Sn-Ag-Cu Solder Joint Microstructure and Orientation Evolution as a Function of Position and Thermal Cycles in Ball Grid Arrays Using Orientation Imaging Microscopy

Thermally cycled plastic ball grid array (PBGA) packages with full arrays of 196 solder joints after various preconditions were examined to observe the microstructure evolution of Sn-Ag-Cu solder joints during aging and thermal cycling, focusing on Sn grain orientation. Each PBGA package was polished to obtain a plan-view cross-section of every solder joint, and characterized using both polarized optical microscopy and orientation imaging microscopy (OIM). By OIM observations, distribution maps were obtained based on Sn crystal c-axis orientations. Each precondition showed a characteristic distribution related to the combined thermal aging and thermal cycling history. This study on Sn grain orientation using OIM provides further understanding about deformation and microstructure evolution processes that occur during thermal cycling, and the impact of isothermal aging as a precondition.     

Methodology for Analyzing Strain States During <Emphasis Type="Italic">In Situ</Emphasis> Thermomechanical Cycling in Individual Lead-Free Solder Joints Using Synchrotron Radiation

To examine how a lead-free solder joint deforms in a thermal cycling environment, both the elastic and plastic stress and strain behavior must be understood. Methods to identify evolution of the internal strain (stress) state during thermal cycling are described. A slice of a package containing a single row of solder joints was thermally cycled from 0°C to 100°C with a period of about 1 h with concurrent acquisition of transmission Laue patterns using synchrotron radiation. These results indicated that most joints are single crystals, with some being multicrystals with no more than a few Sn grain orientations. Laue patterns were analyzed to estimate local strains in different crystal directions at different temperatures during a thermal cycle. While the strains perpendicular to various crystal planes all vary in a similar way, the magnitude of strain varies. The specimens were subsequently given several hundred additional thermal cycles and measured again to assess changes in the crystal orientations. These results show that modest changes in crystal orientations occur during thermal cycling.     

Anisotropic Crystal Plasticity Finite Element Modeling of the Effect of Crystal Orientation and Solder Joint Geometry on Deformation after Temperature Change

The crystal orientation of the tin phase in a Pb-free Sn solder joint has a significant effect on the stress state, and hence on the reliability of the solder joint. A set of crystal plasticity analyses was used to evaluate stress and strain resulting from a 165°C temperature change in a single-crystal joint using two simplified geometries used in practical solder joints. Phenomenological flow models for ten slip systems were estimated based upon semiquantitative information available in the literature, along with known anisotropic elastic property information. The results show that the internal energy of the system is a strong function of the tin crystal orientation and geometry of the solder joint. The internal energy (and presumably the likelihood of damage) is highest when the crystal c-axis lies in the plane of the substrate, leading to significant plastic deformation. When the a-axis is in the plane of the interface, deformation due to a 165°C temperature change is predominantly elastic. The texture of the copper substrate using isotropic Cu elastic properties, or anisotropic elastic properties with [001] substrate normal direction, does␣not have a significant effect on the stress or strain in the Sn phase of the joint.     

Improving the Reliability of Si Die Attachment with Zn-Sn-Based High-Temperature Pb-Free Solder Using a TiN Diffusion Barrier

The thermal fatigue reliability of Si die-attached joints with Zn-30wt.%Sn, high-temperature, Pb-free solder was investigated, focusing on the interfacial microstructure and joining strength of a Cu/solder/Cu joint during thermal cycling. A sound die attachment on an aluminum nitride (AlN) direct-bonded copper (DBC) substrate was achieved by forming Cu-Zn intermetallic compound (IMC) layers at the interface with the Cu of the substrate. During the thermal cycling test performed between −40°C and 125°C, thermal fatigue cracks were induced by the growth of Cu-Zn IMCs at the interface with the Cu. A␣thin titanium nitride (TiN) film was applied to suppress the formation of Cu-Zn IMCs. Adequate joint formation was accomplished by using an Au/TiN-coated DBC substrate, and only the TiN layer was observed at both interfaces. In conjunction with the TiN diffusion barrier, the Si die-attached joint created with Zn-30wt.%Sn solder exhibited a stable interfacial microstructure during thermal cycling. No microstructural changes, such as IMC formation, grain growth or formation of fatigue cracks, were observed, and the joining strength was maintained even after 2000 cycles.     

Influence of crystallographic orientation of Sn-Ag-Cu on electromigration in flip-chip joint

The influence of the crystallographic orientation of Sn-3.0 wt%Ag-0.5 wt%Cu flip-chip joints and underfill on electromigration was investigated. The current density applied in our tests was 15 kA/cm² at 160 °C. Various times to failure of the test samples show a clear dependence of the electromigration behavior on the Sn grain orientations. Different microstructural evolutions were observed in all solder bumps in correlation with the crystallographic orientations of the Sn grains after an electromigration test. The primary failure of the solder joints was caused by dissolution of the Cu electrode at the cathode interface. Rapid dissolution of the Cu electrode occurred when the c-axis of the Sn grains was parallel to the direction of electron flow. On the other hand, slight dissolution of the Cu electrode was observed when the c-axis of the Sn grains was perpendicular to the direction of electron flow. Some grain boundaries interrupt the migration of Cu and the trapped Cu atoms form new grains of intermetallic compounds at the grain boundaries. In addition, underfill inhibits serious deformation of solder bumps during current stressing.     

Electromigration and Thermomechanical Fatigue Behavior of Sn0.3Ag0.7Cu Solder Joints

The anisotropy of Sn crystal structures greatly affects the electromigration (EM) and thermomechanical fatigue (TMF) of solder joints. The size of solder joint shrinkage in electronic systems further makes EM and TMF an inseparably coupled issue. To obtain a better understanding of failure under combined moderately high (2000 A/cm²) current density and 10–150°C/1 h thermal cycling, analysis of separate, sequential, and concurrent EM and thermal cycling (TC) was imposed on single shear lap joints, and the microstructure and crystal orientations were incrementally characterized using electron backscatter diffraction (EBSD) mapping. First, it was determined that EM did not significantly change the crystal orientation, but the formation of Cu₆Sn₅ depended on the crystal orientation, and this degraded subsequent TMF behavior. Secondly, TC causes changes in crystal orientation. Concurrent EM and TC led to significant changes in crystal orientation by discontinuous recrystallization, which is facilitated by Cu₆Sn₅ particle formation. The newly formed Cu₆Sn₅ often showed its c-axis close to the direction of electron flow.     

Impact of Isothermal Aging on Long-Term Reliability of Fine-Pitch Ball Grid Array Packages with Sn-Ag-Cu Solder Interconnects: Surface Finish Effects

The interaction between isothermal aging and the long-term reliability of fine-pitch ball grid array (BGA) packages with Sn-3.0Ag-0.5Cu (wt.%) solder ball interconnects was investigated. In this study, 0.4-mm fine-pitch packages with 300-μm-diameter Sn-Ag-Cu solder balls were used. Two different package substrate surface finishes were selected to compare their effects on the final solder composition, especially the effect of Ni, during thermal cycling. To study the impact on thermal performance and long-term reliability, samples were isothermally aged and thermally cycled from 0°C to 100°C with 10 min dwell time. Based on Weibull plots for each aging condition, package lifetime was reduced by approximately 44% by aging at 150°C. Aging at 100°C showed a smaller impact but similar trend. The microstructure evolution was observed during thermal aging and thermal cycling with different phase microstructure transformations between electrolytic Ni/Au and organic solderability preservative (OSP) surface finishes, focusing on the microstructure evolution near the package-side interface. Different mechanisms after aging at various conditions were observed, and their impacts on the fatigue lifetime of solder joints are discussed.     

Influence of Substrate Temperature on Structural and Thermoelectric Properties of Antimony Telluride Thin Films Fabricated by RF and DC Cosputtering

Antimony and tellurium were deposited on BK7 glass using direct-current magnetron and radiofrequency magnetron cosputtering. Antimony telluride thermoelectric thin films were synthesized with a heated substrate. The effects of substrate temperature on the structure, surface morphology, and thermoelectric properties of the thin films were investigated. X-ray diffraction patterns revealed that the thin films were well crystallized. c-Axis preferred orientation was observed in thin films deposited above 250°C. Scanning electron microscopy images showed hexagonal crystallites and crystal grains of around 500 nm in thin film fabricated at 250°C. Energy-dispersive spectroscopy indicated that a temperature of 250°C resulted in stoichiometric Sb₂Te₃. Sb₂Te₃ thin film deposited at room temperature exhibited the maximum Seebeck coefficient of 190 μV/K and the lowest power factor (PF), S ² σ, of 8.75 × 10⁻⁵ W/mK². When the substrate temperature was 250°C, the PF increased to its highest value of 3.26 × 10⁻³ W/mK². The electrical conductivity and Seebeck coefficient of the thin film were 2.66 × 10⁵ S/m and 113 μV/K, respectively.     

Reliability of Cu nanoparticle joint for high temperature power electronics

Power cycle reliability of Cu nanoparticle joint has been studied for high temperature operation of power devices. Al₂O₃ heater chips and Cu–65 wt% Mo baseplates were joined by Cu nanoparticles and Sn–0.7Cu and power cycle tests of 65/200 °C and 65/250 °C were carried out on the joints. The Cu nanoparticles were prepared by reducing Cu carbonate in ethylene glycol with dodecanoic acid + dodecyl amine (C12) and decanoic acid and decyl amine (C10) as capping agents. A power cycle test of 65/200 °C did not inflict severe damage on the Cu nanoparticle joints so that there were not many cracks formed after 3000 cycles. Vertical cracks were formed in the C12 Cu nanoparticle joint after 3000 cycles of 65/250 °C test, however the maximum temperature during the power cycle test did not change at all because vertical cracks did not have an effect on preventing heat flow. On the contrary, lateral cracks were completely formed in the Sn–0.7Cu soldered joint after 200 cycles of 65/200 °C test and in the C10 Cu nanoparticle joint after 360 cycles of 65/250 °C test. In these experiments, the maximum temperatures were rapidly increased because heat conduction was prevented across the formed lateral cracks.     

The Effect of Sn Orientation on Intermetallic Compound Growth in Idealized Sn-Cu-Ag Interconnects

The work reported here explores the influence of crystal orientation on the growth of the interfacial intermetallic layer during electromigration in Cu||Sn||Cu solder joints. The samples were thin, planar Sn-Ag-Cu (SAC) solder layers between Cu bars subject to a uniaxial current. Electron backscatter diffraction (EBSD) was used to characterize the microstructure before and after testing. The most useful representation of the EBSD data identifies the Sn grain orientation by the angle between the Sn c-axis and the current direction. The tested samples included single-crystal joints with c-axis nearly parallel to the current (“green” samples) and with c-axis perpendicular to the current (“red” samples). At current density of 10⁴ A/cm² (steady-state temperature of ~150°C), an intermetallic layer grew at an observable rate in the “green” samples, but not in the “red” ones. A current density of 1.15 × 10⁴  A/cm² (temperature ~160°C) led to measurable intermetallic growth in both samples. The growth fronts were nearly planar and the growth rates constant (after an initial incubation period); the growth rates in the “green” samples were about 10× those in the “red” samples. The Cu concentrations were constant within the joints, showing that the intermetallic growth is dominated by the electromigration flux. The measured growth rates and literature values for the diffusion of Cu in Sn were used to extract values for the effective charge, z ^*, that governs the electromigration of Cu. The calculated value of z ^* is significantly larger for current perpendicular to the c-axis than along it.     

The Crystal Orientation of β-Sn Grains in Sn-Ag and Sn-Cu Solders Affected by Their Interfacial Reactions with Cu and Ni(P) Under Bump Metallurgy

Recently, it has been reported that the crystal orientation and grain size of the β-Sn phase in Sn-rich solders have profound effects on the reliabilities of Pb-free solder joints, such as thermo-mechanical fatigue and electromigration. Additionally, it is also known that the microstructure of the Sn-rich solders is strongly affected by their alloy composition. In this study the grain size and orientation of the β-Sn phase were investigated in terms of their alloy composition and interfacial reactions with two different under bump metallurgies (UBMs), Cu and Ni(P). Solder balls (380 μm in diameter) of pure Sn, Sn-0.5Cu, Sn-0.5Ag, and Sn-1.8Ag (in weight percent) were reflowed on Cu and Ni(P) UBMs. After the reflow at 250°C for 120 s, the microstructure of the solder joints was analyzed by cross-polarization light microscopy and electron backscatter diffraction. For the compositional analysis of solder joints, electron probe micro-analysis was used and thermodynamics calculations were also performed. During reflow on Cu or Ni(P) UBM, Cu and Ni atoms were dissolved quickly and were saturated to their solubility limits in the solders, causing changes in composition and β-Sn grain orientation.     

Thermal Expansion of n-Type Doped Bi2Te2.88Se0.12 and p-Type Doped Bi0.52Sb1.48Te3 Solid Solutions from ?60°C to +60°C

An automated dilatometric system was developed to measure the thermal expansion of solid materials from −60°C to +400°C. This system was then applied to measure the linear thermal expansions of n-type doped Bi₂Te_2.88Se_0.12 and p-type doped Bi_0.52Sb_1.48Te₃ solid solutions along the a-axis from −60°C to +60°C. The experiments were performed using a vertical “tube/push rod” dilatometer under constant-temperature conditions. The initial alloys were synthesized from Bi, Te, and Se or Sb (each at 99.999 mass% purity) in stoichiometric ratios. The method based on the correlation between the thermal expansion and the heat capacity was proposed to calculate the axial expansion coefficients along the c-axis.     

In?Situ Synchrotron Characterization of Melting, Dissolution, and Resolidification in Lead-Free Solders

Melting and solidification of SAC 305 lead-free solder joints in a wafer-level chip-scale package were examined in situ with synchrotron x-ray diffraction. The chips with balls attached (but not assembled to a circuit board) were reflowed one to three times using a temperature and time history similar to an industrial reflow process. Diffraction patterns from the same joint were collected every 0.5 s during the melting and solidification process. The solidification of the Sn phase in the solder joint occurred between 0.5 s and 1 s. During melting, most of the Sn melted in about 0.5 s, but in some cases took 2–5 s for the Sn peak to completely disappear. In one instance, the Sn peak persisted for 30 s. The Ag₃Sn peaks dissolved in about 1–2 s, but the Cu₆Sn₅ peaks from the interface were persistent and did not change throughout the melting and solidification process. Completely different Sn crystal orientations were always developed upon resolidification.     

Synthesis of <Emphasis Type="Italic">c</Emphasis>-Axis Inclined AlN Films in an Off-Center System for Shear Wave Devices

AlN thin films are of continuing interest for excitation of acoustic waves in surface and bulk acoustic wave devices. We report herein on preparation and characterization of c-axis inclined AlN films by a new method of rotating the substrate holder plate to different angles in an off-center system. The microstructure of the c-axis inclined AlN films was investigated using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The analyses showed that polycrystalline AlN films with c-axis inclination of up to 12° could be obtained using the off-center system. Solidly mounted resonators based on the deposited c-axis inclined and vertical AlN films were successfully realized. The frequency responses showed dual-mode resonance characteristics located at 1.12 GHz and 1.87 GHz, corresponding to shear and longitudinal resonant modes, respectively.     

Thermomechanical Stress and Strain in Solder Joints During Electromigration

Thermomechanical stress and strain in the solder joints of a dummy area array package were studied as electromigration occurred. A current density of 0.4 × 10⁴ A/cm² was applied to this package, constructed with 9 × 9 solder joints in a daisy chain, to perform the electromigration test. After 37 h, the first joint on the path of the electron flow broke off at the cathode, and the first three solder joints all exhibited a typical accumulation of intermetallic compounds at the anode. Different solder joints exhibited dissimilar electromigration states, such as steady state and nonsteady state. Finite element analysis indicated that during steady-state electromigration, although the symmetrical structure produced uniform distributions of current density and Joule heating in all solder joints, the distribution of temperature was nonuniform. This was due to the imbalanced heat dissipation, which in turn affected the distribution of thermomechanical stress and strain in the solder joints. The maximum thermomechanical stress and strain, as well the highest temperature and current crowding, appeared in the Ni/Cu layer of each joint. The strain in the Ni/Cu layer was significant along the z-axis, but was constrained in the x–y plane. The thermomechanical stress and strain increased with advancing electromigration; thus, a potential delamination between the Ni/Cu layer and the printed circuit board could occur.     

Enhancement of the hydrogen gas sensitivity by large distribution of c-axis preferred orientation in highly Ga-doped ZnO polycrystalline thin films

The hydrogen gas sensing properties of highly Ga-doped ZnO (GZO) polycrystalline thin films deposited by radio-frequency magnetron sputtering have been studied. The relationship between the microstructural properties of preferred c-axis oriented thin films and the hydrogen gas sensing properties is described. The crystallite size and the preferred orientation distribution were characterized by X-ray diffraction. The crystallite size increased and the preferred orientation distribution decreased with increasing film thickness. In order to control the crystallite size and the c-axis orientation separately, a highly oriented ZnO template layer with different thickness was employed for deposition of 30-nm-thick GZO films. The c-axis orientation of these films were nearly comparable each other, while the crystallite size increased significantly with increasing thickness of the ZnO templates. The hydrogen gas sensitivity at an operating temperature of 330 °C increased slightly with decreasing crystallite size, while the sensitivity was dramatically enhanced by increasing the preferred orientation distribution. It is therefore proposed that the c-axis orientation plays an important role in determining the sensitivity of the hydrogen gas sensor.     

The Influence of Sn Orientation on Intermetallic Compound Evolution in Idealized Sn-Ag-Cu 305 Interconnects: an Electron Backscatter Diffraction Study of Electromigration

Previous research showed the relationship between Sn grain orientation and the intermetallic growth rate in Sn-Ag-Cu (SAC)305 interconnects. Samples with the Sn c-axis aligned parallel to the current flow have an intermetallic compound growth rate significantly faster than samples with the c-axis perpendicular to the current flow. This study continues the previous research by investigating intermetallic growth in polygranular joints and in joints that have a thin Ni layer at the cathodic or anodic interface of the interconnect. Planar SAC305 interconnects were sandwiched between two Cu pads (sometimes incorporating a thin Ni layer at the interface) and subjected to uniaxial current. The crystallographic orientation of Sn in these samples was characterized with electron backscatter diffraction before and after electromigration testing. The results show that polycrystalline joints have relatively slow intermetallic growth rates, close to those found in single-crystal joints with the c-axis perpendicular to the current. When a Ni layer was present on the anode side, the intermetallic grew at a rate comparable to that in samples without a Ni layer. However, when the Ni layer was on the cathode side, the intermetallic growth was significantly retarded. The measured growth rates of the intermetallic, combined with literature values for the diffusion of Cu in Sn, were used to calculate values for the effective charge, z ^*, which is significantly smaller for samples with current parallel to the c-axis than for either polycrystalline samples or samples with the c-axis perpendicular to the electron flow.     

Low-Resistance Cu-Sn Electroplated–Evaporated Microbumps for 3D Chip Stacking

Low-resistance copper-tin (Cu-Sn) microbumps, with sizes varying from 5 μm × 5 μm to 20 μm × 20 μm and formed by electroplating–evaporation bumping (EEB) technology for three-dimensional integration of large-scale integrated chips, have been evaluated for their microstructure and electrical resistance. It was inferred from x-ray diffraction data that the formation of low-resistance Cu₃Sn intermetallic compound (IMC) is facilitated at higher bonding temperature. Electron probe microanalysis mapping showed that, even before bonding, Cu-Sn IMCs were formed at the interface between Cu and Sn, whereas they were sandwiched between the Cu of the upper and lower microbumps after bonding. Electron backscatter diffraction analysis revealed that the crystal orientation of Sn grains was sharply localized in the (100) orientation for physical vapor deposited (PVD) sample, while electroplated Sn film exhibited a mixed crystal orientation in all (100), (110), and (001) axes. A resistance value of ~35 mΩ per bump was obtained for Cu-Sn microbumps with area of 400 μm², which is several times lower than the resistance value reported for Cu-Sn microbumps fabricated by a pure electroplating method. The low resistance value obtained for EEB-formed Cu-Sn microbumps after bonding is explained by (i) the reduced surface roughness for evaporated Sn, (ii) the high degree of crystal grain orientation resulting from layer-by-layer growth in the PVD Sn, despite their smaller grain size, and (iii) the absence of impurity segregation at grain boundaries.     

Interfacial Reactions of Si Die Attachment with Zn-Sn and Au-20Sn High Temperature Lead-Free Solders on Cu Substrates

The interfacial reaction of Si die attachment with a high temperature lead-free solder of Zn-xSn (x = 20 wt.%, 30 wt.% and 40 wt.%) was investigated, and the currently used high temperature lead-free solder of Au-20Sn was compared. A sound die attachment to a Cu substrate can be achieved with Zn-Sn solder. No intermetallic compound (IMC) phase was observed in the solder layer, and only primary α-Zn and Sn-Zn eutectic phases were observed. At the interface with the Si die, with a metallization of Au/Ag/Ni, an AgAuZn₂, IMC layer was formed along the interface, and the Ni coating layer did not react with the solder. At the interface with the Cu substrate, CuZn₅ and Cu₅Zn₈ IMC layers were confirmed, and their thicknesses can be controlled by soldering conditions. During multiple reflows, the growth of these IMC layers was observed, but no additional voids or cracks were observed. For more reliable die attachment, a titanium nitride (TiN) coating layer was applied to suppress the formation of Cu-Zn IMCs. The Si die attached joint on the TiN-coated Cu was quite stable during the multiple reflows, and no visible IMC phase was confirmed in the interfacial microstructure.     

An Investigation of Microstructure and Microhardness of Sn-Cu and Sn-Ag Solders as Functions of Alloy Composition and Cooling Rate

The microstructure and microhardness of Sn-xAg and Sn-xCu solders were investigated as functions of alloy composition and cooling rate. The Ag compositions examined varied from 0.5 wt.% to 3.5 wt.%, while Cu varied from 0.5 wt.% to 2.0 wt.%. Three cooling rates were employed during solidification: 0.02°C/s (furnace cooling), about 10°C/s (air cooling), and 100°C/s or higher (rapid solidification). Sn grain size and orientation were observed by cross-polarization light microscopy and electron-backscattering diffraction (EBSD) techniques. The microhardness was measured to correlate the mechanical properties with alloy compositions and cooling rates. From this study, it was found that both alloy composition and cooling rate can significantly affect the Sn grain size and hardness in Sn-rich solders. The critical factors that affect the microstructure–property relationships of Sn-rich solders are discussed, including grain size, crystal orientation, dendrite cells, twin boundaries, and intermetallic compounds (IMC).