The shorter the length, the stiffer the flex.

I have an application requiring two rigid PCBs mounted at 90° to each other. I would like to connect them with a flex zone and use rigid-flex construction. How close can rigid areas be to each other; i.e., what is the shortest flex length between rigid areas?

Most manufacturers can form 0.250" flex sections with no issues, and many can get down to less than half that. There are manufacturing and final-use implications of short flex sections between rigid areas on rigid-flex circuitry. The manufacturing issues affect cost, and the final-use issues could cause premature failure if the specified flex length is too small.

Manufacturing Issues

Alignment/misregistration. How a flex area is created in a rigid-flex circuit is important to understand to grasp the challenges. You probably know that copper-clad polyimide substrates are present in both flex and rigid areas of a rigid-flex, not just in the flex areas. The way the manufacturer makes an area “flexible” is by eliminating all rigid materials in those areas. This is done many ways, depending on the manufacturer, but virtually all include a punching operation to form “windows” in the prepreg adhesive and FR-4 layers. These rigid layers then are aligned to the flex layers, so the future flexible areas are prepreg-free. The smaller these areas, the more critical alignment is. For example, consider an application where the distance between rigid areas is 0.1". If the top prepreg layer is skewed to the right by 0.015" and the bottom prepreg is skewed to the left 0.015", the flex zone is now 0.07", not 0.1". This is a 30% reduction in flex area, which could have a significant impact in the final application. However, if the flex zone is an inch wide or more, a 0.03" reduction is inconsequential. Very short distances between rigid areas can have a significant impact on yields and therefore cost.

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