Fluxless Soldering for Hermetic Packaging of MOEMS

Fluxless soldering is desirable for the hermetic packaging of micro-optoelectromechanical (MOEMS) systems, especially those used in harsh environments, or those that require very long shelf lives. An example of such microsystem is a Safe & Arm MEMS device that requires reliable operation over 20 to 30 years. For this application, degradation and out-gassing of the organic materials, such as those contained in fluxes and epoxies could result in the contamination and stiction of the moving microparts.

In this paper we present simulation and experimental results of using a diode laser and a programmable hot-plate to attach and seal fiber optic feed-throughs and a MEMS die into a Kovar carrier package. In order to obtain reliable fluxless solder joints, certain environmental conditions namely, inert and/or reducing gas environment needs to be present during the process. In addition, the solder and substrate surfaces must be free of oxides and organic contaminants. We describe a fluxless soldering process using 100% Indium and 80%-20%Au-Sn solders.

Acceptable process parameters such as the laser power density, spot size, and duration, package geometry and die attach pressure have been determined both experimentally and through simulation. The role of oxygen in the formation of oxide film covering solder performs, and the feeding mechanism has also been experimentally investigated. It has been established that oxygen levels of about 0.025% (250 ppm) obtained inside a glove-box obtained using inert gas (100% N2 or 95%N2, 5% H2) is necessary to achieve adequate joints.