Thick Copper Plating: Current Density and Plating Uniformity

Thick copper plating builds conductor layers of 70 micrometres and above, either to carry large currents or to spread heat away from power devices. The deposition is the same electrochemical process used for ordinary boards, but every tolerance becomes harder to hold: thickness varies more across the panel, the hole walls receive a smaller fraction of the surface copper, and the profile left for etching is rougher. Controlling thick copper plating means controlling the current distribution rather than the chemistry alone.

What Thick Copper Plating Is Used For

The obvious application is current carrying. A power converter may need 2 oz or 3 oz copper on the outer layers to keep resistive losses and temperature rise acceptable, and a motor drive may need more. The second application is thermal: thick copper under a power device spreads heat sideways and lowers the junction temperature for the same board area.

A third application is mechanical. Heavy copper is used for bus bars on the board, for high-current connectors and for boards that have to survive repeated thermal cycling, where a thicker conductor reduces the strain in the joints at either end.

Current Density and Plating Time

Thickness is the product of current density and time, so a 70 micrometre deposit takes roughly twice as long as a 35 micrometre one at the same current density. Plating for longer at a lower current density gives a more uniform and finer-grained deposit, and it is the method of choice for thick copper where distribution matters.

Raising the current density to save time is possible up to a point. Above the recommended value the deposit becomes coarse and nodular, the surface is rougher before etching, and the throwing power into the holes falls. On thick copper work the current density is usually set below the value used for standard panels precisely because the deposit is deep.

Plating Uniformity and the Edge Effect

The edge effect is the tendency of current to concentrate at the perimeter of the panel, and it becomes more damaging as the plating time grows, because the excess accumulates for longer. A panel that plates evenly enough in 40 minutes can show a large difference between centre and edge after 90 minutes.

Uniformity is expressed as the difference between the thickest and thinnest measurement points across the panel. Reducing it means changing the field: shields at the panel edge, larger anode-to-cathode distance, or thieving patterns that consume current in the open areas. The relevant methods are the same as those used for plating uniformity on standard boards, applied with a tighter target.

Surface Profile Before Etching

A thick deposit is rougher than a thin one, and roughness matters because the copper has to be etched afterwards. A coarse, nodular surface etches unevenly, and the nodules can survive as copper residues between adjacent traces. Where the design has fine spacing on the same layer as heavy copper, this is the conflict that decides whether the board can be made at all.

The profile is controlled by the additive system in the bath and by the agitation. Organic additives refine the grain and level the deposit, and they are consumed faster during long plating cycles, so their concentration has to be analysed rather than scheduled. Agitation removes the depleted layer at the surface and keeps the local concentration higher.

Bath Chemistry and Additive Control

Copper sulphate, sulphuric acid, chloride and the organic additives form a system, and each has a working range. Copper concentration sets the conductivity and the deposition rate; acid improves the conductivity of the solution; chloride participates in the additive mechanism at a low concentration, typically tens of parts per million, and both too little and too much changes the deposit appearance.

Analysis at intervals through a long plating run is the practical control. A bath that is correct at the start can be out of range by the end of a heavy cycle, and the boards plated at the end of the run are the ones that show it. The plating thickness record should carry the analysis values alongside the measurements.

Hole Wall Versus Surface Thickness

The hole wall receives less copper than the surface, and the ratio falls as the hole gets deeper or the copper gets thicker. On heavy copper boards the wall thickness is often specified separately, because the surface requirement is set by current carrying and the wall requirement by barrel reliability.

Throwing power is the property that governs the ratio, and it is improved by lower current density, better agitation and the correct additive balance. Where the ratio is unacceptable, the answer is usually to plate the holes first in a separate, lower-current step rather than to increase the total plating time. The throwing power of the bath should be re-established whenever the chemistry is changed.

Handling and Rework Risk

A heavy copper layer is stiff and the panel is heavier, so handling damage becomes more likely. Scratches and dents that would be invisible on a thin layer leave a mark that survives etching, and a dented surface etches into a visible defect. Racks that hold the panel at a few points are a common source of this kind of damage.

Rework is difficult. A plating defect buried under 100 micrometres of copper cannot be repaired by touching up the surface, and a panel rejected for thickness cannot be re-plated without changing the profile further. This is why heavy copper boards are usually run with a coupon that is measured before the lot is released. Microvoids at the interface between the base copper and the plated layer are a particular concern, and the copper microvoid checks that apply to standard plating apply here as well.

Measuring Thickness and Quality

Surface thickness is measured by X-ray fluorescence or by the coulometric method on a coupon, and hole wall thickness by microsection. For thick copper the microsection also shows the grain structure, the interface with the base foil and any voids, so it is the more informative of the two and should be part of the first article.

Measurements should be taken at several positions across the panel, and the spread reported rather than a single mean. The mean alone cannot show an edge effect, and an edge effect is the failure that is most likely to spoil a heavy copper lot.

Records and Process Limits

The record for a heavy copper lot should carry the current density, the plating time, the bath analysis at start and finish, the thickness measurements at each position and the coupon section. Together these describe how far the process is from its limits, which is what matters when the next lot is heavier still.

A process at the edge of its capability should be recognised as such. Where the required thickness is close to the maximum the line can hold uniformly, the design should be reviewed for an alternative such as a copper inlay or a thicker starting foil, because a process that has no margin will eventually produce a rejected lot.

FAQ

How long does thick copper plating take? Roughly twice as long for 70 micrometres as for 35 at the same current density, and longer still if the current density is reduced to improve uniformity. Time is the price paid for even distribution.

Can heavy copper be plated at a higher current density? Only up to the point where the deposit stays fine-grained and the throwing power remains acceptable. Beyond that the surface becomes nodular and the hole walls thin out.

Why is the edge of a heavy copper panel thicker? Because current concentrates at the perimeter and the excess accumulates for the whole plating time. Shields and a larger anode distance are the usual corrections.