Small, temporary, disposable inserts, called "gap welding preforms," have been proposed for use in attaching electrically conductive ribbons to radio-frequency (RF) electronic circuits. As explained below, the use of gap welding preforms would help to ensure consistency of the ribbon connections and would reduce the time necessary for making the connections.
In the situation that inspired the proposal, gold ribbons 0.025 in. (0.64 mm) wide and 0.001 in. (0.025 mm) thick must be formed and attached to RF circuitry under a microscope up to 0.6 in. (15 mm) deep into RF housings. A ribbon must be manually held in place and formed to the required strain-relief arch between connection points by use of tweezers and other small tools, according to the best judgement of a technician. The ribbon is attached by use of a gap welding machine that includes small probes for placing welds. The technician's view of the ribbon through the microscope can be limited, especially inasmuch as side viewing is usually not possible. Sometimes, this difficult forming-and-attachment procedure inadvertently results in excessive strain-relief arch, which can degrade the RF signal and thus make it necessary to rework the connection.
A gap welding preform would be made of a material that could hold a predetermined shape and could be easily removed after use without causing any deterioration of the welded ribbon or the surrounding material. For example, a gap welding preform could be made of a thickened acrylic compound similar to that used for conformal coating of circuit boards. Thickeners and solidifiers could be mixed into the material to facilitate extrusion into various predetermined shapes. A gap welding preform for a given application would be shaped so that a ribbon laid on it during a forming-and-attachment procedure would end up shaped in the strain-relief arch required for that application.
The steps in the use of a gap welding preform would be the following:
- The preform would be cut to a length greater than the width of the ribbon to be attached.
- The preform would be placed between the connection points in the circuit path.
- The ribbon would be formed over the preform (see figure) and one end of the ribbon would be welded in place.
- The ribbon would be examined to determine whether it was in the desired shape and, if not, would be reformed as necessary.
- The ribbon would be welded in place at the end not previously welded.
- The preform would be dissolved by a suitable solvent (e.g., alcohol) and the ribbon connection would be inspected. The use of dissolution (instead of another method) to remove the preform would eliminate the risk of tearing the welded ribbon, reduce the time needed to remove the preform, and reduce the time needed for inspection.
This work was done by Richard F. Davis and Mark A. Hillyer of Hughes Electronics Corp. for Goddard Space Flight Center. GSC-14103