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Target Condition: Designing Unconventional Geometries
Geometric interference: In terms of a PCB design, this is a condition in which a mechanical feature on a finished PCB could come into unwanted contact with a significant counterpart, thereby affecting further assembly or electromechanical performance.
Geometric dimensioning and tolerancing (GDT), as covered in the pages of ASME Y14.5 and IPC-2615, is easy to understand if you speak the language. So is Mandarin. However, there is a void in the PCB design industry when it comes to conveying what is required on fabrication drawings for PCB outlines. It seems that those who speak and document PCB dimensioning and tolerancing have little patience for those who do not. The apparent state of PCB dimensioning and tolerancing knowledge in the industry is likened to the state of the city of Babel where the language is confused. There is an elephant in our industry’s inspection room, and it is not backed up against three mutually perpendicular datum planes.
There are dozens of process steps a PCB goes through before completion. One of the final steps is to de-panel or excise a PCB assembly from its supporting panel structure so it can move on to its next assembly level where it must fit well. Can you imagine building and shipping a thousand simple PCB assemblies to a final assembly plant and getting a message that PCB outlines are interfering with other parts? Now what? Rejection, return, rework, reshipping, and reinstall are profit-killers.
Conversely, imagine a PCB manufacturer put in the position of meeting an incredibly tight tolerance of ±0.005" (0.13 mm) on a PCB design that is 36" (914.40 mm) long. Challenging design constraints do not help a manufacturer’s PCBs to fall neatly off the assembly line. When overly complex design specification challenges a supplier’s ability to interpret manufacturing capability, expensive inspection and quality assurance process phases need to be added, requiring the expertise of additional project stakeholders.
I review hundreds of PCB designs, and I see many PCB designers wasting the valuable time of their PCB project stakeholder counterparts by misunderstanding or misusing the power of dimensioning and tolerancing techniques. PCB dimensioning and tolerancing is a language requiring a sender and a receiver. If there is a mismatch on a PCB fab drawing, failure soon follows. I see that many designers could use some help simplifying their understanding of dimensioning and tolerancing as well as language skills to better convey the performance requirements of their PCB outlines.
PCB outlines are usually defined by our mechanical engineering stakeholders who commonly provide the outline parameters in the formats of IDF, DXF, or STEP.
Unless PCB designers are designing for a “snap fit” board edge requirement or must match a complex enclosure profile—perhaps meant to seal in some potting compound or reduce the leakage of photons in a bright LED display—most PCB design outlines are given ample clearance from other parts. In fact, I think we could get most PCB designers to agree that most PCB design edge surfaces worldwide interface with relatively vast amounts of air (or space in a vacuum). If we can agree, then what can we do to enhance our dimensioning and tolerancing language skills to lower process rejection rates or reduce time wasted inspecting for needlessly wide tolerance specification at PCB fabrication and assembly facilities?
Consider these four tips:
1. Select a next assembly interface point such as a mounting hole for the design layout origin. Documenting the X0, Y0 origin is a simple way of establishing a locational datum point for the entire hole pattern which is processed in the beginning stages of manufacturing with minimal steps. A hole is a powerful datum feature to reference because it shows up on every single process layer.
2. Use X and Y dimensions to simply locate the PCB outline with a relative tolerance to the hole pattern. Remember, these two geometric features (the holes and outlines) are 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. These dimensions and their respective tolerances will definitely be inspected for quality because the dimensions are present.
3. Add overall dimensions to the PCB outline. These dimensions convey more than some PCB designers think. The dimensions need to be present in order to establish a tolerance zone for the entire PCB. The dimensions are nominal values to which a zone of acceptability 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, particularly 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 catastrophic documentation. To keep things simple, 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 cutting the features will be provided in the form of CAM data. If there is little chance for interference, or if the feature will end up suspended in air or space, a generally wide title block tolerance may suffice. Designers need to understand 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 stakeholders 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.
4. Add a simple note to specify outline profile to be inspected within 0.XXX" of design data. PCB designers and their project stakeholders can benefit from a simple technique which draws from the GDT concept of “profile of a surface.” Using this technique helps eliminate countless dimensions from the fabrication print. All that is required is that the PCB designer determine an acceptable range of tolerance. Ask yourself, “If this board edge shrinks or grows by a certain amount, what are the effects?” The tolerance zone created by a profile callout automatically applies to the zone around the PCB profile regardless of its complex shape.
Take a hint from Batman: “Zap” these four steps into your design workflow. Your PCB will “zing” through the manufacturing and inspection phases. Your PCB stakeholder counterparts will love you and your PCB edges suspended in air will be none the worse until you go back and ask that mechanical engineer why you can’t lop off Batman’s pointy ear so you can fit another image on the manufacturing panel.
This column originally appeared in the December 2023 issue of Design007 Magazine.
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