The capability extract shows in thirty seconds what runs as standard and what carries a surcharge.
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To the data check →The base film every flexible circuit is built on, and one of the few organic materials that will sit at 200 °C indefinitely without degrading. It bends, it takes heat, and it absorbs water like almost nothing else in the portfolio — which is why the handling rules are stricter than the design rules.
Polyimide film — the material most engineers know by the Kapton trade name — combines three properties nothing else quite matches: it stays flexible at 25 µm thickness, it tolerates continuous service above 200 °C, and its dielectric constant is lower than FR-4. That is why it is simultaneously the flex material, the high-temperature material and, in thin builds, a usable RF substrate.
Two build variants matter in practice. Adhesive builds bond copper to the film with an acrylic layer; adhesiveless builds cast the polyimide directly onto the copper. Adhesiveless is thinner, more dimensionally stable and better in dynamic bending, and it is what we default to for anything that flexes repeatedly — the acrylic adhesive is the weak layer in a bend and the one that expands most in the z-axis.
With rolled annealed copper and a correct radius, dynamic flex applications reach millions of cycles. No rigid laminate approaches this.
Continuous service to 200 °C and short excursions to 260 °C, without the resin softening that limits epoxy laminates.
Dk around 3.5, below FR-4. In thin flex builds that makes controlled impedance achievable with sensible track widths.
| Property | Value | Note |
|---|---|---|
| Continuous service | up to 200 °C | Short excursions to 260 °C |
| Film thickness | 12.5 – 100 µm (0.5 – 4 mil) | Depends on the material; 25 µm is the common default |
| Dk (1 GHz) | 3.4 – 3.6 | Lower than FR-4, stable with frequency |
| Df | 0.002 – 0.008 | Usable well into the GHz range |
| Water absorption | 1.5 – 3.0 % | An order of magnitude above FR-4 — bake before reflow |
| Copper type | RA 12 – 35 µm | Rolled annealed for dynamic bending |
| Build | adhesiveless / adhesive | Adhesiveless preferred for dynamic flex |
| Tear resistance | moderate | Nicks at the outline propagate — radius all corners |
Guide values for pre-selection. Binding values are confirmed per project against your data.
The points below are what most often comes back as a query after data review. Settling them before layout freeze saves a loop.
Polyimide absorbs one to three percent water by mass. Reflow turns that into steam inside the laminate. This is not a precaution, it is a process step — typically 2 – 4 hours at 120 °C immediately before assembly.
Electrodeposited copper has a columnar grain structure that cracks under repeated bending. Rolled annealed has an elongated grain aligned with the sheet and is the only sensible choice for a moving flex.
Polyimide tears from a notch. A sharp internal corner in the outline is a crack initiation site — give every corner a radius, 0.5 mm minimum.
Stagger tracks between flex layers instead of stacking them, and keep the copper as close to the centre of the bend as the build allows.
The film itself costs many times what glass-epoxy prepreg does, panel utilisation is poor because flex outlines nest badly, and the process has extra steps — coverlay lamination, bake cycles, careful handling. The 80 – 150 % surcharge reflects all three.
Adhesiveless for anything dynamic, thin or impedance-controlled. Adhesive builds are cheaper and perfectly adequate for a static flex that is folded once at assembly.
Yes — glass-reinforced polyimide laminate exists and is used where continuous service temperature is high but no bending is needed. It is a different product from the flex film, with FR-4-like handling but a far higher Tg.
Sealed with desiccant, months. Open on a shelf, days before a bake becomes necessary. Treat the moisture barrier bag as part of the product.
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