Au/Sn Solder

Wafer-Level Vacuum Packaging of Smart Sensors

“…Since Au 0.8 Sn 0.2 bonding layers do not oxidize, one does not need to clean the surface with flux or other chemicals that might be incompatible with released MEMS devices. The inertness of Au also makes the Au 0.8 Sn 0.2 eutectic an ideal solution for devices used in corrosive environments (e.g., in medical applications) [41]. In addition, Au 0.8 Sn 0.2 is a relatively hard, high-yield-strength solder, resistant to thermomechanical fatigue such as creep.…”

“…Once the layers are deposited, the stack is annealed to create the Au 0.8 Sn 0.2 eutectic material. This deposition approach has been demonstrated for both the IC interconnect [41,51,52] and hermetic MEMS packaging [31,32,33,37,38]. Figure 4 depicts an implementation of this approach in the fabrication of a WLVP for an RF resonator device with vertical feedthroughs [35].…”

MEMS and Thermal Management

“…The adhesive fracture on the substrate side was apparently related to delamination of the solder from (or from near) the NiCr adhesion layer on the substrate. For the combinations with Ni/Au on the substrate and TiW as adhesion layer, for both types of solder (7,8), the fracture mode remained adhesive on the chip side, as before thermal aging.…”

“…A second hypothesis was that the Au-richer joints would be less reliable than their counterparts containing less Au. This speculation was based on the mentioned problematic morphology of the AuSn 4 phase [8] expected to form in larger amounts for the Au-richer combinations. The condition for a good bond for this application was an average bond strength of >10 MPa and cohesive fracture in the solder during shear testing.…”

“…There are four stable Au-Sn intermetallic compounds: Au 5 Sn (ξ), AuSn (δ), AuSn 2 (ε), and AuSn 4 (η) [2]. Oppermann and Hutter [8] showed that already at room temperature, AuSn, AuSn 2 , and AuSn 4 form gradually when Au and Sn are in direct contact. These phases are expected to form quickly due to fast diffusion of gold, which can also lead to Kirkendall voids at the original gold surfaces.…”