1. The PCB CAM and fabrication process
To create the creation tools the CAM operator requires a perfect physical style of the bare PCB. As we've said, he client's incoming fabrication data is utilized as digital data to reconstruct that physical model.
It could at first view seem unusual that machine documents such Gerber, Excellon and IPC-D-56A areused because of this, but it actually makes sense. The Gerber format has developed way beyond its roots as a photoplotter drivers to be the perfect vehicle with which to transfer digital image and drill information from CAD to CAM. Which is exactly because of these roots that Gerber data files are so flawlessly suitable for their current role of accurately representing where copper and other materials are. Likewise, Excellon drill documents correctly specify where in fact the drill openings are. This makes these indeed formats with the capacity of describing a PCB.
Therefore the incoming Gerber, Excellon and other data is always read in to the fabricator's CAM system which analyses, reworks and transforms the image and drill information into creation tools. That is an extremely different proposition from using the designer's datasets straight as creation tools in three aspects:
• The data data files aren't standalone items, but part of the interconnected dataset that,with each other, explains a PCB.
• PCB CAM must ‘know’ more in regards to a PCB than simply the image. CAM must ‘know’, for example, which pads are advantage connectors, because these have to be precious metal plated; CAM must ‘know’ which drill openings are vias, because the solder mask is treated differently around via pads than around element pads; etc.
• CAM changes (edit) the image documents before production, including the solder mask openings are usually adapted to match tolerances. The data files therefore have to be editable effectively. Without this limitation a bitmap (TIFF) or contour (PDF, DXF) will be a sensible data format. The problem is these formats aren't editable as a PCB - they may be general image formats. An adequately constructed Gerber document ís editable.
These three factors affect how the designer's PCB fabrication data must be organized. The designer do not need to worry whether his / her files can drive NC creation machines - it's the job of the fabricator’s CAM to control the look data such that it will operate on real creation equipment. The designer's concentrate should be on specifying the finish product accurately, completely and unequivocally.
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