Studies on solder bump electromigration in Cu/Sn–3Ag–0.5Cu/Cu system

This paper aims to understand the solder bump electromigration phenomenon in the Cu/Sn–3Ag–0.5Cu/Cu system. A temperature of 453 K with a current density of 10 kA/cm² was applied. A void nucleated at the highest current density point at the cathode. As the void grew along the cathode side, a solder depletion occurred on the opposite side of the electron entry point, resulting in an open failure. A unique purposely-designed 3D model simulation methodology provides a good understanding of the void nucleation and growth behavior. The temperature of the solder joint during the electromigration test was measured successfully by the resistance change in the junction line between the two joints.     

Evaluation of displacement rate effect in shear test of Sn–3Ag–0.5Cu solder bump for flip chip application

An experimental investigation was combined with a non-linear finite element analysis using an elastic–viscoplastic constitutive model to study the effect of ball shear speed on the shear forces of flip chip solder bumps. A solder composition used in this study was Sn–3mass%Ag–0.5mass%Cu. A low cost bumping process has been employed using electroless Ni and immersion Au followed by solder paste stencil printing. A thin layer of intermetallic compound, (Ni_1−xCu_x)₃Sn₄, was formed by the reaction between the solder and electroless Ni with a thickness of about 1.4 μm, while some discontinuous (Cu_1−yNi_y)₆Sn₅ particles were also formed at the interface. The compositions of the resulting compounds were identified using energy dispersive spectrometer (EDS) and electron microprobe analysis (EPMA). Shear tests were carried out over a shear speed range from 20 to 400 μm/s at a shear ram height of 20 μm. The shear force was observed to linearly increase with shear speed and reach the maximum value at the fastest shear speed in both experimental and computational results. The optimum shear speeds for the shear test of solder bumped flip chip were recommended to be not exceeding 200 μm/s. The failure mechanisms were discussed in terms of von Mises stresses and plastic strain energy density distributions.     

elemental redistribution and interfacial reaction mechanism for the flip chip Sn-3.0Ag-(0.5 or 1.5)Cu solder bump with Al/Ni(V)/Cu under-Bump metallization during aging

In flip chip technology, Al/Ni(V)/Cu under-bump metallization (UBM) is currently applicable for Pb-free solder, and Sn−Ag−Cu solder is a promising candidate to replace the conventional Sn−Pb solder. In this study, Sn-3.0Ag-(0.5 or 1.5)Cu solder bumps with Al/Ni(V)/Cu UBM after assembly and aging at 150°C were employed to investigate the elemental redistribution, and reaction mechanism between solders and UBMs. During assembly, the Cu layer in the Sn-3.0Ag-0.5Cu joint was completely dissolved into solders, while Ni(V) layer was dissolved and reacted with solders to form (Cu_1−y,Ni_y)₆Sn₅ intermetallic compound (IMC). The (Cu_1−y,Ni_y)₆Sn₅ IMC gradually grew with the rate constant of 4.63 × 10⁻⁸ cm/sec^0.5 before 500 h aging had passed. After 500 h aging, the (Cu_1−y,Ni_y)₆Sn₅ IMC dissolved with aging time. In contrast, for the Sn-3.0Ag-1.5Cu joint, only fractions of Cu layer were dissolved during assembly, and the remaining Cu layer reacted with solders to form Cu₆Sn₅ IMC. It was revealed that Ni in the Ni(V) layer was incorporated into the Cu₆Sn₅ IMC through slow solid-state diffusion, with most of the Ni(V) layer preserved. During the period of 2,000 h aging, the growth rate constant of (Cu_1−y,Ni_y)₆Sn₅ IMC was down to 1.74 × 10⁻⁸ cm/sec^0.5 in, the Sn-3.0Ag-1.5Cu joints. On the basis of metallurgical interaction, IMC morphology evolution, growth behavior of IMC, and Sn−Ag−Cu ternary isotherm, the interfacial reaction mechanism between Sn-3.0Ag-(0.5 or 1.5)Cu solder bump and Al/Ni(V)/Cu UBM was discussed and proposed.     

Experimental characterization and mechanical behavior analysis of intermetallic compounds of Sn–3.5Ag lead-free solder bump with Ti/Cu/Ni UBM on copper chip

Due to today’s trend towards ‘green’ products, the environmentally conscious manufacturers are moving toward lead-free schemes for electronic devices and components. Nowadays the bumping process has become a branch of the infrastructure of flip chip bonding technology. However, the formation of excessively brittle intermetallic compound (IMC) between under bump metallurgy (UBM)/solder bump interface influences the strength of solder bumps within flip chips, and may create a package reliability issue. Based on the above reason, this study investigated the mechanical behavior of lead-free solder bumps affected by the solder/UBM IMC formation in the duration of isothermal aging. To attain the objective, the test vehicles of Sn–Ag (lead-free) and Sn–Pb solder bump systems designed in different solder volumes as well as UBM diameters were used to experimentally characterize their mechanical behavior. It is worth to mention that, to study the IMC growth mechanism and the mechanical behavior of a electroplated solder bump on a Ti/Cu/Ni UBM layer fabricated on a copper chip, the test vehicles are composed of, from bottom to top, a copper metal pad on silicon substrate, a Ti/Cu/Ni UBM layer and electroplated solder bumps. By way of metallurgical microscope and scanning-electron-microscope (SEM) observation, the interfacial microstructure of test vehicles was measured and analyzed. In addition, a bump shear test was utilized to determine the strength of solder bumps. Different shear displacement rates were selected to study the time-dependent failure mechanism of the solder bumps. The results indicated that after isothermal aging treatment at 150 °C for over 1000 h, the Sn–Ag solder revealed a better maintenance of bump strength than that of the Sn–Pb solder, and the Sn–Pb solder showed a higher IMC growth rate than that of Sn–Ag solder. In addition, it was concluded that the test vehicles of copper chip with the selected Ti/Cu/Ni UBMs showed good bump strength in both the Sn–Ag and Sn–Pb systems as the IMC grows. Furthermore, the study of shear displacement rate effect on the solder bump strength indicates that the analysis of bump strength versus thermal aging time should be identified as a qualitative analysis for solder bump strength determination rather than a quantitative one. In terms of the solder bump volume and the UBM size effects, neither the Sn–Ag nor the Sn–Pb solders showed any significant effect on the IMC growth rate.     

Soldering-induced Cu diffusion and intermetallic compound formation between Ni/Cu under bump metallization and SnPb flip-chip solder bumps

Nickel-based under bump metallization (UBM) has been widely used as a diffusion barrier to prevent the rapid reaction between the Cu conductor and Sn-based solders. In this study, joints with and without solder after heat treatments were employed to evaluate the diffusion behavior of Cu in the 63Sn-37Pb/Ni/Cu/Ti/Si₃N₄/Si multilayer structure. The atomic flux of Cu diffused through Ni was evaluated from the concentration profiles of Cu in solder joints. During reflow, the atomic flux of Cu was on the order of 10¹⁵–10¹⁶ atoms/cm²s. However, in the assembly without solder, no Cu was detected on the surface of Ni even after ten cycles of reflow. The diffusion behavior of Cu during heat treatments was studied, and the soldering-process-induced Cu diffusion through Ni metallization was characterized. In addition, the effect of Cu content in the solder near the solder/intermetallic compound (IMC) interface on interfacial reactions between the solder and the Ni/Cu UBM was also discussed. It is evident that the (Cu,Ni)₆Sn₅ IMC might form as the concentration of Cu in the Sn-Cu-Ni alloy exceeds 0.6 wt.%.     

Diffusion behavior of Cu in Cu/electroless Ni and Cu/electroless Ni/Sn-37Pb solder joints in flip chip technology

For Cu pads used as under bump metallization (UBM) in flip chip technology, the diffusion behavior of Cu in the metallization layer is an important issue. In this study, isothermal interdiffusion experiments were performed at 240°C for different times with solid-solid and liquid-solid diffusion couples assembled in Cu/electroless-Ni (Ni-10 wt.% P) and Cu/electroless Ni (Ni-10 wt.% P)/ Sn-37Pb joints. The diffusion structure and concentration profiles were examined by scanning electron microscopy and electron microprobe analysis. The interdiffusion fluxes of Cu, Ni and P were calculated from the concentration profiles with the aid of Matano plane evaluation. The values of J_Cu, J_Ni, and J_P decreased with increasing annealing time. The average effective interdiffusion coefficients on the order of 10⁻¹⁴ cm²/s were also evaluated within the diffusion zone. The amounts of Cu dissolved in the intermetallic compounds (IMCs) Ni₃Sn₄ and Ni₃P that precipitate after annealing the Cu/electroless Ni/Sn-37Pb joints were about 0.25 at.% and 0.5 at.%, respectively. For the short period of annealing, it appears that the presence of electroless Ni (EN) with the Sn-Pb soldering reaction assisted the diffusion of Cu through the EN layer.     

Intermetallic compound formation and morphology evolution in the 95Pb5Sn flip-chip solder joint with Ti/Cu/Ni under bump metallization during reflow soldering

Intermetallic compound formation and morphology evolution in the 95Pb5Sn flip-chip solder joint with the Ti/Cu/Ni under bump metallization (UBM) during 350°C reflow for durations ranging from 50 sec to 1440 min were investigated. A thin intermetallic layer of only 0.4 μm thickness was formed at the 95Pb5Sn/UBM interface after reflow for 5 min. When the reflow was extended to 20 min, the intermetallic layer grew thicker and the phase identification revealed the intermetallic layer comprised two phases, (Ni,Cu)₃Sn₂ and (Ni,Cu)₃Sn₄. The detection of the Cu content in the intermetallic compounds indicated that the Cu atoms had diffused through the Ni layer and took part in the intermetallic compound formation. With increasing reflow time, the (Ni,Cu)₃Sn₄ phase grew at a faster rate than that of the (Ni,Cu)₃Sn₂ phase. Meanwhile, irregular growth of the (Ni,Cu)₃Sn₄ phase was observed and voids formed at the (Ni,Cu)₃Sn₂/Ni interface. After reflow for 60 min, the (Ni,Cu)₃Sn₂ phase disappeared and the (Ni,Cu)₃Sn₄ phase spalled off the NI layer in the form of a continuous layer. The gap between the (Ni,Cu)₃Sn₄ layer and the Ni layer was filled with lead. A possible mechanism for the growth, disappearance, and spalling of the intermetallic compounds at the 95Pb5Sn/UBM interface was proposed.     

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.     

The influence of solder volume and pad area on Sn–3.8Ag–0.7Cu and Ni UBM reaction in reflow soldering and isothermal aging

This paper examines various aspects of SAC (Sn–3.8Ag–0.7Cu wt.%) solder and UBM interactions which may impact interconnection reliability as it scales down. With different solder joint sizes, the dissolution rate of UBM and IMC growth kinetics will be different. Solder bumps on 250, 80 and 40 μm diameter UBM pads were investigated. The effect of solder volume/pad metallization area (V/A) ratio on IMC growth and Ni dissolution was investigated during reflow soldering and solid state isothermal aging. Higher V/A ratio produced thinner and more fragmented IMC morphology in SAC solder/Ni UBM reflow soldering interfacial reaction. Lower V/A ratio produced better defined IMC layer at the Ni UBM interface. When the ratio of V/A is constant, the IMC morphology and growth trend was found to be similar. After 250 h of isothermal aging, the IMC growth rate of the different bump sizes leveled off. No degradation in shear strength was observed in these solder bump after 500 h of isothermal aging.     

Shear and Pull Testing of Sn-3.0Ag-0.5Cu Solder with Ti/Ni(V)/Cu Underbump Metallization During Aging

Ti/Ni(V)/Cu underbump metallization (UBM) is widely used in flip-chip technology today. The advantages of Ti/Ni(V)/Cu UBM are a low reaction rate with solder and the lack of a magnetic effect during sputtering. Sn atoms diffuse into the Ni(V) layer to form a Sn-rich phase, the so-called Sn-patch, during reflow and aging. In this study, the relationship between interfacial reaction and mechanical properties of the solder joints with Ti/Ni(V)/Cu UBM was evaluated. Sn-3.0Ag-0.5Cu solder was reflowed on sputtered Ti/Ni(V)/Cu UBM, and then the reflowed samples were aged at 125°C and 200°C, respectively. (Cu,Ni)₆Sn₅ was formed and grew gradually at the interface of the solder joints during aging at 125°C. The Sn-patch replaced the Ni(V) layer, and (Ni,Cu)₃Sn₄ was thus formed between (Cu,Ni)₆Sn₅ and the Sn-patch at 200°C. The Sn-patch, composed of Ni and V₂Sn₃ after reflow, was transformed to V₂Sn₃ and amorphous Sn during aging. Shear and pull tests were applied to evaluate the solder joints under various heat treatments. The shear force of the solder joints remained at 421 mN, yet the pull force decreased after aging at 125°C. Both the shear and pull forces of the solder joints decreased during aging at 200°C. The effects of aging temperature on the mechanical properties of solder joint were investigated and discussed.     

An Embrittlement Mechanism of Impact Fracture of Sn–Ag–Cu Solder Joints on BGA Using Electroplated Ni/Au Surface Finishes

The impact toughness evaluation and fracture mechanism analysis in board level of Sn–3mass%Ag–0.5mass%Cu solder joints of ball grid arrays (BGAs) using electrolysis Ni/Au plating were performed. The cause of impact toughness degradation of BGA solder ball joints is the segregation of impurities to the (Cu, Ni)$_{6}$Sn$_{5}$ intermetallic compound grain boundary formed in the solder joints. The impurities, consisting of Cl and organic matters, are taken in the Ni plating film at the time of Ni plating. The organic matter impurities come primarily from the solder mask of the BGA interposer substrates. To improve the impact toughness of the Sn–3mass%Ag–0.5mass%Cu solder joint of the BGA, it is necessary to lower the concentration of these impurities. This, in turn, places importance on solder mask material selection (to minimize Ni plating bath contamination) as well as contamination prevention and plating bath sanitization.      

High temperature reliability and interfacial reaction of eutectic Sn–0.7Cu/Ni solder joints during isothermal aging

The interfacial reactions and growth kinetics of intermetallic compound (IMC) layers formed between Sn–0.7Cu (wt.%) solder and Au/Ni/Cu substrate were investigated at aging temperatures of 185 and 200 °C for aging times of up to 60 days. After reflow, the IMC formed at the interface was (Cu, Ni)₆Sn₅. After aging at 185 °C for 3 days and at 200 °C for 1 day, two IMCs of (Cu, Ni)₆Sn₅ and (Ni, Cu)₃Sn₄ were observed. The growth of the (Ni, Cu)₃Sn₄ IMC consumed the (Cu, Ni)₆Sn₅ IMC at an aging temperature of 200 °C due to the restriction of supply of Cu atoms from the solder to interface. After aging at 200 °C for 60 days, the Ni layer of the substrate was completely consumed in many parts of the sample, at which point a Cu₃Sn IMC was formed. In the ball shear test, the shear strength decreased with increasing aging temperature and time. Until the aging at 185 °C for 15 days and at 200 °C for 3 days, fractures occurred in the bulk solder. After prolonged aging treatment, fractures partially occurred at the (Cu, Ni)₆Sn₅ + Au/solder interface for aging at 185 °C and at the (Ni, Cu)₃Sn₄/Ni interface for aging at 200 °C, respectively. Consequently, thick IMC layer and thermal loading history significantly affected the integrity of the Sn–0.7Cu/Ni BGA joints.     

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.     

Effect of Ag micro-particles content on the mechanical strength of the interface formed between Sn–Zn binary solder and Au/Ni/Cu bond pads

In this study, addition of Ag micro-particles with a content in the range between 0 and 4 wt.% to a Sn–Zn eutectic solder, were examined in order to understand the effect of Ag additions on the microstructural and mechanical properties as well as the thermal behavior of the composite solder formed. The shear strengths and the interfacial reactions of Sn–Zn micro-composite eutectic solders with Au/Ni/Cu ball grid array (BGA) pad metallizations were systematically investigated. Three distinct intermetallic compound (IMC) layers were formed at the solder interface of the Au/electrolytic Ni/Cu bond pads with the Sn–Zn composite alloys. The more Ag particles that were added to the Sn–Zn solder, the more Ag–Zn compound formed to thicken the uppermost IMC layer. The dissolved Ag–Zn IMCs formed in the bulk solder redeposited over the initially formed interfacial Au–Zn IMC layer, which prevented the whole IMC layer lifting-off from the pad surface. Cross-sectional studies of the interfaces were also conducted to correlate with the fracture surfaces.     

Suppressing Sn-Patch Growth in Ti/Ni(V)/Cu Under Bump Metallization with Sn-Ag-Cu Solder After Reflow and Aging

Sputtered Ti/Ni(V)/Cu under bump metallization (UBM) is widely used in flip chip technology because the metals are nonmagnetic and the consumption of the Ni(V) layer is low. It is noted that V does not react with solders and intermetallic compounds (IMC) during reflow and aging; however, a Sn-patch forms in the Ni(V) layer, and the Sn-patch growth may cause the IMCs to detach from the interface of solder joints. In this study, Sn-3.0Ag-0.5Cu solder was reflowed on Ti/Ni(V)/Cu UBM with different Cu thicknesses at 250°C for 60 s, and then aged at 150°C for various periods of time. It was revealed that the Sn-patch growth could be controlled by increasing the Cu thickness in the Ti/Ni(V)/Cu UBM. A feasible approach to suppress Sn-patch formation after reflow and aging is discussed.     

Effect of nano Al2O3 additions on the microstructure, hardness and shear strength of eutectic Sn–9Zn solder on Au/Ni metallized Cu pads

Nano-sized, nonreacting, noncoarsening Al₂O₃ particles have been incorporated into eutectic Sn–Zn solder alloys to investigate the microstructure, hardness and shear strength on Au/Ni metallized Cu pads ball grid array substrate (BGA). In the plain Sn–Zn solder joint and solder joints containing Al₂O₃ nano-particles, a scallop-shaped AuZn₃ intermetallic compound layer was found at the interfaces. In the solder joints containing Al₂O₃ nano-particles, a fine acicular-shaped Zn-rich phase and Al₂O₃ nano-particles were found to be homogeneously distributed in the β-Sn matrix. The shear strengths and hardness of solder joints containing higher percentage of Al₂O₃ nano-particles exhibited consistently higher value than those of plain solder joint and solder joints containing lower percentage of Al₂O₃ nano-particles due to control the fine microstructure as well as homogeneous distribution of Al₂O₃ nano-particles acting as a second phase dispersion strengthening mechanism. The fracture surfaces of plain Sn–Zn solder joints exhibited a brittle fracture mode with smooth surfaces while Sn–Zn solder joints containing Al₂O₃ nano-particles showed a typical ductile failure with very rough dimpled surfaces.     

Influence of Palladium Thickness on the Soldering Reactions Between Sn-3Ag-0.5Cu and Au/Pd(P)/Ni(P) Surface Finish

This study provides a comparison of the influence of Pd(P) thickness on reactions during soldering with the Sn-3Ag-0.5Cu alloy. Soldering was carried out in an infrared-enhanced conventional reflow oven, and a multiple reflow test method (up to ten cycles) was performed. With increasing Pd(P) thickness, the (Cu,Ni)₆Sn₅ grew more slowly at the solder/Ni(P) interface, while the Ni₂SnP/Ni₃P bilayer became predominant after the first reflow. These three intermetallics, i.e., (Cu,Ni)₆Sn₅, Ni₂SnP, and Ni₃P, gradually coarsened as the number of reflow cycles increased. Furthermore, an additional (Ni,Cu)₃Sn₄ layer appeared between (Cu,Ni)₆Sn₅ and Ni₂SnP, especially for the case of a thicker Pd(P) layer (0.2 μm). The attachment of the (Ni,Cu)₃Sn₄ to the Ni₂SnP, however, was fairly poor, and a series of microcracks formed along the (Ni,Cu)₃Sn₄/Ni₂SnP interface. To quantify the mechanical response of the interfacial microstructures, shear testing was conducted at two different shear speeds (0.0007 m/s and 2 m/s). The results indicated that the interfacial strength and the Pd(P) thickness were strongly correlated.     

Reliability of Pb(Mg,Nb)O3–Pb(Zr,Ti)O3 multilayer ceramic piezoelectric actuators by Weibull method

Expected lifetime and reliability of multilayer ceramic actuators were considered. 5 × 5 × 5 mm³ sized multilayer ceramic actuators with 60 ceramic sheet layers embedded internal electrodes were selected in this experimental analysis. Specially selected rectified AC bias was applied to the multilayer ceramic actuators to remove heating during charging and discharging process. Sixteen sets of multilayer ceramic actuators under the different experimental conditions were tested for statistical analysis. Weibull function method was employed to calculate mean time to failure. Arrhenius model and power law model were considered to simulate experimental data. From the experimental data and theoretical consideration, equation of expected lifetime was estimated.