Flex Circuit Tear Protection

[perts]

I have a flex a circuit that experiences a tear during exterme stress conditions. Is there something I can do to the flex circuit to prevent it from tearing without redoing the design. I am not in the position to do a relayout or change the geomoetry of the flex circuit.

Is there something that I can put on the flex circuit to help prevent the tear from starting.

1. some glue/rtv that I can put on the circuit
2. put tape/nomex on the flex circuit to prevent tear
3. other ideas.

any ideas would be appreciated.

 

[Compositepro]

Laminate it with fiberglass reinforced adhesive tape.

 

[3DDave]

It's tearing because the strain in the material is too much. The strain is a result of deformation. The application of other materials can work if they can limit the deformation, but if the amount of flex is part of the design applied materials may just move the location of the greatest strain and cause tearing somewhere else. If the reinforced part is forced to deform the same amount as before the flex circuit will still tear but the force will be higher to also deform the applied material.

There will be a stress concentration where the reinforcement end so it's important to have a strain relief at that location.

Unfortunately the effective correction is to redesign the path for the flex circuit to decrease the amount of deformation and therefore the amount of strain.

 

[IRstuff]

How often does this stress occur? If often, you may also have issues with work hardening of the copper traces, which may break or open over time, even if you deal with the flex tape itself breaking.

Tearing is problematic, since that implies way more bending or flexing than would result in a reliable system. I agree with all of 3DD's other comments

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[3DDave]

IR - I am reminded of some buds of mine who designed a flex board - a 5 bonded layer flex board. Guess what doesn't flex when it has 5 layers of copper?

 

[Comcokid]

Is the tear occurring at the end of a slit feature in the flex? Stress will concentrate and may result in a tear. Slit features should end in a larger circular radius, or a large circular feature so stress cannot concentrate at a point.

 

[IRstuff]

Yeah, our's was only a single layer but our system was intended for around 35 million cycles, and our first iteration only lasted about 300,000 or so. We had to redo the flex cable with oxygen-free copper, which supposedly had sufficient resistance to work hardening, but it has a higher conductivity to start with, so maybe that.

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[MacGyverS2000]

Does the tear start in a location where you can punch a hole in the poly?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[perts]

Yes, it starts where a slot is punched.

 

[biff44]

so can you stiffen the area around the punched hole? maybe have a small picture frame of copper around the hole? Glue on a 5 mil picture frame of G10 board?