Dee,
Trace width is a function of risk. A happy face button with blinking eyes; 2mil line. A Defense Missile; 8 mil line. The 2mil very short distance like <.2 inch, the 8mil, on flex, about 8 inches. The length on the 2mil is function of PCB process wherein the 8 is more end configuration/usage of part.
Many, many more complexities come to the table; dielectric callouts, robust design, Supplier selection, and the like. Personally I still believe it comes down to annular ring for the process and limiting the number of flexures prior to install. The number of flex layers also plays a large roll in reliability.
I have done sub .005 widths on flex. It is flex-flex not rigid-flex and for short distance, even the cable itself is small and limited layers. To predict your situation: based on 5mil in the flex, the designer didn't have correct input and the vias are to close to the rigid flex area, you have a less than optimum stack up, probably a thickness issue and the pad to PTH size combined with a >90 degree flexure is giving you some intermittent opens.
I would look at the entire configuration from a systems approach perspective, it is probably not one item.
Good luck,
Boston Brad
Bradford Saunders
Packaging Consultant
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