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50 The PCB Design Magazine • June 2015 article In the second scenario, a significant part of the enclosure or mechanical assembly model is brought from the MCAD package into the PCB design software, where the rigid-flex board out- line can be designed specifically to fit with it. Rigid-flex layer stack sections can be defined and then flexible circuit areas have bending lines added. In the PCB design tool's 3D mode, the folds are then implemented to reveal where potential clearance violations and interference occurs. The PCB design can then be interac- tively modified to resolve the problems and check right away—without having to build any further mock-ups or translate design databases from one tool to another. Introduction Rigid-flex can have ample benefits, and many designers who never had to do so before are now considering it. PCB designers are fac- ing higher pressures to build evermore densely populated electronics, and with that come ad- ditional pressures to reduce costs and time in manufacturing. Rigid-flex PCB technology of- fers a solution that is viable for many product designs facing these challenges. However, there are aspects of rigid-flex tech- nology that could potentially be potholes in the road for newcomers. So it's wise to first un- derstand how flex circuits and rigid-flex boards are actually made and the challenges of mak- ing sure everything will fold in the right way, all while maintaining good flex-circuit stability and lifespan. In addition, design hand-off to the fabrica- tor is fraught with risk, especially for those less experienced (either in design or manufacture). First, there must be absolutely clear documen- tation concerning what is required. Such doc- umentation includes layer stack and material definitions, fabrication drawings, and design notes. If these items are not accurate and com- plete there will be production delays at best, and scrap prototypes at worst. Second, the PCB layout and design is still traditionally carried out in 2D CAD systems where it is difficult to vi- sualize or model how the components mounted on the final PCB assembly (PCBA) will fit in 3D space when the rigid-flex circuit is in operation. Third, problems with assembly of the folded flex circuit may not be discovered until boards are in pick-and-place (PNP) or during final prototype assembly into the target enclosure. These issues dramatically increase the risk and potential costs of developing a successful prod- uct around a rigid-flex PCB. Rigid-Flex PCB Construction To best understand the problems discussed in this paper it is necessary to offer a very brief overview of typical rigid-flex PCB construction. This subject matter is treated lightly here but a more thorough discussion can be found in the author's guidebook [1] . The most common method of flexible print- ed circuit production is to begin with polyimide (PI) film sheets, typically 2–4 mils in thickness, which are pre-coated with laminated or electro- deposited copper foil, on one or both sides. Laminated foil is adhered to the film using a thin (typically 2 mils) adhesive layer. The cop- per pattern on this flexible substrate is etched using the same process as rigid PCB substrates, using a photolithographic process. Figure 1 il- lustrates the construction of a typical single lay- er flexible printed circuit (FPC). After etching, additional adhesive sheets and PI film layers are laminated onto the FPC to protect the copper, known as coverlays. Components can be mounted on the FPC, with component land patterns (pads) being ex- posed for soldering through openings in the coverlay film. Thus, coverlay also forms a sol- dermask in most cases. Usually, the areas of the FPC that have components mounted require stiffeners, or fully rigid PCB substrate (using FR- RIGID-FLEx PCB RIGHT THE FIRST TIME – WITHOuT PAPER DOLLS continues Figure 1: single-layer FPC construction.

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