Edited by Stephen J. Mraz
Etched-foil heaters have become the engineer’s choice for advanced thermal management and applications that require heaters but lack space. These heaters are made from metal foil patterned and etched to create a precise conductive element on its surface. The element’s rectangular cross section exposes more of the element’s heating surface and puts more of it in contact with the object being heated, than the alternative, wound-wire heaters.
Advantages of foil heaters
Compared to flexible wound-wire heaters, foil heaters need wider elements to achieve the same resistance and wattage, but the elements can be spaced much closer together. This tight spacing, with traces as close to each other as 0.004 in., translates into even heat distribution, a key reason engineers use foil heaters. Additionally, because the element pattern is created using photolithography, elements can’t contact adjacent elements and short out internally. The photoimaging process also ensures all foil heaters coming off the production line are identical.
Another important feature of foil heaters is that they can be extremely thin. For example, materials such as polyimide (DuPont’s Kapton) can be used to make heaters as thin as 0.005 in., with the foil heating element as thin as 0.0005 in. Being thin lets foil heaters fit in tight spaces, and their flexibility lets them wrap around tight corners and complex shapes.
Foil heaters also let designers add devices to the heaters such as thermistors, fuses, and other electrical components. They can be soldered directly to the heater using traditional soldering methods. The result can be a heater with built-in control logic.
Two of the most commonly used materials for making flexible heaters are polyimide and silicone rubber. The materials serve as both carriers for the foil and as dielectric layers covering the top of the heating element.
Polyimide heaters are typically less than 0.007-in. thick (without the mounting adhesive). This lets them wrap tightly around curved and irregular surfaces and provide short thermal response times. The low profile also lets designers mount heaters in tight spaces, which can be helpful if the heater is added late in the design process. The designer might be able to add the heater without changing the packaging or component layout.
Although polyimide is the material of choice when thinness is a necessity, the adhesive commonly used on polyimide heaters cannot withstand temperatures over 292°F (200°C). Polyimide heaters can only withstand continuous temperatures of 300°F (150°C). But they have quick response times and deliver the desired heat quickly.
Silicone rubber, on the other hand, is durable and withstands continuous temperatures as high as 455°F (235°C). Silicone rubber heaters are typically 0.030-in. thick (without a backing adhesive). This limits its bend radius to approximately 1.5 in. The thicker silicone is also less compliant, so it may not bond well to curved surfaces.
Silicone rubber also adds more thermal insulation compared to polyimide. So a silicone-rubber heater may be slower to reach temperature and a higher wattage heater may be needed.
Making foil heaters
The processes used to manufacture foil heaters are much like those used in the paper/foil-conversion industry with one key exception: Polyimide films are unstable and its dimensions can change in response to changes in heat or humidity. And different lots of the same film can have different thicknesses or inconsistent material stability in the X and Y axes. This injects variability that must be taken into account during fabrication.
Here are the steps in making etched-foil heaters:
Creating the base laminate. The first step is to select and laminate the foil to the base substrate (polyimide in this example). This is often done with a thermosetting adhesive compatible with the two materials. The resulting bond must withstand subsequent fabrication processes, such as chemical etching, as well as any stresses the final application will put on it. For some applications, the bond needs to meet other requirements such as reduced outgassing, UL flame retardancy, and mechanical flexing.
Bonding the foil to the substrate is typically done at elevated pressure and temperature for an extended period of time. The goal is flat and nonstressed laminates with high bond and peel strengths.
Drilling registration holes. Tooling holes drilled into the base laminate serve as registration method for aligning the heater’s layers aligned during fabrication.
Imaging. This step puts the pattern of the conductive element on the base laminate. A photoimageable resist is applied to the foil/polyimide panel. A mask layer is then placed over the resist. This mask is usually created using CAD software. The mask specifies the dimensions and shape of the heater element, which should give it the proper resistance and output.
The resist layer is then exposed to UV light, curing it so it can be used as a chemical-etch resist. This resist protects the heater element while noncured resist gets removed, exposing the foil for etching and its removal.
Etching. The panel goes through a series of chemical etching, stripping, and cleaning cycles to remove foil unprotected by the resist while leaving the heating element bonded to the polyimide. The process is also controlled so that the heater element has the desired thickness and width. Etching creates elements that are within 10% of the proper resistance. This variance is less with wider elements.
Chemicals used in this step depend on the foil being etched. Alkaline-based chemical are used for copper alloys, while other chemicals are used for stainless steel and foils with iron.
The heater’s resistance is tested and the device validated at this step in the fabrication sequence.
Top dielectric lamination. The next step applies a polyimide dielectric film (coverlay) to the panel. This coverlay has thermoset adhesive on one side and has pre-drilled tooling alignment holes and access points. These access points provide wire attachment points, sensor attachment locations, or component mounting positions.
The dielectric layer is aligned with the base panel and pressed onto it so that it conforms without creating voids or air bubbles.
Optional: backside adhesive. To simplify the task of attaching the heater to a heat sink or housing, some customers want a pressure-sensitive adhesive (PSA) placed on the back of the heater.
The most common PSAs are 0.002-in. thick and shipped with release liners customers remove prior to installation. The PSA’s bond strength reaches its final and greatest level about 72 hr after it is pressed into place.
Excising. The final step is to remove the finished part from the panel, a task commonly done with steel-rule dies. These dies can maintain dimensional tolerances within ±0.01 in.
After the heater is removed from the panel, wires, custom marking, and other components can be added.
Flexible heaters are an affordable and effective way to protect devices against freezing, and to control viscosity, condensation, and processes. The thin profile and precise output helps engineers meet a variety of design challenges.
Comparing flexible heaters: foil versus wire
There are two common types of flexible heaters: the older wound-wire devices and foil-based devices. Both use metals with different resistivity and heating characteristics.
Wire heaters use a single or several strands of wire to transfer heat to a tangent point or arc area of its circular heating element. In contrast, the flat surface of foil heaters provide more uniform heating, thanks to significantly more surface area devoted to heat transfer.