Plated Copper Ductility and Elongation Testing Methods

A plated through hole has to survive the expansion and contraction of the board around it, and that means the copper in the barrel has to stretch without breaking. Ductility is the property that allows it, and it is not the same as thickness or purity: a barrel can be more than thick enough and still crack on the first thermal cycle because the deposit has no elongation left. This article explains where ductility comes from, how it is measured, and how the test results relate to what happens in service.

Why Ductility Matters in a Barrel

The laminate expands far more than the copper when the board is heated, so the barrel is stretched along its axis with every thermal cycle. A ductile deposit accommodates that movement by deforming, while a brittle one concentrates the strain at a defect and cracks.

The failure is cumulative. A barrel that survives one cycle may fail after a hundred, which is why the test used to qualify the process is usually a cycling test rather than a single excursion. Ductility sets how many cycles the joint can take before the crack starts, so the mechanical property of the deposit becomes part of the reliability budget rather than a laboratory curiosity.

Where Ductility Comes From

Electroplated copper ductility is controlled by the deposit structure, which in turn is controlled by the bath chemistry and the plating conditions. A fine, equiaxed grain structure with a low level of impurities is ductile, while a columnar structure with included organic material is brittle. The structure is set at the moment of deposition, so no later process step can restore ductility that was never there.

The impurity level comes largely from the organic additives that are used to control the deposit thickness distribution. Those additives are necessary, because without them the plating would not reach into the barrel evenly, but their breakdown products are incorporated into the deposit and reduce the elongation. Our hole copper notes describe how the deposit is specified and measured.

<img src="https://www.gopcba.com/wp-content/uploads/2024/09/pcb-design1.jpg" alt="Tensile test specimen of plated copper ready for testing” />

Organic Additives and Their Effect

The additives work by adsorbing on the surface and slowing the deposition locally, which is what produces a uniform thickness in a hole. The same adsorption means that some of the organic material is trapped in the growing deposit, and the amount trapped depends on the concentration, the temperature and the current density.

An over dosed bath produces a bright, smooth deposit with a low elongation, which is the classic trade in plated copper: cosmetics improve while ductility falls. The bath analysis and the additive dosing schedule are therefore directly linked to the mechanical properties of the barrel. Carbon treatment, which removes the accumulated breakdown products, is one of the few ways to recover ductility without replacing the bath.

Elongation and Tensile Testing

Elongation is measured by pulling a specimen to failure and recording how far it stretched, expressed as a percentage of the original length. The specimen is usually a thin strip of plated copper removed from a coupon, and the result is compared with a specification that states both the tensile strength and the elongation.

The test is destructive and sensitive to the specimen preparation, so the method has to be fixed. The specimen dimensions, the grip arrangement, the pulling speed and the way the elongation is measured all change the result, and a comparison between two laboratories is only valid if the method matches. The specification should therefore name the method as well as the acceptance figure.

Sample Preparation for a Tensile Test

The specimen is usually prepared by plating a coupon, then stripping the copper from the substrate and cutting a strip to a defined width. Damage during stripping, a notch from the cutting tool or a bend in handling will all reduce the measured elongation by concentrating the strain.

The plating conditions for the coupon should match the production conditions as closely as possible, including the current density and the bath age. A coupon plated in a fresh laboratory bath does not represent a production barrel, and the difference is often larger than the tolerance on the specification itself. Our plating thickness guide covers the coupon measurements taken alongside the mechanical test.

Interpreting the Stress Strain Curve

The curve shows the elastic region, the yield point and the plastic region up to failure. The elongation figure is taken at the point of fracture, while the shape of the curve before that point indicates how the material work hardens and whether it necks down.

A curve that reaches a high stress with very little plastic strain describes a brittle deposit, even if the ultimate strength looks impressive. A curve with a long plastic region describes a ductile deposit that will absorb the cyclic strain of service. The shape of the curve is therefore more informative than the failure stress alone.

Thermal Shock as an Indirect Test

A thermal shock test subjects a plated hole to rapid temperature changes and then examines it by microsection for cracks. It is an indirect measure of ductility, because a ductile deposit survives the cycling and a brittle one cracks, and it has the advantage of testing a real hole rather than a test strip.

The test conditions, the number of cycles and the acceptance criteria should be written down, because the result depends on all of them, and a change to any one of them changes the number of cycles that counts as a pass. Our aspect ratio guide explains how the hole geometry changes the strain that the thermal shock applies.

Correlating Test Results With Field Behaviour

The tensile elongation figure and the thermal shock result measure different things, and neither is a complete prediction of service life. The useful approach is to establish the correlation between them on the actual process, and then to use whichever test is more practical for routine control.

That correlation also has to be re-established when the bath chemistry changes, because the relationship between elongation and thermal shock survival depends on the deposit structure rather than on the number alone. Our plating adhesion notes describe the interface that both tests ultimately examine.

Process Control and Bath Maintenance

The controls that protect ductility are the ones that protect the bath: regular analysis, controlled dosing, carbon treatment to remove organic breakdown products, and a defined bath life. A bath that is kept bright by over dosing will pass a visual inspection and fail a thermal shock test.

At gopcb the mechanical result is tracked against the bath history and the acceptance of a plated barrel is judged in our quality documentation, so that a drift in ductility can be traced to a change in the process rather than treated as an isolated event. The bath log is the record that makes that tracing possible.

FAQ

What elongation should plated copper have? The requirement comes from the specification being used, and it is usually stated as a minimum percentage along with a minimum tensile strength. The figure should be set from the thermal cycling the product will see rather than from a generic number.

Can a ductile deposit look different from a brittle one? It often looks duller, because the brightening additives that improve the appearance also reduce the elongation. A bright deposit is not automatically a better one.

Is thermal shock enough on its own? It is a good indicator because it tests the real hole geometry, but it does not give a number that can be trended. Using both tests and correlating them gives the most useful picture.