Two types of laser have been applied to solder reflow—carbon dioxide (CO2) and neodymium-doped yttrium-aluminum-garnet (Nd:YAG). Both generate radiation in the infrared region with wavelengths of about 10.6 μm from the CO2 laser and 1.06 μm for the YAG laser.

The wavelength of 1.06 μm is more effectively absorbed by metal than by ceramics and plastics; the wavelength of 10.6 μm is normally reflected by conductive surfaces (metals) and absorbed by organics.

The main attributes of laser soldering are short-duration heating and highintensity radiation, which can be focused onto a spot as small as 0.002 in (0.050 mm) in diameter. With these inherent attributes, laser reflow is expected to

■ Provide highly localized heat to prevent damage to heat-sensitive components and to prevent cracking of plastic IC packages

■ Provide highly localized heat to serve as the second or third reflow tool for assemblies demanding multiple-step reflow

■ Require short reflow time

■ Minimize intermetallic compound formation

■ Minimize leaching problems

■ Generate fine-grain structure of solder

■ Reduce stress buildup in solder joint

■ Minimize undesirable voids in solder joint

With these attributes in mind, laser soldering is particularly beneficial to soldering densely packed regions, where local solder joints can be made without affecting the adjacent parts, to soldering surface mount devices on printed-circuit boards having heat sinks or heat pipes, and to soldering multilayer boards.

In addition, it also provides sequential flexibility of soldering different components and enhances the high-temperature performance of adhesives used for mounting surface-mount devices.

With respect to reflow time, laser soldering can be accomplished in less than 1 sec, normally in the range of 10 to 800 ms. The laser can be applied to pointto- point connections through pulsation as well as to line-to-line connections via continuous laser beam scan.

The fine-pitch flat-pack devices have been connected to printed wiring boards using YAG continuous laser beam scans on each side of the package.

Both the use of prebumped solder pads and the direct application of solder paste are feasible. In directly reflowing solder paste, although using spattering and heat absorption problems have been observed, they are not incurable.

To eliminate these problems, the preheating and predrying step is necessary. Location of laser beam impringement is another factor. In addition, compatible properties of solder paste have be designed to accommodate fast heating in relation to fluxing and paste consistency, coupled with the proper design of the equipment and its settings.

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