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26 – Design rules for FR4 semi-flex PCBs

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 db electronic Semi-flex PCBs are PCBs made entirely of FR4 material, which have a controlled depth-milling in order to be able to bend them. As FR4 is made of glass fibre fabric these designs are not suitable for constant dynamic stress, but for static operations / installations with a required bending called flex-to-install.

The most important criterion is the minimum distance for drill holes, which is adhere to be absolutely necessary.



Picture left – detailed side view



26.1 – Parameters for semi-flex PCBs with 1 copper layer

The length of the semi-flexible area depends on the bending radius required:
• Bending radius 90 degrees – L ≧ 10 mm
• Bending radius 180 degrees – L ≧ 20 mm
Bending for semi-flex PCBs is only allowed at the depth-milled section (copper on the outside).

Example 1: Semi-flexible area is as width as the PCB

db electronic db electronic db electronic db electronic A constriction on both sides supports the bend and relieves the material.



Example 2: Transition from semi-flexible to rigid areas

db electronic db electronic db electronic db electronic As of rigid-flex PCBs here also a milled radius is recommended in order to prevent tearing at the Semi-flexible area. This radius must be much larger compared to the polyimide rigid-flex solution.
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Example 3: Bending area inside the PCB

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db electronic		 db electronic db electronic db electronic For a bend inside the PCB a milled section is requested.
This can save space during installation. Also here a slight constriction would be better to specify the bending of the material.



Relevant information

• Semi-flex PCBs are not designed for multiple bends, but for one-time bending (flex-to-install).
• The semi-flexible area must be bend very carefully. A controlled pre-bending by using a bending tool is recommended.
• The ridig parts of the PCB must be screwed to the housing or a carrier to relieve the bending area.
• The semi-flexible area has usually a thickness of approx. 0.25 mm.
• The longer the semi-flexible area L, the less it will be stressed.
• The copper layers must always be on the outside – see below.

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