What is the difference between FPC solder mask and overlay?

Have you ever had trouble choosing solder mask and overlay when designing a flexible circuit boardSolder mask and overlay have the same basic function of insulation, do you know the difference between solder mask and overlay? Can the solder mask and overlay be overlapped for insulation? Do you think flexible solder mask is the same as rigid printed circuit solder mask?

1) Design: Actually, coverlay is widely used in flexible circuit boards. Since most FPCs require the addition of PI/FR4 reinforcement and EMI shielding. However, having BGA pads and dense solder mask opening areas limits the minimum size of the openings and the smallest mesh of material that can then remain between adjacent openings. I would like to share with you the overlay and solder mask that the FPCB covers together. Cover the overlay first, then place the solder mask slightly over the overlay to prevent any gaps in the combined layers. This design places higher demands on our production to ensure your functional and reliable design.

2) Thickness: Why did you choose to use green solder mask instead of overlay? The normal overlay thickness is 27.5/50um. The solder mask thickness is 8/12um. As customers need ultra-thin flexible PCBs. Then, we recommend customers to use solder mask instead of overlay.

3) Color: The most popular color of mulch is yellow, we usually prepare yellow, white, black mulch in stock. Solder mask is available in green, black, white, yellow and other colors. Note that FPC solder mask is completely different from PCB. The cost of FPC solder mask is much higher than that of PCB, because the bending resistance and high temperature resistance of FPC solder mask are better than PCB.