I'm struggling to find some guidelines for the thickness to specify for a polyimide stiffener under a surface mount connector on a flex board. I see lots of call us and we'll help you figure it out but no guidelines or explanations. I want to use one under a small 20 pin surface mount connector both to keep things flat for assembly and to provide some relief for multiple removal/insertion cycles (100 or less).

  • \$\begingroup\$ Do you mean a stiffener on flex PCB on the opposite side of gold contacts, that is inserted into an FPC connector? \$\endgroup\$
    – Oliver
    Aug 25, 2017 at 12:10
  • \$\begingroup\$ I would be interested in that too, but here I'm asking about an actual physical connector. Something like this hirose.com/product/en/products/BM15FR \$\endgroup\$
    – confused
    Aug 25, 2017 at 12:17
  • \$\begingroup\$ Generally speaking, stiffness is dependent on the Young's modulus and increases with the cube of thickness. If you can't find guidelines, maybe measure compare actual samples of working connectors found in the wild. \$\endgroup\$ Aug 25, 2017 at 13:27

2 Answers 2


There is no standard value but the typical answer I receive from chinese fabs like Itead and Elecrow is 0.3mm for a polymide stiffener.

Specifically from the Elecrow team it was suggested to me that if I insisted the stiffener thickness to be 0.6mm I should choose Rigid (FR4) material but normally the thickness is 0.3mm.

Now, when it comes to stiffener thickness for FPC cables (underneath the gold contacts) most datasheets suggest a thickness of 0.2mm for the polymide. (see here and here for 2 examples)


This will be dependent on the connector that is used-- mating/unmating forces, the stiffness of the connector itself, and the size of the connector are all variables that play a factor into the stiffener thickness/material.

Any good connector supplier will be able to provide this information (it's in their best interest after all that you know how to use their product). Often this will be in the datasheet or app notes for the connector used. If it isn't, reach out to the supplier and they should have one of their applications engineers provide the information you need.

From the datasheet for the connector that you linked: enter image description here

Notice all of the notes that are given--the manufacturer has already figured out the failure modes and has recommendations for stiffeners.


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