1. Introduction

Rigid‑Flex PCB integrates the structural strength of rigid boards and the bendability of flexible circuits (FPC). It is widely adopted in consumer electronics, automotive electronics, medical devices and other industries, serving as the carrier for component mounting and electrical interconnection.

PCB warpage brings a series of manufacturing and reliability issues, including drilling offset, BGA pad cratering, SMT shifting, cold solder joints and connector contact failure. These defects directly lower assembly yield and long-term reliability of end products. For assemblies with SMT and BGA packaging, the industry generally requires PCB warpage to be controlled within 0.5%.

For rigid-flex PCBs with flexible circuits on the outer layer, the inherently asymmetric stack-up leads to uneven thermal stress release from prepreg (PP) shrinkage after lamination, which stands out as a critical process challenge.

Many projects have tried edge copper flooding and replacement of lamination release materials. These methods only slightly improve substrate rigidity and cannot fundamentally eliminate warpage. Stack-up reconstruction to form a near-symmetric layer structure is the root-cause solution.

2. Warpage Mechanism of Rigid-Flex PCB with Outer-Layer FPC

2.1 Stress Imbalance Caused by Asymmetric Stack-up

Rigid-flex PCBs are laminated with core, prepreg (PP), copper foil and other materials with different coefficients of thermal expansion (CTE). Uneven material stress distribution during cooling easily causes warpage.

Take the 4-layer rigid-flex PCB with outer-layer FPC as an example:

- One side is a thin flexible circuit, and the other side is a thick rigid core. Stress is generated when PP shrinks from high temperature during lamination.

- The rigid core is much thicker than the flexible circuit, shifting the neutral centerline toward the rigid side. PP shrinkage stress accumulates on one side of the centerline.

- Asymmetric stress release finally bends the board toward the flexible side and causes warpage.

> Single-sided build-up structure is a typical warpage-prone design. The build-up side has high resin content and large shrinkage stress, while the opposite core side bears less stress, resulting in bending toward the build-up side. Such boards usually suffer high warpage rejection rates and are a well-known manufacturing difficulty for PCB factories.

2.2 Improvement Strategies

Two major approaches are available to address stress release from asymmetric stack-up:

1. Improve substrate rigidity (edge copper layout): Copper foil anchors the base material. Higher copper retention boosts overall rigidity and balances internal stress. However, copper is ductile, and excessive copper flooding may interfere with manual flattening. This is an auxiliary improvement method.

2. Symmetric stack-up reconstruction (root-cause solution): Symmetric arrangement of prepreg offsets curing shrinkage stress on both sides and minimizes warpage from the source.

3. Design of Warpage Improvement Experiments

A set of comparative tests were carried out to evaluate performance from three dimensions: edge copper pattern design, lamination release material selection and stack-up reconstruction.

3.1 Optimization of Edge Copper Layout

We optimized copper layout on Panel/Set borders. Grid or solid copper patterns were added on L2/L3 inner layer Set edges and panel edges to enhance rigidity:

- ✅ Control group (Grid copper): 0.5 mm trace width, 1.5 mm trace spacing, 90° cross grid

- ✅ Solid copper scheme: Solid copper pattern, with 0.2 mm clearance from routing contour to avoid copper exposure after routing.

> Test data: Grid copper group average warpage: 1.62%~2.35%; solid copper group average warpage: 2.19%. The solid copper test coupons show marginally lower average warpage, yet the overall warpage remains above 1.5%. The improvement is limited and fails to meet the 0.5% specification for high-end packaging.

3.2 Adjustment of Lamination Release Materials

We tested different release materials (3-in-1 composite film, high-temperature release film, silicone) and their effects on PP curing shrinkage and warpage release:

- Scheme 1: 3-in-1 composite film (High-temperature PET release film + elastic buffer lamination film + High-temperature PET release film)

- Scheme 2: High-temperature release film

- Scheme 3: Silicone release material

> Test results: 3-in-1 film average: 2.25%; high-temperature release film average: 2.07%; silicone average: 2.24%. Different lamination release materials deliver negligible improvement. All groups stay around 2.0% warpage. Simply switching lamination release materials cannot effectively resolve this type of rigid-flex warpage.

3.3 Stack-up Optimization (Core Improvement Solution)

Concept: Split the rigid core into core + PP, so PP curing shrinkage occurs on both sides of the lamination neutral centerline to form a near-symmetric stack-up. The total thickness of the split core + PP matches the original rigid core thickness, and the thickness of the split PP is kept close to the PP between rigid and flexible layers.

- Before optimization: Flexible sub-board (PI 50 μm, copper 18 μm) + No-flow PP + Thick FR‑4 rigid core (760 μm)

- After optimization: The original 760 μm rigid core is split into a 680 μm rigid core + 80 μm standard PP. The flexible sub-board and No-Flow PP remain unchanged. Copper on one side of the rigid core inner layer is fully etched away to match the symmetric stack-up design.

> ✅ Test data:

> - Before optimization: Average sampled warpage 2.32%, maximum 2.86% per coupon

> - After stack-up optimization: Average sampled warpage only 0.24%, maximum 0.48%. The finished board warpage is below 0.5%, satisfying SMT/BGA high-end packaging control requirements!

4. Test Summary & Engineering Recommendations

- Edge copper layout (grid / solid copper): Minor improvement, warpage still high. Auxiliary method for rigidity reinforcement.

- Lamination release material replacement (release film / silicone / 3-in-1 film): Barely visible improvement. Not recommended as primary solution.

- ✅ Stack-up reconstruction for near-symmetric layer design: Significant improvement, warpage controllable below 0.5%. Root-cause core solution.

👉 Practical engineering advice:

1. For rigid-flex PCBs with outer-layer FPC, symmetric stack-up design shall be prioritized to offset PP curing shrinkage at the stress source. It is the most effective method for warpage control.

2. Edge copper flooding can be applied together to boost rigidity, while the ductility of copper foil and its impact on manual flattening should be noted.

3. Do not rely solely on lamination release material adjustment to eliminate warpage. Avoid allocating excessive trial-run resources on release material tuning.

4. For complex stack-ups with lamination or plug vias, extra copper flooding can be adopted for rigidity improvement, paired with manual flattening as supplementary control.

5. Conclusion

Warpage is a persistent high-risk defect in rigid-flex PCB manufacturing and SMT assembly, directly impacting product yield and end-system reliability. The comparative tests confirm that stack-up reconstruction eliminates the root inducement of warpage and brings the warpage of outer-layer flexible rigid-flex PCB within the specification for high-end packaging.

Juehui Weiye Circuit provides pre-project stack-up evaluation, DFM for manufacturability and customized process solutions for rigid‑Flex PCBs. Serving consumer electronics, medical, industrial control and automotive sectors, we help customers avoid mass-production risks such as warpage and BGA cold solder joints, and improve yield & delivery stability. Contact our engineering team for rigid-flex PCB evaluation, prototype and mass production inquiries.