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Mechanical Considerations of Flexible Circuits

Static and Dynamic Optimization

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Design GuidelinesMechanical Considerations

Static (One-time Bend)o Rule of Thumb, Bend radius for flex to install is 10x the thickness of the

flex this can often be pushed to 5x.o Can use stiffeners to create a natural folding configuration.o Keep stress concentrators well away from bend areas.o A soft adhesive system may “pucker” on the inside of a tight bend radius.

Dynamic (Multiple Bend)o Rule of Thumb, Bend radius for dynamic flex is 100x the flex thickness.o Conductor thickness and width should remain constant in bending areas.o Conductors should be staggered from layer to layer and not stacked on

top of each other to increase flexibility.o Conductor thickness and width should remain constant in bending areas.

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Design GuidelinesMaterial Selection – Type of Copper

Dynamic – Considerations that effect Flex Lifeo Grain structure and grain size, RA Cu. grain structure is best compared to standard

ED grain structure for Dynamic applications.

o Copper surface roughness, smoother is better.

*Comparison of crystal structure – cross-sectional observation of Cu. foil crystals after annealed

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*Crystal structure & high flexibility mechanism

Design GuidelinesMaterial Selection – Type of Copper

Dynamic – Considerations that effect Flex Life

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Design GuidelinesEngineering & Designing FPC

Make the circuit robust to withstand Flexing

Not Recommended

Bending Zone

Preferred Routing

Bending Zone

o Preferred practice is to route conductors perpendicularto bend and foldareas in a single metal layer if possible.

o Eliminate copper plating on the conductors in the flexing area (dynamic region) by utilizing selective (pads plating/button plating‐only) allowing the circuit to have increased flexibility.

o Conductors should be staggered from layer to layer and not stacked on top of each other to increase flexibility.