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Optical End-Point Detector for Critical Control of Melting or Reflow

IP.com Disclosure Number: IPCOM000039658D
Original Publication Date: 1987-Jul-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Latta, M: AUTHOR [+4]

Abstract

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.

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Optical End-Point Detector for Critical Control of Melting or Reflow

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. The rapid change in speckle pattern is detected by the use of a photodetector in combination with a suitable "mask". Optimized "signal-to- noise" can be attained by suitable choice of the detection direction, the solid angle, as well as the patterns on the mask. Referring to Fig. 2, the soldering laser is turned on at t=0, and off at t=2.85 sec. The abrupt change in the detected optical signal at t=1.179 sec indicates solder melting, and at t=3.677 sec indicates solder re-solidification. Thus, the molten time is 2.498 sec.

(Image Omitted)

The change of phase of solder from solid to liquid and liquid to solid results in a very abrupt change in the speckle...