A PCB stackup

You are trying to make a board that holds parts on more than one floor of copper. The picture at the top is that sandwich cut open: woven fiberglass, sheets of copper, and a plated hole that stitches the sheets together. The list of those sheets — which copper, which insulator, how thick — is a stackup. A PCB is the finished board. The stackup is its recipe. A line you meant to be 50 ohms, 0.20 mm wide over a plane 0.20 mm below, becomes something else when the house’s standard stack puts that plane 0.36 mm below. Layout names have to match this list.

What the cut edge is showing

Each bright copper line in the picture is a layer you will send as its own plot. The pale bands are core or prepreg, the fiberglass and resin that set the distance between coppers. The vertical barrel is a through via: a drilled hole whose walls were plated so the floors can speak. A buried via would stop between inner floors. A microvia would only reach the next sheet. Drawing a via on the wrong pair of layers is walking the stairs into the wrong apartment.

The second picture is the same kind of board before anyone cuts it open. From the outside you see green mask and a thin edge. The stackup is hidden in that thickness. A 1.6 mm finished board with 35 µm copper on the outer floors — often called 1 ounce — does not look like a layer cake until you polish the edge. The editor’s top view will never show that thickness. A technology table or the house’s drawing has to.

Why the names have to match the recipe

How hard a signal fights reflections depends on geometry: trace width, how far the copper sits above a plane, and how thick the insulator is. People call that match impedance. Change the stackup after routing and you have a new board wearing old traces. The photograph of the edge will look tidy either way. The eye diagram will not.

If the editor’s layer order does not match the stackup, a via you thought landed on ground lands on a signal. If you rename an inner copper because “GND_plane” sounded nicer than the fab’s inner-layer name, the plot files go out with the wrong film.

Copper balance is the density rule of fiberglass. A desert of copper on one floor and a solid plane on the other warps the panel in the press. Extra copper dots in the empty regions — thieving — are the board cousin of fill.

Ask the house for a bare coupon of the stack they will actually press, and pick one plated hole your file treats as a connection from the top face to the bottom face. Under a lamp, look at the copper ring around the drill on the top face. Flip the coupon and look at the ring on the bottom face. You want a closed loop of copper on both faces, not a crescent and not bare laminate against the drill. A ring that has vanished on one face is a drill that wandered off the pad.

That wander has a budget: the minimum annular ring in the same quote as the stack. Write the minimum beside the pad diameter, then enlarge the pad until both faces would still show copper if the drill shifts by that amount. Check a hole near the middle of the coupon and a hole near a corner. The panel is registered from tooling holes, and the corner is where that registration is worst. A ring that survives in the middle and breaks at the corner means the pad is legal only where the drill was kind.

Use a loupe. A narrow ring looks intact from arm’s length and is gone once you can see laminate between the drill wall and the copper. Write the top width and the bottom width next to that hole’s coordinate, and keep those two widths in the quote beside the pad diameter so the pad you enlarge is the pad the coupon already missed. If either number is below the minimum you were quoted, the coupon has already shown you a miss, and the pad in the file is not large enough for the process you are buying. The polished edge in the first picture is where a saw later shows the barrel. This ring check is the one you can finish with the coupon in your hands, before that saw and before the zip goes out. A crescent on the bottom face, with a full ring on the top, is already a miss, even when the editor still shows the pad centered on the hole.

0.20 mm to the plane, then the ring

The trace sized for 50 ohms assumed a plane 0.20 mm away. Write that 0.20 mm in the quote next to the stack you are actually buying. A 1.6 mm board with 35 µm outer copper can still put the plane at 0.36 mm if the house presses a different core. The trace outline does not change. The impedance does. The polished edge in the first picture is where you later see the foils. It will not tell you the 50 ohm calculation was for a different spacing.

Before that saw, take the bare coupon and one plated hole your file treats as top-to-bottom. Under a lamp, the copper ring on the top face and the ring on the bottom face should both be closed loops. A crescent on the bottom is a drill that left the pad. Write the minimum annular ring from the quote beside the pad diameter, and enlarge the pad until a shift of that amount still leaves copper on both faces. Check a hole near the middle and a hole near a corner. The corner is where registration is worst.