Issue link: https://iconnect007.uberflip.com/i/1512857
42 DESIGN007 MAGAZINE I DECEMBER 2023 created during entirely different phases of the production cycle. Holes are drilled toward the beginning stages, but board edges are cut or processed as part of the last. ese dimensions and their respective tolerances will definitely be inspected for quality because the dimen- sions are present. 3 Add overall dimensions to the PCB out- line. ese dimensions convey more than some PCB designers think. e dimensions need to be present in order to establish a toler- ance zone for the entire PCB. e dimensions are nominal values to which a zone of accept- ability must be communicated. What are the board outline limits below if the tolerance is identified at ±0.005" (0.13 mm)? What if the board outline itself is not so simple? I've worked on some LED light board designs with very complex geometry, par- ticularly in my days designing boards for the gaming industry. How would a PCB designer dimension the outline for a PCB shaped like Batman's silhouette? How about another large PCB in the shape of the cursive signature of Frank Sinatra? Hitting the "auto-dimension" button on a design with so many arc origins and spline features would surely yield cata- strophic documentation. To keep things sim- ple, we should consider adding dimensions only to critical, next assembly interface points in order to define tolerance zones to define areas of acceptability. PCB edges which hang aimlessly out in air or space may be considered noncritical. Do their complex arcs and spline really need dozens of dimensions? Remember, nominal data for cut- ting the features will be provided in the form of CAM data. If there is little chance for interfer- ence, or if the feature will end up suspended in air or space, a generally wide title block tol- erance may suffice. Designers need to under- stand that overall dimensions are not there to tell a manufacturer how to program a routing operation—the CAD data does that—but are there for overall reference to the stakehold- ers who will be quoting and planning. Final inspection stakeholders have other ways to inspect the cutter's work if the designer offers the option in the form of a fabrication note. Figure 3: Overall dimensions for PCB outline.