In the electric heating industry, there are two choices when it comes to low-profile flat heating applications for surface area heating: thin-film and thick-film heating elements. Both styles are designed with circuit layouts that provide unique features traditional heated-wire or core-heater elements cannot -- namely, the ability to provide specific wattage over a location, as well as the ability to vary that wattage over a surface area. Thin- and thick-film heating elements can be designed to operate with just about any input voltage needed, making customization a real strength for thin- or thick-film heating elements.
What are the basic differences between the two heater types? How are these heaters controlled? First, let us define each class of heaters.
Thin-Film Flexible HeatersThin-film flexible heaters can be constructed using silicone, polyimide or a polyester material. While watt density is dependent on material choices, with these materials, the maximum allowable temperature is 500°F (260°C). That temperature would require a heater designed with polyimide FEP class materials. The advantage of a thin-film flexible heater is that the footprint can mimic the shape needed to warm or heat. Thickness can go as low as 0.00315" (0.08 mm) using polyimide. The overall thermal mass of thin-film heaters provides fast heatup and cooldown for exact control of temperature delivery using a thermal control. Maximum watt density for most thin films, with the exception of Mylar, can be as high as 25°F W/in2.
Zonal control can be incorporated into the circuit, providing different watt densities in various strategic locations of the heater. This comes in handy for applications where you may have a mass over a certain location that needs more wattage to heat up and acts like a heat sink as compared to another area that has a thin surface. By designing the heater to have zonal control, the heat delivery pattern can be uniform and consistent.
Material choices are determined based on temperature and cost. The lowest and most cost-effective type is the Mylar heater construction, but this is limited to lower temperatures of around 140°F (60°C).
Silicone has been used for many applications where its properties can be used to act as a blanket to hold heat and protect the surface. Silicone is robust and can withstand many adverse conditions. Polyimide (often better known by the DuPont brand Kapton) provides the thinnest, highest temperature thin-film construction. Certain classes of polyimide and FEP also provide very low outgassing, needed for applications where the heater cannot contribute to changing pressures or air content.
Thin-film heaters can also be designed to limit their rise and protect a run-away condition, and they can be self limited in certain applications.
Thick-Film Heating ElementsThick-film heating elements are classified as solid structures as compared to more flexible thin-film elements. Watt density can be as high as 350°F W/in2 dependent on application.
The high watt density heaters have a relatively thin structure with a nominal dimension of 0.015" (0.381 mm). They provide fast heatup, low mass and, like thin film, the ability to place the heating element in designs where space is at a premium.
Types of thick film include the ceramic thick-film heater, mica heater and the more durable thick-film element that is bonded directly to a metal plate such as one made of stainless steel. Using materials with the qualities of similar thermal expansion creates a substrate where a specific heating element layout can be designed and implemented for high reliability performance. This provides a robust, low profile heater that, in many cases, also becomes the surface that is heated.
Thick-film design also can incorporate a keep-warm trace for a dual-wattage effect, depending on the application.
Thermal ControlThe danger with high watt density heaters is that fast heating also can lessen the time to react to overheat. Controls must be quick to react and protect the heater -- and the application it is inside.
As their name implies, passive thermal controls are controls that do not need electrical power in order to function. Typical devices include bimetal thermal controls and one-shot fuses for over-limit protection. In most cases, these devices are able to control and provide over-limit safety for thin-film elements. For thick-film devices and the higher watt density that can be required for thick-film applications, passive controls may not be enough. With thick-film heaters, the rise in temperature can be very fast, as much as 392°F (200°C) in a few seconds. Thermistors and RTDs are fast responding for control, but most often cannot manage temperature over a 392°F (200°C) level.
One alternative to ensure adequate temperature control with thick-film heaters is to select a design whereby a sensor grid is applied between two fired layers of dielectric. The detection grid is adjacent to a lower temperature (LT) dielectric layer. When micro-voltage leakage appears anywhere on the surface of the heater, the electronic control picks it up and can turn off the heater instantly. As compared to a local thermostat, this provides full area protection. Also, some thick-film heaters can incorporate a fusible link integrated in the circuit for over-limit safety in case the electronic control fails.
Thin- and thick-film heaters continue to stretch the possibilities with integrated systems of thermal control, new materials and smarter electronic controls. These heating technologies are truly digital electronic heating solutions, providing engineers with last minute add-ons where warming may be needed in an existing design with little space, to revolutionary ways to provide instant hot water or steam.