By Kelly Dack | Published on: December 03, 2015
What a wonderful PCB layout and design world we live in. Whoever thought that a PCB layout – once hand taped on mylar, shaped and cut with X-Acto knives (ouch) – would be so easily designed using modern day CAD tools? Over the past few decades, design requirements have shrunken immensely from .100 grid spacing on component leads to .016 [.4mm] spacing and smaller. Overall packaging constraints have become so tight, machinery producing the parts has trouble holding the dimensional requirements for mating with other parts at their next assembly. Lately, PCB CAD tools have incorporated 3D modeling capability in order give designers a better look at how a PCB design will mate with other parts.
Question: How does 3D PCB design capability or even simple Gerber and Excellon manufacturing data help the PCB fabricator to produce accurately drilled holes, routed board edges and determine yield? Answer: It doesn’t.
So let’s forget about Gerber data and three-dimensional design for a few moments, and just talk about dimensioning and tolerances.
Myth: A dimensioned fabrication drawing is no longer required for PCB fabrication because 3D design capability and the awesome accuracy of Gerber and Excellon manufacturing data.
Reality: CAD data is nominal data and does not take variable manufacturing tolerances into consideration. This remains one of the most important attributes of a PCB designer – the knowledge and ability to ensure a PCB’s dimensional form and fit success by considering allowable tolerances.
Where a PCB design is produced these days is more of a concern than ever. Even without a properly dimensioned fabrication drawing, designers expect to see smooth, beautifully routed board edges, slots and drilled holes from their local prototype supplier. If the prototype fits, they mistakenly assume everything will always be ok because the nominal Gerber and drill data is apparently “good enough.” But after sending the same data to an off-shore production facility, I’ve seen designers jaws drop after a first article inspection noting ragged mechanical features using the same Gerber and drill data. So, without meaningful dimensions and tolerances, are moans to adhere to manufacturing specification “MIL-TFP-S” (Make It Like The %#@! Print, Stupid) at all justified? Without meaningful dimensions which include tolerances -- no way.
I’ve spent the summer of 2015 working closely within the PCB manufacturing community and I have seen a disturbing void in the communication of mechanical allowance and inspection documentation from the PCB design community. Now, more than ever, there is need for continuing education with regard to understanding the purpose for dimensioning and tolerancing on PCB fabrication drawings. I highly recommend that all designers consider IPC’s CID basic training which covers in-depth applications of this subject. If you have not been through the training and might be considering it, contact EPTAC corp. You can look forward to a healthy discussion on the purpose of dimensioning and tolerances on a PCB, how to apply them meaningfully and how they are interpreted by your PCB supplier. You will be surprised to find that dimensions and tolerances are used less to convey to the supplier where to cut the PCB (the Gerber data is good for that) than to how to inspect their work and determine whether the machinery and processes have done a good job producing parts (yield) that will fit as intended by definition.
But for now, here are three dimensional tips (not to be confused with 3D tips) I’ll offer to facilitate mechanical communication between a PCB designer and board supplier:
Now, if your CAD software can design in 3D, rotate it, spin it, flaunt it -- impress your mechanical engineers at the design review. But remember that 3D PCB CAD data is only nominal data which does not communicate min/max design definition and allowance. The ability to consider and express these important values on a fabrication drawing using industry standard dimensioning and tolerancing techniques is part of what puts the “design” in the term designer.
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