Wednesday, September 6, 2017

Polyimide Thermofoil Heaters Used for Space Applications

Thermofoil heaters of an all polyimide (adhesive-less) construction are used for high reliability space applications. They are ideal for extreme temperature fluctuations and have high tear and tensile strength. The specifications for the design and manufacture of these heaters are rigidly controlled, and rightly so. These heaters often perform critical heating application on satellites and other space craft.

Construction

A polyimide (Kapton) is used as the heater base material, also known as the substrate or mounting surface. A second polyimide layer over the heating element provides a protective enclosure.

Heater Element

The heating element is made of a Ni-600 Inconel (nickel-chromium-iron) alloy. Single or dual resistive elements may be used in the design, but the element must be must be of a single layer in cross-section. The element are normally an etched foil design with uniform in cross-section, and they have a minimum trace width of 0.010 in. (0.0254 cm) by design. Spacing between foil traces is tightly controlled and must not be less than 0.010 in. (0.0254 cm). The spacing between the outer foil trace and the heater edge (border trim) also cannot be less than 0.010 in. (0.0254 cm).

Lead Wires

Lead wires must be a minimum of 26 gauge when using high strength copper alloy conductors, and 24 gauge for all other copper conductors. Lead wire insulation must consist of polyimide (Kapton), polytetrafluoroethylene (PTFE/Teflon), or ethylene-tetrafluoroethylene (ETFE) materials.

Lead Termination


The termination of the lead wire has to be welded and contain a minimum of two weld points between the lead wire and landing bond pad. Lead wire terminations also need to be enclosed in a hardened Hysol epoxy potting, in order to secure lead wires to the heater so that lead pull stresses are not transmitted to the weld joint.
Power Rating

Polyimide thermofoil heaters used for space have a maximum power rating of 4.5 W/in2 (0.7 W/cm2) when suspended in still air at 25°C, although this specification is for test purposes only and is not indicative of the maximum power rating in application (with heater mounted to a heat sink). Actual rated power (or voltage) are specified in each application.
Visit this web page to view or download the entire NASA General Specification for Thermofoil Heater, All- Polyimide, Space Applications.