Flexible Circuits: The Basics

Just getting started with flexible circuits?  Here are few “basics” to keep in mind.   Designing flexible circuits is not difficult, but does require adjustments to ensure that you achieve the flexibility and reliability that you are expecting.
Benefits of flexible circuits:

  • Solution to a packaging problem
  • Reduce assembly costs – elimination of connectors and solder joints
  • Replacement for PCB and wires – simplify system design, reduces number of interconnections and eliminates human error.
  • Reduce weight and space
  • Dynamic flexing
  • Thermal Management/ High Temp Applications
  • Aesthetics

Design Considerations:  Conductor Pad Design and Filleting:

  • Pads should have tie-downs (rabbit ears).  Tie-downs are captured by the coverlay to anchor the copper and prevent separation of copper and base materials.
  • Filleting:  All pads should be filleted to reduce stress points during flexing.

Bending and Folding Considerations:

  • Radiused Traces – help to alleviate breaking during folding and bending
  • I-Beam  Effect- when conductors are routed directly on top of one another, stiffness is increased in fold areas.  A better alternative is to stagger conductors, alternating their location to retain maximum flexibility.
  • Fold Lines – tick marks in copper or silkscreen help identify designated location for bending or folding.
  • Circuit Trace Width – this should not change in bend areas and the transition should be at least .030” from fold line.
  • Bend radius – very general guideline of 10 times material thickness will work with most applications.
  • Button Plating – increased flexibility by plating thru holes but not traces.

 Shielding:

  • Solid Copper – this is the most common method of shielding.  This can be done on one or both sides of the circuit or to cover selective conductors.
  • Crosshatched Copper – crosshatching will help the circuit to retain its flexibility and can be done in selective “flexing” areas or entire layers.
  • Conductive Silver – not recommended for dynamic- flexing applications, but can be substituted for copper shield in certain applications.

 Stiffeners:

  • FR4 – normally used to give added rigidity under a connector area.  These can be bonded with pressure sensitive adhesive or thermal set adhesive.  Often used as a carrier panel for automated assembly.
  • Polyimide – often used to give added thickness under conductor fingers to meet ZIF connector requirements.  Can be used to give added strength to high wear areas or to identify “fold” areas.  Outline can blanked or routed at the same time as the flex to meet tight tolerance requirements.
  • Location:  Stiffener and coveraly termaination points should overlay a minimum of .030” to avoid stress points and to reduce the chance of traces breaking.

Please contact us if you have questions or if we can provide any additional information to assist with your flexible circuit needs.
www.omnipcb.com