Can I place the stiffener on the soldering surface of a flex PCB?

No. Because the reinforcing layer will hinder the flow of solder and the deposition of solder paste, resulting in the inability of the solder joints to be fully immersed, thereby affecting the reliability of the soldering. The correct approach is to place the reinforcing plate on the component installation surface or the non-soldering surface. This can provide sufficient mechanical support without interfering with the soldering process.

If due to structural limitations it is indeed necessary to add a reinforcing plate on the soldering surface, it is recommended to confirm with our engineers during the design stage. PCBfast can provide feasible process optimization suggestions based on the actual situation.