When you request a quote for a flexible printed circuit (FPC), one of the first technical decisions your supplier will ask you to make is deceptively simple: which base film should the circuit be built on — polyimide (PI) or polyester (PET)? The choice affects thermal performance, flexing endurance, assembly compatibility, and often 30–50% of your total circuit cost. Pick the wrong one, and you either overpay for performance you don’t need or field-fail a product you can’t afford to recall.
This guide compares the two materials from a buyer’s perspective, so you can make the call — or ask your supplier smarter questions before you commit to a design.
What Polyimide and PET Actually Are
Both materials are thin polymer films used as the substrate (and usually the coverlay) of a flexible circuit, but their properties diverge quickly.
Polyimide (PI) is the industry-standard film for flexible circuits. It retains its mechanical and dielectric properties across an extreme temperature range (roughly -269°C to +400°C short-term, with continuous ratings around 200–260°C depending on the grade). It is dimensionally stable during high-temperature assembly, which is why PI circuits survive reflow soldering and remain the default for any product built with standard SMT processes. It also supports fine-line etching — at Xingbeihai, our polyimide circuits routinely achieve line width and spacing down to 2 mil (0.0508 mm).
Polyester (PET) is the budget alternative. It offers good electrical properties and reasonable flexibility at a significantly lower material cost, but it softens well below soldering temperatures. PET circuits typically cannot pass through reflow; they are assembled with low-temperature conductive adhesives, crimped z-axis connections, or heat-sealed connectors instead. That constraint shapes everything downstream: component choices, assembly process, and field reliability.
Polyimide vs PET: Side-by-Side Comparison
| Property | Polyimide (PI) | Polyester (PET) |
|---|---|---|
| Continuous service temperature | ~200–260°C | ~80–110°C |
| Reflow / wave soldering | Yes | No (low-temp assembly only) |
| Flexural endurance (dynamic flex) | Excellent | Moderate; best for static or low-cycle bends |
| Tensile strength & tear resistance | High | Moderate |
| Moisture absorption | Higher (~1–3%) | Lower (<0.5%) |
| Dimensional stability at fine pitch | Excellent | Limited |
| Relative material cost | Higher | Lower |
| Typical applications | Automotive, medical, phones, wearables, industrial | Low-cost consumer goods, membrane switches, disposable devices |
When Polyimide Is the Right Choice
Choose polyimide when any of the following is true about your product:
- The circuit will be soldered. Any standard SMT or reflow process effectively mandates PI. PET will deform at reflow temperatures.
- The product sees heat. Under-hood or in-cabin automotive FPC applications, industrial power electronics, and LED lighting all push ambient or self-generated temperatures past PET’s comfort zone. Automotive-grade circuits also typically require the tight tolerance and traceability of an IATF 16949-certified process.
- The circuit flexes repeatedly. Hinges, folding phones, printer heads, and moving modules need PI’s superior flexural fatigue life.
- You need fine lines or controlled impedance. High-density and high-speed designs rely on the dimensional stability of PI during etching and lamination. Our engineering team regularly produces 2 mil traces on polyimide for compact modules.
- Long field life matters. Medical devices, industrial equipment, and anything with a warranty worth defending should be on PI.
When PET Makes Sense
PET is a legitimate engineering choice — not just a corner to cut — when:
- Your product is cost-sensitive and low-temperature: toys, simple consumer gadgets, single-use or disposable electronics, and membrane switch panels.
- The circuit is static or flexed only a few times in its life (install-and-forget).
- Assembly uses crimped or heat-sealed connectors rather than solder.
- The operating environment stays below ~80°C with margin.
In these niches, PET can cut substrate cost meaningfully, and a good manufacturer will tell you honestly when you’re in this territory rather than selling you PI reflexively.
Cost: Look Past the Film Price
Material cost is the obvious difference, but total cost of ownership is what matters:
- Assembly cost: PI circuits run through standard reflow lines. PET requires special low-temperature processes, which can erase the material savings at volume.
- Yield: PI’s dimensional stability improves yield on fine-pitch designs; PET struggles as density increases.
- Field failure cost: a PET circuit in a hot enclosure is a reliability time bomb. Compare the cost of one field return batch against the PI premium.
Five Questions to Ask Before You Commit
- What is the maximum continuous temperature the circuit will experience — including nearby heat sources, not just ambient?
- How many flex cycles will the circuit endure over its lifetime, and at what bend radius?
- What assembly process will be used — reflow, wave soldering, or connector-only?
- What trace width and spacing does the design require?
- What’s the real cost of a failure in the field?
If your answers point in different directions, ask your supplier for a design-for-manufacturing (DFM) review before the tooling is cut. Swapping substrate materials after artwork is committed is expensive; choosing correctly upfront is free. If you’re still weighing FPC against rigid boards entirely, see our earlier guide FPC vs Rigid PCB: How to Choose the Right Circuit Board for Your Product.
Need Help Deciding?
Shenzhen Xingbeihai Electronics has manufactured flexible circuits since 2008, with ISO 9001, ISO 14001, and IATF 16949 certification, 2 mil fine-line capability, and a monthly capacity of 25,000 m². Our engineers review customer layouts daily and can recommend the most cost-effective material stack for your application — whether that ends up being polyimide, PET, or something in between.
Browse our FPC product portfolio or contact the team for a material selection review and a quote within 24 hours.