PCB copper is a layer too

You are trying to connect two pads on a green board with a strip of metal, and the metal is a foil, not a chip film. The picture at the top is that foil before anyone paints it: copper traces and round pads on fiberglass, warm metal, no green yet. On a printed circuit board (PCB) people still call the sheet a layer. The metal itself is copper. The picture is the layer made solid. The editor’s colored shapes are the instruction for this foil. Thin copper necks into a pad — thermal spokes — keep soldering heat from vanishing into a lake of that foil.

Green is not the copper

The second picture is the mix-up in one frame. Green solder mask covers the board. Copper shows only where the mask opens, on the pads. Beginners call the whole board “the copper” because the product is green and famous. The green is paint. The copper is underneath, or it is gone where the etch removed foil. A photograph of a finished green board is not the copper layer. The copper layer is the plot that told the etch what to keep.

Chip people use the same idea for films on silicon: one named sheet, one factory step. On a board the conductor is foil glued to fiberglass, etched or plated, often specified by an ounce weight. A 1 ounce foil is about 35 µm thick. That thickness is not in the top view. The top view stores the outline of the traces, the pads, and the pours.

What the sheet contains

Traces are often drawn as a centerline plus a width, the same pencil as a chip path, and then plotted as a filled shape. Pads are the lands a part will sit on. A pour is a wide region of copper, often ground, that must pull back from pads it should not touch. Drill hits punch through the sandwich and, when plated, connect floors. The green outline of the board is not copper. It is the edge.

If two tools disagree on the name of this sheet — top copper in the editor, a different name in the Gerber zip — the factory etches the wrong film. The bare board in the first picture is what you get when the name, the etch, and the foil agreed. Send that sheet as its own file, and do not let the solder-mask file pretend to be the metal.

Walk a single trace from pad to pad on that bare board. The width is current and impedance, not a chip minimum-width copied from a foundry manual. A 0.15 mm trace on 1 ounce copper is a common small signal. A power trace is wider because it is a heater if it is not. The pour beside it has to stop short of the pads, or the etch leaves a short the photograph will show as one lake of metal. That clearance is a number in the design rules of the house, in millimeters, on this sheet.

On a bare board like the one in the first picture, find a pad that is supposed to be ground and look at how the copper enters it. Spokes leave a gap of fiberglass between the pour and most of the pad, with only a few thin bridges crossing that gap. A solid lake all the way around the pad is the failure: the iron sits, the solder will not wet, and the heat has gone into the pour. Add the necks on the copper layer, plot that film again, and look at the same pad on the next bare board before anyone paints it.

The green board in the second photograph hides those necks inside a mask opening. You can approve the green and still have a pad that will not solder. Show the copper alone in the editor, the way the bare board shows it alone, and point at the bridges. If you cannot point at them, they were never drawn. Count the bridges into that pad. A spoke pad shows laminate between a few thin necks. A lake pad shows copper all the way around, with no laminate at the edge of the land. On a scrap, touch an iron to one of each. The lake stays dull because the pour drinks the heat. The spoke pad wets. That wet joint is the check, and the green mask in the second photograph will hide it once the paint is on. Hold the iron on the lake pad for the same count you used on the spoke pad, so a dull joint is the pour drinking heat, not a shorter visit.

One trace, in millimeters, on the foil

On a bare board like the one in the first picture, pick the small-signal trace and measure it against the plot, not against the green board. A 0.15 mm trace on 1 ounce copper — about 35 µm thick — is a common small signal. The thickness is not in the top view. The outline is. If the plot says 0.15 mm and the foil on the bench is a hairline or a pour, the etch followed a different file than the one you are holding. The 35 µm is the foil weight. It does not widen the trace.

Then find the pour beside that trace. It has to stop short of the pads and short of the trace. A gap you can see on the bare board is the clearance the house wrote in millimeters. If the pour and the trace are one lake, the photograph is already the short, and solder mask in the second picture will paint over it without removing it. Open the copper plot alone, the way the bare board shows the foil alone, and point at the 0.15 mm trace from pad to pad. If you can only point at it after the green is on, you are looking at a mask opening, not at the metal. The sheet you send is the copper file. The green file is the paint.