Printed Circuit Design & Fab - December 2007 - (Page 42)

DfM Sunstone’s design rules. Once completed, tested and certified, Sunstone makes these rule decks freely available from its website, and partnering websites. Designers planning to use Sunstone for PCB fabrication can load these rule decks into their CAD systems and design direct to Sunstone’s manufacturing processes. The payoff can be tremendous for designers. “If they’ve got a DRC rule deck they can depend on, our designers are more likely to use denser layouts and really pack in the geometries, even during prototype,” said Stilwell-Baker’s Zaccari, speaking of his highly experienced staff. “That offers fewer design iterations to our customers, shortening their time to market and increasing our overall value-add to the project.” With a certified rule deck driving the CAD tool’s routing constraints, engineers can concentrate instead on other aspects of their design that are equally key to DfM, such as parts research or thermal effects. “It can have a big effect,” responded Duane Benson, of Screaming Circuits when asked about the impact of DfM on downstream PCB manufacturing services, such as assembly. “Physical spacing between the components, for example, is frequently overlooked by automated DRC in the CAD package. Yet, something that looks good on the board, such as putting several components with large thermal mass close together, can make an otherwise good design unmanufacturable.” Benson’s comments illustrate the point that there is a lot more to DfM than the normal route spacing, footprints, drill sizes or thermal issues. Component choices also affect the manufacturability of a design. If a completed design uses a component, for example, that is discontinued by the manufacturer shortly thereafter, the design team will certainly be going through another revision just to accommodate that part’s obsolescence. This is yet another set of tradeoffs designers must make during the design process. Catastrophic Board design errors Functional spec errors PCB layer mapping Assembly errors Design errors Process yield issues Component specs Systemic Parametric Functional spec errors Component mismatch Timing errors in PCB Assembly placement Process window /yield FIGURE 3. Manufacturing failure types, by category. customer service manager at Sunstone Circuits, picking up on Robledo’s point. “It’s a feature of the tool that’s built in and commonly overlooked in the prototype phase. Design engineers don’t often have full knowledge of either the process rule specifics, nor the DRC engine’s rules syntax. Some engineers tell us they’ve given up on the interactive DRC checks because their best efforts to write a rule deck only resulted in a long list of spurious errors. Instead of designing boards, they find themselves in an endless debugging loop in order to get meaningful DRC results.” Robledo’s and Viter’s comments are echoed by PCB engineers elsewhere in the industry. “Writing rule decks is not my preferred activity,” says John Draut, director of hardware design at LightFleet in Camas, WA. “I have multiple vendors with differing rules, and I’m not the one who knows all the subtleties of a particular fabricator’s process. Though I often use the rules checker inside the layout tool, I can’t be certain that the rule settings are applicable to the specific fabrication vendor. Last minute fabrication vendor decisions can have significant impact on my development schedule thanks to differing rules,” says Draut. “So interactive rules checking with multiple ‘vendor certified’ rule decks to choose from during PCB layout would significantly improve the development schedule when compared to batch mode checks run at the fab vendor’s facility.” CAD tool vendors, perhaps understandably, are reluctant to take on the task of writing rule decks. Their core competence is in software development, not process definition. Lee Harding, engineering manager at Sunstone Circuits, finds: “There are dozens of popular or semi-popular PCB design layout tools and something like 350 PCB manufacturing companies in North America alone. If a CAD tool company were to try to support many or most of the fabricators, the task would be huge. That’s why they don’t do it.” Harding adds, “The same holds true for fabricators. They haven’t seen the value in taking the time to write rule decks for all the CAD tools in the field. Most fabricators prefer to receive Gerber files and not interact with the CAD tools that make the Gerbers.” TABLE 1 examines the strengths and weaknesses of the different DfM tool approaches. Statistical Conclusion As tools continue to develop, we can expect that DfM’s current boundaries will expand to more sophisticated checks, possibly even bringing part-specific lifecycle information into the design process in new and innovative ways. This thought leads us full circle to what PCB designers do: innovate. Talk to a PCB designer about his work, and you’ll be reminded that the job is all about making tradeoffs: balancing densities with heat buildup; trading square inches for components; and achieving the same performance with lower cost components. With all these balancing decisions, any room in the prototype phase a designer can capitalize upon to truly experiment with an alternate design approach that could result in a major innovation is valuable. “If the designer is confident in the design rules, then he or she may have the extra bandwidth to try some alternative approaches alongside the tried-and-true ones,” comments Harding. “If our no-cost DfM/DRCs leave more engineering budget available for experimentation, then perhaps we help promote some innovation.” PCD&F NOLAN JOHNSON is CAD/EDA product marketing manager at Sunstone; njohnson@sunstone.com. DECEMBER 2007 The Sunstone Approach Sunstone Circuits is pursuing the DfM rule deck approach, collaborating with PCB software developers to create DRC/ DfM rule decks that accurately and completely implement 42 PRINTED CIRCUIT DESIGN & FAB

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Printed Circuit Design & Fab - December 2007 Contents Our Line Market Watch Around the World Happenings ROI 2007 PCB Designer Salary Survey Interconnect Strategies Positive Plating RF Design Ad Index Noise Reduction Supply Chain DfM Off the Shelf Marketplace The Signal Doctor

Printed Circuit Design & Fab - December 2007

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