A flexible heater circuit is a resistor drawn in copper: the trace is the heating element, and the board is the heater. The design problem is hitting a target resistance on a given area, and keeping that resistance (and the temperature) predictable when the copper heats up. The polyimide base lets the heater wrap around curved parts and survive the temperatures a heater creates, but it does not remove the need to size the trace, protect the circuit, and tell the fab exactly what resistance you want.
This guide covers the sizing math you can check yourself, what changes when copper heats, the material choices that set the temperature limit, sensing and protection, and what to send a supplier. It is general engineering guidance for low-voltage heaters. Mains-powered heaters add safety and certification requirements that are outside this article.
Judgment first: is a flex heater the right tool?
- Use a flex etched-copper heater when you need a thin, conformable heating element on a curved or space-limited part, with a custom pattern, and you can accept the limits of copper as a heating material.
- Consider an etched alloy-foil heater instead when you need very tight resistance control or a much higher operating temperature. Foil alloys have higher resistivity and a lower temperature coefficient than copper, so they stay stable over a wide range.
- Consider a thick-film or wire-wound heater when you need high watt density or temperatures beyond what a laminated flex build is designed for.
If the circuit already needs routing, connectors or sensors on the same flex, a copper heater integrated into the flex is often the simplest option, and that is the case this guide addresses. Our guide to flex PCB thermal management covers the opposite problem, moving heat out of components.
Sizing the heater trace: a worked example
The resistance of a copper trace is R = ρ × L ÷ (w × t), where ρ is the resistivity of copper (about 1.68 × 10⁻⁸ Ω·m at 20 °C), L is the trace length, w is its width and t is its thickness. Power follows from the supply: P = V² ÷ R. So you choose the voltage and power you want, work out the resistance, and then lay out a trace of the right length, width and copper thickness.
This is a calculation for an illustrative case, not a measured result:
| Item | Value |
|---|---|
| Supply voltage | 12 V |
| Heater area | 50 mm × 30 mm (15 cm²) |
| Copper | about 18 µm (roughly 0.5 oz) |
| Trace width and pitch | 0.15 mm trace, 0.30 mm pitch, serpentine |
| Number of 50 mm runs | 100 (30 mm ÷ 0.30 mm) |
| Total length | about 5.0 m (ignoring the turns) |
| Resistance per metre | 1.68 × 10⁻⁸ ÷ (0.15 × 10⁻³ × 18 × 10⁻⁶) ≈ 6.2 Ω/m |
| Resistance at 20 °C | about 31 Ω |
| Power at 20 °C | 12² ÷ 31 ≈ 4.6 W, or about 0.31 W/cm² |
Two things to take from the example. A heater trace has to be long and thin: metres of narrow copper in a palm-sized area, which is why heater layouts are serpentines with tight pitch. And the result is sensitive to the geometry: a ±0.02 mm variation in the width of a 0.15 mm trace is about ±13% in resistance, so the etching tolerance matters. Specify a resistance tolerance and how it is measured, not only the line width.
Copper heats up, and its resistance rises
Copper's resistance increases with temperature at roughly 0.39% per kelvin. For a heater that runs well above room temperature, that matters:
| Rise above 20 °C | Resistance factor | Power at fixed voltage |
|---|---|---|
| +20 K | about 1.08× | about 93% |
| +60 K | about 1.24× | about 81% |
| +100 K | about 1.39× | about 72% |
In the example above, a 60 K rise takes the heater from about 4.6 W to about 3.7 W at the same 12 V. Two consequences: design the power at the operating temperature, not at 20 °C, and expect a copper heater to be mildly self-limiting, which helps stability but means a cold start draws the most power. If you need constant power across a wide range, drive it with a controller rather than relying on a fixed voltage.
Materials: what sets the temperature limit
The polyimide film itself tolerates high temperatures, but a laminated flex is a stack of materials and the weakest one sets the continuous limit. In practice that is usually the adhesive in the lamination or coverlay, not the polyimide. Adhesiveless constructions remove the adhesive layer and raise the limit, at higher cost. Check each layer's datasheet against your maximum operating temperature with margin, and tell the supplier the real temperature, not the nominal one. Our guide to flex PCB materials compares polyimide, PET and LCP, and the guide to coverlay openings covers the covering layer.
Copper weight is a trade-off between resistance and flexibility. Thinner copper gives more resistance per length (good for compact heaters) and bends better; thicker copper carries current but stiffens the circuit. See copper thickness: current versus bend life. If the heater is mounted flat and fixed, the bend-life limits matter less; if it flexes in service, follow the dynamic bend life guide.
Sensing, control and protection
A heater with no feedback runs until it fails. Plan three things:
- A temperature sensor (commonly an NTC thermistor) placed where the temperature actually needs to be controlled, bonded with good thermal contact.
- A controller (on/off or PID) that holds the setpoint and can limit power at cold start.
- Independent over-temperature protection, such as a thermal cut-off or fuse, that works even if the controller or sensor fails. Do not rely on the controller alone.
Corners and tight turns concentrate heat, so round the bends in the trace and keep the pitch uniform. If one region needs more heat, such as an edge that loses heat faster, change the local pitch deliberately rather than leaving it to chance.
Mounting and heat sinking
A heater is only as good as its thermal contact with the part it heats. An air gap under the flex becomes a hot spot that overheats the copper while the part stays cold. Bond the heater to the part with a pressure-sensitive or thermal adhesive suited to the temperature, and use a backing insulation layer on the outside so heat goes into the part rather than the air. Terminations are the other weak point: put a stiffener under solder tabs and connectors and give them strain relief. See the stiffener guide and our flex stiffener service.
What to ask about assembly add-ons
Heater projects often ask the fab for more than an etched circuit: thermistors, magnets, bonded heat spreaders or adhesives. Ask early what is in scope. In one project, a customer ordered a flexible heater finger prototype and asked whether future lots could bond micro magnets at openings on the finger. We cover it below because it shows why this question belongs in the first design review.
Representative project (anonymized)
A customer in Australia ordered a flexible heater-finger prototype board and asked whether later lots could have small neodymium magnets bonded at openings on the finger (about 1.0 mm in diameter and 0.5 mm thick, axially magnetized); the prototype itself did not need magnets. The risks were practical: the magnets were too small to place accurately by hand, the finger was thin so the bond might not hold and a magnet could detach, and handling could damage the finger. We started the prototype order without magnet installation, confirmed with the factory whether magnet bonding was manufacturable, and told the customer clearly that this magnet bonding could not be supported for the current or future lots. The customer acknowledged this and the magnet-free prototype order proceeded. The takeaway is to raise assembly-feasibility questions during the DFM review so the design can change before tooling and orders, instead of discovering the limit later. (Representative, anonymized; details generalized.)
What to send the supplier
- Stack-up: base polyimide, copper weight, adhesive or adhesiveless, coverlay, and any stiffeners.
- Layout: trace width, spacing, serpentine area, and where the terminations are.
- Electrical target: operating voltage, target resistance with a tolerance, a measurement temperature and the measurement points, and the resulting power.
- Thermal requirement: maximum operating temperature, duty cycle, and what the heater is mounted on.
- Sensing and protection: thermistor type and location, and any thermal cut-off.
- Mounting: adhesive type, backing insulation and heat-sink contact.
- Test plan: resistance measurement method (a four-wire measurement is common for low values), insulation test if required, and any thermal imaging or burn-in.
- Out-of-scope check: list any assembly step you expect (magnets, bonded spreaders, thermistor attach) and ask whether it is supported.
- Quantity and ramp: prototype, pilot and production, so tooling and first-article inspection can be planned. See tooling and NRE costs, the first article and PPAP guide and the prototype guide.
Send the package through our custom flex PCB service or start with a flex prototype to validate the resistance and the thermal behaviour before committing to volume.
FAQ
How do I calculate the resistance of a flex heater trace?
Use R = ρ × L ÷ (w × t): copper resistivity (about 1.68 × 10⁻⁸ Ω·m at 20 °C) times trace length, divided by trace width times copper thickness. Then check power with P = V² ÷ R. Always recalculate at the operating temperature, because copper's resistance rises by roughly 0.39% per kelvin.
Why does my flex heater draw less power once it warms up?
Copper has a positive temperature coefficient, so its resistance increases as it heats. At a fixed voltage the current and power fall as the temperature rises, which is why a cold heater draws the most power. Design the power at the operating temperature and use a controller if you need constant output.
What temperature can a polyimide flex heater run at?
It depends on the whole stack, not only the polyimide: the adhesive or coverlay layers usually set the continuous limit. Check each material's datasheet against your real maximum temperature with margin, and consider an adhesiveless construction if you need more headroom.
Do I need a thermistor and a thermal cut-off?
Yes for most designs. A thermistor lets a controller hold the temperature, and an independent over-temperature device protects against a failed controller or sensor. Place the sensor where the temperature actually matters and give it good thermal contact.
Can the PCB supplier bond magnets or heat spreaders to the heater?
Do not assume so. Assembly add-ons depend on what the factory can do reliably, and very small parts on thin flex can be impractical. Ask during the design review which steps are supported, as the project above shows.
Is copper or alloy foil better for a flex heater?
Copper is the practical choice when the heater shares a flex PCB with routing and connectors, but its resistance changes with temperature and the trace has to be long and thin. Etched alloy foils have higher resistivity and a lower temperature coefficient, which gives tighter stability and shorter traces, but they are a different construction. Choose by your resistance tolerance and temperature range.