Torleif A. Tollefsen

Au-Sn SLID Bonding—Properties and Possibilities

Au-Sn solid–liquid interdiffusion (SLID) bonding is a novel and promising interconnect technology for high-temperature applications. This article gives a review over previously published work on Au-Sn SLID bonding. An overview of the crystal phases and the thermomechanical properties of the Au-Sn phases relevant for Au-Sn SLID bonding is given. A summary of the bonding conditions used during Au-Sn SLID bonding is presented together with results from reliability tests. Additional challenges, possibilities, and recommendations for how a reliable high-temperature Au-Sn SLID bonding should be constructed are also discussed.

High Temperature Interconnect and Die Attach Technology: Au–Sn SLID Bonding

Au-Sn solid-liquid interdiffusion (SLID) bonding is a novel and promising interconnect and die attach technology for high temperature (HT) applications. In combination with silicon carbide (SiC), Au-Sn SLID has the potential to be a key technology for the next generation of HT electronic devices. However, limited knowledge about Au-Sn SLID bonding for HT applications is a major restriction to fully realizing the HT potential of SiC devices. Two different processing techniques-electroplating of Au/Sn layers and sandwiching of eutectic Au-Sn preform between electroplated Au layers-have been studied in a simplified metallization system. The latter process was further investigated in two different Cu/Si3N4/Cu/Ni-P/Au-Sn/Ni/Ni2Si/SiC systems (different Au-layer thickness). Die shear tests and cross-sections have been performed on as-bonded, thermally cycled, and thermally aged samples to characterize the bonding properties associated with the different processing techniques, metallization schemes, and environmental stress tests. A uniform Au-rich bond interface was produced (the ζ phase with a melting point of 522°C). The importance of excess Au on both substrate and chip side in the final bond is demonstrated. It is shown that Au-Sn SLID can absorb thermo-mechanical stresses induced by large coefficient of thermal expansion mismatches (up to 12 ppm/K) in a packaging system during HT thermal cycling. The bonding strength of Au-Sn SLID is shown to be superb, exceeding 78 MPa. However, after HT thermal ageing, the ζ phase was first converted into the more Au-rich β phase. This created physical contact between the Sn and Ni atoms, resulting in brittle NixSny phases, reducing the bond strength. Density functional theory calculations have been performed to demonstrate that the formation of NixSny in preference to the Au-rich Au-Sn phases is energetically favorable.

Reliable HT electronic packaging — Optimization of a Au-Sn SLID joint

Au-Sn solid-liquid-interdiffusion (SLID) bonding has proven to be a favorable die attach and interconnect technology for high temperature (HT) applications. In combination with silicon carbide (SiC) devices, Au-Sn SLID bonding has potential to be a key technology in future HT electronic systems. In this paper an optimized HT Au-Sn SLID joint is presented. Finite element analysis (FEA) were performed to design an optimized Au-Sn SLID joint for a HT Cu / Si3N4 / Cu / NiP / Au / Au-Sn / Au / Ni / Ni2Si / SiC package (representing a SiC transistor assembled onto a Si3N4 substrate). The optimized package (minimized residual stress at application temperature) was fabricated and investigated experimentally. The bond strength of the optimized joint was superb, with an average die shear strength of 140 MPa. An optimization of bonding time (1–10 min), temperature (290–350 °C) and atmosphere (ambient air, vacuum) was performed. Superb joints were fabricated at a bonding time of 6 min, and a bonding temperature of 300 °C, demonstrating an efficient, industry-feasible Au-Sn SLID bonding process.

Thermoelectric module for high temperature application

This work has investigated a Bi2Te3 based thermoelectric module for higher temperatures. An analytical model coupling the electrical, thermal and thermoelectrical domains is presented. A corresponding finite element model was developed incorporating the Seebeck, Peltier, Thompson, and Joule effects. Both models were compared with experimental results performed on a commercially available device. A comparison with a CoSb3 based module is also presented. The results from this study have confirmed that modern Bi2Te3 based thermoelectric modules have the potential to be used at temperatures up to 300 °C for electrical power generation, but they will be outperformed by CoSbs-based modules at around 350 °C.

Ni-Sn solid liquid interdiffusion (SLID) bonding — Process, bond characteristics and strength

Ni-Sn joints created by solid liquid interdiffusion (SLID) bonding were investigated. Process parameters were varied between 300 °C and 360 °C with annealing times up to 20 min to study the joint development. Symmetric Ni / Ni-Sn intermetallic compound / Ni joints were fabricated. It was found that a bond profile of 5 min at 300 °C was insufficient to completely solidify the joint. 5 min at 360 °C or 20 min at 300 °C solidified the joint and initiated the homogenization stage to form a stable high temperature compatible Ni / Ni3Sn / Ni joint structure. Early development of scallop shaped structures of Ni3Sn4 between the Ni and Sn layers was observed. Coalescent scallops form solid pillars between die and substrate, which results in large irregular voids in the center of the bond line. Initial shear strength results indicate extremely strong bonds, with a joint strength of more than 200 MPa.

SLID Bonding for Energy Dense Applications – Thermo-Mechanics

Solid-Liquid Inter-Diffusion (SLID) bonding is traditionally a technology used for high performance and high reliable die attach/interconnect applications. The generic properties of SLID allows the bonding to occur at a relatively low process temperature. However, when the bond is completed, the final joint has a melting point well above the process temperature. This makes it well suited as for high performance electronic assemblies. The typical bonding temperature of Cu-Sn SLID and Au-Sn SLID are 250–300 °C and 320–350 °C respectively. These temperatures compare to that of other high temperature (HT) electronic adhesives e.g. Staystik® 101G. The thermal performance of the SLID bond is superior to other electronic interface materials. This is due to the thin joint (∼ 10 μm) and the high thermal conductivity (∼ 60 W/m·K for Au-Sn). Thus, the thermal resistance of a SLID joint, about 2×10−3 cm2·K/W, is significantly lower than most other thermo-mechanical joints suitable for use in electronic assemblies. SL...

Die shear strength as a function of bond frame geometry — Au-Au thermo-compression bonding

The die shear strength has been studied as a function of bond frame geometry for wafer-level Au-Au thermo-compression bonds. The shear strength of samples with a 100 and 200 μm wide bond frame (bond area 1–2 mm2) was higher and more uniform than that of samples with a 400 μm wide bond frame (bond area 4 mm2). Bond frames with rounded corners had higher bond strength than samples with right angled corners (strength increased by 10–20 %). Three different fracture modes were observed. These trends were supported by finite element analysis (FEA) simulations. Conservative bonding parameters (temperature ≥ 400 °C, time ≥ 15 min, pressure 21 MPa) were applied to assure a uniform bond quality for the inspected samples. Shear testing as a method to quantify bond strength was discussed in general and in particular with respect to observed effects of bond frame geometries.