Copper-Nickel Clad Strip: The Bond Line Is the Product

Three products reach the battery and connector market under names that sound interchangeable: pure nickel strip, nickel plated copper strip, and copper nickel clad strip. Only one of them is a bond line product. The other two are a single metal, or a thin deposit on a single metal, and everything that follows depends on which of the three a drawing actually asks for.

The distinction matters because the failure modes do not overlap. Pure nickel strip fails by general wastage of a section that is nickel all the way through. Plated strip fails at the pores in a layer a few micrometres thick. Clad strip fails at the interface between two metals that were pressed together, and that interface is invisible from both surfaces.

This article treats the clad product as the subject and the other two as the baseline. It starts with where the nickel sits, works through the conductivity arithmetic, then spends most of its length on the bond line: how it is made, which defects survive inspection, and what thermal cycling and bending do to it. The last two sections are the four questions that settle a material choice and the five lines that belong on a purchase order.

The first column of any comparison should be the nickel distribution, not the thickness. A 0.2 mm strip can be nickel through its full section, nickel on 2 micrometres of its surface, or nickel across 40 micrometres of one face, and those are three different materials.

The second and third columns are where most specifications go wrong. A plated strip carries one or two micrometres of nickel, which will crack at a tight bend and will be gone after one pass with an abrasive. A clad strip carries tens to hundreds of micrometres, which can be ground, reworked, and welded at the same spot more than once. On a sample both look like nickel on the surface. In service they are not the same material.

Pure nickel strip has its own place, and it is narrower than the marketing suggests. Its conductivity is roughly a quarter that of copper, so a high current joint needs a larger section, and both weight and cost follow. What it offers is uniformity: when corrosion starts at the surface, grinding a layer away still leaves nickel, and a weld cannot punch through a coating into a different substrate. That suits small currents, aggressive environments, and joints that get reworked.

Current in a clad strip flows in parallel through the two layers. The equivalent conductivity is the thickness weighted sum of the two conductivities, which for a nickel fraction f of the total thickness is one minus f times the copper value plus f times the nickel value.

Taking copper as 100 % IACS and nickel as 25 % IACS, the table below follows directly. Its practical use is that a nickel fraction converts immediately into a conductivity penalty and a cost penalty, and both move in the same direction.

Two readings matter. On conductivity, going from 10 % to 30 % nickel costs fifteen points, from 93 % to 78 %. On cost, the nickel content triples while nickel carries a higher price per kilogram than copper. Both readings worsen together, so the thickness ratio is a balance point between weld window and conductivity; treating it as a safety margin makes both conductivity and cost worse.

Battery connectors cluster near 20 % nickel, and the weld window governs that choice, with the conductivity peak a distant second. Too thin a nickel layer and resistance spot welding burns through into the copper, leaving a joint whose strength varies from part to part. Enough nickel and the welding parameters have headroom, while 85 % IACS is already sufficient for a short high current link.

One piece of arithmetic is often missed. At 20 % nickel the clad strip has about 29 % of the resistance of pure nickel, which sounds like a large gain, but it still carries roughly 18 % more resistance than pure copper. The product solves the conflict between welding to nickel and conducting like copper. It does not solve a shortage of copper, and very low resistance still comes from a larger copper section.

The whole technical content of a clad strip sits at the interface. A good bond bends as one material. A poor bond looks identical on the sample and blisters after a few thermal cycles in the equipment.

Roll bonding is the common route. Two coils pass through a mill at a large reduction, the surface oxide breaks up and is extruded, fresh metal meets fresh metal under pressure, and a primary bond forms. An annealing step then lets atoms diffuse across the interface until the joint is metallurgical. The route suits thin strip, allows a wide range of thickness ratios, and is the source of most clad material used in battery connectors.

Explosive bonding uses a shock wave and produces a wavy interface. It suits thick plate and combinations that resist rolling, and it is essentially absent from thin strip. The third route is thick electroplating or electroless plating, which is a deposit and not a cladding. Its bond relies on mechanical keying, its density falls as thickness rises, and through porosity is a real risk. Calling a thick deposit a clad strip leads to acceptance criteria that measure the wrong thing.

A three layer nickel copper nickel stack deserves a note. The symmetrical arrangement balances residual stress, reduces curl, and offers a weldable face on both sides. The price is a second bond line, and a weak bond on either one shows up as an edge crack during slitting.

Clad strip defects share a pattern: invisible at the macro scale, decisive at the micro scale. Visual inspection and a micrometer will not find them, which is why bond coverage has to be measured by ultrasonic scan and confirmed by bend and peel tests.

Of the first four, discrete unbonded spots are both the most common and the easiest to pass. They are invisible on a sample because the surrounding metal holds the faces flat. Only after dozens of thermal cycles does the stress at the edge of an unbonded region tear it open, and by then the strip is in a finished assembly.

Bond coverage therefore belongs in the purchase contract as a number. A common requirement is 98 % or better, and the figure has to travel with the scan resolution and the rejection threshold. Without those two, two inspection labs can disagree about the same coil and both be technically correct.

Copper and nickel expand at different rates, roughly 16.5 to 17 ppm/K for copper and 13.4 ppm/K for nickel, a difference of 3 to 3.6 ppm/K. A strip taken from room temperature to 150 °C, a swing of 130 K, develops a free expansion difference of about 0.045 % across the interface. On a strip 20 mm wide, the two faces would move about 9 micrometres relative to each other.

That number looks small, and once would be. It is applied repeatedly, at the edge of every unbonded region, as a shear each time the joint heats and cools. After enough cycles the small spots link up. This is why a clad strip qualification includes thermal cycling as well as a single lap shear test.

Bending is the second source. Under a bend the outer face is in tension and the inner face in compression, and because the two layers have different stiffness the interface does not sit on the geometric neutral axis. Strain at the bond line is higher than at either surface, and it rises as the bend radius falls. Specifying a bend radius of three to five times the thickness is the cheapest single measure available.

One metallurgical detail belongs here. Copper and nickel are mutually soluble in the solid state, so at temperature the interface gradually disappears as the two compositions blend. That helps bond strength and complicates any inspection that relies on seeing a sharp line under etching. Because the two elements diffuse at different rates, a band of voids can also form on the faster diffusing side, which matters for thick sections held at temperature for long periods.

The weakness of nickel plated strip is not the thinness of the nickel. It is that the nickel is not continuous. Every electroplated layer has pores, their number set by the bath, the current density, the substrate roughness, and above all the pre treatment.

The electrochemistry explains why a pore is worse than its size suggests. Nickel is the less noble metal here, with a standard potential near minus 0.25 V against about plus 0.34 V for copper. Once a pore reaches the copper, the nickel around it becomes a small anode and the copper a large cathode, so the corrosion current concentrates in the pore and enlarges it. Nickel plated steel is worse again, because iron sits below nickel in the series.

Plated strip life therefore tracks porosity, with thickness as a secondary factor. A heavier deposit reduces the number of through pores, but a poorly controlled pre treatment will still leave pathways in a thick layer. An order that specifies three micrometres of nickel without adhesion, porosity, or salt spray acceptance has not specified life.

Clad strip does not have this problem. Its nickel is a rolled, continuous layer with no pores to speak of, and grinding half of it away still leaves nickel. That is the concrete reason a clad product holds up better in humid, salt laden, or electrolyte exposed joints, and the reason it justifies a higher material price.

Four questions, asked in order, converge quickly. They map to welding, current, environment, and maintenance.

A single yes rules out pure nickel strip, because a quarter of copper conductivity forces a larger section. If all four answers are no, meaning a small current, a dry environment, and a joint assembled once, pure nickel strip is the simplest choice: uniform through the section, with no coating to breach and no bond line to inspect.

One misreading is common. The nickel layer of a clad strip is a design variable at 10 % to 30 % of thickness, and losing part of it does not remove the function. The nickel layer of a plated strip is a consumable. The two cannot share an acceptance clause.

Price differences between suppliers usually come from specification wording; process capability rarely explains the gap. Five lines make quotations comparable.

The first line is the one most often dropped. A buyer writes 0.2 mm copper nickel clad, while the split between copper and nickel inside that 0.2 mm decides whether the conductivity is 93 % or 78 %. Two suppliers quoting their own default ratio can differ by twenty percent with no difference in skill.

The fifth line decides the experience on the line. Strip with an out of tolerance burr snags in the feeder and scores the guide rollers, and one stoppage costs more time than the material price difference. Burr height belongs on the drawing as a number, not as a note asking for no visible burr.

DLX operates in Changzhou, Jiangsu, supplying pure nickel strip in grades N4, N6, Ni200 and Ni201, nickel plated strip built to the substrate and coating thickness on the drawing, and copper nickel clad strip built to a specified copper to nickel thickness ratio in single or double sided construction. All three ship as coil, with chemistry, ultrasonic scan maps and peel test records available.

Four items make a quotation accurate: total thickness with the copper to nickel ratio, the acceptance values for coverage and peel strength, the temper required, and coil weight with bore. If the part runs on an automatic line, describe the feeding method and the bend radius as well, since the edge and flatness requirements follow from them.

A well bonded strip does not delaminate under normal bending and thermal cycling. Separation starts at two places: unbonded spots that an ultrasonic scan would have shown, and edge burrs or micro cracks left by slitting. Coverage and edge quality are the two acceptance items that keep it from happening.

At low current, in a dry environment, assembled once, it can, and it costs less. In humid, salt laden, or electrolyte exposed service, the pores become corrosion sites, the nickel dissolves as a small anode, and the pores grow. A clad strip has continuous nickel and no pore mechanism, at a higher material cost.

It lasts longer and costs more conductivity and money. Moving from 10 % to 30 % nickel takes conductivity from 93 % to 78 % and triples the nickel content. The ratio belongs to the weld window and the current density. There is no safety margin rule to fall back on.

Thin strip is measured by four point probe on sheet resistance, or by eddy current against a reference sample, and either can be cross checked against the parallel calculation. The two methods diverge on thin material, so the contract should name the method, the sampling position, and the temperature.

The nickel face. Spot and laser welding have a wider window on nickel, because copper draws heat away faster than the nugget can form. Where a design forces a weld on the copper face, surface preparation or ultrasonic welding is usually needed, with parameters that have nothing in common with the nickel face.

Five: thickness build with tolerances, bond coverage and peel strength, conductivity with its method, temper and mechanical values, and edge plus coil parameters. The first two decide whether the material works. The last three decide whether it runs.

The three forms separate on one line: pure nickel strip is nickel all the way through, plated strip is a thin nickel shell, and clad strip is half a section of nickel bonded to half a section of copper. The first two win on thickness. The third wins or loses at the interface.

Clad conductivity is arithmetic. At 20 % nickel the equivalent is about 85 % IACS, roughly 29 % of the resistance of pure nickel and about 18 % above pure copper. It resolves the conflict between welding to nickel and conducting like copper. It does not make copper unnecessary.

The choice comes down to four questions: does the weld land on nickel, is the current high, is the environment wet, and will the joint be reworked. Four noes point to pure nickel strip. A single yes points to the clad product, and then the bond coverage number matters more than the grade.

Physical property data for copper and nickel, used for the parallel conductivity calculation and the thermal expansion estimate.

The copper nickel binary phase diagram, used to explain mutual solid solubility and the fading of the interface at temperature.

Literature on the bond mechanism in roll bonded dissimilar metals, used for the fresh surface and diffusion anneal description.

Application notes on ultrasonic C scan inspection of clad plate, used for the coverage acceptance method.

Process data for resistance spot welding and laser welding of battery connectors, used for the relation between nickel thickness and weld window.

Studies of porosity and corrosion in electroplated nickel, used to locate the failure site in plated strip.

ASTM B162, Standard Specification for Nickel Plate, Sheet and Strip, used for grade and dimensional wording on the pure nickel product.

Conductivity figures in this article are parallel estimates from the thickness ratio, taking copper as 100 % IACS and nickel as 25 % IACS, and they show orders of magnitude and trends; no specific lot is described. Coverage, peel strength and thickness tolerances vary with supplier process and equipment, and the certificate of compliance and the contract govern. The expansion values and the displacement estimate are calculated from published data; actual stress depends on restraint and assembly preload. Standards are revised, and the version cited in the contract applies. This article supports material selection and enquiry, and it does not replace engineering design or incoming inspection.