Advances in Current Rating Techniques for Flexible Printed Circuits Ron Hayes* Abstract Twist Capsule Assemblies are power transfer devices commonly used in spacecraft mechanisms that  require electrical signals to be passed across a rotating interface. Flexible printed circuits (flex tapes, see Figure 2) are used to carry the electrical signals in these devices. Determining the current rating for a  given trace (conductor) size can be challenging. Because of the thermal conditions present in this  environment the most appropriate approach is to assume that the only means by which heat is removed  from the trace is thru the conductor itself, so that when the flex tape is long the temperature rise in the trace can be extreme. While this technique represents a worst-case thermal situation that yields conservative current ratings, this conservatism may lead to overly cautious designs when not all traces  are used at their full rated capacity. A better understanding of how individual traces behave when they are  not all in use is the goal of this research.   In the testing done in support of this paper, a representative flex tape used for a flight Solar Array Drive  Assembly (SADA) application was tested by energizing individual traces (conductors in the tape) in a vacuum chamber and the temperatures of the tape measured using both fine-gauge thermocouples and infrared thermographic imaging. We find that traditional derating schemes used for bundles of wires do not apply for the configuration tested. We also determine that single active traces located in the center of a flex tape operate at lower temperatures than those on the outside edges.  Introduction Electrical power transmission across rotating interfaces (such as in SADAs) is typically accomplished  using slip rings, twist capsules, or wire wraps depending on the degree of rotational freedom required by the application. Slip rings permit continuous rotation in either direction but can be expensive and complex, which can be considered a liability in terms of reliability in some cases. Some applications do not require continuous rotation and therefore can forgo slip rings in favor of the other, more cost-effective options listed above. While wire wraps can be very simple, they are also difficult to adequately characterize in  terms of parasitic torque and life, with these parameters varying widely depending on many factors in the design. A much more common, more predictable, and higher reliability option is the twist capsule. This device makes use of flexible printed circuit tapes to carry the electrical signals between a rotor and stator. These tapes are typically constructed of a single-layer copper conductor layer sandwiched between layers of Kapton film using acrylic adhesive to bond the whole thing together. A critical aspect to the design of any conductor in a space environment is the temperature rise in that  conductor due to Joule (I 2R) heating; this is especially true for flex tapes used in twist capsules. The  standard means for rating conductors in flex tapes has been to use nomographs derived from the electronics industry such as in MIL-P-50884. These techniques leave much to be desired, as experience has shown that in a vacuum the temperature rise in the conductors is highly dependent on the  unsupported length of the flex tape. The heritage rating technique does not take this into account. A more physically representative technique was developed in the 1990’s that took this length into account, and was based on a temperature-dependent finite-difference model (Hayes and Allen). This model has been  used successfully for two decades, and has been tested and confirmed to be conservative in most cases.  Results from a modified version of this model (modified to approximate radiative losses) are shown in  *Honeybee Robotics Spacecraft Mechanisms Corporation, Longmont, CO   /g51/g85/g82/g70/g72/g72/g71/g76/g81/g74/g86/g3/g82/g73/g3/g87/g75/g72/g3/g23/g21/g81/g71/g3/g36/g72/g85/g82/g86/g83/g68/g70/g72/g3/g48/g72/g70/g75/g68/g81/g76/g86/g80/g86/g3/g54/g92/g80/g83/g82/g86/g76/g88/g80/g15/g3/g49/g36/g54/g36/g3/g42/g82/g71/g71/g68/g85 /g71/g3/g54/g83/g68/g70/g72/g3/g41/g79/g76/g74/g75/g87/g3/g38/g72/g81/g87/g72/g85/g15/g3/g48/g68/g92/g3/g20/g23/g16/g20/g25/g15/g3/g21/g19/g20/g23 161