The Gerber Guide, Chapters 15 and 16

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Chapter 15: The Use of Gerber Viewers

Before sending your Gerber files off to your fabricator, you are often advised to check them using a reputable Gerber viewer such as GC-Prevue. This is excellent advice.

Note that this involves more than just verifying that the viewer displays your intended image: It is important that you check too that the file is valid. Even when handling invalid data, viewers typically try to reverse-engineer the intended image by "reading between the lines." This is perfectly OK, but the file is still invalid and, according to Gerber specification:  An invalid Gerber file is meaningless and does not represent an image.

A file with errors must not be sent to the fabricator as if all is well, even if the intended image is shown. This is because even if your reader has reverse engineered the intended image from the invalid data, another reader may not be so successful. And that reader may be your fabricator's CAM, which will result in scrap. Should this happen, the fault lies squarely with the file. To quote from the Gerber specification: The responsibilities are obvious and plain. Writers must write valid and robust files and readers must process such files correctly. Writers are not responsible for navigating around problems in the readers, nor are readers responsible for solving problems in the writers.

It is therefore extremely important that you check that your files are valid. Invalid files can cause viewers to throw error messages like the one in Figure 1, taken from GC-Prevue.

These messages clearly indicate that there is something very wrong with the file. The question is, what you do if you see such errors? It’s not easy. Low resolution is often the root cause of problems, so it is worth trying to output the file at the resolution recommended in Chapter 10 in this series. 

The only safe solution is to fix the bugs in the Gerber output software. It is therefore essential that you provide detailed information of the problem to your software supplier so that the bug can be fixed for the future. That said, the chances are that your board cannot wait for this fix and you have no way to output a valid file. This is then a conundrum. You could send the invalid data with the necessary caveats and hope that your fabricator's software, like your reader, will reverse engineer the intended image correctly. If it does, all is well. But this is a risk, so if you decide to do this, always include a netlist as a safeguard, as advised in Chapter 8 in this series. You can also ask your fabricator to send you the images he generates in CAM, so that you can check them for errors.

To read this entire article, which appeared in the July 2016 issue of The PCB Design Magazine, click here.


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