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56 The PCB Design Magazine • June 2015 sign (rigid and flexible) must be defined (Figure 8) and then assigned to the areas of the board that will contain those layer stack subsets (Fig- ure 9). Once the board regions have had the various layer stacks assigned the bending and folding areas for the final product are defined also (Figure 9). After the board regions and bending lines are defined the flex regions can be folded and examined in the software model to ensure cor- rect form. At this stage it can quickly become apparent if the flexible extents are too short or too long and adjusted accordingly. The components are then laid out, along with physically bulky objects with STEP mod- els—connectors, heat sinks, LEDs, light pipes, and other parts. At this stage it is beneficial to have the final enclosure model imported into the ECAD environment using a STEP file, and interference checks can then be interactively executed in real-time, or a batch processed de- sign rule check (DRC). Interferences can then assist in the proper relocation of components to rapidly converge on a solution. Figure 10 shows the 3D mode view of the sample stepper motor drive board, with flex re- gions folded into the in-situ shape of the final mechanical assembly. The entire PCBA file can be re-exported as a STEP model back into the MCAD software, for final mating with the me- chanical design. The modified process for rigid-flex design without paper dolls is shown in Figure 11. By enabling layer stack regions and folding simula- tion in the ECAD environment, final clearance/ interference checking can take place visually during the design process, making it possible to do "right first time" design of rigid-flex boards every time. At the end of the design process, further time savings are realized in that a STEP file 3D model can be generated from the entire PCBA in both folded and unfolded states, includ- ing partially folded states. This has great ben- efits in terms of design documentation on the MCAD side, but also allows for clear assembly article RIGID-FLEx PCB RIGHT THE FIRST TIME – WITHOuT PAPER DOLLS continues Figure 7: Process flow with ECAD and MCAD tools having separate database.