Flexible printed circuit boards (FPCs) are highly sensitive to moisture due to their thin substrates and material characteristics. Improper moisture control can lead to defects such as delamination, blistering, and soldering failures during SMT assembly.

Why Moisture Control Is Critical for FPC

FPC materials such as polyimide (PI) and adhesives tend to absorb moisture from the environment. If not properly dried before assembly, rapid heating during reflow soldering can cause:
Delamination between layers
Pad lifting
Blistering or bubbles
Solder joint defects

For high-reliability applications, proper baking is a necessary pre-processing step.

Standard Temperature & Humidity for FPC Baking
1. Recommended Baking Temperature
The industry-standard baking temperature for FPC is:
100°C – 120°C (commonly 105°C or 110°C)
Key considerations:
Avoid exceeding 125°C to prevent material deformation

Maintain stable temperature (±5°C tolerance recommended)

2. Humidity Control Requirements

FPC storage and baking environments should follow strict humidity standards:

Storage humidity: ≤ 30% RH

Workshop humidity: ≤ 40% RH

MSL-sensitive FPC: Follow IPC moisture sensitivity guidelines

Additionally:

Use dry cabinets (≤10% RH) for long-term storage

Minimize exposure time before SMT (ideally < 8 hours)

Baking Time Based on FPC Thickness

The baking time depends mainly on board thickness and layer structure. Thicker FPCs absorb more moisture and require longer baking.

General Baking Time Guidelines:

FPC Thickness Recommended Baking Time (at 105°C)

≤ 0.1 mm 1 – 2 hours

0.1 – 0.2 mm 2 – 4 hours

0.2 – 0.3 mm 4 – 6 hours

≥ 0.3 mm 6 – 8 hours

Additional Factors Affecting Baking Time
Besides thickness, you should also consider:
1. Storage Duration
Long exposure (>1 week): Increase baking time by 1–2 hours
2. Packaging Condition
Vacuum-sealed: Standard baking is sufficient
Opened or damaged packaging: Extend baking time
3. Material Type
Adhesive-based FPC: Higher moisture absorption → longer baking
Adhesiveless FPC: Shorter baking time
4. Multilayer Structure
More layers = more moisture retention
Recommend extending baking time accordingly
Best Practices for FPC Baking
To ensure consistent quality, follow these best practices:
1. Use Professional Baking Equipment
Convection oven with uniform airflow
Temperature calibration required
2. Avoid Overlapping Boards
Ensure even heat distribution
Use trays or racks for spacing
3. Control Cooling Process
Cool down in a low-humidity environment
Avoid immediate exposure to ambient air
4. Bake Before SMT Assembly
Ideally perform SMT within 8 hours after baking
If exceeded, re-baking is recommended
Common Mistakes to Avoid
❌ Excessive temperature causing FPC deformation
❌ Insufficient baking leading to moisture-related defects
❌ Ignoring humidity control after baking
❌ Repeated baking cycles damaging material properties
FPC baking is a critical step in ensuring high-quality PCB assembly. By strictly controlling:
Temperature (100–120°C)
Humidity (≤30–40% RH)
Baking time based on thickness and exposure
manufacturers can significantly reduce defects and improve product reliability.
At PCBAMake, we implement strict moisture control processes and optimized baking profiles to ensure every flexible PCB meets high-reliability standards.