Flexible heaters represent a product technology generically described as thin and flexible heating elements used as attachments to various heat sinks to provide freeze protection, consistent temperatures, and thermal control from -80C to +230C and higher. Options exist in terms of materials and shapes, resulting in custom engineered thermal performance. Thermistors or thermocouples are components for temperature sensing often assembled to heaters as part of a value add option. Soldering is a less common component attachment method as higher temperature heaters can often reflow a solder joint. Similar to flexible circuits, the three primary cost drivers are physical size, volume, and material construction.
The following are some of those considerations when purchasing a heater circuit.
Custom vs. Standard: Standard heater part numbers can be procured in various shapes, sizes, materials and configurations to match a range of thermal requirements. In many cases standard heaters are offered as stocked or catalog parts to be purchased with immediate delivery. There are a number of suppliers that standard heater options, including All Flex. A variation of this theme is an online configuration tool to assist specifying and ordering a “customized standard” heater. Sizes, shapes, and thermal characteristics can be varied within some technical borders. This customized design is offered with either a 5 day (expedited cost) or a 10 day delivery time. Fully customized heater circuits are usually required when special shapes, materials or power requirements are needed and are not available in a standard product format.
In some sophisticated designs, custom heater zones are created within a flexible circuit. This generally limits the heater element to a copper construction which can be limiting due to copper’s conductivity. Selective copper plating is sometimes used to create regions of higher and lower conductivity within a flexible circuit with heater regions.
High Temperature vs Standard Temperature: Most polyimide based heater circuits are specified to perform well with temperatures at about 150C. Top end temperature performance is difficult to state unequivocally as robustness is driven by the thermal characteristics of the heated module. Characteristics of the heat sink, heater attachment methods, and temperature ramp time can all affect how well a flexible heater performs. As temperatures exceed 150 C, alternative materials are recommended for robust performance. All Flex offers a high performance solution that has been tested and successfully performed in applications requiring in excess of 250C.
Polyimide Based vs. Silicone Rubber: Silicone rubber heaters have some advantages over polyimide heaters. Silicone provides better wear protection from abrasion and mechanical damage and provides a moisture barrier enabling heaters to be used in outdoor applications. Barrel heaters are used to keep liquids at controlled temperatures and are generally made of silicone rubber. Silicone rubber tends to be thicker which can impede or slow heat transfer. The added thickness also makes a silicone heater less flexible which is generally limiting when bonding to a tightly curved surface. Finally, silicone rubber can experience some out-gassing which may make it a poor choice in tightly sealed environments or for chemically sensitive applications where out-gassing might cause reliability issues.
Electrical Parameters: Heaters can be specified in terms of resistance, amperage, voltage and power. If two of these parameters are known, Ohm’s Law can be used to calculate the other two. But thermal transfer is a complex equation with ambient temperatures, heat sink materials, and air flow affecting product performance.
A common practice for determining flexible heater design often involves a bit of trial and error experimentation. Fortunately this can usually be done quickly with a sample heater, a representative heat sink, and a variable power source (Variac). Adjusting the power supply to increase or decrease the heater’s voltage is done until the desired thermal performance of the heat sink is achieved. This voltage reading can be used with Ohm’s Law to provide a watt density calculation. On-line calculators make this a simple task. This data provides the information for a custom heater design to achieve the same watt density (i.e. thermal output) with the power source available.
Designers interested in using flexible heater circuits in their application may consult with an application specialist to understand the options. One can use our online schedule a Heater Design Consultation to set up an appointment. All Flex also offers a free online Heater Design Course.
For more information on Heater Circuits please visit the All Flex Heaters Resource Library.