Suggestion 1

How to install a button and a battery holder on a flex PCB:

On a Flex PCB, the opposite side of the welding components must have stiffener;

We suggest that you can try to add the stiffener in the opposite of the button and battery;

Suggestion 2

Component selection recommendation:

Due to the Flex PCB is soft and easy to be folded, while the components are hard, so the components can’t be welded to it directly. If so, the pad will fall off.

We suggest that you should choose SMD package as much as possible.