Optoelectronic Packaging: Solder Assembly

Soldering is a well-known metallurgical bonding technology used for joining metal parts. In the electronic industry, it is widely used for assembling components, whether it is on circuit boards or on silicon wafers, to provide a reliable mechanical joint as well as an electrical interconnection. Flip-chip technology- a method of attaching chips using solder bumps directly on substrates for providing high density electrical interconnection-has emerged as a new assembly tool for providing the fine alignment needed in optoelectronic packages. This technology provides both the needed electrical interconnection and a low cost passive alignment method- a method where in the optical components are aligned without electrically activating them. Solders are also used for attaching various parts within an optoelectronic package, such as laser die attachment, hermetic lid seal at the fiber parts, etc.,. Hence, it is quite common to have a solder hierarchy within a package to prevent disturbing the previously assembled parts during subsequent soldering operations. For example, in a laser package, eutectic Au/Sn solder with melting point (MP) of 280C can be used to attach a laser chip to a substrate, eutectic Sn/Pb solder (MP 183C) for attaching the substrate to the metal package, and eutectic Bi/Sn (138C) for hermetic sealing of the fiber part.