A Hermetic Seal Using Composite Thin-Film In/Sn Solder as an Intermediate Layer and Its Interdiffusion Reaction with Cu

A bonding joint between Cu metallization and evaporated In/Sn composite solder is produced at a temperature lower than 200°C in air. The effects of bonding temperature and duration on the interfacial bonding strength are studied herein. Cross sections of bonding joints processed at different bonding conditions were examined by scanning electron microscopy (SEM). The optimal condition, i.e., bonding temperature of 180°C for 20 min, was chosen because it gave rise to the highest average bonding strength of 6.5 MPa, and a uniform bonding interface with minimum voids or cracks. Good bond formation was also evidenced by scanning acoustic imaging. For bonding couples of patterned dies, a helium leak rate of 5.8 × 10⁻⁹ atm cc/s was measured, indicating a hermetic seal. The interfacial reaction between Cu and In/Sn was also studied. Intermetallic compounds (IMCs) such as AuIn₂, Cu₆Sn₅, and Cu₁₁In₉ were detected by means of x-ray diffraction analysis (XRD), and transmission electron microscopy (TEM) accompanied by energy-dispersive x-ray (EDX) spectroscopy. Chemical composition analysis also revealed that solder interlayers, Sn, and In were completely converted into IMCs by reaction with Cu. All the IMCs formed in the joints have remelting temperatures above 300°C according to the Cu-In, Cu-Sn, and Au-In phase diagrams. Therefore, the joint is able to sustain high service temperatures due to the presence of these IMCs.     

Solid Liquid Interdiffusion Bonding of (Pb,Sn)Te Thermoelectric Modules with Cu Electrodes Using a Thin-Film Sn Interlayer

A (Pb, Sn)Te thermoelectric element plated with a Ni barrier layer and a Ag reaction layer has been joined with a Cu electrode coated with Ag and Sn thin films using a solid–liquid interdiffusion bonding method. This method allows the interfacial reaction between Ag and Sn such that Ag₃Sn intermetallic compounds form at low temperature and are stable at high temperature. In this study, the bonding strength was about 6.6 MPa, and the specimens fractured along the interface between the (Pb, Sn)Te thermoelectric element and the Ni barrier layer. Pre-electroplating a film of Sn with a thickness of about 1 μm on the thermoelectric element and pre-heating at 250°C for 3 min ensures the adhesion between the thermoelectric material and the Ni barrier layer. The bonding strength is thus increased to a maximal value of 12.2 MPa, and most of the fractures occur inside the thermoelectric material. During the bonding process, not only the Ag₃Sn intermetallics but also Cu₆Sn₅ forms at the Ag₃Sn/Cu interface, which transforms into Cu₃Sn with increases in the bonding temperature or bonding time.     

Intermetallic reactions in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Au/Ni surface finishes

The intermetallic compounds (IMCs) formed during the reflow and aging of Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Au/Ni surface finishes were investigated. After reflow, the thickness of (Cu, Ni, Au)₆Sn₅ interfacial IMCs in Sn3Ag0.5Cu0.06Ni0.01Ge was similar to that in the Sn3Ag0.5Cu specimen. The interiors of the solder balls in both packages contained Ag₃Sn precipitates and brick-shaped AuSn₄ IMCs. After aging at 150°C, the growth thickness of the interfacial (Ni, Cu, Au)₃Sn₄ intermetallic layers and the consumption of the Ni surface-finished layer on Cu the pads in Sn3Ag0.5Cu0.06Ni0.01Ge solder joints were both slightly less than those in Sn3Ag0.5Cu. In addition, a coarsening phenomenon for AuSn₄ IMCs could be observed in the solder matrix of Sn3Ag0.5Cu, yet this phenomenon did not occur in the case of Sn3Ag0.5Cu0.06Ni0.01Ge. Ball shear tests revealed that the reflowed Sn3Ag0.5Cu0.06Ni0.01Ge packages possessed bonding strengths similar to those of the Sn3Ag0.5Cu. However, aging treatment caused the ball shear strength in the Sn3Ag0.5Cu packages to degrade more than that in the Sn3Ag0.5Cu0.06Ni0.01Ge packages.     

Intermetallic Compound Formation Mechanisms for Cu-Sn Solid–Liquid Interdiffusion Bonding

Cu-Sn solid?Cliquid interdiffusion (SLID) bonding is an evolving technique for wafer-level packaging which features robust, fine pitch and high temperature tolerance. The mechanisms of Cu-Sn SLID bonding for wafer-level bonding and three-dimensional (3-D) packaging applications have been studied by analyzing the microstructure evolution of Cu-Sn intermetallic compounds (IMCs) at elevated temperature up to 400°C. The bonding time required to achieve a single IMC phase (Cu₃Sn) in the final interconnects was estimated according to the parabolic growth law with consideration of defect-induced deviation. The effect of predominantly Cu metal grain size on the Cu-Sn interdiffusion rate is discussed. The temperature versus time profile (ramp rate) is critical to control the morphology of scallops in the IMC. A low temperature ramp rate before reaching the bonding temperature is believed to be favorable in a SLID wafer-level bonding process.     

Kinetics of Intermetallic Compound Formation at the Interface Between Sn-3.0Ag-0.5Cu Solder and Cu-Zn Alloy Substrates

The growth kinetics of an intermetallic compound (IMC) layer formed between Sn-3.5Ag-0.5Cu (SAC) solders and Cu-Zn alloy substrates was investigated for samples aged at different temperatures. Scallop-shaped Cu₆Sn₅ formed after soldering by dipping Cu or Cu-10 wt.%Zn wires into the molten solder at 260°C. Isothermal aging was performed at 120°C, 150°C, and 180°C for up to 2000 h. During the aging process, the morphology of Cu₆Sn₅ changed to a planar type in both specimens. Typical bilayer of Cu₆Sn₅ and Cu₃Sn and numerous microvoids were formed at the SAC/Cu interfaces after aging, while Cu₃Sn and microvoids were not observed at the SAC/Cu-Zn interfaces. IMC growth on the Cu substrate was controlled by volume diffusion in all conditions. In contrast, IMC growth on Cu-Zn specimens was controlled by interfacial reaction for a short aging time and volume diffusion kinetics for a long aging time. The growth rate of IMCs on Cu-Zn substrates was much slower due to the larger activation energy and the lower layer growth coefficient for the growth of Cu-Sn IMCs. This effect was more prominent at higher aging temperatures.     

A Study on the Thermal Reliability of Cu/SnAg Double-Bump Flip-Chip Assemblies on Organic Substrates

The Cu/SnAg double-bump structure is a promising candidate for fine-pitch flip-chip applications. In this study, the interfacial reactions of Cu (60 μm)/SnAg (20 μm) double-bump flip chip assemblies with a 100 μm pitch were investigated. Two types of thermal treatments, multiple reflows and thermal aging, were performed to evaluate the thermal reliability of Cu/SnAg flip-chip assemblies on organic printed circuit boards (PCBs). After these thermal treatments, the resulting intermetallic compounds (IMCs) were identified with scanning electron microscopy (SEM), and the contact resistance was measured using a daisy-chain and a four-point Kelvin structure. Several types of intermetallic compounds form at the Cu column/SnAg solder interface and the SnAg solder/Ni pad interface. In the case of flip-chip samples reflowed at 250°C and 280°C, Cu₆Sn₅ and (Cu, Ni)₆Sn₅ IMCs were found at the Cu/SnAg and SnAg/Ni interfaces, respectively. In addition, an abnormal Ag₃Sn phase was detected inside the SnAg solder. However, no changes were found in the electrical contact resistance in spite of severe IMC formation in the SnAg solder after five reflows. In thermally aged flip-chip samples, Cu₆Sn₅ and Cu₃Sn IMCs were found at the Cu/SnAg interface, and (Cu, Ni)₆Sn₅ IMCs were found at the SnAg/Ni interface. However, Ag₃Sn IMCs were not observed, even for longer aging times and higher temperatures. The growth of Cu₃Sn IMCs at the Cu/SnAg interface was found to lead to the formation of Kirkendall voids inside the Cu₃Sn IMCs and linked voids within the Cu₃Sn/Cu column interfaces. These voids became more evident when the aging time and temperature increased. The contact resistance was found to be nearly unchanged after 2000 h at 125°C, but increases slightly at 150°C, and a number of Cu/SnAg joints failed after 2000 h. This failure was caused by a reduction in the contact area due to the formation of Kirkendall and linked voids at the Cu column/Cu₃Sn IMC interface.     

Effect of Isothermal Aging and Thermal Cycling on Interfacial IMC Growth and Fracture Behavior of SnAgCu/Cu Joints

The growth behavior of interfacial intermetallic compounds (IMCs) of SnAgCu/Cu soldered joints was investigated during the reflow process, isothermal aging, and thermal cycling with a focus on the influence of these parameters on growth kinetics. The SnAgCu/Cu soldered joints were isothermally aged at 125°C, 150°C, and 175°C while the thermal cycling was performed within the temperature ranges from −25°C to 125°C and −40°C to 125°C. It was observed that a Cu₆Sn₅ layer formed, followed by rapid coarsening at the solder/Cu interface during reflowing. The grain size of the interfacial Cu₆Sn₅ was found to increase with aging time, and the morphology evolved from scallop-like to needle-like to rod-like and finally to particles. The rod-like Ag₃Sn phase was formed on the solder side in front of the previously formed Cu₆Sn₅ layer. However, when subject to an increase of the aging time, the Cu₃Sn phase was formed at the interface of the Cu₆Sn₅ layer and Cu substrate. The IMC growth rate increased with aging temperature for isothermally aged joints. During thermal cycling, the thickness of the IMC layer was found to increase with the number of thermal cycles, although the growth rate was slower than that for isothermal aging. The dwell time at the high-temperature end of the thermal cycles was found to significantly influence the growth rate of the IMCs. The growth of the IMCs, for both isothermal aging and thermal cycling, was found to be Arrhenius with aging temperature, and the corresponding diffusion factor and activation energy were obtained by data fitting. The tensile strength of the soldered joints decreased with increasing aging time. Consequently, the fracture site of the soldered joints migrated from the solder matrix to the interfacial Cu₆Sn₅ layer. Finally, the shear strength of the joints was found to decrease with both an increase in the number of thermal cycles and a decrease in the dwell temperature at the low end of the thermal cycle.     

Effect of volume in interfacial reaction between eutectic Sn-3.5% Ag-0.5% Cu solder and Cu metallization in microelectronic packaging

The Sn-3.5Ag-0.5Cu (wt.%) is the most promising replacement for the eutectic tin-lead solder alloy. Here, an investigation has been carried out to compare the interfacial reactions of the Cu pad of a ball grid array (BGA) substrate with molten eutectic Sn-3.5% Ag-0.5% Cu solder having different volumes. Two different sizes of BGA solder balls were used: 760-μm and 500-μm diameter. Scanning electron microscopy (SEM) was used to measure the consumed thickness of the Cu and also the thickness of the intermetallic compound (IMC). The soldering reaction was carried out at 230°C, 240°C, and 250°C for 1 min, 5 min, 10 min, and 20 min. The Cu consumption was much higher for the Sn-Ag-Cu solder with higher volume. On the other hand, the mean thickness of the intermetallics for solder with smaller volume was thicker than that of the bigger solder balls. The Cu₃Sn compound was also observed at the interface between the Cu₆Sn₅ IMCs and Cu substrate for longer reflow for the both solder balls. Larger Cu₆Sn₅ IMCs were observed in the bulk of the solder with bigger volume. A simplistic theoretical approach is carried out to find out the amount of Cu₆Sn₅ IMCs in the bulk of the solder by measurement of the Cu consumption from the substrate and the thickness of the IMCs that form on the interface.     

Low-Temperature Synthesis of High-Adhesion Cu(Mg) Alloy Films on Glass Substrates

Cu(0.5 at.%Mg) alloy films were deposited on glass substrates, and annealed at 200–400 °C in vacuum. The resistivity of the Cu(Mg) films was reduced to about 3.0 μΩcm after annealing at 200 °C for 30 min, and the tensile strength of adhesion of the Cu(Mg) films to the glass substrates was increased to 30–40 and 35–55 MPa after annealing at 250 and 300 °C, respectively. The reduction in resistivity can be explained as reduced impurity scattering and grain-boundary scattering, since Mg segregation to the film surface and Cu(Mg)/glass interface, and consequent Cu grain growth, were observed. Increased adhesion of the Cu(Mg) films to glass substrates after annealing was also explained by the strong segregation of Mg atoms, and the formation of a reaction layer at the interface. Mg atoms were observed to have reacted with the glass substrates and formed a thin crystalline MgO layer at the interface in the samples annealed at 300 °C, while Mg atoms were highly concentrated above the Cu(Mg)/glass interface without oxide formation at the interface in the samples annealed at 250 °C. Thus, the process temperature and time to obtain low-resistivity and high-adhesion Cu alloy films on glass substrates could be reduced to 250 °C and 30 min using Cu(Mg) films.     

Controlling intermetallic compound growth in SnAgCu/Ni-P solder joints by nanosized Cu6Sn5 addition

Nanosized Cu₆Sn₅ dispersoids were incorporated into Sn and Ag powders and milled together to form Sn-3Ag-0.5Cu composite solders by a mechanical alloying process. The aim of this study was to investigate the interfacial reaction between SnAgCu composite solder and electroless Ni-P/Cu UBM after heating for 15 min. at 240°C. The growth of the IMCs formed at the composite solder/EN interface was retarded as compared to the commercial Sn3Ag0.5Cu solder joints. With the aid of the elemental distribution by x-ray color mapping in electron probe microanalysis (EPMA), it was revealed that the SnAgCu composite solder exhibited a refined structure. It is proposed that the Cu₆Sn₅ additives were pinned on the grain boundary of Sn after heat treatment, which thus retarded the movement of Cu toward the solder/EN interface to form interfacial compounds. In addition, wetting is an essential prerequisite for soldering to ensure good bonding between solder and substrate. It was demonstrated that the contact angles of composite solder paste was <25°, and good wettability was thus assured.     

Synthesis and Optical Properties of Si-Rich Nitride Containing Silicon Quantum Dots

Hydrogenated silicon-rich nitride films were deposited by plasma-enhanced chemical vapor deposition using NH₃ and SiH₄. As-deposited samples were thermally annealed under different conditions in argon ambient. Fourier-transform infrared spectroscopy was carried out to investigate the bonding configurations, and Raman scattering spectroscopy was used to study the microstructures and confirm the presence of Si quantum dots (QDs). We found that a near-stoichiometric silicon nitride matrix was formed after high-temperature processing. When the annealing temperature reached 1100°C, the degree of crystallinity (X _c) increased to 51.6% for the 60-min sample compared with 46.1% for the 30-min sample. Red-light and yellow-light emission were obtained from the samples annealed at 1100°C for 30 min and 60 min, respectively. The emission mechanism is dominated by excitons confined within the Si QDs. The ultra-nanocrystals play an important role in the luminescence blue-shift. We measured the bandgap values from optical absorption studies. The increase of the optical bandgap from 1.80 eV to 1.90 eV with increase of the annealing temperature from 950°C to 1100°C is ascribed to the silicon clusters and nitride matrix.     

Microstructure evolution of gold-tin eutectic solder on Cu and Ni substrates

The microstructures of the eutectic Au20Sn (wt.%) solder that developed on the Cu and Ni substrates were studied. The Sn/Au/Ni sandwich structure (2.5/3.75/2 μm) and the Sn/Au/Ni sandwich structure (1.83/2.74/5.8 μm) were deposited on Si wafers first. The overall composition of the Au and the Sn layers in these sandwich structures corresponded to the Au20Sn binary eutectic. The microstructures of the Au20Sn solder on the Cu and Ni substrates could be controlled by using different bonding conditions. When the bonding condition was 290°C for 2 min, the microstructure of Au20Sn/Cu and Au20Sn/Ni was a two-phase (Au₅Sn and AuSn) eutectic microstructure. When the bonding condition was 240°C for 2 min, the AuSn/Au₅Sn/Cu and AuSn/Au₅Sn/Ni diffusion couples were subjected to aging at 240°C. The thermal stability of Au20Sn/Ni was better than that of Au20Sn/Cu. Moreover, less Ni was consumed compared to that of Cu. This indicates that Ni is a more effective diffusion barrier material for the Au20Sn solder.     

Intermetallic compounds formed during interfacial reactions between liquid Sn-8Zn-3Bi solders and Ni substrates

The morphology and growth kinetics of intermetallic compounds (IMCs) formed at the interfaces between liquid Sn-8Zn-3Bi solders and nickel substrates in the temperature range from 225°C to 400°C are investigated for the applications in bonding recycled sputtering targets to their backing plates. The results show that a continuous single layer of Ni₅Zn₂₁ IMC appears at temperatures below 325°C, while a double layer containing Ni₅Zn₂₁ and Ni₃₅Zn₂₂Sn₄₃ IMCs is formed at temperatures above 325°C. In both cases, the growth kinetics of IMCs is interface-controlled. During the growth of IMCs, their reaction fronts migrate in the direction of the solder much more rapidly than toward the nickel substrate, and erosion of the Ni substrate is quite slight.     

Pressureless Bonding by Use of Cu and Sn Mixed Nanoparticles

Because of high thermal and electrical conductivity, high melting point, and low cost, bonding by sintering of Cu nanoparticles is promising as a new method to replace the Pb-rich solders currently used in high-temperature applications. However, it is difficult to achieve sufficient strength by using this method because, in the absence of applied pressure, oxidized surfaces inhibit sintering. In this study, we report pressureless bonding of Cu plates with Ni layers by use of Cu and Sn mixed nanoparticles. Bonding was achieved without pressure from 250°C to 350°C in hydrogen by use of pure Cu nanoparticles, and by using mixtures of Cu and Sn nanoparticles in which the amount of Sn varied from 10 to 50 wt.%. The highest strength bonds were obtained by use of Cu–10 wt.% Sn mixed nanoparticles, because the sinterability of the Cu nanoparticles is enhanced by diffusion of Sn into Cu to form an appropriate amount of Cu–Sn intermetallic compounds (IMC) and diminish microvoids. However, when the amount of Sn was greater than 10 wt.%, Cu–Sn IMC were formed to such an extent that the significant reduction of Cu-rich layers led to reduced strength. When the bonding temperature was 350°C, Sn diffused into Cu so much that microvoids were formed in the Sn-rich layer. Because the number of microvoids increased as the amount of Sn was increased, the shear strength could not be enhanced by bonding at higher temperature when the amount of Sn was greater than 30 wt.%.     

Investigation of soldering process and interfacial microstructure evolution for the formation of full Cu3Sn joints in electronic packaging

Within electronic products, solder joints with common interfacial structure of Cu/IMCs/Sn-based solders/IMCs/Cu cannot be used under high temperature for relatively low melting points of Sn-based solders (200–300 °C). However, there is a trend for solder joints to service under high temperature because of the objective for achieving multi-functionality of electronic products.With the purpose of ensuring that solder joints can service under high temperature, full Cu₃Sn solder joints with the interfacial structure of Cu/Cu₃Sn/Cu can be a substitute due to the high melting point of Cu₃Sn (676 °C). In this investigation, soldering process parameters were optimized systematically in order to obtain such joints. Further, interfacial microstructure evolution during soldering was analyzed. The soldering temperature of 260 °C, the soldering pressure of 1 N and the soldering time of 5 h were found to be the optimal parameter combination. During soldering of 260 °C and 1 N, the Cu₆Sn₅ precipitated first in a planar shape at Cu-Sn interfaces, which was followed by the appearance of planar Cu₃Sn between Cu and Cu₆Sn₅. Then, the Cu₆Sn₅ at opposite sides continued to grow with a transition from a planar shape to a scallop-like shape until residual Sn was consumed totally. Meanwhile, the Cu₃Sn grew with a round-trip shift from a planar shape to a wave-like shape until the full Cu₃Sn solder joint was eventually formed at 5 h. The detailed reasons for the shape transformation in both Cu₆Sn₅ and Cu₃Sn during soldering were given. Afterwards, a microstructure evolution model for Cu-Sn-Cu sandwich structure during soldering was proposed. Besides, it was found that no void appeared in the interfacial region during the entire soldering process, and a discuss about what led to the formation of void-free joints was conducted.     

Investigation of Sn Whisker Growth in Electroplated Sn and Sn-Ag as a Function of Plating Variables and Storage Conditions

Sn whiskers are becoming a serious reliability issue in Pb-free electronic packaging applications. Among the numerous Sn whisker mitigation strategies, minor alloying additions to Sn have been proven effective. In this study, several commercial Sn and Sn-Ag baths of low-whisker formulations are evaluated to develop optimum mitigation strategies for electroplated Sn and Sn-Ag. The effects of plating variables and storage conditions, including plating thickness and current density, on Sn whisker growth are investigated for matte Sn, matte Sn-Ag, and bright Sn-Ag electroplated on a Si substrate. Two different storage conditions are applied: an ambient condition (30°C, dry air) and a high-temperature/high-humidity condition (55°C, 85% relative humidity). Scanning electron microscopy is employed to record the Sn whisker growth history of each sample up to 4000 h. Transmission electron microscopy, x-ray diffraction, and focused ion beam techniques are used to understand the microstructure, the formation of intermetallic compounds (IMCs), oxidation, the Sn whisker growth mechanism, and other features. In this study, it is found that whiskers are observed only under ambient conditions for both thin and thick samples regardless of the current density variations for matte Sn. However, whiskers are not observed on Sn-Ag-plated surfaces due to the equiaxed grains and fine Ag₃Sn IMCs located at grain boundaries. In addition, Sn whiskers can be suppressed under the high-temperature/high-humidity conditions due to the random growth of IMCs and the formation of thick oxide layers.     

<Emphasis Type="Italic">In Situ</Emphasis> Study of Reduction Process of CuO Paste and Its Effect on Bondability of Cu-to-Cu Joints

A bonding method utilizing redox reactions of metallic oxide microparticles achieves metal-to-metal bonding in air, which can be alternative to lead-rich high-melting point solder. However, it is known that the degree of the reduction of metallic oxide microparticles have an influence on the joint strength using this bonding method. In this paper, the reduction behavior of CuO paste and its effect on Cu-to-Cu joints were investigated through simultaneous microstructure-related x-ray diffraction and differential scanning calorimetry measurements. The CuO microparticles in the paste were gradually reduced to submicron Cu₂O particles at 210–250°C. Subsequently, Cu nanoparticles were generated instantaneously at 300–315°C. There was a marked difference in the strengths of the joints formed at 300°C and 350°C. Thus, the Cu nanoparticles play a critical role in sintering-based bonding using CuO paste. Furthermore, once the Cu nanoparticles have formed, the joint strength increases with higher bonding temperature (from 350°C to 500°C) and pressure (5–15 MPa), which can exceed the strength of Pb-5Sn solder at higher temperature and pressure.     

Thin-film reactions during diffusion soldering of Cu/Ti/Si and Au/Cu/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> with Sn interlayers

The multilayer thin-film systems of Cu/Ti/Si and Au/Cu/Al₂O₃ were diffusion-soldered at temperatures between 250°C and 400°C by inserting a Sn thin-film interlayer. Experimental results showed that a double layer of intermetallic compounds (IMCs) η-(Cu_0.99Au_0.01)₆Sn₅/δ-(Au_0.87Cu_0.13)Sn was formed at the interface. Kinetics analyses revealed that the growth of intermetallics was diffusion-controlled. The activation energies as calculated from Arrhenius plots of the growth rate constants for (Cu_0.99Au_0.01)₆Sn₅ and (Au_0.87Cu_0.13)Sn are 16.9 kJ/mol and 53.7 kJ/mol, respectively. Finally, a satisfactory tensile strength of 132 kg/cm² could be attained under the bonding condition of 300°C for 20 min.     

Contact Resistance of Flip-Chip Joints in Wearable Electronic Textiles

Flip-chip bonding to a Cu lead frame transferred to a fabric was achieved by use of a non-conducting adhesive. Average contact resistance of the flip-chip joints was evaluated on variation of the Cu and Sn thickness of Cu/Sn bumps of size 150 × 220 μm². The total thickness of the Cu/Sn bumps was fixed at 15 μm. The average contact resistance of the flip-chip joints on the fabric was 5.4–10.8 mΩ, depending on the Sn thickness of the Cu/Sn bumps; this was lower than for flip-chip joints on a rigid Si substrate (15.6–26.5 mΩ). The average contact resistance of flip-chip joints on the fabric decreased from 10.8 mΩ to 5.5 mΩ when the chip–bump configuration was changed from 15-μm-thick Sn to 7-μm-thick Cu/8-μm-thick Sn. The contact resistance of flip-chip joints bonded with the 7-μm-thick Cu/8-μm-thick Sn bumps remained below 10 mΩ for up to 750 h in the 85°C/85% relative humidity test and even decreased to below 4 mΩ in the storage test at 125°C for up to 1000 h.     

Diffusion Soldering of Pb-Doped GeTe Thermoelectric Modules with Cu Electrodes Using a Thin-Film Sn Interlayer

Directly coating a GeTe(Pb) thermoelectric device with a Ni barrier layer and an Ag reaction layer and then diffusion soldering with a Cu electrode coated with Ag and Sn leads to breakage at the GeTe(Pb)/Ni interface and low bonding strengths of about 6 MPa. An improved process, precoating with 1 μm Sn film and heating at 250°C for 3 min before electroplating with Ni and Ag layers, results in satisfactory bonding strengths ranging from 12.6 MPa to 19.1 MPa. The precoated Sn film leads to the formation of a (Ni,Ge)₃Sn₄ layer between the GeTe(Pb) thermoelectric material and Ni barrier layer, reducing the thermal stress at the GeTe(Pb)/Ni interface.