Copper Plating Thickness Distribution Across a Panel

Electroplating does not deposit copper evenly, and no practical plating line removes the variation entirely. The copper is thicker at the edges of a panel and around the rack contacts, thinner in the middle and thinner still at the centre of a deep hole. The range across a production panel is a process characteristic that has to be measured and specified rather than assumed.

Why Thickness Varies Across a Panel

The variation has two sources. The first is the distribution of current over the surface, which follows the geometry of the tank, the anodes and the panel; the second is the ability of the solution to plate into a recess, which depends on the chemistry and on how well the solution is moved.

The result is a thickness map with a characteristic shape: heavy at the edges and at the rack contacts, lighter in the centre, and lighter again down the wall of a hole. The range affects the etching process and the impedance of any controlled impedance trace, and it sets how much margin the design really has.

Current Density and the Primary Distribution

Current leaves the anode and prefers the nearest path to the cathode, which means the edges of a panel facing an anode receive more current than the centre. This primary distribution is a geometric effect, and it exists even in a perfectly mixed solution.

Because deposit thickness follows the current, the edges of a panel plate faster. The effect is reduced by moving the anodes further away, by using shields that block the field at the edges, or by splitting the anodes into zones with separate rectifiers.

Throwing Power and Hole Coverage

Throwing power describes how well a solution deposits into a recess, and it depends on the chemistry and on the conductivity of the solution relative to the polarisation of the electrode. A solution with good throwing power plates the walls of a hole more evenly, while a poor one plates the surface well and leaves the centre thin.

The practical limit is the thickness at the centre of the hole rather than the average. A specification that gives a minimum hole copper figure is really a statement about throwing power, aspect ratio and current density together, and it cannot be met by changing one of those three alone. The minimum that applies to a plated barrel is set out in the guide to hole copper.

Aspect Ratio and the Centre of the Hole

Aspect ratio is the depth of the hole divided by its diameter, and it compounds the throwing power problem. A hole with a ratio of one is easy to plate, while a hole with a ratio of ten is not, because the solution inside it is replenished only by diffusion and by whatever agitation reaches the opening.

High aspect ratio panels are therefore plated at a lower current density for a longer time, with stronger agitation and with chemistry chosen for throwing power. The trade is throughput, and it is the reason a deep hole costs more to plate than a shallow one.

Anodes, Shields and Racking

The anode arrangement sets the shape of the distribution. Anodes that are too small relative to the cathode concentrate the current, anodes that are consumed unevenly create a pattern of their own, and an anode that has become passive stops contributing and forces the rest to work harder.

Shielding is the standard correction. A frame of non conductive material around the panel edges blocks part of the field and reduces the excess at the edges, and the shield pattern is normally established for a panel size and then reused across the products that share it.

Agitation and Solution Movement

Agitation brings fresh solution to the surface and into the holes, and it removes the depleted layer that would otherwise form. Air sparging, panel movement, eductor flow and vibration are all used, and the combination has to be strong enough to reach the centre of the deepest hole without causing turbulence that damages a thin pattern.

The uniformity of agitation across the tank matters as much as its strength. A rack position that receives less flow than another will plate thinner, and rotating work through the positions is a simple way of averaging that difference out across a batch.

Measuring Thickness on Panels and Coupons

Surface copper thickness is measured by eddy current gauges, by X-ray fluorescence or on a microsection, and each method has a different relationship to the true average. Hole copper is measured on a section, often at several positions around the barrel and at more than one hole on the panel.

The measurement plan should state where the readings are taken, because a single figure for a whole panel hides the variation the process is trying to control. A coupon plated with the production panel and measured at defined points gives a comparable record from batch to batch.

Adjusting the Process for Uniformity

The levers available are current density, plating time, anode geometry, shield design, agitation and chemistry. Lower current density improves the ratio of hole to surface thickness but takes longer, which is the most common trade made on a difficult board.

Changes should be made one at a time and measured on a coupon, because several of these levers interact. Increasing agitation without touching the current, for example, may improve the holes and slightly reduce the surface thickness, which reads as an improvement on one measurement and a deterioration on the other.

Specification, Records and Verification

A plating specification should give a minimum surface thickness, a minimum hole thickness, the measurement method, the coupon design and the positions where readings are taken. It should also state the permitted variation across a panel, since a value that holds at the edge may not hold in the middle.

Records allow the process to be reviewed when a downstream problem appears. A batch that failed an impedance measurement can be compared with its plating record to see whether the copper was thinner than usual or the laminate spacing had changed. The thickness requirements themselves are described in the guide to plating thickness.

Process Control and Verification

On a design of this kind, current density is the item that decides how the rest of the board is arranged. Keeping a sample from the panel turns a dispute into a measurement, because the same coupon can be re-examined by both parties without rebuilding the batch. Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end.

FAQ

Why is copper thicker at the panel edges? Because current takes the shortest path from the anode, so the edges receive more current per unit area than the centre. The effect is reduced with shields, a greater anode distance or zoned anodes, but it is never removed entirely.

What aspect ratio can be plated reliably? It depends on the chemistry and the equipment, and ratios above about eight to one are normally treated as a special case requiring longer plating and stronger agitation. The limit is set by the hole copper measurement, not by the drawing.

How many thickness readings should be taken? Enough to see the pattern rather than a single number, with readings at the edges, the centre and at least two of the smallest holes. That set is what shows whether a change improved uniformity or merely shifted the average.