4 layers Rigid-flex PCB

This board is change from 4L Rigid-flex board. Why change 4L Rigid-flex to be 4L FPC? When manufactured as 4L Rigid-flex board, some PTH vias (not GND) are connecting outside the solder mask opening to the Stainless steel stiffener. Improvement measure: 1. Suggest to plug vias with resin, and then do the solder mask, in this way, those vias without openings will not connect to stainless steel.( no need to modify Gerber) 2. Assess customer’s suggestion to use 4 layer flex works, vias need to be covered by coverlay, which can prevent those vias with openings connect to stainless steel.( need to modify Gerber) Second issue, due to the weight of Stainless steel stiffener, the Rigid’s Breakouts in panel was very weak, allowed boards fall from panel. After change to 4L FPC, the issues improved a lot.