A method and apparatus for activating fusible links on a circuit substrate. The circuit substrate is supported in a fixture which is cooled to a below ambient temperature. Cooling of the circuit substrate decreases the absorption of energy by the substrate, permitting a smaller spot size laser beam having a lower wavelength to be employed for interrupting the fusible links. The substrate is cooled by a refrigeration coil in heat transfer with the fixture holding the substrate. Moisture formation is avoided by placing the substrate and laser source in a controlled atmosphere.