Au-Sn Intermetallic Compounds Research Articles

Spectroscopic ellipsometry modelling of thin AuSn films and nanostructures as plasmonics materials

A considerable interest in nanostructured thin films from intermetallic compounds of noble metals (Ag and Au) and post-transition metals is raised due to their unique plasmonic properties, which makes them potential materials for application in photonics, catalysis and biosensing systems. In this work the possibility for deposition of polycrystalline AuSn thin films was investigated, as thin films with the same composition and different thicknesses (10-100 nm) were obtained by co-depositing of Au and Sn metals. The chemical composition was determined by X-ray microanalysis. The X-ray diffraction patterns indicated formation of the intermetallic compound AuSn in the thin films. The complex permittivity ε = ε′ - i.ε″ of the thin AuSn films as function of the thickness was investigated by spectroscopic ellipsometry. The possibility for application of nanostructures from the AuSn intermetallic compound as suitable substrates for the needs of surface-enhanced Raman and fluorescence spectroscopy in the spectral interval 2.8 - 4.7 eV was analysed.

Microstructures and mechanical properties of ENIG/Sn-3.5Ag/ENIG joints formed by ultrasonic-assisted solder bonding

The effects of temperature and ultrasonic-assisted time on the microstructures and mechanical properties of the electroless nickel-immersion gold (ENIG)/Sn-3.5Ag/ENIG sandwich structure in ultrasonic chip bonding were investigated. Ultrasonic-assisted solder bonding was performed for 0.5–4 s at 25, 150, and 250 °C. The bonding was successful under all conditions. At 25 °C, Au–Sn intermetallic compounds (IMCs) were formed at the upper interface, and AuSn4 was formed on Ni–Sn IMCs at the lower interface. As the bonding time increased, spalling of AuSn4 was increasingly observed at the upper interface. At 150 °C, a trend similar to that at 25 °C was observed, but as the time increased, the thickness of the joint decreased, Au–Sn IMCs at the upper interface disappeared, and thin Ni–Sn IMCs and Cu6Sn5 were formed. The results observed at 250 °C differed from those observed at 25 and 150 °C because ultrasonic-assisted solder bonding was performed with the solder in a molten state. Ni–Sn IMCs were formed at the upper and lower interfaces, residual Sn and Au–Sn IMCs were present inside, and the thickness of the joint was very small. At 250 °C, the shear strength was high regardless of the bonding time. Observations of the top views and cross-sections of the fracture surfaces showed that, at 25 and 150 °C, the fractures initially occurred at the lower interface but moved to the upper interface as the bonding time increased. At 250 °C, the fractures did not significantly change, regardless of the ultrasonic-assisted time.

Microstructure and Optical Properties of Nanostructural Thin Films Fabricated through Oxidation of Au-Sn Intermetallic Compounds.

AuSn and AuSn2 thin films (5 nm) were used as precursors during the formation of semiconducting metal oxide nanostructures on a silicon substrate. The nanoparticles were produced in the processes of annealing and oxidation of gold–tin intermetallic compounds under ultra-high vacuum conditions. The formation process and morphology of a mixture of SnO2 and Au@SnOx (the core–shell structure) nanoparticles or Au nanocrystalites were carefully examined by means of spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDX). The annealing and oxidation of the thin film of the AuSn intermetallic compound led to the formation of uniformly distributed structures with a size of ∼20–30 nm. All of the synthesized nanoparticles exhibited a strong absorption band at 520–530 nm, which is typical for pure metallic or metal oxide systems.

Microstructure evolution of Au/SnSb-CuNiAg/(Au)Ni during high temperature aging

PurposeThe purpose of this paper is to investigate the morphology evolution and the composition transformation of Au-Sn intermetallic compounds (IMCs) of the new Au/Sn-5Sb-1Cu-0.1Ni-0.1Ag/(Au)Ni solder joint during the high temperature aging.Design/methodology/approachSn-5Sb-1Cu-0.1Ni-0.1Ag solder balls (500 µm in diameter), heat sink with structure of 7.4 µm Au layer on 5 µm Ni-plated Cu alloy and Si chip with 5.16 µm plated Au were used to fabricate micro-solder joints. The joints were performed in a furnace at 150°C for 150, 250 and 350 h aging. The samples were polished and deep etched before analyzed by metallographic microscope and scanning electron microscopy, respectively. Energy dispersive x-ray spectroscopy was used to identify the composition of the IMCs.Findingsß-(Au,Ni,Cu)10Sn phase is formed during the soldering process. The IMCs evolution has two periods during the aging. The first is theξ-(Au,Ni,Cu)5Sn,ξ-(Au,Cu)5Sn andδ-AuSn were formed and grew to form a full-compound joint after about 150 h aging. The second is the conversion of the full-compound joint. The IMCs converted toξ′ phase when the aging time extends to 250 h, and transformed toε-(Au,Ni,Cu)Sn2andη-(Au,Ni,Cu)Sn4after 350 h aging. The thicker gold layer and thinner solder joint can promote the growth of the IMCs.ß-(Au,Ni,Cu)10Sn emerged in Au/SnSb-CuNiAg/(Au)Ni in this research, which is not usually found.Originality/valueThe results in this study have a significant meaning for the application of the new Sn-5Sb-1Cu-0.1Ni-0.1Ag in harsh conditions.

The Evolution of IMCs in Single Crystal Sn3.0Ag0.5Cu and Sn3.0Ag3.0Bi3.0In BGA Solder Joints with Au/Ni/Cu Pads Under Current Stressing

The growth behavior of intermetallic compound (IMC) in single crystal Sn3.0Ag0.5Cu (SAC305) and Sn3.0Ag3.0Bi3.0In (SABI333) ball grid array solder joints with Au/Ni/Cu pads under 104 A/cm2 current stressing was investigated. Characterization by scanning electron microscopy and electron backscattered diffraction mapping were utilized to identify the microstructure and crystal orientation of solder joints. The AuSn4 IMC particles in the SAC305 solder matrix were formed along the electron flow direction and c-axis direction of Sn. On the other hand, it was observed that the Au(Sn0.83, In0.17)4 IMC particles generated in SABI333 solder matrix decomposed from needle-like morphology into small pieces of Au(Sn0.23, In0.77)2 during current stressing. This phenomenon depends on the different diffusion rate and opposite migration direction of Sn and In atoms, which have different driving forces. Moreover, the results showed that the growth behavior of IMC particles in SAC305 solder joints was significantly dependent on the c-axis direction of Sn, while that of IMCs in SABI333 was almost maintained without polarization effect. This study indicated that SABI333 solder joints under EM service could exhibit a better performance than that of SAC305 solder joints.

The dissolution and reprecipitation behavior of the AuSn4 intermetallic compound in a solder joint during liquid-state and solid-state reactions

The present study investigated the reaction behavior of the AuSn4 intermetallic compound in the Ni/Sn1.8Ag+Sn3Ag0.5Cu/Au/Pd/Cu solder joint when subjected to multiple reflow for five cycles and solid-state aging at 150 °C for 1000 h. Multiple reflow induced grain refining of the AuSn4 in the solder matrix. The grain refining was ascribed to the repeated dissolution and reprecipitation of the compound that occurred during multiple reflow. The dissolution of the AuSn4 was driven by the chemical potential gradient at the AuSn4/molten solder interface. The dissolution behavior enriched the solder matrix with the constituent elements that were partly involved in the formation of the Cu6Sn5 at the solder/Cu interface and the subsequent reprecipitation as the refined AuSn4 particles when the solder was solidified. Solid-state aging, however, induced complete dissolution of the AuSn4 and the migration of the constituent elements toward the solder/Cu interface to form a more stable (Cu,Ni,Pd,Au)6Sn5 phase due to the thermodynamic stability competition that occurred among the compounds. Consequently, no AuSn4 reprecipitation behavior occurred during solid-state aging. An understanding of the reaction behavior of the AuSn4 intermetallic compound is important for practical applications due to its brittle characteristics. The refining of the compound throughout the solder matrix may also strengthen the solder joint, leading to a better performance.

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.

Harsh service environment effects on the microstructure and mechanical properties of Sn–Ag–Cu-1 wt% nano-Al solder alloy

This paper investigates the electrical and mechanical performances of eutectic Sn-3Ag-0.5Cu (wt%) solder with the addition of Al nanoparticles. The study revealed that the elastic moduli, electrical resistivity and damping properties of such solder alloy were improved. Further, interfacial reaction phenomena on Au/Ni-plated Cu pad ball grid array substrate during isothermal aging and thermal cycle was evaluated in terms of the formation and growth kinetics of intermetallic compound (IMC) layer. A structural analysis confirmed that at their interfaces a ternary (Cu, Ni)-Sn IMC layer was adhered at the substrate surface. The thickness of this IMC layer was increased with increasing the duration of the isothermal aging and thermal cycle without any defects. In addition, the formation of Ag3Sn, Cu6Sn5, Sn–Al–Ag and AuSn4 IMC phases were evenly distributed in the solder matrix which acts as the second phase reinforcement. The measured shear strength and microhardness indicated that the exposure of the solder joints to the thermal cycles make the joints degraded faster than the situation in isothermal aging.

Interfacial Reaction Characteristics of Au Stud/Sn/Cu Pillar Bump During Annealing and Current Stressing.

In this work, intermetallic compound (IMC) growth behavior in Au stud/Sn/Cu pillar bumps was investigated under annealing and current stressing conditions. AuSn2 and AuSn4 IMCs formed at the interface between the Au studs and Sn after bonding. The AuSn2 phase grew significantly as the stressing time increased, causing micro-voids to form near the (Cu, Au)6Sn5, AuSn2 and AuSn4 IMC interfaces. The interfacial reactions resulting from current stressing took place quicker than observed for pure annealing. The apparent activation energies for the growth of the AuSn2 phase during annealing and current stressing were 0.52 eV and 0.47 eV, respectively, which may be closely related to the acceleration of the interfacial reaction by electron wind forces during current stressing.

The Spontaneous Growth of Pb Whisker from the Pb<sub>60</sub>Sn<sub>40</sub> Solder Layer on the Au Coating

In a recent failure analysis case of relay, the pure Pb whisker growth is found on the Pb60Sn40 solder layer on the Au coating of the spring leaf. The content analysis by the EDX shows that the Pb whisker grows from the Pb grain, which is surrounded by the AuSn intermetallic compound. It is thought that the elastic force of the spring leaf and the compressive stress from the intermetallic compound contributes to the Pb whisker growth, and the later factor is confirmed by the FIB (focused ion beam) result. Besides, the statistics result shows that the longer Pb whisker corresponds to the thicker Au coating, which could be explained by the effect of Au: forming AuSn intermetallic compound and Pb rich area. This work enriches our understandings on the metal whisker growth and consequently potential failure mode of SnPb solder.

Effect of Au-Sn IMCs’ formation and morphologies on shear properties of laser reflowed micro-solder joints

Purpose – The purpose of this paper is to investigate the effect of typical morphologies of Au-Sn IMCs (intermetallic compounds) at the interfaces of solder and pads on shear properties of laser reflowed micro-solder joints. Design/methodology/approach – Sn-2.0Ag-0.75Cu-3.0Bi (SnAgCuBi) solder balls (120 μm in diameter), pads with 0.1, 0.5, 0.9 or 4.0 μm thickness of Au surface finish, and different laser input energies were utilized to fabricate micro-solder joints with Au-Sn IMCs having different typical morphologies. The joints were performed by a shear test through a DAGE bond test system. Fracture surfaces of the joints were analyzed by scanning electron microscopy and energy-dispersive X-ray spectrometry to identify fracture modes and locations. Findings – Morphologies of Au-Sn IMCs would affect shear properties of the joints remarkably. When needle-like AuSn4 IMCs formed at the interfaces of solder and pads, almost entire surfaces presented the manner of ductile fracture. Moreover, shear forces of this kind of solder joints were higher than those of joints without Au-Sn IMCs or with a nearly continuous/continuous Au-Sn IMCs layer. The reason was that the shear performance of the solder joints with needle-like AuSn4 IMCs was enhanced by an interlocking effect between solder and needle-like AuSn4 IMCs. As a nearly continuous or continuous Au-Sn IMCs layer formed, the fracture surfaces presented more character of brittle than ductile fracture. However, if an Au layer still remained under Au-Sn IMCs, the shear performance of the joints would be enhanced. Originality/value – The results in this study can be used to optimize microstructures and shear properties of laser reflowed micro-solder joints.

Effect of the Silver Content of SnAgCu Solder on the Interfacial Reaction and on the Reliability of Angle Joints Fabricated by Laser-Jet Soldering

The silver content of lead-free solders affects their microstructure, the interfacial reaction, and the performance of the joints in reliability tests. In this study, Sn3.0Ag0.5Cu (wt.%, SAC305) and Sn1.0Ag0.5Cu (wt.%, SAC105) solder balls of diameter 55 μm were reflowed on gold surface pads by laser-jet soldering. It was found that four types of layered intermetallic compound (IMC) were formed at the interfaces; these were Au5Sn/AuSn, AuSn, AuSn2, and AuSn4 from the pad side to the solder matrix. The Au5Sn/AuSn eutectic region, thickness 400 nm, formed because of the high cooling rate induced by the laser-jet soldering. During high-temperature storage tests, the silver became segregated at the interfaces between the Au–Sn IMC and the solder matrix, resulting in inhibition of IMC growth in SAC305 joints, the shear strengths of which were higher than those of SAC105 joints. In mechanical drop tests, however, percentage failure of the SAC305 joints was twice that of the SAC105 joints.

Low-Temperature Solid-State Bonding Using Hydrogen Radical Treated Solder for Optoelectronic and MEMS Packaging

Many optoelectronic and MEMS devices require hermetic sealing for protection of the optoelectronic elements and movable fragile parts to ensure stable and reliable performance characteristics. Most commonly used bonding techniques for hermetic sealing are glass-frit bonding, anodic bonding, solder bonding, and eutectic bonding. For many applications, low temperature bonding is desirable because the temperature sensitive materials or components degrade and thermally induced stresses can be created during bonding. Furthermore, since the hermetic sealing is typically the final processing step in device fabrication, bonding method that is tolerant to surface roughness is desirable.In this study, a low-temperature solid-state bonding process at bonding temperature lower than 200 °C using hydrogen radical treated Sn-3.0Ag-0.5Cu (wt%) solder without flux is investigated for potential application as hermetic packaging. The interfacial reaction between solder and Au is studied by means of scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX). The bonding quality is evaluated by means of die-shear test and leak rate test.The re-oxidation of hydrogen radical treated Sn was studied by X-ray photoelectron spectroscopy (XPS). Fig. 1 shows the typical XPS spectra of Sn plate (purity: 5N) treated with hydrogen radicals (a) and argon fast atom beam (FAB) (b). The XPS Sn (3d2/5) spectra have been used to follow the surface oxide growth on Sn surfaces as a function of exposure time to air after removal of surface oxide. Two peaks are due to the presence of Sn metal and SnOx. The surface of Sn plate treated with Ar FAB contained a large amount of SnOxthan that with hydrogen radicals after exposure time to air of over 20 h. These results indicate differences between the removal of surface oxide by chemical reaction of hydrogen radicals and that by physical bombardment using fast atom beams. The inhibitory effect of re-oxidation of hydrogen radical treated solder [1] was confirmed by XPS measurement.Fig. 2 shows the process flow of the low-temperature solid-state bonding. Sn-Ag-Cu solder paste (melting point: 217 °C) is deposited by screen printing technique. Then, the surface oxides of Sn-Ag-Cu particles in the paste are reduced by hydrogen radical treatment at 180 ºC. After that, the solder is reflowed at 230 °C for 90 s with additional hydrogen radical treatment for 3 sec. During the reflow process, the Sn-Ag-Cu particles completely coalesce and solder pattern is formed on the wettable metal parts of the substrate [2]. Then, the Sn-Ag-Cu patterns are exposed to air and bonded with Au films without flux at temperatures below the melting point.Successful bonding was obtained at a bonding temperature above 150 °C for 10 min. Fig. 3 shows the SEM image and EDX map of the cross section of the bonded interface under the application of a bonding pressure of 150 MPa and held at 170 °C for 30 min. The Sn-Ag-Cu solder pattern was deformed to achieve enough contact with Au thin film (thickness: 500 nm) and Au-Sn intermetallic compounds were formed during bonding process as shown in Fig. 3(b). No clear interface corresponding to the original Au surfaces is visible. To evaluate the bonding strength, die-shear test was performed and the die shear strength was over 30 MPa. Fracture mainly occurred in the inside of solder layer, which indicates that solder layer was well bonded with Au.The Sn-Ag-Cu sealing ring pattern (width: 100 μm, size: 2.4 mm × 2. 4 mm, thickness: 5-10 μm) was formed on a glass substrate (2.8 × 2.8 mm2) by hydrogen radical reflow process for hermetic leak rate testing. The cavity structure (cavity volume: 1.9 × 10-3 cm3) with Ti (thickness: 50 nm)/Au (thickness: 500 nm) sealing ring was also fabricated on a Si substrate. Hermeticity was evaluated by measuring the resonance characteristics of photothermally excited microcantilevers inside the cavity [3]. The samples bonded at the low temperature of 170 °C under the application of a bonding pressure of 150 MPa showed leakage rates of less than 1.0 × 10−9 Pa·m3 s−1, which is the rejection limit defined by the MIL-STD-883G specification.In conclusion, hydrogen radical treated Sn-Ag-Cu solder patterns were bonded with Au thin films at a lower temperature than the melting point of Sn-Ag-Cu without flux. This process is expected to be applicable for chip-size packaging of optoelectronic and MEMS devices.

The effect of intermetallic compound evolution on the fracture behavior of Au stud bumps joined with Sn-3.5Ag solder

The microstructure and joint properties of Au stud bumps joined with Sn-3.5Ag solder were investigated as functions of flip chip bonding temperature and aging time. Au stud bumps were bonded on solder-onpad (SOP) at bonding temperature of 260°C and 300°C for 10 s, respectively. Aging treatment was carried out at 150°C for 100 h, 300 h, and 500 h, respectively. After flip chip bonding, intermetallic compounds (IMCs) of AuSn, AuSn2, and AuSn4 were formed at the interface between the Au stud bump and Sn-3.5Ag solder. At a bonding temperature of 300°C, AuSn2 IMC clusters, which were surrounded by AuSn4 IMCs, were observed in the Sn-3.5Ag solder bump. After flip chip bonding, bonding strength was approximately 220.5mN/bump. As aging time increased, the bonding strength decreased. After 100 h of aging treatment, the bonding strength of the joint bonded at 300°C was lower than that bonded at 260°C due to the fast growth rate of the AuSn2 IMCs. The main failure modes were interface fractures between the AuSn2 IMCs and AuSn4 IMCs, fractures through the AuSn2 IMCs and pad lift. Initial joint microstructures after flip chip bonding strongly affected the bonding strengths of aged samples.

First principles study of Au-Sn intermetallic compounds

A first-principles plane-wave pseudopotential method based on the density functional theory is used to investigate the energies, electronic structures, and elastic properties of intermetallic compounds of Au-Sn system. The enthalpies of formation, the cohesive energies, and elastic constants of these compounds are estimated from the electronic structure calculations, and their structural stabilities are also analyzed. The results show that the Au5Sn compound is unstable with respect to other compounds, and the bonding effects of AuSn2 and AuSn4 are stronger than those of AuSn and Au5Sn, for there are the strong hybridizations between Au and Sn atoms in AuSn2 and AuSn4 compounds. The main bonding effect of AuSn is Sn–Sn bonding interaction, and due to the Au content being maximal in Au5Sn the bonding of p electrons in Sn conduction band is suppressed by the covalent bonding of Au–Au.

Oxidation and Au‐Sn reaction of laser reflowed micro‐solder joints protected by N2 or exposed to air atmosphere

PurposeThe purpose of this paper is to investigate oxidation and the Au‐Sn reaction of laser reflowed (LR) micro‐solder joints when different protective atmospheres were applied.Design/methodology/approachA N2 atmosphere at room temperature, 60°C, 100°C and 130°C, or an air atmosphere at room temperature were utilized in this study. The solder balls were composed of Sn‐2.0Ag‐0.75Cu‐3.0Bi, and 120 μm in diameter. The surface finish of one pad of the joints was 4.0 μm Au/0.1 μm NiFe/0.01 μm Ta, another pad was made of Cu plated with 3.0 μm Au. The laser reflow process time was controlled to within 10 ms. Auger Electron Spectroscopy (AES) was used to identify the oxidation condition of LR solder joints with or without protection from a N2 atmosphere at room temperature. The appearance and cross‐sections of the joints protected by a N2 atmosphere at different temperatures were evaluated using SEM analysis.FindingsOxidation of LR solder joints from an air atmosphere was extremely severe, and the surfaces of solder were rough as compared with joints protected by a N2 atmosphere. Au‐rich phases and needle‐like AuSn4 intermetallic compounds (IMCs) formed at the interfaces of the solder and the pads. As the temperature of the N2 atmosphere was increased above 100°C, almost all of the Au‐rich phases disappeared. More needle‐like AuSn4 IMCs formed at the interfaces, as compared with that in joints protected by a N2 atmosphere at room temperature and 60°C. In addition, the orientation of the IMCs had clearly changed.Originality/valueThe results may provide a guide for controlling oxidation and the Au‐Sn reaction of micro‐solder joints during the LR process, and improving the properties of joints between solder and pads with Au surface finishes, by regulating the protective atmosphere.

Au 스터드 범프와 Sn-3.5Ag 솔더범프로 플립칩 본딩된 접합부의 미세조직 및 기계적 특성

The effect of flip chip bonding parameters on formation of intermetallic compounds (IMCs) between Au stud bumps and Sn-3.5Ag solder was investigated. In this study, flip chip bonding temperature was performed at <TEX>$260^{\circ}C$</TEX> and <TEX>$300^{\circ}C$</TEX> with various bonding times of 5, 10, and 20 sec. AuSn, <TEX>$AuSn_2$</TEX> and <TEX>$AuSn_4$</TEX> IMCs were formed at the interface of joints and (Au, Cu)<TEX>$_6Sn_5$</TEX> IMC was observed near Cu pad side in the joint. At bonding temperature of <TEX>$260^{\circ}C$</TEX>, <TEX>$AuSn_4$</TEX> IMC was dominant in the joint compared to other Au-Sn IMCs as bonding time increased. At bonding temperature of <TEX>$300^{\circ}C$</TEX>, <TEX>$AuSn_2$</TEX> IMC clusters, which were surrounded by <TEX>$AuSn_4$</TEX> IMC, were observed in the solder joint due to fast diffusivity of Au to molten solder with increased bonding temperature. Bond strength of Au stud bump joined with Sn-3.5Ag solder was about 23 gf/bump and fracture mode of the joint was intergranular fracture between <TEX>$AuSn_2$</TEX> and <TEX>$AuSn_4$</TEX> IMCs regardless bonding conditions.

The growth of intermetallic compound in Cu/Sn3.5Ag/Au solder joints under current stressing

In this study, the interfacial reactions and polarity effect on intermetallic compounds (IMCs) formation of the Cu/Sn3.5Ag/Au solder joints were investigated under 100 °C, 2.6 × 103 A/cm2 of current stressing. With electron flow from the Cu toward the Au, both Sn and Cu atoms migrated from the cathode to the anode, these phenomena also accompanied with the crack formation at the (Cu,Au)6Sn5/solder interface and the rapid dissolution of Cu consumed. The sequence of Au–Sn IMCs formed at the Au side upon electromigration (EM) is AuSn4/AuSn2/AuSn/Au5Sn while as AuSn4/AuSn2/AuSn for aging. At cathodic Au side, the voids were formed in the AuSn4 and at the AuSn4/solder interface due to the migration of Sn toward the anode. The voids formation in AuSn on both cathode and anode sides is due to the consumption of Sn by transformation of AuSn into Au5Sn.

Effect of isothermal aging on intermetallic compounds and Kirkendall void growth kinetics of Au stud bumps

Intermetallic compounds (IMCs) and Kirkendall void growth kinetics at various interfaces in Au stud bumps were studied in terms of isothermal aging at 120 °C, 150 °C, and 180 °C for 300 h. Phases of AuSn, AuSn2, and AuSn4 form at the interface of Au studs and Sn after a reflow. The thickness of an Au-Sn IMC was quantified with an image analyzer as a function of aging temperature and time. The growth of the Au-Sn IMC increases linearly with the square root of the aging time at 120 °C, 150 °C, and 180 °C. The growth of the Au-Sn IMC at 180 °C differs from the growth of the Au-Sn IMC at 120 °C and 150 °C because of excessive Au consumption by the Au studs. Kirkendall voids form at the interface of the Au stud and the Au-Sn IMC and increase linearly with the square root of the aging time. The apparent activation energies for the growth of the Au-Sn IMC and the Kirkendall voids were determined to be 0.57 eV and 0.2 eV, respectively, from measurement of the thickness of the Au-Sn IMCs and the width of the Kirkendall voids in relation to the aging temperature and time.

Microstructure, Intermetallic Growth, and Reliability of Rapidly Solidified Pb-Free Solder Joints Formed via Solder Ball Jetting

The majority of research on Pb-free solders has been done with reflow times sufficient to allow significant intermetallic growth at the liquid solder/pad interface. With the need for small, fine-pitched solder joints and to avoid damage to heat-sensitive read/write sensors, the hard disk drive industry predominantly uses solder jet bonding for electrical connections of the heads. This technique does not use flux and has solidification times on the order of a millisecond, four orders of magnitude smaller than that of conventional solder technology. Therefore, the intermetallic formation is highly nonequilibrium and is localized near the pad interface. Surface finish thicknesses and composition have a significant influence on intermetallic growth and composition of solder joints. The intermetallic microstructure, along with voids, can significantly impact joint reliability. In this paper, results of wettability and mechanical solder ball shear measurements on as-plated and aged samples are reported and correlated with microstructure and growth of intermetallic compounds (IMCs) at the solder/pad interface. A wide range of Au-Sn IMCs are found at the pad interface, and Au-embrittlement effects are not found until Au exceeds 7 wt.%.