This was very thin dimensionally, similar in size and shape to a quarter, with a hole in the center leaving about 1/8th inch of the rim. It was designed for use as an antenna that would fit laminated between two plastic sheets, to make a type of “smart card”. The greatest challenge of this project was in meeting the narrow dimensions required for the coil.
The customer specified electrical criteria were met by using #38 AWG bondable wire which is just under .005” in diameter. The allowable space in which the coil had to fit was .010” and a coil two wire diameters thick would allow for enough turns. However, the inside lead wire, which is the start-of-winding wire, had to cross over the windings, and there was not room for that one strand. Even though it just crossed over from inside to outside, in one area, that one strand actually made the coil three wire diameters thick.
We were able to solve this problem, with an innovative tooling solution from the tool makers, precision work by production, and the use of our CNC controlled winding machines. By winding a coil that effectively started at the outside, wound down to the center of the coil in a width of one wire, like a watch spring or roll of tape, and then wound out again with the same single-wire thickness, we were able to solve this daunting production issue. The result was a coil two layers thick with both the start and finish lead wires entering and exiting from the outside—so no crossover.
