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Laser-assisted Fluxless Soldering
December 31, 1969 |Estimated reading time: 3 minutes
By Randy Heyler
Debuting in precision optoelectronic assembly, laser-assisted fluxless soldering provides performance and cost advantages when compared to traditional laser welding and soldering techniques.
High-precision optical assembly has always been one of the highest cost elements in the packaging of an optoelectronic or photonic component. The submicron assembly tolerances often require ultra-high precision alignment of each of the micro-optical elements while the device (for example, a laser diode) is "active," or generating a light signal to provide alignment feedback. Furthermore, shifting of the parts during bonding (be it adhesive, laser welding or soldering) can destroy the alignment optimization that was attained so carefully (and expensively). This can result in high scrap or rework costs as well.
Additionally, there are many applications in which either the optoelectronic components themselves (and their coatings) are sensitive to organic elements, or reliability requirements over high temperatures and relative humidity preclude the use of adhesives. This leaves laser welding and soldering as the only means to affect the precision attachment of the components. Consequently, much work has gone into developing these processes and proving their viability from a manufacturing perspective.
The more popular of these two "adhesive-free" processes — laser welding — has been used successfully in a variety of applications, but most ubiquitously in optical amplifier pump lasers, in which a single fiber with a lensed tip is aligned within just a few microns of the laser facet. The laser welding technique, however, requires the addition of metal housings to each optical element (in this case, a cylindrical ferrule around the fiber tip) since the weld typically will penetrate several hundred microns deep, beyond the thickness of any type of metallization coating. Other additional assembly components also are required in a laser welding joint, such as a support channel or "clip," which provide the mechanical structure between the metal ferrule and the baseplate to which it attaches. These additional components add both size and cost, and the resulting bond typically needs some mechanical re-adjustment (through deformation or re-bending) after the welding is complete. Both these factors adversely affect overall process yields and assembly cost.
Alternatively, traditional soldering techniques require either the use of flux (to enhance wetting and reduce oxidation between elements), or are limited in their ability to adjust and maintain the relative position between the components in three dimensions (e.g., die bonding, which allows for adjustment only in one plan, would not be suitable for cylindrical elements such as fibers).
Recently, a new type of precision soldering technology has been commercialized that provides both performance and cost advantages when compared to the traditional laser welding and soldering techniques. This technology, known popularly as "laser soldering" but more accurately as "laser-assisted fluxless soldering," brings together the benefits of an "adhesive-free" process, while lowering component costs, reducing cycle time and improving yields. Furthermore, it enables a much smaller assembly footprint that can lead to further miniaturization of the device package.
In this process, the need for flux is eliminated via the use of a very well-controlled inert-gas atmosphere, in concert with a combination of laser illumination and resistive heating energy to effect a very consistent, strong and oxide-free bond. A minimum of solder volume is required, yet there is enough material to enable the fiber to be aligned actively within an appropriate tolerance range before solidification. Finally, although the shrinkage is fairly predictable, fast reflows are possible to help achieve best-in-class assembly tolerances of ±0.2 µm. Although the fibers must be metallized, no additional structural components are required and a much smaller assembly volume can be achieved.
Assembly cycle times have been shown to be about half of comparable laser welding assembly times, equipment costs are lower (primarily because of the lower costs of the soldering laser vs. a welding laser), and demonstrated yields (>95 percent) consistently are higher as well.
Further details of this new technique were published in the proceedings of the SMTA conference on Fiber Automation held in San Jose, Calif., in December 2002. Copies of the paper or more information are available from the author.
Randy Heyler, vice president of business development, may be contacted at Newport Corp., Fiber Optics and Photonics Div., 1821 E. Dyer Rd., Santa Ana, Calif. 92705; (949) 862-3465; E-mail: rheyler@newport.com.