We have modelled a few rigid-flex pcbs and we are still developing the process but we do it as follows:
-The flexible "core" (around 0.3 mm) is modelled with a sheetmetal part. It can be created as sheet metal or as solid and converted. This part has 2 instances, one bended and one flatten. Control Y parameter or bend table (quite complex) to have the right bended-flatten lengths.
-Rigid areas (thickness around 2-3 mm) are modeled as independent solid parts
-The sheet metal and the rigid parts are mounted in an assembly with standard mate-align restrictions. This assy has again 2 instances, for bended and flatten.
-The ECAD areas to output are added to the sheetmetal "core" part after the "unbend" feature (this is, flatten). Note that "flat pattern" does not allow to to it that way, but the "unbend" feature does.
Several things can go wrong, mainly the rigid parts not following the constraints when the assy is flatten, but with the right restrictions it works!
Creo does not have the right tool to design these pcbs but I guess they will make it soon, and it will be somehow similar to the sheetmetal module. I think Catia 6 already has it.
Hope it helps