# @stem_antics on Instagram

- **Type:** Video
- **Original URL:** https://www.instagram.com/p/DbbYYcJvrqM
- **Gondola URL:** https://gondola.cc/posts/68532554-stem-antics-instagram
- **Thumbnail:** https://img.gondola.cc/tr:w-,h-,fo-auto/postThumbnails/55b48723d9.jpg
- **Posted:** 2026-07-30T18:55:22.000+00:00
- **Account Owner:** Stem Antics (@stem_antics) — https://gondola.cc/stem_antics

## Caption

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 is brittle and crazes when flexed.
- Adhesiveless laminates bond copper directly to the polyimide. The acrylic adhesive is the thickest, softest, highest expansion part of the stack, and deleting it improves both bend life and Z axis stability.
- Stiffeners of FR4, polyimide or stainless steel are bonded locally under connectors and component fields, so the flexing happens only where it was designed to.
- Panels move. Polyimide shrinks after etching and lamination, so artwork is pre-scaled against measured shrinkage rather than drawn nominal.

Design rules follow the mechanics. Traces cross the bend perpendicular, never parallel, never with a via in the bend zone. Corners are curved. In double sided flex the copper is staggered rather than stacked, keeping neither layer far from the neutral axis where strain falls to zero.

The strongest structures are not the stiffest. They are the ones that know where to yield.

#stemantics #pcbdesign #electronicsmanufacturing #materialsscience #engineering

## Stats

- **Views:** 8,973
- **Likes:** 111
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## Tags

engineering, electronicsmanufacturing, materialsscience, pcbdesign, stemantics

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