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You can find the flex PCB guidelines in the CapSense Design Guide.
The constraints are on the sensor length. For a standard PCB, the trace length can be as big as 300mm whereas for a flex PCB, the maximum trace length can only be 50mm. Other parameters of the sensor design are almost the same.
The point to note in case of flex PCB would be the parasitic capacitance. In order to lower the Cp, the ground clearance can be increased, the trace length can be reduced (as much as possible with 50mm being maximum).
Additionally, if a two layer flex PCB is used, there should not be ground hatch or ground plane right below the sensor. This too can cause increased Cp which may affect performance.
Thanks and regard