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On a Flex Printed Circuit (FPC) made out of Kapton polyimide, will anything bad happen if I put a VIA in a part of the FPC that has to bend?

  • VIA size: 0.2 mm hole diameter in 0.4 mm copper diameter
  • FPC bend radius: 0.7 mm
  • Kapton thickness: 0.2 mm
  • Copper weight: either 2 oz or 1 oz (I haven't decided yet)
AlcubierreDrive
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    The location of the via inherently becomes a weak point. Given that flexing already creates stress on the copper layers, this is a conduction breakage waiting to happen. You will notice that if there ever are vias on fpc, they are found on the parts unlikely to bend... The copper used would also be the thinnest possible, to minimize risk of fracture. – Anindo Ghosh Aug 18 '13 at 08:33
  • @AnindoGhosh Thanks! Interesting recommendation to use the thinnest possible copper. I was actually leaning toward 2 oz copper because my traces are quite narrow (0.12 mm), and I don't want too much trace resistance, especially in places where the traces are manufactured thinner than nominal (such as acid traps). Given these additional considerations, do you still recommend 1 oz copper instead of 2 oz? – AlcubierreDrive Aug 18 '13 at 09:00
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    In general, one tries to avoid high current over thin traces on FPC: Thermal conduction in polyimide is so low that other than radiation off the surface, heat offtake is poor, ergo hot spots and circuit damage. Try using thicker traces on the current carrying tracks, perhaps by putting them as the outermost ones on each side. Yes, I would still suggest the lowest copper thickness available. A fractured trace won't even carry as much current as thin copper will, evidently :-). – Anindo Ghosh Aug 18 '13 at 10:12

1 Answers1

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The question does not specify the expected number of flexing cycles across the device lifetime. For applications where the FPC will be flexed just once at installation, such as connections between LCD panel and attached backpack controller board, pretty much anything goes. At worst, infantile death of the FPC if it happens, will be detected in a test run.

Assuming multiple flexing cycles:

  • The location of the via inherently becomes a weak point. Given that flexing already creates stress on the copper layers, this is a conduction breakage waiting to happen. You will notice that if there ever are vias on FPC, they are found on the parts unlikely to bend.
  • The copper used must also be the thinnest possible, to minimize risk of fracture. This means 1 ounce copper, or thinner even, if the application can stand it.
  • (Ref: mention of minimizing trace resistance because of high current, from OP's comments) In general, one tries to avoid high current over thin traces on FPC: Thermal conduction in polyimide is so low that other than radiation off the surface, heat offtake is poor, ergo hot spots and circuit damage. Try using thicker traces on the current carrying tracks, perhaps by putting them as the outermost ones on each side. I would still suggest the lowest copper thickness available. A fractured trace won't even carry as much current as thin copper will
Anindo Ghosh
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