Before I built the rev B PCB I noticed that OSH Park had reduced their minimum drill size from 13mils to 10mils and their minimum annular ring size from 7mils to 4mils. I dropped the size of all of my vias on bityExpress to the smaller size (18mil total via size vs 27mils). At this smaller size I can start considering more interesting additions to the card for later revisions.
I never really thought twice about it until I started inspecting the cards today. Based on what happened on my first rev B build, I decided to check for continuity on all critical nets to the FPGA before loading the components and I found that TDI was open! That means that my problem on the first card might not have been my soldering but with the board itself. Looking at the PCB under the microscope showed that the annular rings are very thin around the vias. It looks like the 10mil drills were actually drilled at something larger (13mils?).
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| A via from the gerbers. The inner circle is the drill hole and the annular ring is the additional radius beyond the drill. |
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| A via from the bityExpress-B PCB. The drill is a much larger proportion of the total via diameter on the PCB |
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