A fabrication package is a stack of single-purpose files. Each Gerber file describes exactly one layer of the finished board, and each layer type has one job: copper conducts, solder mask insulates and protects, silkscreen informs the humans, solder paste feeds the assembly process, and the outline defines where the board ends. Confusing the types — or failing to notice one is missing — is behind a large share of ordering mistakes, because a wrong layer still renders beautifully; it just renders as the wrong thing. This guide explains each layer type, what it looks like in a viewer, and how to recognise it when filenames are unclear. It pairs with what is a Gerber file and the naming guide, Gerber file extensions explained. The viewers used for the recognition routine below are compared in the best free Gerber viewers roundup.
Copper layers: the circuitry itself
Copper layers are the board’s wiring: tracks, pads, vias and poured planes, etched from the copper foil. A simple board has two — top and bottom — and multi-layer boards add inner copper layers that you will never see on the finished product but which carry signals and power planes inside the stack. In a viewer, copper is the densest, busiest artwork: fine tracks routing between pads everywhere the design goes. Copper is the reference layer for the whole set — drills must land on its pads, mask openings must expose the right parts of it — which is why alignment checks like loading and aligning Gerber layers always start from copper.
Solder mask: the protective skin
Solder mask is the coloured coating — most often green, but any colour you have seen — that covers the board and insulates the copper, leaving openings only where solder joints belong: pads, vias the design wants exposed, test points. Its Gerber file therefore looks like a near-solid sheet with holes punched in it, the photographic negative of the pad pattern. That inversion confuses first-time viewers: the mask file shows mostly what will be covered, and its information lives in the openings. Every opening should sit over copper that is meant to be solderable; mask accidentally covering pads is a classic assembly disaster that a stacked view exposes instantly, since the pads simply vanish under the sheet.
Silkscreen: the printed legend
Silkscreen — also called legend — is the white (usually) printed text and graphics on the board’s surface: component outlines, reference designators like R12 and U3, polarity marks, logos and revision notes. It carries no electrical function at all, which makes it the easiest layer to recognise: sparse ink floating over empty space, readable as language and symbols rather than circuitry. Its checks are human ones. Text must not print over pads (ink on a solder joint is a defect), designators should sit beside their components rather than under them, and the bottom-side legend, if any, should read correctly for the side it is on — a mirrored silkscreen is a frequent export slip that only a look at the actual layer will catch.
Solder paste: the assembly layer
The paste layer describes where solder paste is deposited through a stencil before surface-mount components are placed and reflowed. In the viewer it is the most minimal of the set: only the surface-mount pads appear, as neat arrays of rectangles and circles matching the footprints — nothing for through-hole parts, nothing for tracks. A board with no surface-mount components legitimately has no paste layer, which is a fact to know before reporting one missing. Where paste exists, its check is correspondence: every surface-mount pad on the copper should have a matching paste opening, in the same place, at a sensible size.
Outline: where the board ends
The outline — also called the profile, edge or board shape layer — is a simple closed line (or set of lines) defining the board’s boundary and any internal cutouts or slots. It looks empty compared to the others, and it is one of the most important files in the package: the fab cuts to it, quotes from its dimensions, and judges whether everything else fits inside it. Check that the profile is truly closed, that it measures to the designed size — the measurement routine is in measuring distances in Gerbv — and that no copper, text or stray shape sits outside it.
Recognising layers in Gerbv
Put together, recognition is a visual skill with reliable tells: copper is dense and everywhere, mask is a solid sheet with pad-shaped holes, silkscreen is sparse readable marks, paste is only footprint pads, outline is a bare closed profile, and the drill file — Excellon, not Gerber — is a field of circles that must register with the copper. Load the set in Gerbv (free from the download page), toggle layers over the top copper one at a time, and each type announces itself. The Gerber viewer hub ties the whole inspection together into a pre-order routine.
Frequently asked questions
How many copper layers does a board have?
At minimum the two outer sides, top and bottom. Multi-layer boards add inner copper layers for signals and power planes — each is its own Gerber file in the package, recognised by dense track-and-pad artwork like the outer copper.
Why does the solder mask file look almost solid?
Because the mask covers nearly everything: the file describes the coating, and its meaningful features are the openings left over pads and other solderable points. It is effectively the negative of the pad pattern, which is normal and correct.
Which layer defines the board’s size and shape?
The outline (profile) layer — a closed line the fab cuts to. Measure it in the viewer against your design dimensions, confirm the profile is closed, and check that no artwork from any other layer falls outside it.