An Introduction to Rigid-flex Design Best Practices

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Designers increasingly face project requirements for densely populated electronic circuits, including pressures to reduce manufacturing times and costs. To meet these requirements, design teams have increasingly turned to 3D rigid-flex circuits to meet their project’s performance and production requirements. As a first step, rigid-flex designs require closer collaboration between the designer and fabricator than traditional board-and-cable designs. The trade-offs required to produce a successful rigid-flex design translate to a set of design rules the designer can develop with the fabricator’s input. These considerations include the number of layers in the design, materials selections, recommended sizes for traces and vias, adhesion methods, and dimensional control.

In the past, seasoned PCB designers bypassed rigid-flex circuit design by connecting two rigid PCBs with a flexible cable. This approach worked well for short-run designs. However, this approach adds the cost of connectors on each board, the cost of assembling the connectors to the board, and the flexible cable. In Figure 1, the chart maps the cost trade-off between a traditional rigid PCB-and-cable assembly and a 3D rigid-flex design.

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 four-layer PCB with two inner flex layers in a rigid-flex 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.Read the full article here.Editor's Note: This article originally appeared in the January 2014 issue of The PCB Design Magazine.


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