Flex circuit micro facture midigation

[Steve-Flextrapower]

We are working on a flex circuit embedded in an insole. The components on the flex have been holding up well, but we have persistent problems with micro fractures developing on the traces over time causing signal failure. Being embedded in an insole (sandwiched between layers of foam), the flex is subjected to dynamic forces from the user's foot. Current manufacturing parameters of the flex are in the attached. The opens have been developing in the middle of the traces, not at vias or at the soldered components. The goal is to have the lifespan of the flex last at least 4 months, but it has been lasting only about 4-5 days. Any advice on how to increase the durability would be appreciated.

 

[BradtheRad]

Perhaps if the conducting material is carbon rather than copper? Example, one type of spark plug wire has a flexible string (nylon, fiberglass) embedded with carbon or graphite particles.

Carbon nanotubes are manufactured as wire. Strong, elastic.

To solder carbon is a problem. Connections could be made by flexible conductive paste.

Circuit boards are possible via 3D-printing. For your flexible backing I guess the 'ink' shall be a graphite slurry similar to that which goes into resistors and potentiometers.

To make your own conductive rubber:

www.instructables.com/id/Conductive-Rubber-Make-Touch-Sensitive-Robot-Skin/

To make your own conductive glue, paint, ink:

www.instructables.com/id/Conductive-Glue-And-Conductive-Thread-Make-an-LED/

 

[Steve-Flextrapower]

Perhaps if the conducting material is carbon rather than copper? Example, one type of spark plug wire has a flexible string (nylon, fiberglass) embedded with carbon or graphite particles.

Carbon nanotubes are manufactured as wire. Strong, elastic.

To solder carbon is a problem. Connections could be made by flexible conductive paste.

Circuit boards are possible via 3D-printing. For your flexible backing I guess the 'ink' shall be a graphite slurry similar to that which goes into resistors and potentiometers.

To make your own conductive rubber:

www.instructables.com/id/Conductive-Rubber-Make-Touch-Sensitive-Robot-Skin/

To make your own conductive glue, paint, ink:

www.instructables.com/id/Conductive-Glue-And-Conductive-Thread-Make-an-LED/

Thank you for your suggestions! We were hoping the solution could be something that could be implemented on a commercial scale, but you definitely gave me some things to think about.

 

[marce]

Without an image of the Flex and where the breaks are it is hard to determine a solution. From waht you describe these are stress fractures caused by localised extreme flexing.
Determine the fracture positions and see if it is possible to re-route avoiding this area. Done similar for a project harvesting energy from walking etc. ended up with a strange looking flexi, also the only time I use curved routing is on certain flexible circuits.
Again looking at the design would really be required. If it is under an NDA that makes it hard to post on a public forum.

 

[Steve-Flextrapower]

Without an image of the Flex and where the breaks are it is hard to determine a solution. From waht you describe these are stress fractures caused by localised extreme flexing.
Determine the fracture positions and see if it is possible to re-route avoiding this area. Done similar for a project harvesting energy from walking etc. ended up with a strange looking flexi, also the only time I use curved routing is on certain flexible circuits.
Again looking at the design would really be required. If it is under an NDA that makes it hard to post on a public forum.

Thank you for your reply. We did do some re-routing where there were physical tears in the flex, which helped somewhat, but the micro-fractures keep occurring in various areas. When you were working on your project, did you do a force analysis to determine where and how to re-route your traces? If the flex goes inside the shoe, under the foot, the applied forces could change dramatically depending on the gait of the user.

Unfortunately, I cannot post an image of the flex due to NDA concerns.

 

[dick_freebird]

Have you done failure analysis to say whether
this is conductor fatigue from cycles or from
a strain?

I don't see it said, what the trace material is.
You'd want maximum ductility and fatigue
cycle endurance, I'd think offhand, but it might
be that you want a trace that's -stronger-
than its substrate so the substrate gives, not
the wire?

Might an ink filled with nanoparticles (Au?)
be resilient to mechanical elongation?

 

[senilicus]

Have you considered the use of Stretchable Copper Board (SCB) technology?

**broken link removed**

 

[Steve-Flextrapower]

Have you done failure analysis to say whether
this is conductor fatigue from cycles or from
a strain?

I don't see it said, what the trace material is.
You'd want maximum ductility and fatigue
cycle endurance, I'd think offhand, but it might
be that you want a trace that's -stronger-
than its substrate so the substrate gives, not
the wire?

Might an ink filled with nanoparticles (Au?)
be resilient to mechanical elongation?

Thank you for your reply and suggestions. We have not done stringent failure analysis, but believe the failures are more due to strain than cycles.

The trace material being used in the latest iteration was 1.5oz Cu on a .2mm Polyimide substrate. We had tried lighter weights previously of the Cu and on thinner substrates.

 

[Steve-Flextrapower]

Have you considered the use of Stretchable Copper Board (SCB) technology?

**broken link removed**

Haven't heard of this technology -thanks!! We'll check it out.

 

[marce]

I did the initial layout and a couple of re-runs, I then moved on from the project. One advantage we had was the footwear was pretty solid (boots) so less problematic than say trainers. Unfortunately due to redundancy I never got to know the final outcome.
Something else to think about is how the copper is deposited on the flex, refer to the following tome for more detail...
I use this (as well as other references):

https://www.flexiblecircuittechnology.com/third.php

There are numerous other guides, when I log back onto my work system I can get a better list, just recuperating at the moment, so it wont be for a few days I'm afraid.

--- Updated Aug 16, 2020 ---

do a search for wearable electronics as well, there may be solutions in the info.

 

[dick_freebird]

Copper will work-harden and fatigue pretty easily
if there's flexure.

 

[Steve-Flextrapower]

Have you considered the use of Stretchable Copper Board (SCB) technology?

**broken link removed**

Haven't heard of this technology -thanks!! We'll check it out.

I did the initial layout and a couple of re-runs, I then moved on from the project. One advantage we had was the footwear was pretty solid (boots) so less problematic than say trainers. Unfortunately due to redundancy I never got to know the final outcome.
Something else to think about is how the copper is deposited on the flex, refer to the following tome for more detail...
I use this (as well as other references):

https://www.flexiblecircuittechnology.com/third.php

There are numerous other guides, when I log back onto my work system I can get a better list, just recuperating at the moment, so it wont be for a few days I'm afraid.

--- Updated Aug 16, 2020 ---

do a search for wearable electronics as well, there may be solutions in the info.

Very good resource -thanks! BTW, there is a newer edition of that book here... https://iconnect007.com/index.php?cID=788
Feel better!

--- Updated Aug 16, 2020 ---

Copper will work-harden and fatigue pretty easily
if there's flexure.

I see. Thanks.