AuSn Solder Research Articles

Transient liquid phase bonding of copper and ceramic Al2O3 by Al/Ni nano multilayers

Transient liquid phase (TLP) diffusion bonding has been achieved for the dissimilar joining of ceramic Al2O3 and copper using Al/Ni multilayers as interlayers at low temperature. The freestanding Al/Ni nano-multilayers were deposited by direct current magnetron sputtering and their properties were studied by X-ray diffraction, differential scanning calorimetry and scanning electron microscopy (SEM). The joining experiments were carried out at 550, 650 and 750°C for a holding time of 30min without extra pressure under nitrogen protection. The interfacial microstructure and microstructural characterization of the diffusion welded joints were investigated by SEM, energy dispersive X-ray spectroscopy and element mapping. The effectiveness of different temperatures for the mechanical properties of the joints was evaluated by shear strength. In the present work, an AuSn solder was added to expand the surface contact of the welding. The results show that the applications of Ni/Al multilayers and the AuSn solder reduce the bonding temperature and improve the joining quality.

The Thermal-Aging Effect on the Microstructure Evolution and Shear Strength of the Sn-Rich Au-Sn Soldering between Altic and Si Substrate in Microelectronics

The Au-Sn soldering alloys are commonly used in microsoldering process for microelectronic industry due to fluxless process and relatively low melting temperature with good eutectic microstructures. This study investigated the microstructures of Au-Sn soldering between AlTiC and Si substrates with Ti/Pt/Au under bump metallization (UBM). The microstructures of the solder samples under three conditions: before bonding, after bonding and after thermal-cycle aging, were investigated. The shear strength values of pre-aging and post-aging soldering were compared. The thermal-cycling temperatures were ranged from -40 to 125 °C for 300 cycles. The intermetallic compounds (IMCs) of the AuSn solders consist of AuSn, AuSn2, and AuSn4. After thermal-cycle aging, the bonding strength was increased due to the improved IMC bonding between solders and UBM; the shear surfaces were rougher due to the growth of AuSn and AuSn2.

Nonlinear and Temperature-Dependent Material Properties of AU/SN Alloy for Power Module

AbstractIn recent years, the material Au-20Sn eutectic solder, which is resistant to high temperatures, is used for electric interconnections in high-power modules, the material properties such as temperature and strain rate dependent stress-strain curve are critically needed for reliability assessment of Au-20Sn solder joint. Thus, this study was performed to determine the material properties of Au-20Sn eutectic solder under various strain rates and temperature loads. Many researches using shear test to determine the shear resistance of solder joint, however, the mechanical strength as measured by the shear test is the maximum shear strength of the package joint, but this measurement does not represent the stress-strain behavior of Au-20Sn material. To identify the material properties of Au-20Sn eutectic solder, the tensile test was performed to measure its mechanical strength and nonlinear material properties. The strain rate effect was examined in terms of the influence of the mechanical strength on the Au-20Sn eutectic solder at different tensile rates. The temperature-dependent material properties of Au-20Sn solder were also measured under various thermal loadings, and material properties of Au-20Sn obtained in this research can be applied to the simulation model, the thermomechanical behavior and reliability of the power module can be further analyzed and evaluated.

Growth behavior and microstructure of intermetallics at interface of AuSn20 solder and metalized-Ni layer

The AuSn20/Ni joints were prepared by the reflow soldering technology and then annealed at solid-state temperature to form diffusion couples. The interfacial reactions and the growth kinetics of the intermetallic compounds (IMC) at the AuSn20/Ni soldering interface were investigated by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The results show that, the (Ni,Au)3Sn2 phases are formed at the AuSn20/Ni interface after soldering at 583 K. The thickness l of the IMC layer monotonically increases with increasing annealing time t according to the relationship l=k(t/t0)n, where the exponent n is 0.527, 0.476 and 0.471 for 393, 433 and 473 K annealing, respectively. This indicates that the volume diffusion contributes to the growth of the IMC layer at the AuSn20/Ni interface at solid-sate temperature. The pre-exponential factor K0=1.23×10−7 m2/s and the activation enthalpy QK=81.8 kJ/mol are obtained from the results of the parabolic coefficient K by a least-squares method.

Design and characterization of a 3D encapsulation with silicon vias for radio frequency micro-electromechanical system resonator**Project supported by the National Natural Science Foundation of China (Grant Nos. 61234007, 61404136, and 61504130), the Fund from the Ministry of Science and Technology of China (Grant No. 2013YQ16055103), the Key Research & Development Program of Jiangsu Province, China (Grant No. BE2016007-2), and the

In this paper, we present a three-dimensional (3D) vacuum packaging technique at a wafer level for a radio frequency micro-electromechanical system (RF MEMS) resonator, in which low-loss silicon vias is used to transmit RF signals. Au–Sn solder bonding is adopted to provide a vacuum encapsulation as well as electrical conductions. A RF model of the encapsulation cap is established to evaluate the parasitic effect of the packaging, which provides an effective design solution of 3D RF MEMS encapsulation. With the proposed packaging structure, the signal-to-background ratio (SBR) of 24 dB is achieved, as well as the quality factor (Q-factor) of the resonator increases from 8000 to 10400 after packaging. The packaged resonator has a linear frequency–temperature (f–T) characteristic in a temperature range between 0 °C and 100 °C. And the package shows favorable long-term stability of the Q-factor over 200 days, which indicates that the package has excellent hermeticity. Furthermore, the average shear strength is measured to be 43.58 MPa among 10 samples.

Interfacial reactions in the Au/Sn-xZn/Cu sandwich couples

Abstract The interfacial reactions in the Au/Sn-Zn/Cu sandwich couples were investigated. The Sn-xZn/Cu couples were reflowed at 270 °C for 1 h and then clamped together over an Au foil. These Au/Sn-Zn/Cu sandwich couples were aged at 160 °C for various periods of time. The AuSn, AuSn2, and AuSn4 phases were formed at the Sn-1Zn/Au interface when the Au/Sn-1 wt% Zn/Cu sandwich couple was aged for 20 h, the Cu6Sn5 phase was also observed between Au-Sn intermetallic compounds (IMCs) and solder. This result revealed that Cu atoms rapidly diffused through the Sn-1Zn solder to form Cu-Zn IMC on the Au side. The Au-Sn IMCs, Au-Zn IMCs, and an Au-Sn-Zn ternary metastable phase were formed at the interface in the Sn-5Zn/Au and Sn-9 wt% Zn/Au couple. When Zn contents were more than 20 wt%, only the Au-Zn IMCs, AuZn, Au3Zn7, and AuZn3 phase were formed at the Sn-xZn/Au interface. On the Cu side, when the Sn-Zn/Cu couple was reflowed at 270 °C for 1 h, the result indicated that the (Cu, Zn)6Sn5, (Cu, Sn)Zn phases were found at the Sn-1 wt.% Zn/Cu interface, the Cu5Zn8 phase was found at the Sn-5Zn/Cu interface and the (Cu, Sn)5Zn8, (Cu, Sn)Zn5 phases were found at the Sn-xZn/Cu interfaces (x = 9, 20 and 40 wt%). After 1000-h aging, the CuZn and Cu6Sn5 phases were formed at the Sn-1Zn/Cu interface, and only the Cu5Zn8 phase was observed at the Sn-(5 to 40) wt% Zn/Cu interfaces. The results indicated that no Au-Cu ordered phases were observed at both Au and Cu sides, and IMC formation in the Au/(Sn-xZn)/Cu sandwich couple strongly depended on the Zn concentration effect in the Sn-xZn alloys.

Interfacial reactions and mechanical strength of Sn-3.0Ag-0.5Cu/Ni/Cu and Au-20Sn/Ni/Cu solder joints for power electronics applications

The mechanical strength of Sn-3.0Ag-0.5Cu (SAC305) and Au-20Sn solder joints and their interfacial reaction with a Ni-plated ceramic substrate were evaluated to assess their suitability for use as die attach materials in power module applications. The compatibility between the two solder alloys and the Ni substrate was assessed during isothermal long-term aging, while the mechanical strength of the two solder joints was measured by die shear testing. A higher intermetallic compound (IMC) growth rate and Ni consumption rate was observed in the SAC305 solder joint, with the formation of a thick IMC layer and weak interface resulting in brittle fracture. The Au-20Sn solder joint, on the other hand was found to exhibit superior high temperature interfacial stability and joint strength.

A low feed-through 3D vacuum packaging technique with silicon vias for RF MEMS resonators

This paper presents a wafer-level three-dimensional (3D) vacuum packaging technique for radio frequency microelectromechanical systems (RF MEMS) resonators. A Sn-rich Au–Sn solder bonding is employed to provide a vacuum encapsulation as well as electrical conductions. Vertical silicon vias are micro-fabricated by glass reflow process. The optimized grounding, via pitch, and all-round shielding effectively reduce feed-through capacitance. Thus the signal-to-background ratios (SBRs) of the transmission signals increase from 17 dB to 20 dB, and the quality factor (Q) values of the packaged resonators go from around 8000 up to more than 9500. The measured average leak rate and shear strength are (2.55 ± 0.9) × 10−8 atm-cc s−1 and 42.53 ± 4.19 MPa, respectively. Furthermore, thermal cycling test between −40 °C and 100 °C and high temperature storage test at 150 °C show that the resonant-frequency drifts are less than ±7 ppm. In addition, the SBRs and the Q values have no obvious change after the tests. The experimental results demonstrated that the proposed encapsulation technique is well suited for the applications of RF MEMS devices.

A zero-level vacuum encapsulation technique with less parasitic effect for VHF MEMS resonators

This paper presents a zero-level vacuum encapsulation technique with the development of effective parasitic optimization methods for VHF MEMS resonator. Sn-rich Au-Sn solder bonding is used to achieve vacuum hermeticity packaging with high shear strength. The measured average leak rate and shear strength are (2.2±0.9)×10−8atm-ccs−1 and 40.65±8.25MPa among 12 packaged chips, respectively. An equivalent circuit model of the packaged resonator is established to analyze the parasitic effect caused by encapsulation. A significant reduction of the parasitic effect is achieved via grounding sealing ring, and decreasing parasitic capacitances between the sealing ring and signal feedthroughs. The measured frequency spectrum of a packaged resonator is not distorted by parasitic effect, which is well consistent with the simulation result. Besides, the Q value of the resonator increases from 6400 to 10,300, as well as the transmission gain at harmonic peak from −73.92dB to −70.36dB due to an excellent electrical performance of the encapsulation structure and a high vacuum in this packaging cavity.

Power electronic assemblies: Thermo-mechanical degradations of gold-tin solder for attaching devices

The eutectic Au80Sn20 solder alloy has been applied in semiconductor assemblies and other industries for years. Due to some superior physical properties, Au/Sn alloy gradually becomes one of the best materials for soldering in electronic devices and components packaging but the voids growth in AuSn solder joints is one of the many critical factors governing the solder joint reliability. Voids may degrade the mechanical robustness of the die attach and consequently affect the reliability and thermal conducting performance of the assembly. Severe thermal cycles [−55°C/+175°C] have highlighted degradations in AuSn die attach solder. The inspection of as-prepared die-attachments by X-ray and SEM (observation of cross-section) shows that the initial voids sizes were increased and a propagation of transverse cracks inside the joint between voids has appeared after ageing, it was featured also the existence of the IMC typical scallop-shape morphology with the phase structure of (Ni, Au)3Sn2 on as-reflowed joints. In this paper, we evaluate the origin of these degradations and ways to address them.

Experimental Method and Finite-Element Simulation Model for Investigation Into Flip-Chip-on-Film Inner Lead Bonding Parameters

One of the key factors to design reliable eutectic Au-Sn solder joints used in flip-chip-on-film (FCOF) packaging is the bonding temperature distribution at the interface of the gold bump and the copper lead on the inner lead bonding (ILB) process. Furthermore, determining an appropriate bonding force is essential in achieving a suitable compromise between the bonding strength of the ILB process and the gap size in the subsequent resin potting process. Accordingly, this paper proposes a method based on a noncontact laser spot sensor for determining the inner lead interface temperature. The tests are performed using materials from FCOF products at various temperatures in the range of 300 °C-440 °C. The experimental results are used to construct an ANSYS finite-element model of the FCOF system, including the silicon chip, the polyimide film, the copper lead, and the gold bump to simulate a temperature-dependent ILB process. A good agreement is observed between the numerical and experimental results for the deformed profile of the polyimide film/copper lead following the bonding process and the variation of the bump sinking value with the bonding force. Finally, the numerical results are used to construct an FCOF/ILB parameter design chart for selecting the bonding force required to obtain a given gap size and bump sinking value in the FCOF/ILB process.

Effect of multiple flip-chip assembly on the mechanical reliability of eutectic Au–Sn solder joint

Eutectic Au–Sn solder has widely been used for high temperature bonding since it enables fluxless soldering and provides mechanically stable solder joint. However, effect of multiple bonding process on the mechanical reliability has not been studied in detail. In this study, microstructure evolution of Au–Sn solder joint and its effect on mechanical reliability were systematically investigated. During the multiple reflow process, the eutectic phase gradually transformed into ζ phase, which resulted in loss of solderable area. The phenomenon turned out to be responsible for degradation of bonding strength of the solder joint.

Microstructure and mechanical properties of Sn-rich Au-Sn solders designed using cluster-plus-glue-atom model

Novel Sn-rich Au-Sn solder alloys were designed using the cluster-plus-glue-atom (CPGA) model, and the microstructure, melting behavior, wettability and mechanical properties of the designed Sn-Au-Ag(Ni) solders were systematically investigated. The cluster formula of [Sn11Au2]Sn3, a stable Sn-centered cluster (11 Sn atoms and 2 Au atoms) plus 3 Sn glue-atoms, is applicable to substitute Ag and Ni for Au. The [Sn11AuAg]Sn3 and [Sn11Au0.5Ag1.5]Sn3 solders are near-eutectics with a narrow melting temperature range of 206.89–207.25°C; while the Sn-Au-Ni solders are non-eutectics with a wide melting temperature range of 215.33–255.44°C, due to a large difference in chemical property between Ni and Au. The microstructure of the Sn-Au-Ag solders consists of AuSn4 and Ag3Sn phases dispersed in β-Sn matrix; while that of the Sn-Au-Ni solders consists of AuSn4 and Ni3Sn4 phases dispersed in β-Sn matrix. The wettability of the Sn-Au-Ag(Ni) solders on Ni substrate is better than that on Cu substrate because of the severe interfacial reaction on Cu substrate. The shear strength of the [Sn11Au0.5Ag1.5]Sn3 and [Sn11Au0.5Ni1.5]Sn3 solder joints is 50.59MPa and 36.28MPa, respectively, which is significantly higher than that of eutectic Au-30at% Sn solder joints (24.5MPa).

Comparative study of Au–Sn and Sn–Ag–Cu as die‐attach materials for power electronics applications

A comparative study of Sn–3.0Ag–0.5Cu (SAC305) and Au–20Sn solders as die‐attach materials for high‐temperature power electronic applications was performed. The solid‐state interfacial reactions of SAC305 and Au–20Sn solders with a Ni‐plated Si chip and a direct‐bonded‐copper substrate, as well as the growth of interfacial intermetallic compound layers and interfacial stability, were investigated and compared during aging at 150 and 200 °C for up to 2000 h. The SAC305 solder exhibited a higher interfacial intermetallic compound growth rate and a higher consumption rate of the Ni layer than the Au–20Sn solder. The Au–20Sn solder had a superior interfacial stability for high‐temperature power electronic applications. Copyright © 2016 John Wiley & Sons, Ltd.

Interfacial reaction mechanism and kinetics between Au–20Sn and Sn

Sn is a promising solderable layer to join Au–20Sn solder alloy with common substrates at low temperature. The interfacial reactions between Au–20Sn and Sn during liquid/solid state diffusion and solid state diffusion were investigated. AuSn2 and AuSn4 layers formed at the interface of Au–20Sn/Sn. During liquid/solid state diffusion reaction (LSDR), Au atoms diffuse continuously into liquid Sn. After solidification, lath-like AuSn4 grains gradually precipitated adjacent to the interface in the Sn side. The solidified Sn matrix gradually transformed from eutectic microstructure to hypereutectic microstructure, and finally to peritectic microstructure. Potential mechanisms of LSDR at different temperatures were explained. Growth kinetic analysis of solid state diffusion reaction indicated that the growth rate of AuSn2 is lower than that of AuSn4. The time exponent, n ≈ 0.5, indicates that volume diffusion is the dominant growth mechanism of AuSn2 layer, while grain boundary diffusion is the rate-controlling mechanism for the thickening of AuSn4 with n close to 0.33. The activation energies for the growth of AuSn2 and AuSn4 layers are determined to be 10.97 kJ/mol and 7.79 kJ/mol, respectively.

Au–Sn bonding material for the assembly of power integrated circuit module

Insulated gate bipolar transistor (IGBT) chips are the key components in high-temperature power electronic modules, which have to efficiently convert electricity between direct and alternating current. In this study, the eutectic Au–Sn (20 wt.% Sn) is successfully used to assemble IGBT chips and direct-bond-copper substrates by using solid liquid interdiffusion (SLID) bonding. During subsequent isothermal aging at 150, 200, and 240 °C, the microstructure evolution and growth kinetics of intermetallic compounds are investigated. Excellent thermal stability and mechanical strength are observed. It is concluded that the eutectic Au–Sn solder is ideal to assemble high-temperature IGBT by using the SLID process.

Maintaining Low Voiding Solder Die Attach for Power Die While Minimizing Die Tilt

This paper addresses work to minimize voiding and die tilt in solder attachment of a large power die, measuring 9.0 mm × 6.5 mm × 0.1 mm (0.354″ × 0.256″ × 0.004″), to a heat spreader. As demands for larger high power die continue, minimizing voiding and die tilt is of interest for improved die functionality, yield, manufacturability, and reliability. High-power die generate considerable heat, which is important to dissipate effectively through control of voiding under high thermal load areas of the die while maintaining a consistent bondline (minimizing die tilt). Voiding was measured using acoustic imaging and die tilt was measured using two different optical measurement systems. 80Au-20Sn solder reflow was achieved using a batch vacuum solder system with optimized fixturing. Minimizing die tilt proved to be the more difficult of the two product requirements to meet. Process development variables included tooling, weight and solder preform thickness.

Flip Chip Bonding of a Quartz MEMS-Based Vibrating Beam Accelerometer.

In this study, a novel method to assemble a micro-accelerometer by a flip chip bonding technique is proposed and demonstrated. Both the main two parts of the accelerometer, a double-ended tuning fork and a base-proof mass structure, are fabricated using a quartz wet etching process on Z cut quartz wafers with a thickness of 100 μm and 300 μm, respectively. The finite element method is used to simulate the vibration mode and optimize the sensing element structure. Taking advantage of self-alignment function of the flip chip bonding process, the two parts were precisely bonded at the desired joint position via AuSn solder. Experimental demonstrations were performed on a maximum scale of 4 × 8 mm2 chip, and high sensitivity up to 9.55 Hz/g with a DETF resonator and a Q value of 5000 in air was achieved.

Ternary intermetallic compounds in Au–Sn soldering systems—structure and properties

The intermetallic compound AuSn is essential for the wetting of Au-rich AuSn solders. On the addition of In or Sb, pseudo-binary compounds AuSn1−x In x (x ≤ 0.33) and AuSn1−y Sb y (y ≤ 0.17) are formed. Both adopt the AuSn structure type (P63/mmc, Z = 2). This single-crystal X-ray diffraction study reveals that there is an absence of superstructure ordering upon long-time annealing (years). This behavior constitutes a surprising contrast to the related compounds in the systems Cu–In–Sn and Cu–Sn–Sb. As subsequent total energy calculations disclose, superstructure ordering is neither expected at 0 K nor by the application of high pressure. Hence, decomposition to AuSn and its neighboring phases in the ternary phase diagrams is the only way to release chemical pressure quickly. However, the ternary phases are formed at the expense of the binary compound AuSn, when moving away from the ideal composition. To give an idea how additions of In and Sb will affect the performance of AuSn in the solder joint, we studied the physical properties such as magnetism and resistance as well as mechanical properties such as hardness, elastic modulus, and fracture behavior.

Degradation of high power single emitter laser modules using nanosilver paste in continuous pulse conditions

This paper is mainly reported to a pulse reliability investigation of high power single emitter laser modules with nanosilver paste. Comparative experiments in continuous pulse conditions for the laser modules packaged with nanosilver paste, indium and AuSn solders were conducted. The results indicate that the laser modules attached by nanosilver paste have a longer-term lifetime than those with indium and AuSn solders in continuous pulse conditions. Transient thermal behavior and coupled thermo-mechanical behavior in continuous pulse conditions are simulated by finite element method (FEM). A semi-empirical model based on Arrhenius relationship is established to provide relative reliability assessments for laser modules by combining with the simulating results.