PCB manufacturers require Gerber and Excellon format files to make printed circuit boards (PCBs). The Gerber and Excellon files are usually created by exporting from a PCB layout program or CAD program. The drawing and positioning commands within these files are industry standard, and are different than the proprietary files used for everyday storage by PCB layout and schematic programs.
Regardless of your PCB layout program, before you place your board order, you should always inspect the exported Gerber and Excellon files. Not only does this step independently verify that the board layout has been properly generated, but you sometimes catch last minute design mistakes because you're seeing each layer from a new perspective.
There are many free Gerber viewer applications, and most display Excellon files as well. I use Pentalogix ViewMate, GraphiCode GC-Prevue, and OSH-Park (they have a viewer when you start an order but before committing). My favorite PCB viewer is Paragon Robotics Gerber Viewer because it is:
Open your PCB layout file in Copper Connection. For this example, I'm using the Sandwich version 5 robot board that is provided with Copper Connection. Choose Example PCBs from the Help menu if you can't locate that file in your Gallery.
Next, choose "Export to Gerber and Excellon" from the File menu. Choose the "(default)" manufacturer for this example. Then, click the Export button.
Default Export of Gerber and Excellon files in Copper Connection
You'll be asked where to store the exported files. I usually use an Export folder on my desktop for convenience, but you can save the exported files wherever you want.
In your browser, go to http://paragonrobotics.com/free-tools/gerber-viewer/en-US.html and click the little plus icon in the upper left.
Paragon Robotics Gerber Viewer adding files
You will be asked to choose the file to upload. Click the Browse button, select Sandwich.zip (or whatever is the name of your project), and click open. Then click OK.
Paragon Robotics upload window
The zip file contains all of the Gerber files and the Excellon file. You could upload the individual files, but that's less convenient.
The left side of the screen lists all of the files/layers by name. The checkboxes allow you to toggle between showing and hiding a layer. Experiment with checking and unchecking different boxes.
Paragon Robotics Gerber Viewer showing Sandwich robot PCB layers
Copper Connection exports files with the name of your board and a unique filename extension for each particular layer. You can see which extensions correspond to which layer by going back to the Copper Connection Export window and clicking on the File Extensions tab.
Configurable Export File Extensions in Copper Connection
In this example, if you turn off all of the layers except for Sandwich-v5.txt, you'll see only the Excellon file -- which represents all of the drill holes. Of course, if you named the file something different, like Sandwich-v5.drl, then you'll still see the drill holes but the file will have a different name.
That brings us to an important topic...
The Gerber and Excellon specifications describe the drawing commands for the contents of the files. But, the specifications don't say what the files should be named. As you can imagine, this can really confuse a PCB manufacturer. By default, Copper Connection follows the most popular naming conventions and even includes comments inside each file describing which layer it belongs to. Nevertheless, if a manufacturer claims "I see part outlines and writing on the top copper and I see pads on the silkscreen", then they've probably mixed up the copper and silkscreen files.
Another example of the problem of a lack of file naming standardization is the coloring of layers in a Gerber viewer. When you look at the files in the Paragon Robotics Gerber viewer, the colors of the layers will be different than Copper Connection. This doesn't mean anything is broken -- it is purely aesthetic in the viewer.
In fact, the layer colors in the viewer seem to change from project to project. This occurs for all Gerber viewers because they don't know or care what the layers represent. They don't try to match a particular color (red) to a particular layer (top copper) because they don't know what the top copper layer file is named. You can take the time to adjust each layer color in the viewer application or you can just remember that the colors have no meaning other than to separate the layers visually.
The naming convention problem goes one step further as the Gerber viewer may display the layers in an illogical order. For example, they may put the top silkscreen on the bottom and the top copper on the top and the bottom copper in the middle. Again, that's because the Gerber viewer doesn't know how to relate the filenames to a PCB layer. The Gerber viewer only knows how to draw the contents of each file -- not how the files combine together. You can use the Up and Down arrow icons in the upper left of the Paragon Robotics Gerber Viewer to properly sort the layers on the screen.
Most manufacturers have seen so many Gerber files that the operator can arrange them instinctively. If your manufacturer prefers a different set of names, you can set them in Copper Connection.
Your goal when inspecting the Gerber and Excellon files is to determine if anything looks crazy. For example, does anything extend beyond the edge of the board? Do the lines and traces seem the wrong size or angle? Are elements missing? Do the holes not align with the pads?
Turn off layers and then view each layer individually. Without looking at the filenames, can you recognize the layer? ("Yup, that's the top copper.")
If there is something tiny or complicated on your board, take a few minutes to zoom in on that detail in the Gerbers.
Usually, everything looks great. Not only does this give you peace of mind, but it can help you communicate with a manufacturer if they are having trouble processing the files themselves.
If you find an issue when viewing the Gerber and Excellon files or if a manufacturer still isn't able to produce the boards, please contact Robot Room Copper Connection support.