The nanoindentation characteristics of Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript>, Cu<Subscript>3</Subscript>Sn,and Ni<Subscript>3</Subscript>Sn<Subscript>4</Subscript> intermetallic compounds in the solder bump

In general, formation and growth of intermetallic compounds (IMCs) play a major role in the reliability of the solder joint in electronics packaging and assembly. The formation of Cu-Sn or Ni-Sn IMCs have been observed at the interface of Sn-rich solders reacted with Cu or Ni substrates. In this study, a nanoindentation technique was employed to investigate nanohardness and reduced elastic moduli of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ IMCs in the solder joints. The Sn-3.5Ag and Sn-37Pb solder pastes were placed on a Cu/Ti/Si substrate and Ni foil then annealed at 240°C to fabricate solder joints. In Sn-3.5Ag joints, the magnitude of the hardness of the IMCs was in the order Ni₃Sn₄>Cu₆Sn₅>Cu₃Sn, and the elastic moduli of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ were 125 GPa, 136 GPa, and 142 GPa, respectively. In addition, the elastic modulus of the Cu₆Sn₅ IMC in the Sn-37Pb joint was similar to that for the bulk Cu₆Sn₅ specimen but less than that in the Sn-3.5Ag joint. This might be attributed to the strengthening effect of the dissolved Ag atoms in the Cu₆Sn₅ IMC to enhance the elastic modulus in the Sn-3.5Ag/Cu joint.     

High-Temperature Resistant Intermetallic Compound Joints for Si Chips and Cu Substrates

A thin-film joining method utilizing evaporated films as the joining material was newly developed for power semiconductor die attachment. When the evaporated films are completely transformed into intermetallic compounds (IMCs) with high melting points, the joint can exhibit the required high-temperature strength. In this study, a joint consisting of Cu₆Sn₅, (Ag,Cu)₃Sn, and Cu₃Sn IMCs was achieved at 573 K after 30 s. Results of nanoindentation tests revealed the hardness and elastic moduli of each IMC. In accelerated tests, a high-temperature strength of at least 15 MPa was shown for 3.6 Ms at 423 K or 500 cycles between 223 K and 403 K. These results suggest that the IMC joint has great potential as a die-attach material.     

Mechanical properties of intermetallic compounds associated with Pb-free solder joints using nanoindentation

Mechanical properties of intermetallic compound (IMC) phases in Pb-free solder joints were obtained using nanoindentation testing (NIT). The elastic modulus and hardness were determined for IMC phases associated with insitu FeSn particle reinforced and mechanically added, Cu particle-reinforced, composite solder joints. The IMC layers that formed around Cu particle reinforcement and at the Cu substrate/solder matrix interface were probed with NIT. Moduli and hardness values obtained by NIT revealed were noticeably higher for Cu-rich Cu₃Sn than those of Cu₆Sn₅. The Ag₃Sn platelets that formed during reflow were also examined for eutectic Sn-Ag solder column joints. The indentation modulus of Ag₃Sn platelets was significantly lower than that of FeSn, SnCuNi, and CuSn IMCs. Indentation creep properties were assessed in localized microstructure regions of the as-cast, eutectic Sn-Ag solder. The stress exponent, n, associated with secondary creep differed widely depending on the microstructure feature probed by the indenter tip.     

Drop impact reliability of Sn–1.0Ag–0.5Cu BGA interconnects with different mounting methods

The poor drop-shock resistance of near-eutectic Sn–Ag–Cu (SAC) solder interconnects drives the research and application low-Ag SAC solder alloys, especially for Sn–1.0Ag–0.5Cu (SAC105). In this work, by dynamic four-point bend testing, we investigate the drop impact reliability of SAC105 alloy ball grid array (BGA) interconnects with two different surface mounting methods: near-eutectic solder paste printing and flux dipping. The results indicate that the flux dipping method improves the interconnects failure strain by 44.7% over paste printing. Further mechanism studies show the fine interfacial intermetallic compounds (IMCs) at the printed circuit board side and a reduced Ag content inside solder bulk are the main beneficial factors overcoming other negative factors. The flux dipping SAC105 BGA solder joints possess fine Cu₆Sn₅ IMCs at the interface of solder/Cu pads, which increases the bonding strength between the solder/IMCs and the fracture resistance of the IMC grains themselves. Short soldering time of flux dipping joints above the solder alloy liquidus mitigates the growth of interfacial IMCs in size. In addition, a reduced Ag content in flux dipping joint bulk causes a low hardness and high compliance, thus increasing fracture resistance under higher-strain rate conditions.     

Microstructure characterization of SnAgCu solder bearing Ce for electronic packaging

The creep-rupture lives of Sn3.8Ag0.7Cu and Sn3.8Ag0.7Cu0.03Ce lead-free solder joints for electronic packaging were investigated, respectively. And the relationship between creep behavior and intermetallic compound (IMC: Ag₃Sn, Cu₆Sn₅, CeSn₃) particles in SnAgCu/SnAgCuCe solder joints has been obtained. Meanwhile, rare earth Ce concentration gradient and retardation effect of Ce on the IMC layer have been observed at the solder/Cu interface. Moreover, aging reaction of Sn and Cu, and the effect mechanism of rare earth Ce on two IMCs (Cu₆Sn₅ and Cu₃Sn) are reported.     

Experimental investigation of the failure mechanism of Cu–Sn intermetallic compounds in SAC solder joints

A detailed experimental study on the fracture mechanism of Cu–Sn intermetallic compounds (IMCs) in the Pb-free solder was presented in this paper. The growth behaviors of the Cu₆Sn₅ and Cu₃Sn IMCs were inspected and the respective evolution pattern of their microstructures was investigated. Then, a detailed fractographic analysis on brittle fractured solder joints was conducted after the high speed ball pull test. The fracture locations in the Cu–Sn IMC layers during different periods of aging process were identified. The fracture modes of Cu₆Sn₅ and Cu₃Sn were determined as well. Afterwards, the fracture energies of different Cu–Sn IMC materials were directly compared using the Charpy impact test with a specially designed specimen. It was found that the grain boundary of Cu₃Sn is the weakest link in the Cu–Sn IMC system. Finally, based on these three parts of study, a mechanism to explain the thermal degradation of Cu–Sn IMCs was proposed.     

EBSD Investigation of Cu-Sn IMC Microstructural Evolution in Cu/Sn-Ag/Cu Microbumps During Isothermal Annealing

The microstructural evolution of Cu/Sn-Ag (~5 μm)/Cu Cu-bump-on-line (CuBOL) joints during isothermal annealing at 180°C was examined using a field-emission scanning electron microscope equipped with an electron backscatter diffraction (EBSD) system. Cu₆Sn₅ and Cu₃Sn were the two key intermetallic compound (IMC) species that appeared in the CuBOL joints. After annealing for 24 h (= t), the solder had completely converted to Cu-Sn IMCs, forming an “IMC” joint with Cu/Cu₃Sn/Cu₆Sn₅/Cu₃Sn/Cu structure. EBSD analyses indicated that the preferred orientation of the hexagonal Cu₆Sn₅ (η) was $ (2\bar{1}\bar{1}3) $ , while the preferred orientation was (100) for the monoclinic Cu₆Sn₅ structure (η′). Upon increasing t to 72 h, Cu₆Sn₅ entirely transformed into Cu₃Sn, and the IMC joint became Cu/Cu₃Sn/Cu accordingly. Interestingly, the grain size and crystallographic orientation of Cu₃Sn displayed location dependence. Detailed EBSD analyses in combination with transmission electron microscopy on Cu₃Sn were performed in the present study. This research offers better understanding of crystallographic details, including crystal structure, grain size, and orientation, for Cu₆Sn₅ and Cu₃Sn in CuBOL joints after various annealing times.     

Influence of Dopant on Growth of Intermetallic Layers in Sn-Ag-Cu Solder Joints

The interfacial interaction between Cu substrates and Sn-3.5Ag-0.7Cu-xSb (x = 0, 0.2, 0.5, 0.8, 1.0, 1.5, and 2.0) solder alloys has been investigated under different isothermal aging temperatures of 100°C, 150°C, and 190°C. Scanning electron microscopy (SEM) was used to measure the thickness of the intermetallic compound (IMC) layer and observe the microstructural evolution of the solder joints. The IMC phases were identified by energy-dispersive x-ray spectroscopy (EDX) and x-ray diffractometry (XRD). The growth of both the Cu₆Sn₅ and Cu₃Sn IMC layers at the interface between the Cu substrate and the solder fits a power-law relationship with the exponent ranging from 0.42 to 0.83, which suggests that the IMC growth is primarily controlled by diffusion but may also be influenced by interface reactions. The activation energies and interdiffusion coefficients of the IMC formation of seven solder alloys were determined. The addition of Sb has a strong influence on the growth of the Cu₆Sn₅ layer, but very little influence on the formation of the Cu₃Sn IMC phase. The thickness of the Cu₃Sn layer rapidly increases with aging time and temperature, whereas the thickness of the Cu₆Sn₅ layer increases slowly. This is probably due to the formation of Cu₃Sn at the interface between two IMC phases, which occurs with consumption of Cu₆Sn₅. Adding antimony to Sn-3.5Ag-0.7Cu solder can evidently increase the activation energy of Cu₆Sn₅ IMC formation, reduce the atomic diffusion rate, and thus inhibit excessive growth of Cu₆Sn₅ IMCs. This study suggests that grain boundary pinning is one of the most important mechanisms for inhibiting the growth of Cu₆Sn₅ IMCs in such solder joints when Sb is added.     

Morphology and mechanical properties of intermetallic compounds in SnAgCu solder joints

This paper presents the studies to determine hardness and elastic modulus of intermetallic compound (IMC) layers in lead-free solder joints using nanoindentation technique. Two types of surface finishes, i.e., organic solderability preservative (OSP) and electrolytic Ni/Au on Cu pad, with Sn3.5Ag0.5Cu solder balls of 330 μm in diameter are studied, and the intermetallic layers are identified to be Cu₆Sn5, Cu₃Sn and (Ni_y,Cu_1−y)₃Sn₄. The thicknesses of these IMC layers are only few microns at reflowed conditions (less than 2.3 μm). Therefore, probing mechanical properties of thinner IMCs using nanoindentation techniques poses immense difficulties and challenges. In this study, taper-mounted samples are used rather than standard cross-sectional mounted for solder joints. This taper sample gives a larger area for nanoindentation measurements. The elastic modulus and hardness of IMC layers are determined based on the parameter P/S² (load/stiffness²) as a function of the indentation depth to minimise the effects of underlying UBM or solder materials. The modulus of Cu₆Sn₅, Cu₃Sn, (Cu_x,Ni_1−x)₆Sn₅ and (Ni_y,Cu_1−y)₃Sn₄ layer are found to be 112.0 ± 5.1 GPa, 135.5 ± 4.3 GPa, 165.0 ± 11.3 GPa and 136.8 ± 5.8 GPa; whereas the hardness values are found to be 6.8 ± 0.4 GPa, 6.6 ± 0.5 GPa, 7.2 ± 0.9 GPa and 8.2 ± 1.0 GPa, respectively. Thus, the IMC layers in the order of increasing hardness and modulus are found to be Cu₆Sn₅, Cu₃Sn, (Cu_x,Ni_1−x)₆Sn₅ and (Ni_y,Cu_1−y)₃Sn₄.     

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.     

Interfacial reaction and failure mode analysis of the solder joints for flip-chip LED on ENIG and Cu-OSP surface finishes

The interfacial reactions and failure modes of the solder joints for flip-chip light emitting diode (LED) on electroless nickel/immersion gold (ENIG) and Cu with organic solderability preservatives (Cu-OSP) surface finishes were investigated in this study. The experimental results demonstrate that the interfacial reactions in the Au/Sn–Ag–Cu(SAC)/ENIG and Au/SAC/Cu systems are different but the failure mechanisms of the two types of solder joints are similar during the shear test. For the Au/SAC/ENIG system, the Au layer on the surface finish of the diodes dissolved into the molten solder and transformed into a continuous (Au, Ni)Sn₄ IMC layer at the diode/solder interface during reflow and the interfacial IMC at the solder/ENIG interface is dendritic Ni₃Sn₄ IMC grains which are surrounded by (Au, Ni)Sn₄. For the Au/SAC/Cu system, however, no IMC layers can be observed at the diode/solder interface. The interfacial IMC at the solder/Cu interface is (Cu, Au)₆Sn₅ and a Cu₃Sn IMC layer at the (Cu, Au)₆Sn₅/Cu interface. Tiny (Au, Cu)Sn₄ IMC grains distribute in the solder layer and surround the (Cu, Au)₆Sn₅ grains. For the two types of systems, the primary failure mode for the cathode is due to the broken of the Si-based insulation layer which led to a high residue stress and poor connection between the Si-based layer and the solder layer. Meanwhile, the failure of the solder joint for the anode is mainly because of the failure of the solder layer under the conductive via. The crack generally forms at this area and then propagated along the diode or the diode/solder interface.     

Mechanical Characterization of Lead-Free Sn-Ag-Cu Solder Joints by High-Temperature Nanoindentation

The reliability of Pb-free solder joints is controlled by their microstructural constituents. Therefore, knowledge of the solder microconstituents’ mechanical properties as a function of temperature is required. Sn-Ag-Cu lead-free solder alloy contains three phases: a Sn-rich phase, and the intermetallic compounds (IMCs) Cu₆Sn₅ and Ag₃Sn. Typically, the Sn-rich phase is surrounded by a eutectic mixture of β-Sn, Cu₆Sn₅, and Ag₃Sn. In this paper, we report on the Young’s modulus and hardness of the Cu₆Sn₅ and Cu₃Sn IMCs, the β-Sn phase, and the eutectic compound, as measured by nanoindentation at elevated temperatures. For both the β-Sn phase and the eutectic compound, the hardness and Young’s modulus exhibited strong temperature dependence. In the case of the intermetallics, this temperature dependence is observed for Cu₆Sn₅, but the mechanical properties of Cu₃Sn are more stable up to 200°C.     

Influence of trace alloying elements on the ball impact test reliability of SnAgCu solder joints

In this work, we present ball impact test (BIT) responses and fracture modes obtained at an impact velocity of 0.8 m/s on SAC (Sn–Ag–Cu) package-level solder joints with a trace amount of Mn or RE (rare earth) additions, which were bonded with substrates of OSP Cu and electroplated Ni/Au surface finishes respectively. With respect to the as-mounted conditions, the Ni/Au joints possessed better impact fracture resistance than those with Cu substrate. Subsequent to aging at 150 °C for 800 h, multi-layered intermetallic compounds emerged at the interface of the Ni/Au joints and gave rise to degradation of the BIT properties. This can be prevented by RE doping, which is able to inhibit the growth of interfacial IMCs during aging. As for aged Cu joints, the Mn-doped samples showed the best performance in impact force and toughness. This was related to the hardened Sn matrix, and most importantly, a greater Cu₃Sn/Cu₆Sn₅ thickness ratio at the interface. Compared to Cu₆Sn₅, Cu₃Sn with a similar hardness but greater elastic modulus possessed better plastic ability, which was beneficial to the reliability of solder joints suffering high strain rate deformation if no excess Kirkendall voids formed.     

Mechanical and Electrical Properties of Cu/Sn-3.5Ag/Cu Ball Grid Array (BGA) Solder Joints after Multiple Reflows

The microstructural evolution, die shear strength, and electrical resistivity of Cu/Sn-3.5Ag (wt.%)/Cu ball grid array (BGA) solder joints were investigated after 1 to 10 reflows using scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron probe microanalysis (EPMA), bonding testing, and a four-point probe station. A Cu₆Sn₅ intermetallic compound (IMC) was formed at both the upper and lower interfaces after one reflow. The IMC thickness increased at the lower interface with increasing reflow number, whereas the IMC morphology and thickness remained virtually unchanged at the upper interface, irrespective of the reflow number. The amount of Cu₆Sn₅ IMC contained in the solder ball increased with increasing reflow number. These microstructural evolutions with increasing reflow number strongly affected the mechanical and electrical properties of the solder joint.     

Interfacial reactions and shear strengths between Sn-Ag-based Pb-free solder balls and Au/EN/Cu metallization

The morphological and compositional evolutions of intermetallic compounds (IMCs) formed at three Pb-free solder/electroless Ni-P interface were investigated with respect to the solder compositions and reflow times. The three Pb-free solder alloys were Sn3.5Ag, Sn3.5Ag0.75Cu, and Sn3Ag6Bi2In (in wt.%). After reflow reaction, three distinctive layers, Ni₃Sn₄ (or Ni-Cu-Sn for Sn3.5Ag0.75Cu solder), NiSnP, and Ni₃P, were formed on the electroless Ni-P layer in all the solder alloys. For the Sn3.5Ag0.75Cu solder, with increasing reflow time, the interfacial intermetallics switched from (Cu,Ni)₆Sn₅ to (Cu,Ni)₆Sn₅+(Ni,Cu)₃Sn₄, and then to (Ni,Cu)₃Sn₄ IMCs. The degree of IMC spalling for the Sn3.5Ag0.75Cu solder joint was more than that of other solders. In the cases of the Sn3.5Ag and Sn3Ag6Bi2In solder joints, the growth rate of the Ni₃P layer was similar because these two type solder joints had a similar interfacial reaction. On the other hand, for the Sn3.5Ag0.75Cu solder, the thickness of the Ni₃P and Ni-Sn-P layers depended on the degree of IMC spalling. Also, the shear strength showed various characteristics depending on the solder alloys and reflow times. The fractures mainly occurred at the interfaces of Ni₃Sn₄/Ni-Sn-P and solder/Ni₃Sn₄.     

Impact of Ni concentration on the intermetallic compound formation and brittle fracture strength of Sn–Cu–Ni (SCN) lead-free solder joints

Cu₆Sn₅ and Cu₃Sn are common intermetallic compounds (IMCs) found in Sn–Ag–Cu (SAC) lead-free solder joints with OSP pad finish. People typically attributed the brittle failure to excessive growth of IMCs at the interface between the solder joint and the copper pad. However, the respective role of Cu₆Sn₅ and Cu₃Sn played in the interfacial fracture still remains unclear. In the present study, various amounts of Ni were doped in the Sn–Cu based solder. The different effects of Ni concentration on the growth rate of (Cu, Ni)₆Sn₅/Cu₆Sn₅ and Cu₃Sn were characterized and compared. The results of characterization were used to evaluate different growth rates of (Cu, Ni)₆Sn₅ and Cu₃Sn under thermal aging. The thicknesses of (Cu, Ni)₆Sn₅/Cu₆Sn₅ and Cu₃Sn after different thermal aging periods were measured. High speed ball pull/shear tests were also performed. The correlation between interfacial fracture strength and IMC layer thicknesses was established.     

Morphology and growth kinetics of intermetallic compounds in solid-state interfacial reaction of electroless Ni-P with Sn-based lead-free solders

A comparative study of solid/solid interfacial reactions of electroless Ni-P (15 at.% P) with lead-free solders, Sn-0.7Cu, Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and pure Sn, was carried out by performing thermal aging at 150°C up to 1000 h. For pure Sn and Sn-3.5Ag solder, three distinctive layers, Ni₃Sn₄, SnNiP, and Ni₃P, were observed in between the solder and electroless Ni-P; while for Sn-0.7Cu and Sn-3.8Ag-0.7Cu solders, two distinctive layers, (CuNi)₆Sn₅ and Ni₃P, were observed. The differences in morphology and growth kinetics of the intermetallic compounds (IMCs) at the interfaces between electroless Ni-P and lead-free solders were investigated, as well as the growth kinetics of the P-enriched layers underneath the interfacial IMC layers. With increasing aging time, the coarsening of interfacial Ni₃Sn₄ IMC grains for pure Sn and Sn-3.5Ag solder was significantly greater than that of the interfacial (CuNi)₆Sn₅ IMC grains for Sn-0.7Cu and Sn-3.8Ag-0.7Cu solders. Furthermore, the Ni content in interfacial (CuNi)₆Sn₅ phase slightly increased during aging. A small addition of Cu (0.7 wt.%) resulted in differences in the type, morphology, and growth kinetics of interfacial IMCs. By comparing the metallurgical aspects and growth kinetics of the interfacial IMCs and the underneath P-enriched layers, the role of initial Cu and Ag in lead-free solders is better understood.     

Intermetallics Characterization of Lead-Free Solder Joints under Isothermal Aging

Solder interconnect reliability is influenced by environmentally imposed loads, solder material properties, and the intermetallics formed within the solder and the metal surfaces to which the solder is bonded. Several lead-free metallurgies are being used for component terminal plating, board pad plating, and solder materials. These metallurgies react together and form intermetallic compounds (IMCs) that affect the metallurgical bond strength and the reliability of solder joint connections. This study evaluates the composition and extent of intermetallic growth in solder joints of ball grid array components for several printed circuit board pad finishes and solder materials. Intermetallic growth during solid state aging at 100°C and 125°C up to 1000 h for two solder alloys, Sn-3.5Ag and Sn-3.0Ag-0.5Cu, was investigated. For Sn-3.5Ag solder, the electroless nickel immersion gold (ENIG) pad finish was found to result in the lowest IMC thickness compared to immersion tin (ImSn), immersion silver (ImAg), and organic solderability preservative (OSP). Due to the brittle nature of the IMC, a lower IMC thickness is generally preferred for optimal solder joint reliability. A lower IMC thickness may make ENIG a desirable finish for long-life applications. Activation energies of IMC growth in solid-state aging were found to be 0.54 ± 0.1 eV for ENIG, 0.91 ± 0.12 eV for ImSn, and 1.03 ± 0.1 eV for ImAg. Cu₃Sn and Cu₆Sn₅ IMCs were found between the solder and the copper pad on boards with the ImSn and ImAg pad finishes. Ternary (Cu,Ni)₆Sn₅ intermetallics were found for the ENIG pad finish on the board side. On the component side, a ternary IMC layer composed of Ni-Cu-Sn was found. Along with intermetallics, microvoids were observed at the interface between the copper pad and solder, which presents some concern if devices are subject to shock and vibration loading.     

Interfacial reaction and shear strength of SnAgCu-xNi/Ni solder joints during aging at 150 °C

The interfacial microstructure and shear strength of Sn3.8Ag0.7Cu-xNi (SAC-xNi, x = 0.5, 1, and 2) composite solders on Ni/Au finished Cu pads were investigated in detail after aging at 150 °C for up to 1000 h. The interfacial characteristics of composite solder joints were affected significantly by the weight percentages of added Ni micro-particles and aging time. After aging for 200 h, the solder joints of SAC, SAC-0.5Ni and -1Ni presented duplex intermetallic compound (IMC) layers regardless of the initial interfacial structure on as-reflowed joints, whose upper and lower IMC layers were comprised of (CuNi)₆Sn₅ and (NiCu)₃Sn₄, respectively. Only a single (NiCu)₃Sn₄ IMC layer was ever observed at the SAC-2Ni/Ni interface on whole aging process. Based on the compositional analysis, the amount of Ni within the IMC regions increased as the proportion of Ni addition increased. The IMC (NiCu)₃Sn₄ layer thickness on the interface of SAC and SAC-0.5Ni grew more slowly when compared to that of SAC-1Ni and -2Ni, while for the (CuNi)₆Sn₅ layer the reverse is true. Except the IMCs sizes are increased with increased aging time, the interfacial IMCs tended to transfer their morphologies to polyhedra. In all composite joints testing, the shear strengths were approximately equal to non-composite joints. The fracturing observed during shear testing of composite joints occurred in the bulk solder, indicating that the SAC-xNi/Ni solder joints had a desirable joint reliability.     

Mechanical Strength and Failure Characterization of Sn-Ag-Cu Intermetallic Compound Joints at the Microscale

Continuous miniaturization of microelectronic devices has led the industry to develop interconnects on the order of a few microns for advanced superhigh-density and three-dimensional integrated circuits (3D ICs). At this scale, interconnects that conventionally consist of solder material will completely transform to intermetallic compounds (IMCs) such as Cu₆Sn₅. IMCs are brittle, unlike conventional solder materials that are ductile in nature; therefore, IMCs do not experience large amounts of plasticity or creep before failure. IMCs have not been fully characterized, and their mechanical and thermomechanical reliability is questioned. This study presents experimental efforts to characterize such material. Sn-based microbonds are fabricated in a controlled environment to assure complete transformation of the bonds to Cu₆Sn₅ IMC. Microstructural analysis including scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD) is utilized to determine the IMC material composition and degree of copper diffusion into the bond area. Specimens are fabricated with different bond thicknesses and in different configurations for various tests. Normal strength of the bonds is measured utilizing double cantilever beam and peeling tests. Shear tests are conducted to quantify the shear strength of the material. Four-point bending tests are conducted to measure the fracture toughness and critical energy release rate. Bonds are fabricated in different sizes, and the size effect is investigated. The shear strength, normal strength, critical energy release rate, and effect of bond size on bond strength are reported.