TEM Observations of the Growth of Intermetallic Compounds at the SnBi/Cu Interface

The microstructure of the eutectic SnBi/Cu interface was investigated by transmission electron microscopy to study the growth mechanisms of the intermetallic compounds (IMCs). Although the growth kinetics of the total IMC layer were similar, the individual Cu₃Sn layer grew faster on polycrystalline Cu than on single-crystal substrates. It was found that, on polycrystalline Cu, newly formed Cu₃Sn grains with a smaller grain size nucleated and grew at both the Cu/Cu₃Sn and Cu₃Sn/Cu₆Sn₅ interfaces during reflow and solid-state aging. The consumption of Cu₆Sn₅ to form Cu₃Sn was faster at the Cu₃Sn/Cu₆Sn₅ interface. While on single-crystal Cu new Cu₃Sn grains nucleated only at the Cu/Cu₃Sn interface, the directional growth of the initial columnar Cu₃Sn controlled the advance of the Cu₃Sn/Cu₆Sn₅ interface.     

Effect of Electromigration on Interfacial Reactions in 90Sn-10Sb Pb-Free Solder Joints

The electromigration effect on interfacial reactions in Cu/90Sn-10Sb/Cu Pb-free solder joints was investigated under electric current stressing. The growth of the Cu₃Sn and Cu₆Sn₅ intermetallic compound (IMC) layers was enhanced at the anode but inhibited at the cathode, compared with the no-current case. The growth of the IMC at the anode followed a parabolic law. Upon increasing the temperature to about 140°C, the thickness of the Cu₆Sn₅ IMC at the anode increased significantly. Sn₃Sb₂ IMC coarsened in the Cu₆Sn₅ IMC at the anode and in the β-Sn at the cathode. The possible mechanism of the electromigration effect is discussed.     

Effect of Intermetallic on Electromigration and Atomic Diffusion in Cu/SnAg3.0Cu0.5/Cu Joints: Experimental and First-Principles Study

Electromigration phenomena in a one-dimensional Cu/SnAg_3.0Cu_0.5/Cu joint were investigated with current stressing. The special effect of intermetallic compound (IMC) layers on the formation of serious electromigration damage induced by nonuniform current density distribution was discussed based on experimental results. Meanwhile, hillocks were observed both at the anode and near the cathode of the joint, and they were described as the result of diffusion of atoms and compressive stress released along grain boundaries to the relatively free surface. Moreover, the diffusion behavior of Cu at the cathode was analyzed with the electromigration equation, and the stability of Ag atoms in the solder during electromigration was evaluated with a first-principles method.     

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.     

Electromigration behavior in Cu/Sn-8Zn-3Bi/Cu solder joint

Electromigration behavior in a one-dimensional Cu/Sn-8Zn-3Bi/Cu solder joint structure was investigated in ambient with a current density of 3.5 × 10⁴ A/cm² at 60 °C. Due to the compressive stress induced by volume expansion resulting from Cu-Zn intermetallic compound (IMC) growth, Cu₅Zn₈ IMC layers were squeezed out continuously along IMC/Cu interfaces at both the anode and the cathode with increasing the current stressing time, which was not only driven by the concentration gradient, but also accelerated by the electromigration. And a few voids propagated and formed at the anode and the cathode solder/IMC interfaces during electromigration. Additionally, Sn hillocks occurred in the bulk solder, and Sn hillocks formed at the anode side were larger than those at the cathode side.     

Microstructure, thermal analysis and hardness of a Sn–Ag–Cu–1 wt% nano-TiO2 composite solder on flexible ball grid array substrates

Sn-Ag-Cu composite solders reinforced with nano-sized, nonreacting, noncoarsening 1 wt% TiO₂ particles were prepared by mechanically dispersing TiO₂ nano-particles into Sn-Ag-Cu solder powder and the interfacial morphology of the solder and flexible BGA substrates were characterized metallographically. At their interfaces, different types of scallop-shaped intermetallic compound layers such as Cu₆Sn₅ for a Ag metallized Cu pad and Sn-Cu-Ni for a Au/Ni and Ni metallized Cu pad, were found in plain Sn-Ag-Cu solder joints and solder joints containing 1 wt% TiO₂ nano-particles. In addition, the intermetallic compound layer thicknesses increased substantially with the number of reflow cycles. In the solder ball region, Ag₃Sn, Cu₆Sn₅ and AuSn₄ IMC particles were found to be uniformly distributed in the β-Sn matrix. However, after the addition of TiO₂ nano-particles, Ag₃Sn, AuSn₄ and Cu₆Sn₅ IMC particles appeared with a fine microstructure and retarded the growth rate of IMC layers at their interfaces. The Sn-Ag-Cu solder joints containing 1 wt% TiO₂ nano-particles consistently displayed a higher hardness than that of the plain Sn-Ag-Cu solder joints as a function of the number of reflow cycles due to the well-controlled fine microstructure and homogeneous distribution of TiO₂ nano-particles which gave a second phase dispersion strengthening mechanism.     

Cu Substrates with Different Grain Sizes

Cu₆Sn₅ and Cu₃Sn are easily formed at the interface between Sn and Cu during reflow and aging processes. Thick Cu-Sn compounds at the interface become brittle, reducing the mechanical strength of solder joints and increasing the consumption of under bump metallization (UBM). It is noted that intermetallic compound (IMC) growth and substrate consumption are affected by factors such as substrate fabrication, substrate orientation, and substrate microstructure. In this study, to determine the effects of substrate grain size on IMC growth and substrate consumption, pure Sn solder was reflowed on annealed Cu substrates with different grain sizes at 250°C for 30 s to 600 s. It was revealed that Cu substrates with smaller grain sizes exhibited reduced IMC growth. In addition, the interdiffusion coefficients of Cu₆Sn₅ and Cu₃Sn were decreased for the Cu substrate with the smaller grain size. The influence of the Cu substrate grain size on IMC growth and substrate consumption is discussed.     

Solid-state intermetallic compound layer growth between copper and 95.5Sn-3.9Ag-0.6Cu solder

Long-term, solid-state intermetallic compound (IMC) layer growth was examined in 95.5Sn-3.9Ag-0.6Cu (wt.%)/copper (Cu) couples. Aging temperatures and times ranged from 70°C to 205°C and from 1 day to 400 days, respectively. The IMC layer thicknesses and compositions were compared to those investigated in 96.5Sn-3.5Ag/Cu, 95.5Sn-0.5Ag-4.0Cu/Cu, and 100Sn/Cu couples. The nominal Cu₃Sn and Cu₆Sn₅ stoichiometries were observed. The Cu₃Sn layer accounted for 0.4–0.6 of the total IMC layer thickness. The 95.5Sn-3.9Ag-0.6Cu/Cu couples exhibited porosity development at the Cu₃Sn/Cu interface and in the Cu₃Sn layer as well as localized “plumes” of accelerated Cu₃Sn growth into the Cu substrate when aged at 205°C and t>150 days. An excess of 3–5at.%Cu in the near-interface solder field likely contributed to IMC layer growth. The growth kinetics of the IMC layer in 95.5Sn-3.9Ag-0.6Cu/Cu couples were described by the equation x=x_o+Atⁿexp [−ΔH/RT]. The time exponents, n, were 0.56±0.06, 0.54±0.07, and 0.58±0.07 for the Cu₃Sn layer, the Cu₆Sn₅, and the total layer, respectively, indicating a diffusion-based mechanism. The apparent-activation energies (ΔH) were Cu₃Sn layer: 50±6 kJ/mol; Cu₆Sn₅ layer: 44±4 kJ/mol; and total layer: 50±4 kJ/mol, which suggested a fast-diffusion path along grain boundaries. The kinetics of Cu₃Sn growth were sensitive to the Pb-free solder composition while those of Cu₆Sn₅ layer growth were not so.     

Effects of different printed-circuit-board surface finishes on the formation and growth of intermetallics at thermomechanically fatigued,small outline J leads/Sn-Ag-Cu interfaces

The effects of printed-circuit-board (PCB) surface finish and thermomechanical fatigue (TMF) on the formation and growth of intermetallic compounds (IMCs) between small outline J (SOJ) leads and Sn-3.0Ag-0.5Cu solder were investigated. The thickness of the IMC layer formed initially at the as-soldered SOJ/Sn-Ag-Cu interface over a Ni/Au PCB surface finish was about 1.7 times of that over the organic solderability preservative (OSP) PCB surface finish. The parabolic TMF-cycle dependence clearly suggests that the growth processes are controlled primarily by solid-state diffusion. The diffusion coefficient for the growth of the total IMC layer at the SOJ/Sn-Ag-Cu interface over the Ni/Au PCB surface finish is the same as that over the OSP PCB surface finish, and thus, the total IMC layer at the SOJ/Sn-Ag-Cu interface over the Ni/Au PCB surface finish is thicker than that over the OSP PCB surface finish. Using the Cu-Ni-Sn ternary isotherm, the anomalous phenomenon that the presence of Ni retards the growth of the Cu₃Sn layer while increasing the initial growth of the Cu₆Sn₅ layer can be addressed.     

Reaction of Liquid Sn-Ag-Cu-Ce Solders with Solid Copper

Small amounts of the rare-earth element Ce were added to the Sn-rich lead-free eutectic solders Sn-3.5Ag-0.7Cu, Sn-0.7Cu, and Sn-3.5Ag to improve their properties. The microstructures of the solders without Ce and with different amounts (0.1 wt.%, 0.2 wt.%, and 0.5 wt.%) of Ce were compared. The microstructure of the solders became finer with increasing Ce content. Deviation from this rule was observed for the Sn-Ag-Cu solder with 0.2 wt.% Ce, and for the Sn-0.7Cu eutectic alloy, which showed the finest microstructure without Ce. The melting temperatures of the solders were not affected. The morphology of intermetallic compounds (IMC) formed at the interface between the liquid solders and a Cu substrate at temperatures about 40°C above the melting point of the solder for dipping times from 2 s to 256 s was studied for the basic solder and for solder with 0.5 wt.% Ce addition. The morphology of the Cu₆Sn₅ IMC layer developed at the interface between the solders and the substrate exhibited the typical scallop-type shape without significant difference between solders with and without Ce for the shortest dipping time. Addition of Ce decreased the thickness of the Cu₆Sn₅ IMC layer only at the Cu/Sn-Ag-Cu solder interface for the 2-s dipping. A different morphology of the IMC layer was observed for the 256-s dipping time: The layers were less continuous and exhibited a broken relief. Massive scallops were not observed. For longer dipping times, Cu₃Sn IMC layers located near the Cu substrate were also observed.     

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.     

Interfacial Reaction Effect on Electrical Reliability of Cu Pillar/Sn Bumps

Thermal annealing and electromigration (EM) tests were performed with Cu pillar/Sn bumps to understand the growth mechanism of intermetallic compounds (IMCs). Annealing tests were carried out at both 100°C and 150°C. At 150°C, EM tests were performed using a current density of 3.5 × 10⁴ A/cm². The electrical failure mechanism of the Cu pillar/Sn bumps was also investigated. Cu₃Sn formed and grew at the Cu pillar/Cu₆Sn₅ interface with increasing annealing and current-stressing times. The growth mechanism of the total (Cu₆Sn₅ + Cu₃Sn) IMC changed when the Sn phase in the Cu pillar/Sn bump was exhausted. The time required for complete consumption of the Sn phase was shorter during the EM test than in the annealing test. Both IMC growth and phase transition from Cu₆Sn₅ to Cu₃Sn had little impact on the electrical resistance of the whole interconnect system during current stressing. Electrical open failure in the Al interconnect near the chip-side Cu pillar edge implies that the Cu pillar/Sn bump has excellent electrical reliability compared with the conventional solder bump.     

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.     

Effect of additions of ZrO2 nano-particles on the microstructure and shear strength of Sn-Ag-Cu solder on Au/Ni metallized Cu pads

Nano-sized, nonreacting, noncoarsening ZrO₂ particles reinforced Sn-3.0 wt%Ag-0.5 wt%Cu composite solders were prepared by mechanically dispersing ZrO₂ nano-particles into Sn-Ag-Cu solder. The interfacial morphology of unreinforced Sn-Ag-Cu solder and solder joints containing ZrO₂ nano-particles with Au/Ni metallized Cu pads on ball grid array (BGA) substrates and the distribution of reinforcing particles were characterized metallographically. At their interfaces, a Sn-Ni-Cu intermetallic compound (IMC) layer was found in both unreinforced Sn-Ag-Cu and Sn-Ag-Cu solder joints containing ZrO₂ nano-particles and the IMC layer thickness increased with the number of reflow cycles. In the solder ball region, AuSn₄, Ag₃Sn, Cu₆Sn₅ IMC particles and ZrO₂ nano-particles were found to be uniformly distributed in the β-Sn matrix of Sn-Ag-Cu solder joints containing ZrO₂ nano-particles, which resulted in an increase in the shear strength, due to a second phase dispersion strengthening mechanism. The fracture surface of unreinforced Sn-Ag-Cu solder joints exhibited a brittle fracture mode with a smooth surface while Sn-Ag-Cu solder joints containing ZrO₂ nano-particles ductile failure characteristics with rough dimpled surfaces.     

Electromigration-Induced Interfacial Reactions in Cu/Sn/Electroless Ni-P Solder Interconnects

The effect of electromigration (EM) on the interfacial reaction in a line-type Cu/Sn/Ni-P/Al/Ni-P/Sn/Cu interconnect was investigated at 150°C under 5.0 × 10³ A/cm². When Cu atoms were under downwind diffusion, EM enhanced the cross-solder diffusion of Cu atoms to the opposite Ni-P/Sn (anode) interface compared with the aging case, resulting in the transformation of interfacial intermetallic compound (IMC) from Ni₃Sn₄ into (Cu,Ni)₆Sn₅. However, at the Sn/Cu (cathode) interface, the interfacial IMCs remained as Cu₆Sn₅ (containing less than 0.2 wt.% Ni) and Cu₃Sn. When Ni atoms were under downwind diffusion, only a very small quantity of Ni atoms diffused to the opposite Cu/Sn (anode) interface and the interfacial IMCs remained as Cu₆Sn₅ (containing less than 0.6 wt.% Ni) and Cu₃Sn. EM significantly accelerated the dissolution of Ni atoms from the Ni-P and the interfacial Ni₃Sn₄ compared with the aging case, resulting in fast growth of Ni₃P and Ni₂SnP, disappearance of interfacial Ni₃Sn₄, and congregation of large (Ni,Cu)₃Sn₄ particles in the Sn solder matrix. The growth kinetics of Ni₃P and Ni₂SnP were significantly accelerated after the interfacial Ni₃Sn₄ IMC completely dissolved into the solder, but still followed the t ^1/2 law.     

Temperature Effect on Intermetallic Compound Growth Kinetics of Cu Pillar/Sn Bumps

The in situ intermetallic compound (IMC) growth in Cu pillar/Sn bumps was investigated by isothermal annealing at 120°C, 150°C, and 180°C using an in situ scanning electron microscope. Only the Cu₆Sn₅ phase formed at the interface between the Cu pillar and Sn during the reflow process. The Cu₃Sn phase formed and grew at the interfaces between the Cu pillar and Cu₆Sn₅ with increased annealing time. Total (Cu₆Sn₅ + Cu₃Sn) IMC thickness increased linearly with the square root of annealing time. The growth slopes of total IMC decreased after 240 h at 150°C and 60 h at 180°C, due to the fact that the Cu₆Sn₅ phase transforms to the Cu₃Sn phase when all of the remaining Sn phase in the Cu pillar bump is completely exhausted. The complete consumption time of the Sn phase at 180°C was shorter than that at 150°C. The apparent activation energy for total IMC growth was determined to be 0.57 eV.     

Influence of Cu content on compound formation near the chip side for the flip-chip Sn-3.0Ag-(0.5 or 1.5)Cu solder bump during aging

Sn-Ag-Cu solder is a promising candidate to replace conventional Sn-Pb solder. Interfacial reactions for the flip-chip Sn-3.0Ag-(0.5 or 1.5)Cu solder joints were investigated after aging at 150°C. The under bump metallization (UBM) for the Sn-3.0Ag-(0.5 or 1.5)Cu solders on the chip side was an Al/Ni(V)/Cu thin film, while the bond pad for the Sn-3.0Ag-0.5Cu solder on the plastic substrate side was Cu/electroless Ni/immersion Au. In the Sn-3.0Ag-0.5Cu joint, the Cu layer at the chip side dissolved completely into the solder, and the Ni(V) layer dissolved and reacted with the solder to form a (Cu_1−y,Ni_y)₆Sn₅ intermetallic compound (IMC). For the Sn-3.0Ag-1.5Cu joint, only a portion of the Cu layer dissolved, and the remaining Cu layer reacted with solder to form Cu₆Sn₅ IMC. The Ni in Ni(V) layer was incorporated into the Cu₆Sn₅ IMC through slow solid-state diffusion, with most of the Ni(V) layer preserved. At the plastic substrate side, three interfacial products, (Cu_1−y,Ni_y)₆Sn₅, (Ni_1−x,Cu_x)₃Sn₄, and a P-rich layer, were observed between the solder and the EN layer in both Sn-Ag-Cu joints. The interfacial reaction near the chip side could be related to the Cu concentration in the solder joint. In addition, evolution of the diffusion path near the chip side in Sn-Ag-Cu joints during aging is also discussed herein.     

Investigation of the Growth of Intermetallic Compounds Between Cu Pillars and Solder Caps

In flip chip applications, Cu pillars with solder caps are regarded as next-generation electronic interconnection technology, because of high input/output density. However, because of diffusion and reaction of Sn and Cu during the high-temperature reflow process, intermetallic compounds (IMC) are formed, and grow, at the interface between the cap and the pillar. Understanding the growth behavior of interfacial IMC is critical in the design of solder interconnections, because excessive growth of IMC can reduce the reliability of connections. In this study, the growth of IMC during thermal cycling, an accelerated method of testing the service environment of electronic devices, was studied by use of focused ion beam–scanning electron microscopy. Under alternating high and low-temperature extremes, growth of Cu₆Sn₅ (η-phase) and Cu₃Sn (ε-phase) IMC was imaged and measured as a function of the number of cycles. The total IMC layer grew significantly thicker but became more uniform during thermal cycling. The Cu₃Sn layer was initially thinner than the Cu₆Sn₅ layer but outgrew the Cu₆Sn₅ layer after 1000 cycles. It was found that, with limited Cu and Sn diffusion, consumption of Cu₆Sn₅ for growth of the Cu₃Sn layer can result in a thinner Cu₆Sn₅ layer after thermal cycling.     

Microstructure Evolution of Cu-Cored Sn Solder Joints Under High Temperature and High Current Density

This work investigated the microstructure evolution of Cu-cored Sn solder joints under high temperature and high current density. The Cu₆Sn₅ phase formed at both the Cu core/Sn interface and Cu wire/Sn interface right after reflow and grew with increasing annealing time, while the Cu₃Sn phase formed and grew at the Cu/Cu₆Sn₅ interfaces. Intermetallic compound (IMC) growth followed a linear relationship with the square root of annealing time due to a diffusion-controlled mechanism. Under high current density, the thickness of the interfacial IMCs of the Cu core/Sn interface at the cathode side increased and the Cu core/Sn interface at the anode side exhibited an irregular and serrated morphology with prolonged current stressing time. Finite-element simulation was carried out to obtain the distribution of current density in the solder joint. Since Cu has lower resistivity, the electrical current primarily selected the Cu core as its electrical path, resulting in current crowding at the Cu core and the region between the Cu core and Cu wire. Compared with the conventional solder joint, the electromigration (EM) lifetime of the Cu-cored solder joint was much longer.     

Electromigration in Cu-Cored Sn-3.5Ag-0.7Cu Solder Interconnects Under Current Stressing

The effect of electromigration in Cu-cored Sn-3.5Ag-0.7Cu solder interconnects under current stressing was investigated. After current stressing at a density of 2 × 10⁴ A/cm², some Cu₆Sn₅ intermetallic compounds accumulated abnormally on the surface of the solder interconnect. The abnormal accumulation phenomenon was explained by the mechanism that thermomigration dominated the migration process. Furthermore, some Cu₆Sn₅ extrusions appeared on the cathode-side surface of the solder interconnect due to the compressive stresses induced by electromigration and thermomigration.