<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.     

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.     

Effect of Amount of Cu on the Intermetallic Layer Thickness Between Sn-Cu Solders and Cu Substrates

In the present study, Sn-Cu solders were synthesized using pure tin with varying weight percentage of nanosized copper particles (0 wt.%, 0.25 wt.%, 0.43 wt.%, 0.86 wt.%, and 1.35 wt.%) by a powder metallurgy route incorporating microwave-assisted sintering. Intermetallic compound (IMC) layer formation between Sn-Cu solders and Cu substrates was investigated following a reflow process. Isothermal aging studies were also conducted on selected Sn-Cu solders at 150°C for up to 4 weeks. Results revealed that the average IMC layer thickness decreases with the addition of nanocopper up to 0.43 wt.%. Beyond 0.43 wt.% Cu addition, the IMC layer thickness started to increase, and the maximum IMC layer thickness was found for Sn with 1.35 wt.% Cu addition. An attempt was made in this study to correlate the effect of nanocopper additions and aging time on the IMC layer thicknesses.     

Chip to Chip Bonding Using Cu Bumps Capped with Thin Sn Layers and the Effect of Microstructure on the Shear Strength of Joints

Chip to chip bonding techniques using Cu bumps capped with thin solder layers have been frequently applied to 3D chip stacking technology. We studied the effect of joint microstructure on shear strength. Joints were formed by joining Sn/Cu bumps on a Si die and Sn/Cu layers on another Si die at 245–330°C using a thermo-compression bonder. Three different types of microstructures were fabricated in the joints by controlling the bonding temperature and time, (1) a Sn-rich phase with a Cu₆Sn₅ phase at the Cu interfaces, (2) a Cu₆Sn₅ phase in the interior with a Cu₃Sn phase at the Cu interfaces, and (3) one single Cu₃Sn phase throughout the whole joint. The joint having a single Cu₃Sn phase had the highest shear strength. Specimens were aged up to 2000 h at 150°C and 180°C. During aging, the microstructures of all joints were transformed in a single Cu₃Sn phase. The shear strength of the joints was very sensitive to the formation of Cu₃Sn and microvoids. Microvoids formed in the solder joints with a Cu₆Sn₅ phase with and without a Sn-rich phase during aging and decreased the shear strength of the joints. Conversely, aging did not induce the formation of microvoids in the joints which originally had only a Cu₃Sn phase and the shear strength was not decreased.     

Interfacial Reactions of Sn, Sn-3.0Ag-0.5Cu, and Sn-9Zn Lead-Free Solders with Fe-42Ni Substrates

Interfacial reactions between Sn, Sn-3.0 wt.%Ag-0.5 wt.%Cu (SAC), and Sn-9 wt.%Zn (SZ) lead-free solders and Fe-42 wt.%Ni (alloy 42) substrates at 240°C, 255°C, and 270°C were investigated in this study. FeSn₂, (Fe,Ni, Cu)Sn₂, and (Ni,Fe)₅Zn₂₁ phases were formed, respectively, at the interface in the Sn/alloy 42, SAC/alloy 42, and SZ/alloy 42 couples. As the reaction time and temperature were increased, the layered intermetallic compound (IMC) assumed two distinct structures, i.e., a thicker layer and a pillar-shaped IMC, in all couples. The IMC thickness of these couples increased with the increase of reaction time and temperature. The IMC thickness was also proportional to the square root of the reaction time. The interfacial reaction mechanism of these couples was diffusion controlled.     

Interfacial Reactions Between Pb-Free Solders and In/Ni/Cu Multilayer Substrates

This study investigates the interfacial reactions between Sn-3.0wt.% Ag-0.5wt.%Cu (SAC) and Sn-0.7wt.%Cu (SC) on In/Ni/Cu multilayer substrates using the solid–liquid interdiffusion bonding technique. Samples were reflowed first at 160°C, 180°C, and 200°C for various periods, and then aged at 100°C for 100 h to 500 h. The scalloped Cu₆Sn₅ phase was formed at the SAC/In/Ni/Cu and SC/In/Ni/Cu interfaces. When the reflowing temperatures were 160°C and 180°C, a ternary Ni-In-Sn intermetallic compound (IMC) was formed when the samples were further aged at 100°C. This ternary Ni-In-Sn IMC could be the binary Ni₃Sn₄ phase with extensive Cu and In solubilities, or the ternary Sn-In-Ni compound with Cu solubility, or even a quaternary compound. As the reflow temperature was increased to 200°C, only one Cu₆Sn₅ phase was formed at the solder/substrate interface with the heat treatment at 100°C for 500 h. Mechanical test results indicated that the formation of the Ni-In-Sn ternary IMC weakened the mechanical strength of the solder joints. Furthermore, the solid–liquid interdiffusion (SLID) technique in this work effectively reduced the reflow temperature.     

Effect of trace platinum additions on the interfacial morphology of Sn–3.8Ag–0.7Cu alloy aged for long hours

While the Sn–Ag–Cu (SAC) family of solders are considered good candidate as lead-free solder replacement materials, their relatively short processing history and application result in a host of materials as well as reliability problems. For good metallurgical bonding and electrical connection, a thin, even layer of intermetallic compound (IMC) is required but excessive growth of the IMC layer will cause various reliability problems. This is especially critical for miniaturized solder pitches in very large scale integration circuits. This work adopts the composite approach of adding 0.15 and 0.30 wt.% of Pt into Sn–3.8Ag–0.7Cu alloy to study the effect of these additions to the IMC layer thickness between the solder and substrate. Alloys were isothermally aged at 150 °C for up to 1000 h to observe contribution of Pt in suppressing excessive IMC growth. It was found that when more Pt was added to the alloy, the IMC layer became more even and continuous. Voids and IMC layer thickness were reduced. This is attributed to the role of Pt in replacing Cu in the solder and thus impeding excessive diffusion.     

Effects of microstructural evolution and intermetallic layer growth on shear strength of ball-grid-array Sn-Cu solder joints

The shear strength of ball-grid-array (BGA) solder joints on Cu bond pads was studied for Sn-Cu solder containing 0, 1.5, and 2.5 wt.% Cu, focusing on the effect of the microstructural changes of the bulk solder and the growth of intermetallic (IMC) layers during soldering at 270°C and aging at 150°C. The Cu additions in Sn solder enhanced both the IMC layer growth and the solder/IMC interface roughness during soldering but had insignificant effects during aging. Rapid Cu dissolution from the pad during reflow soldering resulted in a fine dispersion of Cu₆Sn₅ particles throughout the bulk solder in as-soldered joints even for the case of pure Sn solder, giving rise to a precipitation hardening of the bulk solder. The increased strength of the bulk solder caused the fracture mode of as-soldered joints to shift from the bulk solder to the solder/IMC layer as the IMC layer grew over a critical thickness about 1.2 m for all solders. The bulk solder strength decreased rapidly as the fine Cu₆Sn₅ precipitates coarsened during aging. As a consequence, regardless of the IMC layer thickness and the Cu content of the solders, the shear strength of BGA solder joints degraded significantly after 1 day of aging at 150°C and the shear fracture of aged joints occurred in the bulk solder. This suggests that small additions of Cu in Sn-based solders have an insignificant effect on the shear strength of BGA solderjoints, especially during system use at high temperatures.     

Optimized Cu-Sn Wafer-Level Bonding Using Intermetallic Phase Characterization

The objective of this study is to optimize the Cu/Sn solid–liquid interdiffusion process for wafer-level bonding applications. To optimize the temperature profile of the bonding process, the formation of intermetallic compounds (IMCs) which takes place during the bonding process needs to be well understood and characterized. In this study, a simulation model for the development of IMCs and the unreacted remaining Sn thickness as a function of the bonding temperature profile was developed. With this accurate simulation model, we are able to predict the parameters which are critical for bonding process optimization. The initial characterization focuses on a kinetics model of the Cu₃Sn thickness growth and the amount of Sn thickness that reacts with Cu to form IMCs. As-plated Cu/Sn samples were annealed using different temperatures (150°C to 300°C) and durations (0 min to 320 min). The kinetics model is then extracted from the measured thickness of IMCs of the annealed samples.     

Effects of thermal aging on Cu nanoparticle/Bi-Sn solder hybrid bonding

The effects of high-temperature aging on a novel hybrid bonding layer were investigated. The hybrid layer, which consisted of Cu nanoparticles and a eutectic Bi-Sn solder powder, was formed by a sintering reaction of the solid-phase Cu nanoparticles and a chemical reaction involving the liquid-phase Bi and Sn in combination. The layer was used to bond a SiC chip to a direct bonded aluminum substrate. A conventional Cu nanoparticle-based bonding layer was also prepared as a reference. Samples with these bonding layers were evaluated using the thermal aging test (225 or 250 °C, 100 h). Bonding strength and synchrotron radiation computed laminography (SRCL) measurements were performed both before and after the thermal aging test. It was observed that thermal aging greatly decreased the bonding strength of the conventional layer. In contrast, the bonding strength of the hybrid layer was reduced only slightly by the thermal aging treatment. SRCL images showed that the conventional layer exhibited numerous cracks, which acted as passages for oxidation. On the other hand, in the hybrid layer, the liquid-phase Bi and Sn densified the Cu sintering phase through the formation of an alloyed Cu-Sn phase. As a result, the hybrid layer contained fewer passages for oxidation as compared to the conventional layer and maintained its bonding strength even against thermal aging. Therefore, the hybrid layer, which is highly stable against thermal aging, will be useful for the high-temperature operation of intelligent power modules.     

AlN-to-Metal Direct Bonding Process Utilizing Sintering of Ag Nanoparticles Derived from the Reduction of Ag<Subscript>2</Subscript>O

Bonding between AlN and metals conventionally requires a surface modification process at high temperature such as metallization. The AlN-to-metal direct bonding process by sintering of Ag nanoparticles derived from in situ reduction of Ag₂O microparticles mixed with diethylene glycol is examined. Bonding was conducted at 300–500 °C after a preheating process at 100 °C, and the shear strength exceeded 20 MPa for the joint bonded at 500 °C. The role of preheating in the direct bonding of AlN and Ag was identified. The Ag nanoparticles generated during preheating at 100 °C cover the AlN surface, and they are converted into a thin Ag film. The thin film promotes the formation of a bonding layer, owing to the effective adhesion of the sintered Ag to the film, and contributes to the bonding of Ag and AlN. No interfacial reaction layer is observed. The same bonding process can be applied to other ceramics with poor wettability.     

A Low-Temperature Bonding Process Using Mixed Cu–Ag Nanoparticles

A low-temperature bonding process to form joints with high strength and ionic migration resistance using mixed Cu–Ag nanoparticles was studied. Although it was difficult to obtain strong joints using Cu nanoparticles, with the addition of Ag nanoparticles to the Cu nanoparticles the bonding strength of the Cu-to-Cu joints increased. The joints formed by the mixed Cu–Ag nanoparticles at 350°C exhibited a high bonding strength of ~50 MPa. Counterelectrodes made of the mixed Cu–Ag nanoparticles had four times higher ionic migration resistance compared with counterelectrodes made only of Ag nanoparticles.     

Intermetallic Compound Growth and Reliability of Cu Pillar Bumps Under Current Stressing

Fine-pitch Cu pillar bumps have been adopted for flip-chip bonding technology. Intermetallic compound (IMC) growth in Cu pillar bumps was investigated as a function of annealing or current stressing by in situ observation. The effect of IMC growth on the mechanical reliability of the Cu pillar bumps was also investigated. It is noteworthy that Sn exhaustion was observed after 240 h of annealing when current stressing was not applied, and IMC growth rates were changed remarkably. As the applied current densities increased, the time required for complete Sn consumption became shorter. In addition, Kirkendall voids, which would be detrimental to the mechanical reliability of Cu pillar bumps, were observed in both Cu₃Sn/Cu pillars and Cu₃Sn/Cu under-bump metallization interfaces. Die shear force was measured for Cu pillar samples prepared with various annealing times, and degradation of mechanical strength was observed.     

Low-Temperature Cu-Cu Bonding Using Silver Nanoparticles Fabricated by Physical Vapor Deposition

Silver nanoparticles (Ag NPs) fabricated by physical vapor deposition (PVD) were introduced in Cu-Cu bonding as surface modification layer. The bonding structure consisted of a Ti adhesive/barrier layer and a Cu substrate layer was fabricated on the silicon wafer. Ag NPs were deposited on the Cu surface by magnetron sputtering in a high-pressure environment and a loose structure with NPs was obtained. Shear tests were performed after bonding, and the influences of PVD pressure, bonding pressure, bonding temperature and annealing time on shear strength were assessed. Cu-Cu bonding with Ag NPs was accomplished at 200°C for 3 min under the pressure of 30 MPa without a post-annealing process, and the average bonding strength of 13.99 MPa was reached. According to cross-sectional observations, a void-free bonding interface with an Ag film thickness of around 20 nm was achieved. These results demonstrated that a reliable low-temperature short-time Cu-Cu bonding was realized by the sintering process of Ag NPs between the bonding pairs, which indicated that this bonding method could be a potential candidate for future ultra-fine pitch 3D integration.     

Effect of Ag Content and the Minor Alloying Element Fe on the Mechanical Properties and Microstructural Stability of Sn-Ag-Cu Solder Alloy Under High-Temperature Annealing

This study compares the high-Ag-content Sn-3Ag-0.5Cu with the low- Ag-content Sn-1Ag-0.5Cu solder alloy and the three quaternary solder alloys Sn-1Ag-0.5Cu-0.1Fe, Sn-1Ag-0.5Cu-0.3Fe, and Sn-1Ag-0.5Cu-0.5Fe to understand the beneficial effects of Fe on the microstructural stability, mechanical properties, and thermal behavior of the low-Ag-content Sn-1Ag-0.5Cu solder alloy. The results indicate that the Sn-3Ag-0.5Cu solder alloy possesses small primary β-Sn dendrites and wide interdendritic regions consisting of a large number of fine Ag₃Sn intermetallic compound (IMC) particles. However, the Sn-1Ag-0.5Cu solder alloy possesses large primary β-Sn dendrites and narrow interdendritic regions of sparsely distributed Ag₃Sn IMC particles. The Fe-bearing SAC105 solder alloys possess large primary β-Sn dendrites and narrow interdendritic regions of sparsely distributed Ag₃Sn IMC particles containing a small amount of Fe. Moreover, the addition of Fe leads to the formation of large circular FeSn₂ IMC particles located in the interdendritic regions. On the one hand, tensile tests indicate that the elastic modulus, yield strength, and ultimate tensile strength (UTS) increase with increasing Ag content. On the other hand, increasing the Ag content reduces the total elongation. The addition of Fe decreases the elastic modulus, yield strength, and UTS, while the total elongation is still maintained at the Sn-1Ag-0.5Cu level. The effect of aging on the mechanical behavior was studied. After 720 h and 24 h of aging at 100°C and 180°C, respectively, the Sn-1Ag-0.5Cu solder alloy experienced a large degradation in its mechanical properties after both of the aging conditions, whereas the mechanical properties of the Sn-3Ag-0.5Cu solder alloy degraded more dramatically after 24 h of aging at 180°C. However, the Fe-bearing SAC105 solder alloys exhibited only slight changes in their mechanical properties after both aging procedures. The inclusion of Fe in the Ag₃Sn IMC particles suppresses their IMC coarsening, which stabilizes the mechanical properties of the Fe-bearing SAC105 solder alloys after aging. The results from differential scanning calorimetry (DSC) tests indicate that the addition of Fe has a negligible effect on the melting behavior. However, the addition of Fe significantly reduces the solidification onset temperature and consequently increases the degree of undercooling. In addition, fracture surface analysis indicates that the addition of Fe to the Sn-1Ag-0.5Cu alloy does not affect the mode of fracture, and all tested alloys exhibited large ductile dimples on the fracture surface.     

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.     

Effects of Phenolic Resin Addition on the Electrical Conductivity and Mechanical Strength of Nano-Copper Paste Formed Cu-Cu Joints

Cu-Cu joints bonded by nano-copper pastes with different amounts of phenolic resin were developed at a low temperature (240°C) and a low pressure (1 MPa). The shear strengths of the joints were measured and the bonding layers were observed by scanning electron microscopy. The electrical conductivity of the joints was tested by a parameter analyzer. The results indicated that 2% phenolic resin in the pastes doubled the shear strength of Cu-Cu joints compared to joints made with 0%, 5% and 8% phenolic, because it filled the voids and coated sintered Cu nanoparticles in the joints without affecting the electrical conductivity of the Cu-Cu joint. This was because 2% phenolic paste prevented Cu nanoparticles from oxidation. However, 8% phenolic resin led to an increase of electrical resistivity due to the insulating property of phenolic resin, and a decrease of shear strength due to the hindrance to sintering between the nanoparticles.     

An investigation of the reliability of solderable ICA with low-melting-point alloy (LMPA) filler

Conductive adhesives play a major role in the electronic packaging industry as an alternative to solder due to their potential advantages that include mild processing conditions and superior thermo-mechanical performance. In a conductive adhesive interconnection, adequate mechanical and electrical performance and long-term reliability are critical.In this paper, the reliability of solderable isotropic conductive adhesive (ICA) interconnections was investigated. Reliability testing was performed via thermal shock (−55 to 125 °C, 1000 cycles) and high-temperature and high-humidity tests (85 °C, 85% RH, 1000 h). The interfacial microstructure of the solderable ICA was also investigated. Additionally, the fracture mode was investigated via mechanical pull strength testing before and after the reliability test. The electrical resistance of the solderable ICA interconnection showed improved stability compared to conventional ICAs, and similar stability to conventional solder paste (Sn–3Ag–0.5Cu and Sn–58Bi) due to the metallurgical interconnection formed by the molten LMPA fillers between the corresponding metallization layers. After the reliability tests, the grown IMC layer was composed of Cu₆Sn₅ (η-phase) and Cu₃Sn (ε-phase), and the scallop-type IMC transformed into a layer-type IMC. The fracture propagated along the Cu–Sn IMC/SnBi interface and the fracture surface showed a semi-brittle fracture mode mixed with cleavage and ductile tear bands.     

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.     

A Low-Temperature Pressureless Bonding Process Using a Trimodal Mixture System of Ag Nanoparticles

A low-temperature pressureless bonding process using a mixture of trimodal-sized Ag nanoparticles was proposed to form excellent Cu-to-Cu joints. Cu-to-Cu joints formed using the mixed Ag nanoparticles at 350°C for 5 min showed bonding strength of 13.7 MPa, in spite of the bonding process without pressure. Elongated dimples observed on the fracture surface of the Cu-to-Cu joint strongly support the effect of the trimodal mixture system of Ag nanoparticles in the low-temperature pressureless bonding process.