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article an introduction to rigid-flex design best practices continues Figure 1: Quote-generated cost comparison of rigid-flex design vs. rigid-cable-rigid PCB assembly. This chart relies on simulated manufacturing costs based on real PCB fab quotes for a fourlayer PCB with two inner flex layers in a rigidflex board. The alternative, rigid boards using flexible cable and connectors between them, is also based on quotes from PCB fabricators. In the latter case, the calculation totals the costs of two separate four-layer boards plus the costs of connectors and cable including assembly costs for both alternatives. This calculated "what-if" scenario does not take into account the improvements in reliability and overall higher product quality of the rigid-flex circuit. Among other reasons, the individual boards with flexible connectors and cable can form electrically "cold" joints, causing malfunction. By comparison, the rigid-flex circuit obviously eliminates these joints. Surprisingly, as soon as the project involves any volume over 100 units, a rigid-flex circuit design becomes the obvious choice. Why? Because rigid-flex designs eliminate connectors, connector assembly, and increase reliability and process yields. 28 The PCB Design Magazine • January 2014 Until recently, when a traditional PCB designer first tackled a rigid-flex design, the available PCB design tools did not support board design in 3D. Existing tools also did not support defining and simulating bends and folds in the flex portion of the design. Worse yet, they didn't even support defining different layer stacks in different parts of the design or even what areas constituted the flex part of the design. As a result, rigid-flex designers had to manually determine how to translate a 3D design of the rigid and flexible sections into a flat, 2-D representation suitable for fabrication. This further required them to manually document all areas that were flexible. They also had to double-check to ensure that they did not place components or vias near the transitions between rigid and flexible portions of the design. This also included many additional rules which were mostly not supported in the PCB design software at the time. As suggested earlier, any successful rigid-flex design requires that the design team work closely with the fabricator. The following Golden