Flex PCB Trace Calculator
Polyimide | LCP | Rigid-Flex | FPC
Calculate trace width and impedance for flexible PCB designs. Polyimide-based flex circuits require different considerations than rigid FR4 boards—get the right parameters for reliable flexible electronics.
Flex PCB Materials
| Material | Dk | Df | Flexibility |
|---|---|---|---|
| Polyimide (Kapton) | 3.2-3.5 | 0.002-0.003 | Excellent |
| LCP | 2.9-3.1 | 0.002 | Good |
| PET | 3.0-3.2 | 0.003-0.005 | Moderate |
| Coverlay (PI) | 3.3-3.5 | 0.002-0.003 | Excellent |
Flex PCB Design Considerations
Bend Radius
Traces in bend areas experience stress. Keep traces perpendicular to the bend, use curved traces instead of angles, and follow minimum bend radius guidelines.
Trace Geometry
Use wider traces in flex areas for reliability. Avoid sharp corners—use curves. Stagger traces on different layers to reduce stress concentration.
Coverlay vs Solder Mask
Flex circuits use polyimide coverlay instead of solder mask. Coverlay affects impedance— include it in calculations (Dk ~3.3-3.5).
Flex vs Rigid PCB Differences
Flex PCB Characteristics
Key Differences from FR4
• Lower Dk than FR4 (3.2-3.5 vs 4.3-4.5)
• Thinner total thickness typically
• Lower loss tangent than FR4
• Coverlay instead of solder mask
• Adhesive layers affect impedance
Types of Flex Circuits
Single/Double Layer Flex
- • 1 or 2 copper layers
- • Most flexible option
- • Lowest cost flex type
- • Common for simple interconnects
- • Easy impedance calculation
Multi-Layer Flex
- • 3+ copper layers
- • Reduced flexibility
- • Higher density routing
- • Controlled impedance possible
- • Complex stackup design
Rigid-Flex
- • Flex + rigid sections
- • Components on rigid areas
- • Flex for interconnection
- • Eliminates connectors
- • Complex impedance design
Calculate Flex PCB Trace Dimensions
Use our calculator with Dk=3.2-3.5 for polyimide flex. Account for coverlay in your impedance calculations. Work with your flex fab for exact stackup parameters.
Flex PCB FAQ
What Dk for flex PCB calculations?
Use Dk=3.2-3.5 for polyimide base material. Remember to account for coverlay and adhesive layers in your impedance model.
How does bending affect traces?
Bending stresses traces. Use wider traces in bend areas, route perpendicular to bend axis, and follow minimum bend radius (typically 6x thickness).
What copper type for flex?
Rolled annealed (RA) copper is more flexible and fatigue-resistant than electrodeposited (ED). Use RA for dynamic flex applications.
Can I do controlled impedance on flex?
Yes, but it's more challenging than rigid PCB. Work with an experienced flex fabricator. Multi-layer flex and rigid-flex are better for impedance control.
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