Issue link: https://iconnect007.uberflip.com/i/1219242
76 DESIGN007 MAGAZINE I MARCH 2020 a significant impact and result in a reduction in the number of parts per production panel. A flex design that is 4.1" in length can be sig- nificantly more expensive than a flex that is 3.8" in length. That is a generalization and de- pends on other factors of the overall size, but the takeaway is to always keep your fabrica- tor's panel sizes in mind and, if given the op- portunity, best utilize the production panel to reduce cost. Materials When looking at trade-offs and decisions, materials can fit squarely in that category. Sometimes, there is no choice; a certain dielec- tric type, dielectric thickness, or copper thick- ness is required. These requirements may lead to minimum lot costs for your fabricator, driv- ing up overall cost and often also driving lead- time. But, if there is flexibility in material se- lection, it is recommended to work with your fabricator for the stack-up. Giving the fabrica- tor the ability to use commonly stocked mate- rials will keep materials cost as economical as possible. Flex With Stiffener vs. Rigid-flex The terminology for these two products is often confused. Using FR-4 stiffeners to sup- port a heavy component or dense area of com- ponents on a flexible circuit is highly recom- mended. This provides mechanical support for the component but does not require an electri- cal connection between the flex and FR-4 lay- ers. Meanwhile, rigid-flex also provides sup- port for dense component areas, but most of- ten also has routing layers in those FR-4 layers and requires an electrical connection between all layers. Rigid-flex construction requires a much more complex fabrication process and will be more expensive. This is often necessary, especially if there are dense component areas on both sides of the circuit, and can resolve complex routing and packaging challenges. However, it is worth reviewing to be sure that flex with a stiffen- er approach will not suffice before beginning a rigid-flex construction. Technology Roadmap Understanding your fabricator's technology roadmap is extremely important when looking for ways to be as cost-effective as possible with flex design. Typical technology roadmaps have categories for standard, advanced, and emerg- ing technology. It stands to reason that if stan- dard technology costs less than advanced, and advanced costs less than emerging, common sense would dictate designing to your best abil- ity within the standard category. It also stands to reason that as a design attribute moves along the continuum, there will be associated yield impacts and increased costs. What may not be so intuitive is that when a design pushes multiple areas of technology, the yield loss, and associated cost impact can be exponential. The phrase "everything but the kitchen sink" is not a good thing with flex de- sign, at least in terms of pricing. It is fine to push the limits of trace width and space, the hole size, or the drill-to-copper distance, but pushing all of these in one design will certainly increase the cost. It is recommended to work closely with your fabricator to help navigate the trade-offs and yield impacts to be certain that the impacts of pushing the limits in the de- sign are fully understood, especially with flex- ible materials. Conclusion This list is by no means intended to be an all-inclusive discussion of items to be aware of when trying to navigate the cost impacts of the many design challenges and trade-offs, but I hope this will be a place to start. As is often recommended, the common theme with flex and rigid-flex is to work with your fabricator early in the design; take advantage of their ex- pertise because they are always willing to sup- port their customers and share their knowl- edge. FLEX007 Tara Dunn is the president of Omni PCB, a manufacturer's rep firm specializing in the PCB industry. To read past columns or contact Dunn, click here.