Foldable PCB Manufacturing Guide

Manufacturing Process

The manufacturing process of foldable PCBs builds upon that of traditional PCBs but adds enhanced flexibility control to ensure stable mechanical and electrical performance. Below are the core stages and critical parameters:

1. Substrate Pretreatment – Improve Adhesion and Stability

PI substrates easily absorb moisture, which can affect lamination quality. Portanto, they should be pre-baked at 120°C for 2 hours to remove humidity.
Enquanto isso, the copper surface undergoes micro-etching (Ra 0.3–0.5 μm) to increase surface roughness and promote better bonding with the adhesive layer.

2. Image Transfer and Etching – Precision Defines Reliability

A dry film photolithography process is used, as it’s more suitable for flexible materials than wet film. Exposure accuracy must be controlled within ±0.02 mm.
Acidic etchants such as copper chloride solution are used at a slower rate (around 30% slower than rigid PCB etching) to prevent over-etching that can weaken narrow traces.

3. Coverlay Lamination – Precision in Temperature and Pressure

This step is crucial to maintaining flexibility and durability.
Lamination parameters:

• Temperatura: 180–200°C

• Pressure: 0.3–0.5 MPa

• Tempo: 60–90 seconds
  These settings ensure full curing of the adhesive without bubbles — bubbles can cause delamination or copper damage during bending.

4. Reinforcement Lamination and Forming – Strengthening Rigid Zones

Reinforcement plates (usually FR4 or stainless steel) are laminated in rigid areas under:

• Temperatura: 160–180°C

• Pressure: 0.2 MPa
  Final forming uses laser cutting, which provides smoother edges and prevents stress concentration compared to die punching.

5. Final Testing – Simulating Real Use Conditions

In addition to standard electrical tests (continuity and insulation resistance), special mechanical and environmental reliability tests are required:

• Bending Life Test: Bend radius (Por exemplo, 5 mm), at 10 cycles/min, para 100,000 cycles. The resistance change rate must be ≤15%.

• Environmental Test: 500-hour temperature and humidity cycle from -40°C to +85°C. No delamination or cracking is allowed.

Testing and Quality Assurance

Foldable PCBs (Rigid-Flex PCBs) must undergo comprehensive reliability verification targeting bending strength, lamination stability, and stress tolerance. Even minor internal defects can cause copper cracking or layer delamination during folding.

A robust testing and quality assurance system ensures consistent long-term reliability.

1. Visual and Structural Inspection

Aoi (Inspeção óptica automatizada):
Performed both after inner-layer imaging and final assembly, using high-resolution cameras to detect opens, shorts, missing copper, or misalignment.
For flexible zones, AOI systems use low-tension conveyors to avoid deformation.

X-Ray Alignment Check:
Used to inspect interlayer registration accuracy, buried/blind via continuity, and solder joint integrity.
For multilayer foldable PCBs, X-ray inspection ensures precise alignment and reliable interconnects.

2. Teste elétrico

Open/Short Circuit Test:
Verifies all nets using high-precision probe testers to ensure perfect continuity after repeated bending.

Impedance Control Test:
For high-speed circuits, impedance must stay within ±10% of the design target.
Since Dk and layer thickness variations affect signal quality, tight dielectric control and sampling verification are required.

3. Mechanical Reliability Tests

Dynamic Flex Life Test:
Simulates repeated folding cycles.
Typical standard: ±90° bending for ≥10,000 cycles without open circuits or impedance drift.
Boards using RA copper generally endure higher cycles.

Peel Strength Test:
Measures adhesion between copper and substrate to prevent delamination under stress.
Requirement: ≥0.7 N/mm under 180° peel condition.

Drop & Shock Test:
Assesses structural integrity under mechanical impact during assembly or use.

4. Environmental & Reliability Tests

Thermal Cycling Test:
Cycles between -40°C and +125°C to simulate thermal stress and evaluate layer adhesion.
Usually conducted for 100–500 cycles, followed by functional verification.

Damp Heat Test:
85° c, 85% RH for 168 horas, ensuring the PI film and adhesive maintain stable bonding under humid conditions.

Solder Shock Test:
260°C for 10 seconds × 3 cycles, to verify the heat resistance of pads and surface finishes.

5. Functional Testing (Fct)

After assembly, the final Functional Circuit Test (Fct) ensures full circuit performance under folded conditions.
This includes verification of:

• Signal delay and noise interference

• Power output and power integrity

• MCU programming and functional validation

Mass Production Considerations

After successful prototyping, scaling up production requires addressing the following:

• Material Batch Consistency: Use the same supplier and batch for PI and copper foil to avoid variation in flexibility that affects yield.

• Process Automation: Introduce automated laser cutting and online bending test equipment — manual lamination often reduces yield by 20%.

• Cost Optimization: For non-critical zones, single-sided copper can replace double-sided copper (reducing cost by ~40%). Line width can be narrowed to 0.15 mm where flexibility allows.

Conclusão

Manufacturing a foldable PCB isn’t about making it as flexible as possible, but about balancing mechanical flexibility with reliability and performance.
Different applications — lightweight wearables, high-cycle foldable displays, or high-reliability medical devices — require distinct material, projeto, and process strategies.

By following this guide’s principles, starting from small-batch validation and gradually optimizing toward mass production, you can transform flexibility into a true competitive advantage in your product design.