Introduction
Flexible printed circuit boards (FPCs) enable reliable interconnects and circuits in applications where rigid boards are impractical. Producing high quality FPCs requires specialized fabrication processes tailored for flexible substrates.
This article provides an in-depth look at the end-to-end FPC manufacturing process. We’ll explore the step-by-step sequence from material preparation through final fabrication. Understanding the considerations at each stage allows designers to optimize designs for manufacturability and achieve consistent results.
By the end, the full progression for transforming raw materials into complete FPC assemblies will be clear.
FPC Board Materials
Creating the flex board begins with selecting suitable substrate and coverlay materials:
Base Dielectric
Bonding Adhesives
- Acrylic or epoxy adhesives
- Thermally activated bonding films
Coverlay
- Liquid photoimageable solder mask (LPI)
- Adhesive coated polyimide laminates
Stiffeners
- Polyimide, FR4, aluminum inserts
Materials are certified to IPC specs ensuring consistent quality and performance.
Copper Clad Laminates
Rolls of copper clad flex laminate formed through adhesive bonding:
- Available in single or double sided
- Standard 1/2 to 2 oz copper foils
- Available on quick-turn rolls or panels
- Cut to size for specific designs
Large volume cost savings result from maximizing material utilization.
Inner Layer Preparation
Multilayer FPCs require individually imaging inner layers:
- Copper patterning using lithography
- Etch away unwanted copper
- Strip photoresist masks
- Visually inspect layer quality
- Electrical testing checks shorts and opens
Completed inner layers are interleaved during layup and lamination.
Layup and Lamination
FPC material layers are stacked and bonded together:
- Cut materials to size for each design
- Clean all layers to remove debris
- Precisely align films and foils
- Load into thermal presses
- Apply heat and pressure cycle
- Cool under controlled pressure
Result is a solid laminate with all layers fused into a monolithic board.
Drilling
Holes drilled through the laminated stack:
- Tooling holes for alignment
- Through vias for interconnection
- Depth controlled vias in multilayer boards
- Precise process prevents barreling or tearing
- Deburring cleans up hole walls
Hole walls prepared for subsequent plating process.
Hole Metallization
Coat drilled holes with conductive material using electroless and electrolytic plating processes:
- Electroless copper builds initial seed layer
- Electrolytic copper plates up conductive hole barrels
- Copper thickness from 0.5 to >25 microns
- Optional tin or gold finish over copper
This creates electrical connections between layers through drilled vias.
Patterning
With substrates fully prepared, photolithographic imaging defines circuit conductors:
- Apply photoresist layer onto copper
- Expose with UV through patterning artwork
- Develop to selectively remove resist
- Etch exposed copper regions
- Strip remaining resist after etch
- Repeat for double sided circuits
Result is the complete desired conductor pattern on the flex board.
Solder Mask
Solder mask is applied to prevent solder bridging and protect traces:
- Liquid photoimageable mask (LPI) typically used
- Screens away mask from desired exposed pads
- Cures mask into tough permanent layer
- Optional selective openings for test points
Provides electrical and environmental insulation to the circuitry.
Silk Screening
Printed silkscreen legends help identify components and connectors:
- Ink applied through patterned screens
- Denotes polarity, part numbers, text
- Highly durable epoxy ink resists wear
- White legend on black mask is common
- Also used for board outlines/scoring
Silkscreen guides assembly and identifies the board.
Stiffener Attachment
Optional stiffeners added to reinforce boards:
- Cut metal or laminate layers to size
- Bond in place with adhesive films
- Improves connector durability
- Located only in required high stress areas
Stiffeners prevent flexing damage but increase cost.
Electrical Testing
Each board validated electrically after completion:
- Tests check for short and open circuits
- Validates design connectivity
- Detects any fabrication defects
- Testing may also include loaded capacitance and impedance measurements
Confirms properly functioning boards before shipment and assembly.
Final Processing
FPCs undergo final steps before shipment:
- Route scores for break-apart boards
- V-score flexing joints
- Edge bead removal along routed edges
- Cleaning removes residues
- Package boards to avoid damage during shipment
Resulting finished FPCs are ready for customer assembly.
Conclusion
While requiring tight process control, the sequence of FPC fabrication steps enables reliable flexible printed circuits. Understanding the progression from raw materials through finished boards allows designers to optimize designs for manufacturability. The specialized fabrication processes produce high performance FPCs able to withstand dynamic mechanical environments.
Frequently Asked Questions
Q: What are typical FPC substrate and copper thicknesses?
A: Polyimide dielectric films commonly range from 1 to 5 mils. Copper foil is usually 0.5 to 2 oz (18 to 70 microns).
Q: How many FPC circuit layers can be fabricated?
A: Practical limits are typically around 12 layers. More than 20 requires special processes with limited suppliers.
Q: What minimum trace/space is achievable on FPCs?
A: 3/3 mil lines/spaces are typical on outer layers. 5/5 mil tolerances for buried traces. Even smaller features possible with advanced equipment.
Q: What types of connectors mount to FPC boards?
A: Common connectors are pressure-contact ZIF types or flex-tail soldered terminals into plated through holes.
Q: What are recommended design for assembly guidelines for FPCs?
A: Allow tolerance for misalignment, provide strain relief, keep components small and low-mass, and minimize mechanical stress points.
