After the PCB is designed, what do I need to check in the Gerber files?
The main point for me about looking at the Gerbers outside my primary CAD is to make sure everything looks OK. I put a lot of trust in my main CAD package, and use the Gerber viewer as a qualitative verification.
Things I look for:
- All layers are aligned
- All layers are present (file exists)
- All layers have data (not just vias)
- Board outlines have the correct dimensions
- Fill polygons have the right isolation, orphan settings
- Make sure your soldermask is correct near high-density parts (tented vias, etc.)
Making sure that Eagle merges the layers correctly is my biggest worry when I'm checking the Gerbers, because if you aren't using a 100% verified CAM flow, what you see on the page might not be what you get in the Gerbers. Other than that, everything should be the same. Think of it as looking at a printer's proof before ordering a lot of copies.
If you have the components on hand already, print the outer layers out at 1:1 and place all the parts on there. Ordering parts and boards at the same time is a little faster, but this would have saved me a couple board respins.
Your layout software should have checked the design rules. It's unlikely you'll find design rule errors on the gerbers that the PCB software missed. On the other hand, using gEDA-pcb's photorealistic rendering output, I've caught a few errors before fab, mostly soldermask.
Your fab will have a checklist of capabilities on their website. Go through this line-by-line.
EEVblog #127 discusses such things as panelization and fiducials. Worth watching, especially if you are designing for machine assembly.
A few gotcha's:
Mirrored/Rotated Layers:
Make sure that the top and bottom layers are oriented the same way. An easy way to do this is to check that the soldermask for the top and bottom line up on a few through-hole vias and pads.
Gerber vs. Drill alignment:
Sometimes, the drill holes and the Gerbers will be grossly misaligned. Perhaps the gerbers are centered on the origin, and the drills have their bottom left corner at the origin.
Font not rendering correctly:
Unlike your PCB editor, Gerbers don't have a font library. Make sure to use a vector font, which can be defined as a series of vectors in the gerber, rather than a proportional font, which might be different (and may not) on the gerbers. This may only show up as a difference in character widths, which may or may not cause trouble.
These are easy to see in a Gerber viewer like GC-Prevue, but hard to detect in your export settings.