Issue link: https://resources.ema-eda.com/i/1008704
82 Pictorial CAD data must be accompanied by textual fabrication specifications which are best documented in an engineering drawing. The PCB fabrication drawing serves less to indicate how a PCB is to be manufactured and more to specify how the PCB shall be laid up. CAD data can define nominal values for features such as drill geometry and the PCB outline. Automated machinery does a remarkable job of performing drilling and imaging until it doesn't—sometimes automated machinery fails. A particle of dust may land on the material during imaging or etching, and plating chemistry may weaken finished copper. How will the final inspection stakeholder know what is intended and what to expect without a clear, pictorially-rich document which specifies the design intent? A complete fabrication drawing includes a pictorial view representing the PCB outline. Within the outline, holes are represented using symbology. Each hole position is marked with a symbol which is coordinated with a drill chart on the drawing. Drill Symbols Drill symbols are shapes or textual characters generated by the PCB layout software to graphically show a hole's position on the PCB. Being a symbol, it can simultaneously be represented in a corresponding drill chart to convey more information. It is important when selecting symbols for drill sizing to always use a unique symbol for each type of hole containing the same attributes. For instance, there may be several .023 [0.58] diameter holes required on a PCB. If the holes are alike with regards to plating requirements and hole diameter tolerance they may be assigned the same symbol. However, if there is a different attribute, such as a non-plated condition or a different tolerance requirement, the hole of the same size will require a different symbol which will need to be reflected on the drawing accordingly. Within the outline, holes are represented using symbology. Click to Enlarge
