Interfacial reactions of Cu-containing lead-free solders with Au/NiP metallization

Cu-containing solder alloys have been used to identify their interfacial reactions with electroless NiP. As-reflowed, AuSn₄ intermetallic compounds (IMCs) are formed in the Sn-Cu and Sn-Ag-Cu solders, but in the cases of Sn-Ag-Cu-In, In-Sn-Au IMCs are formed and are uniformly distributed in the solder. Different types of IMCs such as high-Cu (>30 at.%), medium-Cu (30-15 at.%), and low-Cu (<15 at.%) containing IMCs are formed at the interface. High-Cu and medium-Cu containing ternary intermetallic compounds (TIMCs) are found in the Sn-Cu and Sn-Ag-Cu solder joints, respectively. Medium-Cu containing quaternary intermetallic compounds (QIMCs) are found in the Sn-Ag-Cu-In joints. Initially, TIMCs and QIMCs have higher growth rates, resulting in the entrapment of some Pb-rich phase in the high-Cu containing TIMCs and some In-Sn-Au phase in the QIMCs. High-Cu containing TIMCs have a lower growth rate and consume less of the NiP layer. The spalling of medium-Cu containing TIMCs in the Sn-Ag-Cu solder increases both the growth rate of TIMCs and the consumption rate of the NiP layer. Low-Cu containing QIMCs in the Sn-Ag-Cu-In solder are stable on P-rich Ni and reduce the dissolution rate of the NiP layer. Consumption of the NiP layer can be reduced by adding Cu or In, because of the changes of the interfacial IMCs phases, which are stable and adhere well to the P-rich Ni layer during reflow.     

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.     

Intermetallic reactions in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Au/Ni surface finishes

The intermetallic compounds (IMCs) formed during the reflow and aging of Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Au/Ni surface finishes were investigated. After reflow, the thickness of (Cu, Ni, Au)₆Sn₅ interfacial IMCs in Sn3Ag0.5Cu0.06Ni0.01Ge was similar to that in the Sn3Ag0.5Cu specimen. The interiors of the solder balls in both packages contained Ag₃Sn precipitates and brick-shaped AuSn₄ IMCs. After aging at 150°C, the growth thickness of the interfacial (Ni, Cu, Au)₃Sn₄ intermetallic layers and the consumption of the Ni surface-finished layer on Cu the pads in Sn3Ag0.5Cu0.06Ni0.01Ge solder joints were both slightly less than those in Sn3Ag0.5Cu. In addition, a coarsening phenomenon for AuSn₄ IMCs could be observed in the solder matrix of Sn3Ag0.5Cu, yet this phenomenon did not occur in the case of Sn3Ag0.5Cu0.06Ni0.01Ge. Ball shear tests revealed that the reflowed Sn3Ag0.5Cu0.06Ni0.01Ge packages possessed bonding strengths similar to those of the Sn3Ag0.5Cu. However, aging treatment caused the ball shear strength in the Sn3Ag0.5Cu packages to degrade more than that in the Sn3Ag0.5Cu0.06Ni0.01Ge packages.     

Effect of substrate metallization on interfacial reactions and reliability of Sn-Zn-Bi solder joints

The scope of this paper covers a comprehensive study of the lead-free Sn-Zn-Bi solder system, on Cu, electrolytic Ni/Au and electroless Ni(P)/Au surface finishes. This includes a study of the shear properties, intermetallic compounds at the substrate-ball interface and dissolution of the under bump metallization. The Sn-8Zn-3Bi (wt.%) solder/Cu system exhibited a low shear load with thick IMCs formation at the interface. The dissolution of the Cu layer in the Sn-Zn-3Bi solder is higher than that of the other two Ni metallizations. It was found that the formation of a thick Ni-Zn intermetallic compound (IMC) layer at the solder interface of the electrolytic Ni bond pad reduced the mechanical strength of the joints during high temperature long time liquid state annealing. The solder ball shear-load for the Ni(P) system during extended reflow increased with an increase of reflow time. No spalling was noticed at the interface of the Sn-Zn-3Bi solder/Ni(P) system. Sn-8Zn-3Bi solder with electroless Ni(P) metallization appeared as a good combination in soldering technology.     

Morphology and Growth of Intermetallics at the Interface of Sn-based Solders and Cu with Different Surface Finishes

Several types of surface finishes have been applied on Cu substrates in an effort to facilitate bonding and improve the reliability of lead-free solder joints. In the current research, the effects of printed circuit board surface finishes on the reliability of the solder joints were investigated by examining the morphology and growth behavior of the intermetallic compounds (IMCs) between Sn-based solders and different surface finishes on Cu. Three types of Cu substrates with different surface finishes were fabricated in this study: organic solderability preservative (OSP)/Cu, Ni/Cu, and electroless nickel immersion gold (ENIG)/Cu. Sn-3.5Ag and Sn-3.0Ag-0.5Cu were used as the solders. In the experiment, the solder joint specimens were aged isothermally at 150°C for up to 1000 h. Experimental results revealed that the OSP surface finish promoted the interdiffusion between Cu and Sn during soldering. The composition and morphology of the IMC layer at the solder/Ni/Cu interface were sensitive to the Cu concentration in the solder. Meanwhile, the solder joints with different morphological features of the IMCs exhibited significant differences in shear strengths. The Au-containing ENIG surface finish affected the shear strength of the solder joint significantly at the initial stage of isothermal aging.     

Inhomogeneous Consumption of the Electroless Ni-P Layer at the Solder Joint Formed with Sn-3.5Ag-0.7Cu

This report investigates the phenomenon of interfacial serration at the joint between Sn-3.5Ag-0.7Cu solder (SAC) and electroless Ni-P underbump metallization (UBM) during aging. For comparison, the interface between SAC and electrolytic Ni UBM was also examined under similar aging conditions. The joint in the electrolytic Ni/SAC sample showed a smooth, flat interface between the Ni and intermetallic compounds, even after a long aging time. In contrast, electroless Ni-P/SAC exhibited an irregular interface, the irregularity of which became more severe with increased aging time. This observed roughness was initiated by large breaks across the P-rich layer. The breaks were filled with the overlying Ni-Sn-P phase material, which provided an easier interdiffusion path for Ni and Sn in the opposite direction. As a result, the local consumption of the Ni(-P) layer was accelerated, and the interface was serrated. The local maximum consumption rate of the Ni-P layer, as calculated from the minimum thickness of this layer in each sample with aging time, was similar to that of an electrolytic Ni layer.     

Effects of Zn-Containing Flux on Sn-3.5Ag Soldering with an Electroless Ni-P/Au Surface Finish: Microstructure and Wettability

The microstructure resulting from Sn-3.5Ag soldering on an electroless Ni-P/Au pad using flux containing Zn(II) stearate was investigated. The content of zinc compound in the flux was 0 wt.% (Z-0), 20 wt.% (Z-20) or 50 wt.% (Z-50). A study of the interfacial microstructure revealed that both Z-20 and Z-50 fluxes yielded a thinner P-rich layer at the interface than did the Z-0 flux. In addition, compared with the bulky Ni–Sn intermetallics of the Z-0 joint interface, refined interfacial intermetallic compounds (IMCs) were observed when using Zn-containing fluxes, Z-20 and Z-50. Based on qualitative analyses of both Z-20 and Z-50 joint interfaces, it was presumed that their intermetallic layers would consist of Ni, Zn, and Sn. Additionally, the Ni content in the IMC layer of the Z-50 joint was lower than that of the Z-20 joint. Electron probe microanalysis (EPMA) of the initial Z-50 joint interface revealed Zn in the interfacial reaction layer, suggesting that Zn participated in the reaction between solder and the surface finish at an early stage of soldering. Consequently, the supply of Zn from the flux diminished Ni diffusion into the molten solder during heating. This effect may have caused a thin P-rich layer to form at the joint interface.     

Joint Strength and Microstructure for Sn-Ag-(Cu) Soldering on an Electroless Ni-Au Surface Finish by Using a Flux Containing a Cu Compound

The effect of a flux containing Cu(II) stearate (barrier flux) on the strength of soldered joints between an electroless Ni-Au surface finish and two Pb-free soldering systems, Sn-3Ag-0.5Cu and Sn-3.5Ag (wt.%), was examined. Pull and shock tests showed that barrier flux gave a higher joint strength for both solder compositions than did a flux containing no Cu compounds. Interface analysis revealed that a thin P-rich layer and refined Cu-Ni-Sn intermetallic compounds were formed at the joint interface when barrier flux was used. It is assumed that the supply of Cu from barrier flux suppresses diffusion of Ni into the solder.     

Isothermal aging effects on the microstructure and solder bump shear strength of eutectic Sn37Pb and Sn3.5Ag solders

The reliability of the eutectic Sn37Pb (63%Sn37%Pb) and Sn3.5Ag (96.5%Sn3.5%Ag) solder bumps with an under bump metallization (UBM) consisting of an electroless Ni(P) plus a thin layer of Au was evaluated following isothermal aging at 150 °C. All the solder bumps remained intact after 1500 h aging at 150 °C. Solder bump microstructure evolution and interface structure change during isothermal aging were observed and correlated with the solder bump shear strength and failure modes. Cohesive solder failure was the only failure mode for the eutectic Sn37Pb solder bump, while partial cohesive solder failure and partial Ni(P) UBM/Al metallization interfacial delamination was the main failure mode for eutectic Sn3.5Ag solder bump.     

Degradation of electroless Ni(P) under-bump metallization in Sn3.5Ag and Sn37Pb solders during high-temperature storage

The interfacial reaction between electroless Ni(P) under-bump metallization (UBM) and solders is studied. A P-rich layer forms in the UBM along the solder side after reflow and thermal aging. Crack formation inside the P-rich layer can sometimes penetrate throughout the entire UBM layer structure. The Ni(P) UBM degradation occurs earlier in the Sn3.5Ag solder than in Sn37Pb because of its higher reflow temperature. Despite the formation of a P-rich layer and cracks inside the UBM, it still keeps its original function within the high-temperature storage period in this study. Explanations for the formation of the P-rich layer and cracks in the UBM are outlined along with explanations for the Ni(P) UBM degradation process.     

Effects of Cu contents in Pb-free solder alloys on interfacial reactions and bump reliability of Pb-free solder bumps on electroless Ni-P under-bump metallurgy

Using the screen-printed solder-bumping technique on the electroless plated Ni-P under-bump metallurgy (UBM) is potentially a good method because of cost effectiveness. As SnAgCu Pb-free solders become popular, demands for understanding of interfacial reactions between electroless Ni-P UBMs and Cu-containing Pb-free solder bumps are increasing. It was found that typical Ni-Sn reactions between the electroless Ni-P UBM and Sn-based solders were substantially changed by adding small amounts of Cu in Sn-based Pb-free solder alloys. In Cu-containing solder bumps, the (Cu,Ni)₆Sn₅ phase formed during initial reflow, followed by (Ni,Cu)₃Sn₄ phase formation during further reflow and aging. The Sn3.5Ag solder bumps showed a much faster electroless Ni-P UBM consumption rate than Cu-containing solder bumps: Sn4.0Ag0.5Cu and Sn0.7Cu. The initial formation of the (Cu,Ni)₆Sn₅ phase in SnAgCu and SnCu solders significantly reduced the consumption of the Ni-P UBM. The more Cu-containing solder showed slower consumption rate of the Ni-P UBM than the less Cu-containing solder below 300°C heat treatments. The growth rate of the (Cu,Ni)₆Sn₅ intermetallic compound (IMC) should be determined by substitution of Ni atoms into the Cu sublattice in the solid (Cu,Ni)₆Sn₅ IMC. The Cu contents in solder alloys only affected the total amount of the (Cu,Ni)₆Sn₅ IMC. More Cu-containing solders were recommended to reduce consumption of the Ni-based UBM. In addition, bump shear strength and failure analysis were performed using bump shear test.     

Interfacial reactions and compound formation of Sn-Ag-Cu solders by mechanical alloying on electroless Ni-P/Cu under bump metallization

Electroless Ni-P under bump metallization (UBM) has been widely used in electronic interconnections due to the good diffusion barrier between Cu and solder. In this study, the mechanical alloying (MA) process was applied to produce the SnAgCu lead-free solder pastes. Solder joints after annealing at 240°C for 15 min were employed to investigate the evolution of interfacial reaction between electroless Ni-P/Cu UBM and SnAgCu solder with various Cu concentrations ranging from 0.2 to 1.0 wt.%. After detailed quantitative analysis with an electron probe microanalyzer, the effect of Cu content on the formation of intermetallic compounds (IMCs) at SnAgCu solder/electroless Ni-P interface was evaluated. When the Cu concentration in the solder was 0.2 wt.%, only one (Ni, Cu)₃Sn₄ layer was observed at the solder/electroless Ni-P interface. As the Cu content increased to 0.5 wt.%, (Cu, Ni)₆Sn₅ formed along with (Ni, Cu)₃Sn₄. However, only one (Cu, Ni)₆Sn₅ layer was revealed, if the Cu content was up to 1 wt.%. With the aid of microstructure evolution, quantitative analysis, and elemental distribution by x-ray color mapping, the presence of the Ni-Sn-P phase and P-rich layer was evidenced.     

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.     

Pb-free Sn/3.5Ag electroplating bumping process and under bump metallization (UBM)

Pb-free solder is one of the biggest issues in today's electronic packaging industry. This paper introduces a newly developed Sn/3.5Ag alloy plating process for wafer level bumping. The effects of Under Bump Metallization (UBM) on the process, interfacial reaction, and mechanical strength have been investigated. Four different types of sputtering-based UBM layers-TiW/Cu/electroplated Cu, Cr/CrCu/Cu, NiV/Cu, and TiW/NiV-were fabricated with eutectic Pb/63Sn and Sn/3.5Ag solder. The result shows that the Sn/Ag solder gains Cu or Ni from UBM's and becomes Sn/Ag/Cu or Sn/Ag/Ni during reflow process. Sn/Ag solder has higher reactivity with Cu and Ni than Pb/63Sn. The Intermetallic Compound (IMC) spalling from the interface between UBM/solder has been observed on Cr/CrCu/Cu and TiW/NiV UBM's. However, the IMC spalling phenomena did not decrease the bump shear strength with a bump size of 110 /spl mu/m, whereas a size of 60 /spl mu/m brought a decrease in shear value and failure mode change.     

Interfacial microstructure evolution in Pb-free solder systems

The Sn-3.5Ag and Sn-3.0Ag-0.5Cu ball-grid-array solder balls bonded onto Ni/Au metallization exhibited different interfacial morphology after both wetting and solid-state reactions. In contrast to the eutectic-SnPb solder system, both Pb-free systems showed higher solder-ball shear strength after annealing. Reprecipitation of Au as (Au,Ni)Sn₄ at the interface, as shown in the eutectic-SnPb solder system, was not observed in both Pb-free solder systems. Instead, Ni₃Sn₄ and Cu-Sn-Ni-Au intermetallic compounds (IMCs) were found in the SnAg and SnAgCu systems, respectively. In the SnAgCu system, a thick, acicular-Cu-Sn-Ni IMC formed after wetting, but a faceted-Cu-Sn-Ni-Au phase was found with longer annealing. The growth of this interfacial phase in the Sn-3.0Ag-0.5Cu solder system was also slightly inhibited by the addition of Cu, with a formation energy of about 200 kJ/mol.     

Correlation between interfacial reactions and mechanical strengths of Sn(Cu)/Ni(P) solder bumps

The correlation between interfacial reactions and mechanical strengths of Sn(Cu)/Ni(P) solder bumps has been studied. Upon solid-state aging, a diffusion-controlled process was observed for the interfacial Ni-Sn compound formation of the Sn/Ni(P) reaction couple and the activation energy is calculated to be 42 KJ/mol. For the Sn0.7Cu/Ni(P), in the initial aging, a needle-shaped Ni-Sn compound layer formed on Ni(P). Then, it was gradually covered by a layer of the Cu-Sn compound in the later aging process. Hence, a mixture layer of Ni-Sn and Cu-Sn compounds formed at the interface. For the Sn3.0Cu/Ni(P), a thick Cu-Sn compound layer quickly formed on Ni(P), which retarded the Ni-Sn compound formation and resulted in a distinct Cu-Sn compound/Ni(P) interface. The shear test results show that the mixture interface of Sn0.7Cu bumps have fair shear strengths against the aging process. In contrast, the distinct Cu-Sn/Ni(P) interface of Sn3.0Cu solder bumps is relatively weak and exhibits poor resistance against the aging process. Upon the reflowing process, the gap formation at the Ni(P)/Cu interface caused a fast degradation in the interfacial strength for Sn solder bumps. For Sn0.7Cu and Sn3.0Cu solder bumps, Ni₃P formation was greatly retarded by the self-formed Cu-Sn compound layer. Therefore, Sn(Cu) solder bumps show better shear strengths over the Sn solder bump.     

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.     

Early Interfacial Reaction and Formation of Intermetallic Compounds in the Sn-3.5Ag/Cu Soldering System

The early interfacial reaction in the Sn-3.5Ag/Cu soldering system and the system’s premelting behavior were found and characterized by differential scanning calorimetry incorporated into the reflow process. The results show that the early interfacial reaction occurs by way of melting and wetting of the solder layer adjacent to the Cu substrate at a temperature nearly 4°C below the actual melting point of Sn-3.5Ag solder due to solid-state diffusion of Cu atoms into the Sn-3.5Ag binary solder. Consequently, the early interfacial reaction brings about formation of Cu-Sn intermetallic compounds (IMCs) at a temperature below the melting point of Sn-3.5Ag, and a prolonged early interfacial reaction can lead to change of the Cu-Sn IMC morphology from planar-like to scallop-like and promote excessive growth of IMCs at the interface.     

Studies of the mechanical and electrical properties of lead-free solder joints

The mechanical and electrical properties of several Pb-free solder joints have been investigated including the interfacial reactions, namely, the thickness and morphology of the intermetallic layers, which are correlated with the shear strength of the solder joint as well as its electrical resistance. A model joint was made by joining two “L-shaped” copper coupons with three Pb-free solders, Sn-3.5Ag (SA), Sn-3.8Ag-0.7Cu (SAC), and Sn-3.5Ag-3Bi (SAB) (all in wt.%), and combined with two surface finishes, Cu and Ni(P)/Au. The thickness and morphology of the intermetallic compounds (IMCs) formed at the interface were affected by solder composition, solder volume, and surface finish. The mechanical and electrical properties of Pb-free solder joints were evaluated and correlated with their interfacial reactions. The microstructure of the solder joints was also investigated to understand the electrical and mechanical characteristics of the Pb-free solder joints.     

Studies of electroless nickel under bump metallurgy—Solder interfacial reactions and their effects on flip chip solder joint reliability

The electroless-deposited Ni-P under bump metallurgy (UBM) layer was fabricated on Al pads for Sn containing solder bumps. The amount of P in the electroless Ni film was optimized by controlling complexing agents and the pH of plating solution. The interfacial reaction at the electroless Ni UBM/solder interface was investigated in this study. The intermetallic compound (IMC) formed at the interface during solder reflowing was mainly Ni₃Sn₄, and a P-rich Ni layer was also formed as a by-product of Ni-Sn reaction between the Ni-Sn IMC and the electroless Ni layer. One to four microns of Ni₃Sn₄ IMC and a 1800–5000 Å of P-rich Ni layer were formed in less than 10 min of solder reflowing depending on solder materials and reflow temperatures. It was found that the P-rich Ni layer contains Ni, P, and a small amount of Sn (～7 at.%). Further cross-sectional transmission electron microscopy (TEM) analysis confirmed that the composition of the P-rich Ni layer was 75 at.% Ni, 20at.%P, and 5at.%Sn by energy-dispersive x-ray spectroscopy (EDS) and the phase transformation occurred in the P-rich Ni layer by observing grain size. Kirkendall voids were also found in the Ni₃Sn₄ IMC, just above the P-rich Ni layer after extensive solder reflow. The Kirkendall voids are considered a primary cause of the brittle fracture; restriction of the growth of of the P-rich Ni layer by optimizing proper processing conditions is recommended. The growth kinetics of Ni-Sn IMC and P-rich Ni layer follows three steps: a rapid initial growth during the first 1 min of solder reflow, followed by a reduced growth step, and finally a diffusion-controlled growth. During the diffusion-controlled growth, there was a linear dependence between the layer thickness and time^1/2. Flip chip bump shear testing was performed to measure the effects of the IMC and the P-rich Ni layers on bump adhesion property. Most failures occurred in the solder and at the Ni₃Sn₄ IMC. The brittle characteristics of the Ni-Sn IMC and the Kirkendall voids at the electroless Ni UBM-Sn containing solder system cause brittle bump failure, which results in a decreased bump adhesion strength.