Panel Plating Versus Pattern Plating In Fabrication

Two plating sequences are used to build the copper on a printed circuit board, and the difference between them is the order in which the plating and the imaging happen. In one, the copper is plated over the whole panel and then etched away where it is not wanted; in the other, the copper is plated only where the pattern will remain. The two sequences produce different line widths, different thickness distributions, and different cost structures, and the fabricator chooses between them for each layer.

This article explains both sequences, how they differ in the features they can produce, where the plating distribution becomes a problem, and what the designer should specify.

Panel Plating

The panel is drilled, seeded with a thin electroless copper, and then plated with copper over its whole area, including the walls of the holes and the flat surfaces. A thin layer of tin or a similar resist is plated on top, the pattern is imaged in a photoresist, and the exposed plating is etched away, after which the tin is stripped to leave the copper pattern behind.

The consequence is a long etch. The etchant has to remove the full thickness of the plated copper in the open areas, which takes time, and the sideways undercut grows with the etch time. The line width that can be held is therefore coarser than in the other sequence, and the tolerance on the width is wider, but the thickness of the resulting copper is uniform across the panel because the plating was uniform to begin with.

Pattern Plating

The panel is drilled and seeded with a thin electroless copper, and the pattern is then imaged in a plating resist. Copper is plated only into the openings, which are the traces and the hole walls, and the resist is stripped afterwards. The thin seed layer that remains between the traces is etched away in a short etch, which is what allows a fine line to be produced, because the sideways undercut is limited by the small amount of copper that has to be removed.

The thickness distribution is the trade. The plating current prefers the areas of lowest resistance, so a large isolated pad, a heavy plane, or a feature at the edge of the panel receives more copper than a narrow trace in the middle. The result is a variation in thickness across the panel and between features, which affects the resistance of a trace and, on a controlled impedance layer, the impedance itself. The fabricator manages the variation with the bath chemistry, the current profile, and the addition of thieving patterns that equalise the current density.

Comparing The Two

Panel plating produces a uniform thickness and a coarser line, and it is well suited to a heavy copper layer where the thickness matters more than the width, or to a design with generous features where the etch tolerance is not the limiting factor. Pattern plating produces a fine line and a variable thickness, and it is the usual choice for a high density layer with small features, where the width tolerance is what decides whether the board works.

The hole wall is affected as well. In panel plating, the barrel is plated with the same current as the surface, so the thickness in the hole is a known fraction of the surface thickness. In pattern plating, the current has to reach into the hole, and the throwing power of the bath determines how much copper arrives at the centre of a deep hole, which is why the aspect ratio and the plating chemistry become critical together. The additives that control that distribution are described under electroplating additives.

The Distribution Problem In Practice

The variation in thickness is a design issue as well as a process one. A layer that carries a large plane on one side and a fine bus on the other will be plated unevenly, and the fine bus may end up thinner than the current calculation assumes. The copper balance of the layer, which is the same property that controls the etch uniformity, therefore also controls the plating distribution, and the two effects work in the same direction.

The consequence for the designer is the tolerance on the finished copper thickness. A trace that is specified with a thickness and a width has a resistance that depends on both, and the current capacity that the trace provides depends on the same two figures. The calculation that relates them is described under trace width and current calculation, and the thickness figure that goes into it should be the worst case minimum rather than the nominal value that the drawing quotes.

How The Choice Is Made

The choice is the fabricator answer to a set of requirements rather than a decision the designer makes directly. The minimum line width and spacing, the copper weight, the layer count, the hole size and its aspect ratio, and the quantity all point in one direction or the other, and a fabricator with both processes will use pattern plating for the fine layers and panel plating for a heavy copper layer that has to carry a large current.

What the designer can do is specify the requirement rather than the method. The drawing should state the finished copper thickness with its tolerance, the minimum line width, and the impedance where it applies, and the fabricator then selects the sequence that meets those figures. Specifying a plating method directly, without the requirement behind it, invites a process that meets the method and misses the requirement. The fabrication sequence in full is described under PCB design and fabrication.

Process Control and Verification

On a design of this kind, copper thickness 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.

Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

Process Control and Verification

On a design of this kind, copper thickness 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

Which process gives a finer line? Pattern plating, because the etch that removes the unwanted copper is short and the sideways undercut is small. Panel plating has to etch through the full plated thickness, which widens the undercut.

Which process gives a more uniform thickness? Panel plating, because the copper is deposited over the whole panel before the pattern is formed. Pattern plating deposits more copper where the current prefers to flow.

Can the two be mixed on one board? Yes. Many boards use pattern plating for the fine inner layers and panel plating for a heavy outer layer, and the fabricator will normally propose the combination.