Panel Plating vs Pattern Plating: Choosing the Copper Process
Every plated through hole board needs copper on the surface and inside the barrel, and there are two established ways to put it there. Panel plating covers the whole panel with copper before the circuit is defined; pattern plating deposits copper only where the conductors will be. The choice changes the artwork polarity, the number of process steps, the achievable line width and the way copper thickness is controlled in the hole.
What Each Process Does
Panel plating adds copper across the entire surface and through every hole, typically immediately after the electroless copper step that makes the originally insulating barrel conductive. The circuit is then formed by etching away the unwanted copper, so what remains is a conductor of the full plated thickness.
Pattern plating images the circuit first, then plates additional copper into the openings of the resist, followed by a thin layer of tin. The resist and the tin are stripped later, and a short etch removes only the thin original copper that was never plated. The final conductor is therefore built up in the pattern rather than carved out of a blanket layer.
The Panel Plating Sequence
Drilling comes first, followed by desmear and electroless copper. The panel is then mounted on a rack and plated in a copper bath until the surface and the barrel reach the finished thickness. Micro-etch, acid dip, plating and rinsing stages follow one another on an automated line.
After plating, the panel is imaged with a resist, developed and etched, and the resist is stripped. Because the copper being etched is the full thickness of the conductor, the etch has to remove a relatively thick layer, and the etchant works sideways as well as downward. The resulting sidewall undercut is the main reason panel plating struggles with very fine lines.
The Pattern Plating Sequence
Pattern plating begins with a thin layer of electroless copper, enough to make the barrel conductive but far thinner than the finished surface copper. A dry film resist is laminated, exposed and developed so that only the conductor pattern is open. Copper is then plated into those openings to the required thickness, and tin is plated immediately on top of it.
The resist is stripped and the panel is etched. This etch has to remove only the thin original copper, so it is short and the undercut is small. The tin acts as the mask that protects the conductors, and it is stripped after etching to leave the finished pattern.
Why Artwork Polarity Matters
The two processes use opposite artwork conventions. Panel plating with a print and etch flow uses a negative image, where the resist stays on the copper that will remain. Pattern plating uses a positive image, where the resist is removed where copper has to be added.
Mixing the two conventions is one of the classic errors in a fabrication package, and it produces either a board with no conductors or a solid copper plane. The drawing should state the flow explicitly and the layer names should reflect it, because a fabricator receives data from many customers with many different conventions. Copper distribution across the panel, the additives used in the bath and the resulting micro-structure are covered in the notes on electroplating additives.
Tin as an Etch Resist
Tin plating is what makes pattern plating practical. It is deposited immediately after the copper in the same plating line, using a separate tank, and it resists the alkaline etchant that removes the thin base copper. The tin layer is thin, typically a few micrometres, and it has to be continuous across the whole conductor, because a pinhole becomes a broken trace.
Tin also protects the copper surface from oxidation until the next step, which is why the sequence should not be interrupted. After etching, the tin is removed in a dedicated stripping bath, and the copper underneath should appear clean and bright with no residual film.
Copper Thickness in the Hole
Barrel thickness is a separate requirement from surface thickness, and the drawing should state both. A common specification is 25 µm of copper in the hole with a defined surface weight such as 1 oz or 2 oz.
Panel plating produces a barrel thickness close to the surface thickness, with the ratio depending on the throwing power of the bath and the aspect ratio of the hole. Pattern plating lets the two be adjusted more independently, because the copper deposited in the pattern can be tuned while the barrel is plated to its own requirement. In both cases the plated distribution has to be checked by cross section, since current density varies between the panel centre and the edges. The failure modes that appear when the distribution is wrong are described in the review of copper plating defects.
Where Pattern Plating Wins
Fine conductors are the clearest case. With only a thin layer to etch, pattern plating holds lines and spaces below 100 µm that panel plating cannot reproduce consistently, because the thick etch undercuts the resist. Boards with fine pitch ball grid arrays, dense routing or controlled impedance requirements usually come from a pattern plated flow.
The second case is a tight thickness tolerance on the surface copper. Building the conductor up in a defined pattern gives better control than blanket plating followed by a heavy etch, which tends to leave a tapered profile.
Where Panel Plating Still Fits
Panel plating is simpler, uses fewer process steps and costs less, which makes it the natural choice for coarse boards with wide traces and generous spacing. Heavy copper boards above 3 oz are often produced this way, because plating the whole surface to that weight is more straightforward than building it up in a pattern.
Thick panels with high aspect ratio holes also favour panel plating, since the barrel needs a long plating time and the etch afterwards is not the limiting factor. The current rating of the finished conductors, whether they come from one process or the other, follows the calculation described in trace width and current calculation.
Defects and Inspection
Common defects differ between the flows. Panel plating produces undercut sidewalls, copper thickness variation and occasionally a partially etched sliver between two conductors. Pattern plating can leave plating nodules, tin residues, and thin spots where the resist opening was too narrow to plate evenly.
Verification is similar for both. A coupon on every panel is cross sectioned to measure barrel copper and surface copper, line width is measured on a test pattern, and the finished board is electrically tested for continuity and isolation. Registration is checked against the drill pattern, because a conductor that drifts away from its hole reduces the annular ring on one side.
Choosing Between Them
Work from the finest feature and the required copper weight. If the design needs lines below 100 µm, or a tight surface thickness tolerance, pattern plating is the right flow. If the board is coarse, heavy or thick, panel plating delivers it at lower cost and with fewer steps to control.
State the choice on the fabrication drawing along with the finished copper thickness on the surface and in the barrel, the minimum line and space, and the artwork polarity. A board that is built with the wrong flow can pass visual inspection and still be out of specification in ways that only a cross section reveals.
FAQ
Which process gives better hole copper? Both can meet a 25 µm barrel specification. The difference lies in how independently the hole and surface thicknesses can be controlled, and in how much the etch that follows disturbs the conductor profile.
Can a design be switched between the two flows? It can, but the artwork polarity, the copper weights and sometimes the minimum line width all change. A switch should be reviewed by the fabricator rather than assumed.
Why is tin used rather than the resist itself? The resist cannot survive the etchant and is removed before etching. Tin is conductive, etchant resistant and plated in the same line as the copper, so it protects the pattern without extra handling.
