1. Start with the Right Materials (Foundation Matters)

• Flexible layer: High-grade PI + thick coverlay: Use polyimide (PI) with low water absorption (less than 1.5%) for the flexible section. Pair it with a 25μm+ thick coverlay (not thin film) to seal copper traces—this stops moisture from seeping into the flex bends.
• Rigid layer: FR-4 with moisture resistance: Choose FR-4 rated for “high humidity” use (look for IPC 4101/21 specs). Avoid cheap FR-4 that soaks up moisture like a sponge.
• Adhesive: No ordinary glue: Use epoxy adhesive (not acrylic) to bond rigid and flexible layers. Epoxy forms a tight seal that moisture can’t penetrate, even in 90% humidity.

2. Seal the “Weak Spots” (Where Moisture Sneaks In)

• Rigid-flex transition area: This is where the stiff FR-4 meets the flexible PI, and gaps often form. Extend the coverlay from the flexible section 2–3mm onto the rigid layer, then add a thin layer of solder mask over the overlap. It’s like adding a “weatherstripping” seal.

• Holes and edges: Drill holes (for components or mounting) are moisture magnets. Use plated-through holes (PTH) with full copper plating, and apply solder mask around the hole edges. For the PCB’s outer edges, use a “rounded” cut instead of sharp corners—sharp edges crack more easily, letting moisture in.
  

3. Design for Drainage (Don’t Trap Moisture)

• Avoid closed cavities: Don’t create small, sealed spaces between the rigid and flexible layers. Add tiny 0.5mm “vent holes” in non-critical areas to let moisture evaporate.
• Slope traces away from joints: Route copper traces on the flexible layer so they slope slightly toward the vent holes, not toward the rigid-flex transition. This keeps moisture from pooling at the bond.

4. Finish with Protective Coating (Extra Shield)

• Conformal coating: Spray a thin layer of silicone or urethane conformal coating over the entire board—focus on the rigid-flex transition and component solder joints. It’s invisible, flexible, and blocks moisture while letting the PCB bend.
• Avoid over-coating: Don’t cover connector pins or test points—mask them off first. Over-coating can make assembly or repairs hard later.

Turn These Tips into a Reliable Rigid-Flex PCB

• We source only low-moisture materials (PI, FR-4, epoxy adhesive) and test every batch for water absorption.
• Our production team seals transition areas and edges with precision, no rushed gaps.
• We offer custom conformal coating options, matched to your environment (silicone for flexibility, urethane for chemical resistance).