Optical End-Point Detector for Critical Control of Melting or Reflow
Original Publication Date: 1987-Jul-01
Included in the Prior Art Database: 2005-Feb-01
Precise monitoring of the onset of melting and solidification times of a solder ball on a flexible cable is shown in Fig. 1 for laser-soldering. The scheme described here is also applicable for hot-gas soldering or other soldering techniques. Solder reflow is attained by the use of a 4-watt carbon dioxide laser beam which can be switched on and off by a shutter. The on-set of melting and of solidification is detected by a gently focussed helium neon laser beam on the solder surface. Before melting, the non-specular scattered He Ne laser light exhibits a fine speckle pattern. However, the scattered light suddenly shows a different and more coarse speckle pattern at the moment of melting and an equally discontinuous change upon re-solidification.