The standard IPC answer is 10 times the thickness of the material. There is a section in IPC-2223 that offers reasonable information on bend radius calculations.
But there are other factors that need consideration when designing a flex circuit for high reliability.
- Grain direction in the copper is important for tight bend radii
- The stiffness of a flex is primarily driven by the copper weight but kapton thickness is also critical
- Where is the bend located?
Here are some other thoughts of how to increase reliability for a flex circuit application.
Reduce overall thickness in the flex area
- Reduce the base copper weight (and the corresponding adhesive thicknesses) or reduce the dielectric thickness.
- Use adhesiveless base materials. Adhesiveless materials will usually reduce the starting thickness of each substrate by 12-25um (0.0005”- 0.0010”) when compared to adhesive based substrates.
- Eliminate copper plating on the conductors in the flexing area (dynamic region) by utilizing selective (pads plating/button plating-only) allowing the circuit to have increased flexibility.
Make the circuit robust to withstand flexing
- Conductors should be staggered from layer to layer and not stacked on top of each other to increase flexibility.
- Conductor thickness and width should remain constant in bending areas.
- Balance the conductor weights and material thicknesses on each side of the neutral bend axis.
- Bend radius of a flex should be approximately 10 times the material thickness and at least 500um (0.020”) away from the plated through hole.
- Incorporate tear stops or reliefs for slits in the circuit. The end of the slit represents a vulnerable point for a tear to start and to propagate.