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  • Rare-Earth Solders Make Better Bonds

Photonics Spectra
May 2002
Stuart M. Hutson

Diamonds may be "forever," but the bonds they form with metal solders often are not. While typical solders can form strong bonds with metals, they adhere weakly to oxide surfaces such as diamond, quartz or fused silica. However, researchers at Agere Systems/Lucent Technologies in Murray Hill, N.J., have found that melding small amounts of highly oxidizing rare-earth metals with the solders will make them stick to both types of surfaces.

Researchers bonded oxide-surface objects to wires of different metals to make a mobile that displays the versatility of the rare-earth-metal solder bond.

Weak bonding can be problematic when solders are used in optoelectronic devices to hold components like optical fibers in place -- especially when one considers that a shift as small as a few microns of such a component can render a high-performance device useless. Typically, overcoming the weak bonding requires lacing the oxide surface with a thin film of a metal such as nickel to which the solder can adhere. This metallization process introduces its own reliability and cost issues -- not to mention an extra, time-consuming step in the production of optoelectronic devices.

"We didn't want to reinvent the wheel," said Ainissa G. Ramirez, a member of the research team that developed the solder. "We wanted to enhance solders that are already out there so that this extra metallization step can be cut out but the overall process doesn't have to be radically altered."

The researchers added a small amount of lutetium -- 0.5 percent to 2 percent by weight -- to Sn-Ag and Au-Sn solders. These solders were chosen because rare-earth metals have a poor solubility in solder elements such as tin and lead, but some metals, such as gold and silver, have chemical structures that offer limited solubility. This limit actually benefits the process because the easily oxidized rare-earth metals conglomerate into micron-scale islands within the cooled form of the mixture, where they are protected from exposure to environmental oxygen.

Thermal testing of the rare-earth-laden solder revealed that the lutetium had little or no effect upon its melting point.

During the 200 to 250 šC soldering process, the lutetium confined in the islands disperses and is drawn toward the oxide-solder barrier, where it reduces SiO2 in contact with the solder. This creates a chemically bonding, 1- to 5-nm-thick region. The researchers conducted tests to determine the tensile strength of the solder when bonded to optical fibers. The solders continued to remain strong up to a stress of 16.5 MPa, the point at which the fibers broke.

The researchers noted that one application for which the solder may be well suited is attaching temperature-compensating materials and other components in optical fiber gratings. However, the rare-earth mixture may also find a more aesthetic use by fashion designers who may someday use it as a means to build elaborate jewelry.

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