I think GPs idea was to make the PCB no longer long, but have two smaller PCBs with flat flex between them. Then when the phone bends, the PCBs wont bend but let the flat flex take the stress instead.

> (To see the problem, place a coin on a credit card and bend the card - note that the coin no longer touches across its whole surface)

Now cut the card in half and connect it with half a cm of tape and bend it - the two halves will be flat and you'll get a V shape instead. Put the coin on one side and no problem.

I guess it depends on how much open space is in the phone or if the whole PCB is completely flush with the bent casing.

There are already polymer glue solder alternatives. They are nowhere near as good as solder, however.

The normal solution is to make the PCB stiffer... Fundamentally a long flat thin object is going to be bendy or brittle

What about changing the layout, such that you can introduce many, many voids in the circuit board? If you can divide up the board into many separate "compartments" then each individual segment can be proportionally stiffer and less flexy. (Long, thin things are "bendy" because the material can act as a lever against itself, so many short stubby things can be very stiff locally.)