Precision-etched circuits with varied trace width across the outline, for temperature uniformity better than ±1 °C on instrument and optical assemblies. Etched Foil Polyimide Heaters are selected when an assembly requires heating a flat or gently curved surface through a quarter of a millimetre of film, where the heater has to add almost no thickness, almost no mass, and reach temperature in seconds. A photo-etched copper or Inconel foil circuit laminated between two polyimide films, 0.2 to 0.4 mm total. The trace width is varied across the outline during design, so the watt density is profiled to the part — denser at the edges, sparser under a thermal mass — rather than uniform across it. This application guide covers concrete end uses, the specification checklist, the protective device the design needs, and where an alternative construction performs better. This construction is built up to 300 x 500 mm (one piece), 0.2 mm thick to your outline; the full capability table is further down the page. It is rated at 0.8 W/cm² in free air and 7.5 W/cm² bonded full-area to metal, and the difference between those two figures is the difference between a heater that lasts and one that does not.
Typical Applications for Etched Foil Polyimide Heaters
The assemblies and sectors where this construction is specified:
- OEM equipment manufacture: designed-in heating on new equipment, specified from the surface, the target temperature and the available power at the design stage rather than retrofitted afterwards.
- Retrofit and equipment upgrades: replacing a failed or unsuitable heater on existing equipment, where the outline, the voltage and the mounting all have to match what is already there.
- Medical diagnostics and analysers: reagent and cuvette warming, sample blocks, incubation chambers and flow cells, where a degree of drift changes an assay result and the heater has to sit inside a cartridge.
- Optics and imaging: lens, mirror and window de-fogging on cameras, sensors and instruments, bonded to the glass around the clear aperture where 0.25 mm of film is all the space there is.
- Semiconductor and vacuum equipment: small chuck, stage and viewport heating inside chambers, where low outgassing and a heater thin enough not to disturb the assembly both matter.
How to Specify Etched Foil Polyimide Heaters for Your Application
- Outline drawing: There is no catalogue size to order from: this part is made to your outline. Send a drawing or a DXF showing the heated area, every hole and cutout, which face is bonded, and where the leads leave the part — the process builds up to 300 x 500 mm (one piece), 0.2 mm thick. Everything else on this list is quoted against that drawing.
- The baseline stack: An etched circuit between two polyimide films, no adhesive, no sensor: the construction to quote against when the assembly provides its own clamping or bonding and you want the thinnest possible part.
- Uniformity, and what it costs: A profiled etch holds a tighter surface uniformity than any wound construction, because the trace width is set point by point on the artwork. State the uniformity you actually need across the heated face: it is the single specification that decides whether the circuit is a simple serpentine or a designed pattern, and it drives the tooling cost more than the size does.
- Thickness in the stack-up: The film is 0.2 to 0.4 mm before adhesive, and adhesive adds roughly 0.05 to 0.13 mm. Where the heater goes into a designed gap, state the total including the adhesive and the tolerance you can accept, because that stack, not the heater, is what has to fit.
- Corner radii and the outline: Polyimide tears from a nick and the tear propagates. Radius every internal and external corner — 1.5 mm or more where the design allows — and keep mounting holes away from the circuit rather than through it.
- Supply voltage: State the supply voltage: this construction is wound or printed for 5 V DC, 12 V DC, 24 V DC, 36 V DC, 48 V DC, 110-120 V AC or 220-240 V AC, and it is built for the one voltage ordered rather than being switchable between them. At low voltage the current is the problem — a 120 W heater at 12 V draws 10 A, and the lead gauge, the connector and the switching device all have to carry it. At mains voltage the insulation, the earthing or double insulation and the creepage distances become part of the construction, and a low wattage at 230 V means a very fine, very long element that is harder to build reliably than its low-voltage equivalent.
- Wattage and watt density: This construction is rated at 0.8 W/cm² in free air and 7.5 W/cm² bonded full-area to metal. Those are not two options: they are the same heater under two cooling conditions, and the second figure is only true while the heat has somewhere to go. Design at the bonded figure and then leave the heater unbonded, air-gapped, or stuck to plastic or foam, and the element runs at roughly 9 times the density its own surface can shed — the insulation reaches its limit in minutes and the failure is permanent. Give the wattage you need and what the heater is bonded to in the same sentence, and the watt density can be checked against the construction before anything is built.
- Temperature limit: The continuous limit is 200 °C, and it belongs to polyimide film rather than to the element. Nichrome, Inconel and fired resistor pastes all survive far beyond it; what fails first is the material holding them, and no derating of the wattage moves that number. State the maximum temperature the assembly can reach under fault as well as the working set point, because it is the fault case that has to stay inside this number.
- Control and protection: This construction does not limit itself in any way. At rated voltage into a surface that has lost contact, run dry or lost its coolant, it goes on taking full power until the insulation fails, so a thermostat or a thermal cutoff sized to the assembly — not to the heater — belongs in the design and in the schematic. Where a fault must not be allowed to repeat itself, a one-shot thermal cutoff goes behind the resettable device, because a bimetal thermostat cycling on a genuine fault will cycle for as long as the power is on.
- Attachment method: State how the heater is held against its surface: free (mechanically clamped or strapped), supplied with a pressure sensitive adhesive backing and a liner, or vulcanised or bonded to a part you send in. Adhesive is convenient and it is also a thermal layer and a temperature limit of its own, so it is a design decision rather than a packaging option.
- Sensor and where it sits: A sensor can be built into the part rather than fitted next to it: NTC, PT100 or PT1000, a thermocouple, or a bimetal device. Say which, and mark its position on the drawing — a sensor under the densest part of the circuit, a sensor at the coldest corner and a sensor next to the load are three different control schemes, and the drawing is where that gets decided.
- Certification of the construction: The constructions behind this product are built and tested to UL 499, IEC/EN 60335, UL 94 V-0, RoHS & REACH, CE Marking, ISO 13485. That is a statement about a construction — its insulation system, its creepage and clearances, its earthing, its marking and its tests — and not a claim that a particular made-to-drawing outline carries an approval of its own. Say at the enquiry which standard your end product is certified to, because it constrains the build rather than being added to it afterwards.
- Leads and termination: State the wire type, gauge, length and exit direction, and the termination — bare, ferrule, ring, spade or a specific connector housing. The lead exit is a mechanical design decision: it is where a flexible heater is most often folded during assembly, and the joint behind it cannot be inspected once the part is built.
Control, Protection and What Happens at a Fault
This construction does not limit itself in any way. At rated voltage into a surface that has lost contact, run dry or lost its coolant, it goes on taking full power until the insulation fails, so a thermostat or a thermal cutoff sized to the assembly — not to the heater — belongs in the design and in the schematic. Where a fault must not be allowed to repeat itself, a one-shot thermal cutoff goes behind the resettable device, because a bimetal thermostat cycling on a genuine fault will cycle for as long as the power is on.
- The control device: A sensor on the load and a controller — on/off where the load has thermal mass and a few degrees of swing is acceptable, PID where the set point has to be reached without overshoot. This is what holds the temperature; it is not what protects the assembly.
- The limit device: A bimetal thermostat or a thermal cutoff, sensing the heater or the hottest point of the assembly, wired so that it interrupts the heater regardless of what the controller is doing. It has to be independent of the control sensor: a broken sensor wire that the controller reads as cold is one of the commonest ways a heating assembly ends up at full power.
- The fault case to design against: Ask what happens if the heater loses contact with its load, if the fluid stops, if the fan stops, or if the controller output sticks on. In every one of those the heater goes on delivering full power into a surface that is no longer carrying it away, and the answer has to be a device that acts, not a calculation showing it would take a long time.
- Where the sensor goes: On the surface whose temperature is in the specification, in direct thermal contact with it. A sensor reading the air near a heated part reads a temperature that part passed through seconds earlier, and a loop closed on the lagging measurement overshoots the leading one every time it starts.
Bonding & Installation Practice
- A polyimide heater must be bonded over its whole area. With very low mass and a bonded rating several times its free-air one, an unbonded PI heater at design power reaches its limit faster than any other flexible construction here — there is nothing in it to absorb the energy.
- Clean with isopropanol, apply from one edge with a roller, and work every bubble out to an edge. On glass and on machined aluminium the bond quality is visible through the film, which is a genuine advantage over silicone during assembly.
- Remember that the adhesive, not the polyimide, sets the working limit of a PSA-backed heater. The film runs to 200 °C; a standard acrylic PSA does not, and specifying a 200 °C duty with a self-adhesive backing quietly caps the assembly at the adhesive rating.
- Respect the minimum bend radius and never crease the film. Wrapping a cylinder is normal; a fold is a fracture in the foil, and a fracture in an etched circuit is an open circuit.
- Anchor the leads at the termination tab. The solder joint between the flying lead and the etched foil is the mechanical weak point of the whole part, and it is inside the laminate where it cannot be inspected.
Common Specification Mistakes with Etched Foil Polyimide Heaters
- Designing the wattage from the bonded watt density and then bonding to FR4, plastic or a partially machined pocket. Polyimide has the largest ratio between its free-air and bonded figures of any construction in this catalog, which makes it the least forgiving of that error.
- Sharp internal corners on the outline. A square cutout in a polyimide heater is a crack starter, and the tear runs into the circuit.
- Putting the sensor next to the heater rather than under it. A 0.25 mm heater has almost no thermal mass, so it responds far faster than the part it is heating; a sensor reading the wrong side of that time constant produces a loop that overshoots every time it is switched on.
- Specifying polyimide for a wet or steam-exposed duty. The film absorbs moisture and hydrolyses under continuous steam, and the failure is delamination rather than an obvious burn.
- Quoting only one of the two watt density figures in an internal specification. They differ by roughly 9 times on this construction, and a figure copied without the bonding condition attached to it is the single most reliable way to destroy one of these heaters.
- Bonding a self-adhesive heater to an unprepared surface. A pressure sensitive adhesive bonds to what it can touch: machining oil, release agent, a rough finish or a cold surface all leave it with a fraction of its contact area, and the heater then runs partly unbonded while being powered as though it were fully bonded.
- Treating a mains-voltage flexible heater as an equivalent of the low-voltage version. It is a different construction: the insulation system, the creepage distances, the earthing arrangement and the element geometry all change, and the safety standard for the end product applies to the assembly it is fitted into.
Where Etched Foil Polyimide Heaters Are Not the Right Choice
- Not for compound curves. Polyimide will wrap a cylinder cleanly, but it will not conform to a dome, a saddle or a doubly curved surface without wrinkling — that is silicone territory.
- Not for continuous steam, immersion or condensing service. The film hydrolyses, and the heater fails by delamination rather than by burning out.
- Not where the heater will be handled, abraded or walked on. At a fifth of a millimetre it has no mechanical protection of its own, and a nick becomes a tear.
- Not above 200 °C, and not above the adhesive rating where a self-adhesive backing is specified — whichever of the two is lower is the real limit of the assembly.
- Flexible is not the same as conformable. This construction wraps a cylinder cleanly and will not take a compound curve without wrinkling — for a doubly curved surface, silicone rubber is the construction that follows it.
- Not for continuous steam, condensing or immersed service. The insulation absorbs moisture and hydrolyses, and the part fails by delamination rather than by an obvious burn-out.
- Not as its own protection. This construction has no self-limiting behaviour, so it is never the right answer for a design that has nowhere to put a thermostat or a thermal cutoff.