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Design007-Oct2019

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90 DESIGN007 MAGAZINE I OCTOBER 2019 places to get a hands-on introduction to flex design because the suppliers fill their booths with hundreds of flex samples for you to ex- amine and ask questions about; some are even free for the taking. As an EMS provider, our interest is in help- ing the designer to see that the overall design must meet specific assembly constraints to flow down our assembly lines without prob- lems. This is the end game in determining the viability of a flex circuit requiring parts to be assembled on to it. The overall shape and size of the flex circuit are important considerations. If it is large and has as many legs as an oc- topus, the part will be unwieldy on the pro- duction line and will have to be constrained overall, which requires special manufacturing panel array or palletization (tooling) consider- ations and adds cost. If the flex is of a micro-scale, there will be no space for optical targets on the part itself. Fiducials will have to be added outside of the parts and onto a manufacturing panel. Process- ing panel size is not normally a concern for designers, but how these complex parts will be assembled is entirely planned on the basic starting criteria of each layout, including the size, shape, and part mounting requirements. Each EMS provider has unique and sometimes proprietary processes for assembling your flex design, so contacting and establishing a work- ing relationship with them before starting the layout is of paramount importance. Shaughnessy: Is this the approach you've tak- en since you started designing with flex? What are some things to watch out for? Dack: I've always talked to the fabricators, but not so much with the assembly operations until I went to work for an EMS provider. I've since learned that in design, there are a thousand ways to kill your product in both worlds. I have an interesting flex story that I tell when I am teaching my CID classes with re- gard to something to watch out for in speci- fying flex circuitry; it has to do with the foil that's used in flex. In the '90s, I had a require- ment for a dynamic, flexible circuit that pro- vided power to an actuator that moved back and forth for control of a critical-care life sup- port system. The flex circuit had to be de- signed to roll. We worked with a provider who gave us some very simple flex circuit design guidelines, which we followed. The flex circuit was designed, went into prototyping, and was tested for millions of cycles, meaning it would roll back and forth millions of times without fail. After testing, it went into production and passed all of its requirements for agency ap- provals. A year or two after the project had gone into full production, the company started getting alerts that some of the units in the field were failing. Patient alarms were going off in the units and putting people in danger. After a full investigation, the cause was determined to be the flex circuit. But it was not in the flex de- sign as specified; it was due to an alternative manufacturing process, which was approved to improve cost. Remember, in the late '80s, flex was so expensive—and the limited avail- ability of thinner foils made things worse— that someone in project management made the determination to allow a supplier to use their thicker, more readily available, and more cost-effective copper in the design. More cop- per should be better, right? Well, it turned out that the thicker copper in the application began to work-harden and crack, which is what caused the failure. Here's the point of the story: Familiarity with mate- rials is one thing, but familiarity with the ap- plication is an entirely different thing. Consul- tation and buy-in from all of the design and manufacturing stakeholders are especially im- portant when selecting materials and process- ing for flex. Shaughnessy: What advice would you give OEMs and designers just getting into flex? Dack: It's the same for any PCB designer, for that matter: You have to go out and find the educa- tion and training that you need. Similar to most all aspects of PCB design, there are no universi- ty classes covering flex design. Few classes cov-

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