Fluxless Hermetic Lid Sealing Using Electroplated Sn-Rich Solder

Abstract

A new fluxless hermetic sealing technique using electroplated Sn-rich soft solder is reported. Specific glass (SCG72) is chosen as lid material to seal alumina packages. It has nominal coefficient of thermal expansion of 7 ppm/ C, close to that of alumina. Alternatively, sapphire can also be used as the lid material. A thick Sn layer is plated over the Cr/Au patterned glass wafer, followed immediately by thin Au layer. This outer Au layer prevents the inner Sn from oxidation. In bonding, the glass lid is placed over the package rim which has a metallization structure of W/Ni/Au. The fluxless sealing process is performed in vacuum (50 millitorrs) to suppress tin oxidation. Compared to bonding in air, the oxygen content is reduced by a factor of 15 200. Fluxless bonding is valuable in many hermetic sealing applications such as microelectromechanical systems (MEMS), sensors, photonic devices, and imaging devices. Nearly void-free sealed joints are achieved with Sn-rich composition. Helium leakage tests are performed to evaluate hermetic quality. Scanning electron microscope (SEM) with energy dispersive X-ray spectroscopy (EDX) is used to evaluate the composition and microstructure of the sealing joints. The results show how the Sn-rich solder reacts with the W/Ni/Au on the package. This new sealing process can be applied to nearly all devices and packages that require hermetic sealing.

Low temperature Sn-rich Au—Sn wafer-level bonding

“…Then, the wafers were rinsed in DIwater and dried with nitrogen. The bonding of the cap and substrate wafers was performed in SB6e bonder system (SUSS) at different temperatures (180-360 ı C) and under different static pressures (3)(4)(5)(6)(7)(8)(9)(10)(11). The bonded solder joints were characterized with XRD, and scanning electron microscopy (SEM).…”

A wafer-level Sn-rich Au–Sn intermediate bonding technique with high strength

“…It can be seen from figure 1 that there are two stable phases in the Sn-rich region of the phase diagram: AuSn 2 (ε) and AuSn 4 (δ). Much research has been conducted on the Au-Sn solder with 71-92 wt% Sn [4,6,7]. The bonding was performed above the eutectic temperature of 217 • C and under a pressure of 0.586 MPa.…”

A Wafer-Level Sn-Rich Au—Sn Bonding Technique and Its Application in Surface Plasmon Resonance Sensors

“…Compared with the Au-rich Au-Sn hard solder bonding, the Sn-rich Au-Sn soft solder bonding has promising advantages: low residual stresses, low cost, and high shear strength. [6,7] However, the Sn-rich Au-Sn solder is easier to be oxidized due to a large Sn content. Therefore, a Au/Sn multi-layer capped with a Au film is adopted to prevent Sn oxidization.…”

“…Much research has been conducted on the Au-Sn solder with 71-92 wt% Sn. [6,7,9] However, the dependence of the Sn-rich Au-Sn solder bonding upon the conditions is not yet clear. In this Letter, the Sn-rich Au-Sn solder bonding under various conditions such as the Au-Sn composition and temperature are systematically studied.…”