A Gerber viewer is a great tool to use before you submit a PCB design to fabrication. This way, you will see exactly what you are sending to the fab house and you might find potential problems that aren’t easily spotted in the layout editor.
Free online viewer
There are many free viewers on the web. I like to use online viewers so that I don’t have to download and install any software on my computer. And this way, it will work independently of my operating system as well.
Here is a viewer I have used a few times, gerber-viewer.com. It does not require any registration or anything. Just upload your files and view them right away.
What to check for
In general, you use a Gerber viewer to find errors on your board. Perhaps you have used the wrong footprint for a component or that your drill holes somehow has become misplaced on your board.
In the beginning, you don’t know what to check for, but after a few PCBs, you will start to realize which mistakes you tend to repeat. Check for these ;)
For now, here is a couple of things you can check:
Check that all your layers align on the board. This is usually not a problem if all your Gerber files are created from one process.
Board size and footprints
Print a scaled version of the Gerber files on paper and check that the board size is correct. Also, you might want to lay out some components (if you have them) on the print-out to check if you have used the correct footprints.
Even though many fab houses check the files for obvious errors before they start fabrication, you should always check on your own first. This way, you speed up the process so that you and the fab house don’t have to send files back and forth.