Flex and Rigid-Flex boards

Please keep in mind the following Design-Rules for your Flex- or Rigid-Flex circuit boards. 

For designers of flexible circuit boards we recommend the IPC-2223 Guideline / Design guidelines for flexible circuit boards, which is available at the IPC online store or in German, at the FED website.

General Design Rules for flexible PCBs

If possible, limit the number of flex layers to 1 or 2, for maximum mechanical flexibility and cost savings.

Pay attention to a symmetrical stack-up of the printed circuit board.

The flex layers continue within the rigid part as inner layers and can there be used for conductor routing.

Distinguish between dynamic (regular) and stable flexion (bend-to-install).

The minimum bending radius is usually between 1mm and 5mm. The dynamic bending stress can only be reliably ensured with single- and double-layer flexible printed circuit boards.

 Dynamic BendSemi-DynamicStable Bend
 BendingDynamic BendfrequentSemi-Dynamicmax. 20xStable Bend„Bend-to-Install“
 LayersDynamic Bend1-2L recommendedSemi-Dynamic1-4L recommendedStable Bend1-10L possible
 Covering*Dynamic BendPI CoverlaySemi-DynamicPI Coverlay or solder-stopStable BendPI Coverlay or solder-stop
 Min. bending radiusDynamic Bend100-150 x h flexSemi-Dynamic> 20x h flexStable Bend10 - 20 x h flex
 Copper type**Dynamic BendRA copperSemi-DynamicED or RA copperStable BendED or RA copper

h = height

* Flexible solde-stop may break or peel off after 5-10x bending

** RA = Rolled copper, suitable for dynamic, flexible applications; ED = Electrolytically deposited copper , only suited for stable and semi-dynamic applications 

Construction examples for flexible PCBs

Flexible circuit board construction examples

Layout guidelines

  • Make track-width and –spacing within the flexible part as wide as possible
  • At best, the transitions from wide to narrow tracks are continually rejuvenated
  • Rasterize ground planes (as recommended also for rigid printed circuit boards)
  • From 2 flex layers, shifted placement of tracks on the PCB top and bottom
  • Make soldering surfaces and annular rings as large as possible
  • Make connections of tracks and solder pads in a tear-drop, rounded style
  • Stiffeners (partial mechanical reinforcements, for example, in the plug-in or mounting area) can achieve final thicknesses of 0.2 mm - 1 mm

Flex

 < 4 layers4-6 layers7-8 layers
 min. distance copper - contour< 4 layers200µm4-6 layers200µm7-8 layers200µm
 min. distance via - copper< 4 layers150µm4-6 layers200µm7-8 layers300µm

Rigid-Flex

ScopeMinimum
ScopeFlex area lengthMinimum4mm
ScopeDistance PTH (Via) <> Flex areaMinimum1.5mm
ScopeDistance NPTH <> Flex areaMinimum0.5mm
Rigid-Flex PCB Design-Aid

Covering of flexible PCBs

Depending on the application, solder-stop or polyimide (PI) coverlay is recommended as a cover for the flexible circuit board. At best, the maximum possible values for bridge and clearance are used.

 PI CoverlaySolder-stop
 Min. bridgePI Coverlay350µmSolder-stop100µm
 Min. clearancePI Coverlay200µmSolder-stop50µm
 ColorPI CoverlayamberSolder-stopgreen
 ApplicationPI Coverlaydynamic, stableSolder-stopsemi-dynamic, stable
  PI CoverlayFPCB covering coverlaySolder-stopFPCB covering solder-stop

Coverlay is not recommended for QFP components unless they are completely exposed!

Pads and Vias on flexible PCBs

Flexible PCB teardrops, anchore, clearance

In general, the copper adhesion in flexible circuit boards is worse than in circuit boards with standard FR4 material. It is therefore recommended to make the pads / annular rings as large as possible. To improve adhesion, anchors and teardrops can be used.

In order to increase the stability of vias on flexible circuit boards, you can implement the following measures:

  • Give anular rings the maximum size
  • Bind vias using teardrops
  • Use anchors to increase the film adhesion
  • Do not place any vias in the bending area

Calculation of the bending radius

The minimum bending radius r results from the desired application (stable/dynamic) and h, the overall height of the flexible part.

Bending radius acc. to IPC-2223

 StableDynamic
 1LStable10:1Dynamic100:1
 2LStable10:1Dynamic150:1
 MLStable20:1Dynamicnot recommended*
Flexible PCB bending radius acc. to IPC2223

*The dynamic bending can only be reliably ensured with single- and double-layer flexible printed circuit boards.

Bending radius examples

Examples of the minimum bending radius of flexible printed circuit boards with assumed thickness (see flexible circuit board layer buildup).

1 layer e.g. 90µm thicknessStableSemi-DynamicDynamic
1 layer e.g. 90µm thicknessmin. ratio (r/h)Stable10:1Semi-Dynamic20:1Dynamic100:1
1 layer e.g. 90µm thicknessmin. bending radiusStable0.9mmSemi-Dynamic1.8mmDynamic9mm
2 layers e.g. 190µm thicknessStableSemi-DynamicDynamic
2 layers e.g. 190µm thicknessmin. ratio (r/h)Stable10:1Semi-Dynamic20:1Dynamic150:1
2 layers e.g. 190µm thicknessmin. bending radiusStable1.9mmSemi-Dynamic3.8mmDynamic29mm
4 layers e.g. 290µm thichnessStableSemi-DynamicDynamic
4 layers e.g. 290µm thichnessmin. ratio (r/h)Stable20:1Semi-Dynamic50:1Dynamicnot recommended
4 layers e.g. 290µm thichnessmin. bending radiusStable5.8mmSemi-Dynamic15mmDynamicnot recommended

Design of bending areas

The bending area should have parallel, equal-width tracks with the same insulation resistance which are perpendicular to the bending line.

Divide wide conductor traces into narrower conductor traces within the bending area.

Flexible PCB bending area 1

Fill open regions in the bending area with blind conductors.

Flexible PCB bending area 2

Ensure a perpendicular path of the conductor traces to the bending axis. Avoid pads plated-through holes in the bending area.

FPCB bending area 3

The bending area may be optimized depending on the application.

Flex PCB frequently or durable bending

Dynamic bending:
spontaneous and frequent bending
Optimization for dynamic bending:

  • drill holes or slots
  • copper edges at the bending location
  • copper stiffeners as bending aids

Stable bending:
single occurrence bending, e.g. "bend-to-install"
Optimization for stable bends:

  • drill holes or slots
  • Contour narrowing
  • Stabilization through more copper

For 2 or more flex layers, please ensure a shifted placement of the conductor traces on the front and back sides of the flexible portion.

2 layer flexible circuit board copper balance

Use curves instead of corners in the conductor trace path.

Flex PCB curves

Ground plane

Flexible PCB ground plane

Continuous ground planes in circuit boards should always be rastered due to the copper balance. This also applies to flexible circuit boards. Especially in the flexible bending area, ground planes have to be rastered, since otherwise they break.

Your layout program normally provides a function for rastering ground planes. Find an example for rasterization in the EAGLE program here.

Processing guidelines for flexible PCBs

Because of the high moisture absorption of polyimide, flexible circuit boards must be dried (approx. 4 h at 120 ° C) prior to the placement and soldering process and processed within 8 hours!

The soldering parameters known from rigid circuit boards can usually be used.

No guarantee! Please always clarify the final parameters with your assembly partner!

Additional information: