bend radiusThe most frequently asked question we receive regarding flex circuits is “how much can I bend a flex?”

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

Broken plated through hole

Broken plated through hole

  • 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.

 

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