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by Sajeel on Fri May 04, 2018 4:34 pm

I have designed my own custom 8x8 matrix LED panel in eagle software, with neopixels and kept leds very close to each other. I have ordered one same kind of panel about an year ago. I want to know few design principles. As these panels are flexible so I want to know some tips and tricks which usually Adafruit uses. I need to know what should be the flexible material (polymide or kapton ??), thickness of copper??, thickness of polymide material ?? Coverlay thickness ??
I am a learner and learning these kind of PCB making stuffs, this would be a great help !


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by adafruit_support_mike on Sat May 05, 2018 12:34 am

Every component you place on a flexible PCB makes the space between its solder joints rigid. That sudden change in flexibility turns the solder joints into stress raisers. The flexible substrate will bend the most at the edges of the solder joints because the material beneath a component can't bend at all.

Solder work hardens easily, so repeated flexing will eventually make it brittle enough to crack. Once microscopic cracks form, the sides act like levers to concentrate force at the tip of the crack, making that metal break too. Sooner or later, a crack will propagate all the way through the metal.

That happens to all metals. The standard test for repeated stress failure is to put a 1" rod into a lathe, apply force to the end, and run the lathe until the rod breaks. The rotation continually flexes the metal when it's on the same side as the applied force, and compresses the metal when it's on the side opposite the applied force. The force itself can be small.. only a few pounds.. it just has to be swapped back and forth over and over again.

We deal with that on our flexible LED matrices by leaving enough space between the LEDs to keep the forces on the joints low. In very broad terms, reducing the distance between pixels by half makes the force applied to the joints about four time stronger.

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Please be positive and constructive with your questions and comments.