The main challenge is the mounting of the vibration motors on the flexible PCB. Becuase the vibration motor is a constantly moving component, soldering the motor on the flexible pcb by any means will likely fail very quickly. My solution to this is to use a spring loaded contact connector to connect the motor to the PCB. However, the contact point between the motor and the spring contact will need to be held at a constant distance to maintain contact. So I designed a custom bracket to hold the motor and maintain its vertical distance to the board. I also designed the flexible PCB to be a rigid-flex PCB, meaning that there are four rigid sections within the flexible PCB that can support the stress of the motor contact point.