Reactive bonding of an integrated CMOS strain sensor to steel by using hard and soft solders

In this project, the suitability of reactive joining using a novel high-energy material system based on the elements zirconium, silicon and aluminum (Zr/Si/Al) for the mounting of a strain sensor on a steel carrier was investigated. For this purpose, CMOS-integrated silicon-based sensor chips were first connected to a flexible printed circuit board and electrically contacted. Subsequently, the chips were metallized on the backside and soldered onto standardized stainless steel tensile specimens using three different methods, and the joint connections were characterized. In the first method, a soft solder paste based on 96.5% tin, 3.0% silver and 0.5% copper was used to solder the chips to the steel tensile specimens using a standard reflow soldering process. As a second process, a Ni/Al-based reactive foil with tin as soft solder on both sides was used for reactive joining of the chips. Third, hard solders consisting of an aluminum-silicon alloy (AlSi12) and a Zr/Si/Al-based reactive multilayer system were deposited directly onto the chip backsides and onto the tensile specimens. Subsequently, the chips were reactively bonded to the tensile specimens. Tensile forces up to 1200 N were applied to the tensile specimens in the longitudinal direction on a tensile testing device for all three joining methods. For each of the 24 individual PMOS sensors, which are sensitive to the normal stresses σxx-σyy, the signals were recorded. The signal differences compared with the unloaded state provide direct information about the respective mechanical coupling. The lowest coupling was achieved with the soft solder paste, and the highest with the combination of Zr/Si/Al and hard solder.