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26 DESIGN007 MAGAZINE I JULY 2021 Cherie Litson: ere is no "loosey-goosey" hole placement for Class 3 annular rings. ere must be 1-mil minimum annular ring around the hole aer drilling. No breakout is allowed. Holes get drilled before the etching of the cop- per. us, there is no "centering the drill" on a hole. at's a typical misconception from anyone who thinks it's a mechanical issue. It's not. It's about having enough annular ring to account for the fabrication tolerances. If you're not specifying them in your notes, then the "standard" tolerance is taken from the drawing format title block and that will usually be ±5 mils. Most fabricators can hold a tolerance of ±3 mils on material movement during fabrica- tion. Heidi Barnes: I am not directly familiar with this standard, but from a high level there is always a challenge of over-specifying the engineering details at the expense of over-constraining the fabrication process. Building PCBs does take a lot of coordination between the design engi- neer and the fabricator to understand what tolerances need to be specified without esca- lating fabrication costs. It is exciting to see how far PCB fabrication has advanced with the push to higher densities, higher power, and faster speeds. However, I believe that there is still many an engineer out there who feels like it is magic that a PCB works. Statisti- cally it works out, but if one were to add up all the worst-case fabrication tolerances, it would not. Carl Schattke: Class 2 allows some breakout; Class 3 does not. Obviously, there is drill dri and registration tolerances that come into play. Work with your manufacturer to see what pad ring sizes will be needed for your application. ey will give you that based on their process capability. DESIGN007 Do you have any ideas on interpreting IPC-6012 Class 3 annular ring standards, especially for DFM? It seems very loosey-goosey, re: hole placement, etc. Q

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