Flexible heaters are low-cost, ultra-thin heating elements designed to conform to the shape of the surface they are applied to. They deliver precise, reliable heat in applications where space, weight, or geometry constraints make traditional heating methods impractical.
Their versatility has made them indispensable in a wide range of industries, from medical devices that require pinpoint temperature control, to consumer electronics where cost-effective heating is a necessity, to aerospace and defense systems that need ruggedized, high-reliability performance.
The thinnest flexible heater designs use etched foil heating elements, typically made from specialty alloys of copper, nickel, and iron. The etched foil pattern is produced to create a custom uniform, precise heating element that can be laminated between an electrically insulating substrate. The two most common constructions of laminated flexible heaters are made with polyimide and silicone.
Polyimide Flexible Heaters
A polyimide heater is a high-performance, ultra-thin heating solution that uses a polyimide film as the substrate, also referred to as Kapton®. The etched foil element is laminated between polyimide layers with a pressure-sensitive adhesive that withstands a wide range of temperatures while offering excellent dielectric strength.
These heaters are prized for their minimal thickness- as thin as 0.25 mm, and their ability to maintain consistent heat output in demanding environments. Polyimide heaters are frequently used equipment where ultra-thin space constraints exist, or outgassing requirements are critical.
Various polyimide flexible heaters in a variety of shapes and sizes.
Silicone Flexible Heaters
Silicone heaters use etched foil or wire-wound elements embedded in a fiberglass-reinforced silicone rubber sheet. This construction offers higher durability, moisture resistance, and mechanical robustness than polyimide, making it a go-to solution for industrial and outdoor environments.
Silicone heaters are thicker than polyimide heaters (typically starting at 1.0 mm), but they can handle more physical abuse, larger footprints, and more complex mounting configurations. These heaters also help improve temperature uniformity and reduce hotspots that are common in higher power applications.
Silicone flexible heater shown next to a penny.
Establishing System Requirements Before Downsizing
Reducing the size of your flexible heater starts with understanding your system’s requirements.
Before beginning any design, engineers should have clarity on:
- Wattage – The total power the heater is designed to operate at, which is a function of the supply voltage and the heater’s fixed resistance.
- Maximum operating temperature – Influences material selection of the substrate, adhesive, and lead wire. The maximum operating temperature also drives the thermal design of the heater.
- Duration at maximum operating temperature – Heaters can survive short-term exposure to extreme heat, meaning that they can operate above their rated temperatures for brief intervals. This duration needs to be clearly defined to properly design the heater.
- Required footprint – The size of the heater is necessary since this drives the heating element shape, size, and design.
- Maximum allowable thickness – Critical for tight enclosures or layered assemblies, the required thickness of the heater will drive its element and substrate design.
- Attachment method – Adhesive backing, mechanical fasteners, or clamping systems can all influence the thickness and flexibility of the eventual heater.
- Wire attachment – Usually, the thickest part of the heater is the wire attachment point and associated strain relief. Designing the heater such that this thick region is away from critical keep-out zones is an important design consideration.
General Minimum Sizing Guidelines
Once the functional and performance requirements are established, designers can work to create a heater within the physical constraints of the application.
Typical minimum sizes and thicknesses for etched foil heaters are:
- Polyimide Heater Thickness: 0.25 mm ± 0.05 mm minimum
- Silicone Heater Thickness: 1.0 mm ± 0.05 mm minimum
- Silicone Heater with UL Rating: 1.5 mm ± 0.05 mm minimum
- Wire Strain Relief (Epoxy): ~3.0 mm minimum
- Wire Strain Relief (Silicone Patch): ~2.0 mm minimum
- Polyimide Heater Footprint: 15 mm × 15 mm minimum
- Smallest Wire Size: 22 AWG (~1.7 mm diameter)
If your application’s maximum allowable thickness is close to these limits, careful coordination between your design team and the heater manufacturer is essential.
Ultra-thin Polyimide Heater with 22AWG wire and Silicone Patch Strain Relief.
Strain Relief and Wire Size Considerations
When engineers push for the thinnest possible flexible heater, wire, and strain relief often become the true limiting factors. Even if the heater element itself is just a fraction of a millimeter thick, the addition of a standard 22 AWG lead wire with epoxy strain relief can increase the height to around 3.0 mm.
Switching to a silicone patch strain relief can save about a millimeter, but it may come with trade-offs in durability or attachment strength. Reducing the wire gauge can also help, though at the cost of higher electrical resistance and reduced current capacity.
Maximum Size Capabilities
While this article focuses on reducing the flexible heater size, it’s worth understanding the upper limits. For etched foil designs, the maximum size is determined by the foil panel dimensions used during manufacturing. When the application calls for a heater much larger than a single panel, wire-wound heating elements offer an alternative. In this method, long runs of heating wire are laid into multiple silicone panels, which are then stitched and vulcanized together. This allows for heaters exceeding one meter in length, ideal for industrial process heating or frost prevention on large surfaces.
Summary
Reducing the size of a custom flexible heater involves a careful balance between electrical performance, thermal efficiency, and mechanical design constraints. Polyimide and silicone are the two most common materials used, with polyimide offering ultra-thin profiles for compact spaces and silicone providing enhanced durability for demanding environments.
Engineers must first define key system requirements such as wattage, operating temperature, footprint, and thickness to guide the design. Wire gauge, strain relief, and attachment methods often determine the minimum achievable dimensions. While heater elements can be extremely thin, wire terminations typically set the height limit.
Working closely with the flexible heater manufacturer ensures the design meets all performance needs while staying within the required size constraints.
Key Takeaways
- Optimize wire and strain relief design: Choose lower-profile strain reliefs or smaller-gauge wires where feasible.
- Select the thinnest suitable substrate: Polyimide offers the slimmest profile; silicone offers durability and uniform heating at a slightly greater thickness.
- Carefully design the heater power: Increasing watt density can make a more powerful heater, but it can also drive the heater to a premature failure. reduce footprint but must be balanced against maximum temperature and heat uniformity.
- Evaluate attachment methods: Adhesive-backed heaters may eliminate mounting hardware, saving space.
- Work with your manufacturer early: Collaboration in the design stage allows for creative solutions that minimize size without sacrificing performance.
