| abstract |
A heater (20,40,50,70,80) is secured to or embedded in a circuit board (10,30,75,86) and is selectively energizable by current to melt solder in a connection or disconnection operation. The heater is spaced from circuit board contact pads (13,34) by circuit board structure material that transmits the generated thermal energy to the contact pads to melt solder or other fusible material. The heater is a self-regulating heater in the form of a copper substrate having a thin surface layer of magnetically permeable, high resistance material. An alternating current of constant amplitude and high frequency is passed through the heater and concentrated in the surface layer at temperatures below the surface layer Curie temperature. At higher temperatures the current is distributed through the lower resistance substrate to limit further heating. A current return path (21,43,53,73,85) or bus is disposed in closely-spaced parallel relation to the surface layer and establishes an electric field between the substrate and the return bus to bias current in the substrate toward the surface layer. The return bus may be embedded in the circuit board or provided as part of external tooling employed to deliver energizing current. |