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This is the first time I'm designing flex PCB and I wonder do I need to put "stiffeners" under DFN parts which are soldered on flex PCB? Stiffeners would be on opposite side of flex PCB than DFN parts.

Flex cable is 4mm wide and 200mm long and has 3pcs of 2.5mm x 2.5mm DFN parts.

This is a prototype run and DFN parts will be hand assembled.

Design rules seem to propose using stiffeners but I would like to design the flex cable without stiffeners if possible.

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  • \$\begingroup\$ I'm not gonna make a full answer since I'm not really an expert on (rigid)flex, but from what I have read on the topic before it mostly depends on application and reliability. A rigid section is usually better, but if the maximum bend radius will be low, and only limited number of flexed (eg, a cable that goes into a device and after that doesn't move anymore) it could be fine without stiffner. \$\endgroup\$
    – Joren Vaes
    Aug 21 '20 at 11:20
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FPC curvature causes coplanarity stresses that make solder bonds at risk under vibration or excess curvature.

Use FR4 or PI bonded with CA to underside to achieve mechanical support needed as required.

Also at more complexity thicker FPC may be used with a gnd plane or FR4 or SS for BGA.

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