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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