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 -80°C to +230°C 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, so brazing and welding connection processes have been adopted. Like flexible circuits, the three primary cost drivers are physical size, volume, and material construction.
The following are some of those considerations when purchasing a flexible 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, these standard heaters are offered as stocked or catalog parts to be purchased with immediate delivery. Several suppliers offer standard heater options. An alternative for a higher level of customization is a configuration tool used to specify and order a “customized standard” heater. This allows size, shape, and thermal characteristics to be varied within some technical borders. 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.
As design sophistication increases, multiple heat zones are defined within a single heating element. This helps adjust for heat loss around the edge of a heat sink or create unique thermal properties for a specific application. Additional function and complexity can occur as custom heated 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
While medical instrumentation applications targeting 98.6°F are common, most polyimide based heater circuits are specified to perform well with temperatures at less than 150°C. Top-end temperature performance is difficult to state unequivocally as performance robustness is driven by the thermal characteristics of the heated module. Characteristics of the heat sink, heater attachment methods, element pattern, and temperature ramp time can all affect how a flexible heater performs. As temperatures exceed 150°C, alternative materials are recommended. Flexible polyimide based heaters operating at 250°C offer high performance in an increasing array of applications.
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 delivers 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 limiting when bonding to a tightly curved surface. Finally, silicone rubber can experience a degree of out-gassing which may make it a poor choice in tightly sealed environments or for chemically sensitive applications where contamination might cause reliability issues.
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 with some 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.
Alternatively, finite element analysis (FEA) is used to model thermal performance sans sample parts. Collection of material properties for data input, combined with sophisticated software and engineering time, are input variables. This data provides an accurate model of an application’s thermal characteristics with the heat output from a customized flexible heater.
Dave Becker is vice president of sales and marketing at All Flex Flexible Circuits LLC.