear, DIN 43671 for busbars. CUPAL's material properties — a 70/30 aluminium/copper ratio, 41.9 m/(Ohm·mm²) conductivity, 4.6 g/cm³ density — give predictable, documentable figures for these design calculations.

The long-term stability of a pasted or bare bolted joint is installation-dependent and hard to demonstrate; a CUPAL transition, by contrast, is backed by a material certificate. An EN 10204 certificate can be supplied with the shipment — the difference between a 2.2 and a 3.1 certificate is explained in the EN 10204 material certificate article.

Summary

Of the four common ways to build a copper–aluminium transition, paste only delays, the CCA cladding is vulnerable, and the bare bolted joint is the failure itself. Only bimetal solutions deliver a genuine, permanent metallurgical bond — the fixed-geometry cable lug for cable ends, and the cut-to-size CUPAL sheet, washer and strip for surface-mount and custom-geometry transitions. Where a joint has to last for decades with no maintenance, CUPAL is the defensible choice.

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