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June 2015 • The PCB Design Magazine 53 article RIGID-FLEx PCB RIGHT THE FIRST TIME – WITHOuT PAPER DOLLS continues the PCB layout design is cut out of the paper and folded in the way that the flexible portions of the circuit should go in the final product installation. For a more accurate feel of how things will fit, cardboard which approximates the thickness of the rigid portions of the final board should be cut out and glued to the areas of the paper doll which represent the rigid areas of the design. This approach is tedious and time-consum- ing, and it is very difficult to make the paper bend naturally in the same way that PI film of the final product will bend. Therefore, it's diffi- cult to get a clear idea about the fatigue or natu- ral folding properties of the design. Additionally it does not allow for proper checking of the various layer stack regions, and hence there's no guarantee that the design— even if the paper doll fits and folds correctly—is actually manufacturable. To save time and guarantee a proper fit, be- fore the expense of costly prototypes, it is de- sirable to model the rigid-flex design in CAD software. Since the product is initially designed using mechanical CAD (MCAD) tools, it is not a great deal of additional effort for skilled MCAD operators to use sheet metal models to repre- sent the rigid-flex PCB. Sheet Metal Model Method in MCAD In the sheet metal method, the process is relatively straightforward but has many steps. The initial MCAD model of the product is de- signed along with a sheet metal component, which forms part of the assembly. One or more fixed tabs are used to model rigid sections (or flex with stiffener). Figure 5 shows a subset as- sembly of a product where the rigid-flex PCB substrate sits atop a stepper motor model. This is a nice clean way of discovering what area is available for the PCB substrate, but it is still necessary to get this shape into the PCB editor's workspace. Generally, MCAD software packages that model sheet metal have an "un- bend" or "unfold" feature which is usually used for generating the sheet metal stamping out- puts needed for sheet metal fabrication. In this case however we need to generate a model suit- able for import into the PCB editor. The unfold- ed sheet metal model of the board is shown in Figure 4: A paper doll of the rigid-flex design of a stepper motor controller to be mounted in a movable part of the mechanical assembly.