Panel Plating Thickness Distribution: Where the Copper Lands

Panel plating is the step in which copper is deposited over the whole surface and through every hole of the panel before the outer layer pattern is defined. It builds the hole wall and the surface copper in one operation, and it decides whether the minimum thickness in the barrel and the target thickness on the surface can both be met.

The difficulty is that a plating tank does not deposit uniformly. Current crowds to the edges and the corners, the ends of the panel face the anodes more directly than the middle does, and the result is a thickness distribution with a measurable spread rather than a single number.

What Panel Plating Does to a Panel

In panel plating the whole surface is conductive when the panel enters the tank, so copper is deposited on the foil as well as through the holes. The hole wall requirement comes first, which for a Class 3 product is an average of 25 µm with a minimum of 20 µm, and the surface thickness then follows from the same current.

Because the surface receives copper everywhere, the etching step afterwards has to remove more metal than it would after pattern plating. That changes the etch profile and the achievable line width, which is why panel plating is normally paired with a coarser line width or a thicker outer foil.

Where the Thickness Variation Comes From

Three effects set the spread. The primary current distribution is geometric and depends on the distance from each point on the panel to the anode, the secondary distribution depends on the local current density and the electrolyte, and the tertiary distribution depends on the flow of fresh electrolyte past the surface.

On a large panel the edge can plate to twice the thickness measured at the centre when the anodes are distant and the panels are spaced widely. The same effect appears vertically, with the bottom of the panel sitting in a different solution flow from the top.

Current Density and Deposit Rate

Acid copper deposits roughly 0.2 µm per minute at 1 A/dm², so a 25 µm requirement needs about 125 minutes at that rate, and most shops run closer to 2 to 3 A/dm² to keep the cycle commercial. Raising the current density shortens the time but worsens the spatial spread, because the geometric effect grows with current.

The area used to calculate current is the total plated area, including the panel border that no product uses. When that border is thick, the calculated current density is optimistic and the real density on the product is higher, which is one reason a shop that measures thickness rather than trusting the set point ends up with a more stable process.

Anode Spacing, Shielding and Thieving

Anode distance is the most powerful single adjustment. Increasing the anode-to-cathode spacing improves geometric uniformity, because the relative difference in distance between the centre and the edge of the panel becomes smaller, and the price is a longer cycle at the same current.

Shields and thieves are the second lever. An insoluble shield placed between the anode and the panel edge reduces the current that reaches the edge, and thieving bars on the panel border give that current somewhere to go other than the product. Both are tooling decisions for one panel size, and both have to be re-checked when the panel layout changes.

Measuring the Distribution

The distribution is measured on coupons placed with the product, not on a separate sample plated at another time. Coupons at the four corners and at the centre, sectioned and measured under a microscope or with an X-ray fluorescence gauge, give five points that show the shape of the distribution.

Edges within about 25 mm of the panel border are normally excluded from the product specification but should still be recorded, because the edge reading tells the operator which way to move the anode spacing. A single coupon at the centre produces a number that cannot distinguish a flat tank from a badly graded one.

Throwing Power in High Aspect Ratio Holes

Throwing power is the ratio of the copper thickness at the centre of the hole to the thickness on the surface. A hole with an aspect ratio of 8 to 1 needs a throwing power near 0.5 to reach the minimum wall thickness, and the ratio falls as the hole becomes deeper or narrower.

Organic additives, agitation and bipolar waveforms such as the ones described in the notes on reverse pulse plating are the levers used to raise it, and the effect is measurable on a sectioned coupon. Where a design pushes the aspect ratio up, the plating window narrows and that coupon becomes the evidence that the product is inside it.

Tank Chemistry and Agitation

Copper sulphate, sulphuric acid and chloride are the bulk constituents, and the additives that control grain and levelling are present in parts per million. Additive concentration drifts with the plated area, so it is analysed and dosed on a cycle rather than on a calendar.

Agitation is what delivers fresh electrolyte into the hole and removes the boundary layer that limits the deposit rate, and its effect is tied to the roughness of the hole wall and the adhesion of the plating. Air sparging and panel movement both contribute, and a blocked sparger changes the distribution without changing any reading on the rectifier.

Setting a Tolerance the Shop Can Hold

A realistic specification is written as a minimum on the hole wall and a range on the surface, measured at defined coupon positions, rather than as a single thickness with a tight tolerance, because plating uniformity is a property of the tank and the panel together. A minimum wall thickness plus a surface window describes the process that has to be held.

Where the customer requires a distribution figure, the measurement positions and the edge exclusion must be stated with it. A distribution quoted as plus or minus 10 percent means nothing until the measurement points, the panel size and the current density that produced it are all known.

When the Distribution Cannot Be Fixed

Some boards cannot be plated uniformly. A panel that mixes a dense array in one corner with an open area in the opposite corner presents two different local current densities, and no anode adjustment removes the difference. The practical response is to accept a wider distribution and set the minimum so that the thinnest point still passes.

Design choices made earlier decide how much of the problem reaches the tank. Balanced copper coverage, a uniform border and a layout that does not concentrate product in one region all make the distribution easier to hold, and the guidance on aspect ratio and plating applies at the tank as much as at the drill.

FAQ

Why is the copper thicker at the panel edge? Because the current distribution is geometric. Points near the panel edge are closer to the anode in electrical terms, so they receive more current and deposit more copper unless shields or thieves are used.

How many points should be measured? At least five per panel, at the four corners and the centre, and the coupons have to be plated with the product rather than prepared separately. Edge readings within 25 mm should be recorded but excluded from the specification.

Can a wider distribution be accepted? Yes, if the minimum wall thickness and the surface window are still met at the thinnest point. That is usually cheaper than forcing the tank to hold a distribution the panel layout does not allow.