Soldering of optical components to CTE-matched substrates for space applications

Abstract

Optical components in space applications require a bonding method that allows for accurate and stable fixation under mechanical, thermal and radiation exposure. These requirements cannot be met by organic glues, that are typical for this purpose under ambient conditions, due to outgassing and contamination under conditions in space. The suggested method for bonding under these circumstances is soldering which is required to be executed here without oxide-removing flux. Also, only lead-free solders should be applied due to the toxicity of lead-containing materials. For this reason, several low-melting solder alloys were investigated on their compatibility with optical and mounting materials with sputtered-on Ti/Pt/Au layers. Experiments included the measurement of wettability and spreading behaviour, coefficient of thermal expansion and thermal conductivity and the analysis of the solder-substrate compounds microstructure. It was found that next to the most promising SAC305 also several other Sn- and Bi- based solder alloys demonstrated perfect wettability and preferable properties in experiments that should simulate the Solderjet Bumping process. Microstructures displayed that the formation of intermetallic compounds near the substrate interface depends strongly on the thermal properties of the applied materials. These findings contribute to a deeper understanding of soldering mechanisms and the suitability of low-melting, lead-free solders for space applications.