Flexible circuits do not simply bend. They survive bending because every layer is chosen to fail last. The substrate is almost always polyimide. Kapton and its equivalents hold their properties from cryogenic temperatures to past 200 degrees Celsius, absorb solder reflow without deforming, and stay stable at thicknesses of 12 to 50 micrometres. FR4 is glass cloth in epoxy. Bend it and the fibres fracture. Polyimide has no fibres to break. Copper choice matters more than copper thickness. Electrodeposited copper grows as columnar grains that crack along their boundaries under repeated flexure. Rolled annealed copper is worked into elongated, overlapping grains that slide rather than split, and it survives dynamic bending by orders of magnitude more cycles. Print heads and hinges use rolled annealed. Where the process departs from rigid board work: - Coverlay replaces solder mask. A separate polyimide film is laser cut and laminated over the traces, because liquid photoimageable mask...