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.     

Au−Ni−Sn intermetallic phase relationships in eutectic Pb−Sn solder formed on Ni/Au metallization

Recent work has shown that a Au−Ni−Sn ternary compound with a nominal composition of Au_0.5Ni_0.5Sn₄ redeposits and grows at the interface between eutectic Pb−Sn solder and Ni/Au metallization during aging at 150°C. The present work verifies the existence of the Au_0.5Ni_0.5Sn₄ phase by examining the Sn-rich corner of the Au−Ni−Sn ternary phase diagram. The reconfiguration mechanism of the AuSn₄ from the bulk solder is also discussed, with detailed observations of the Au_0.5Ni_0.5Sn₄ microstructure. The results show that the Ni solubility limit in the AuSn₄ phase is approximately 12 at.% at 150°C and thus, the Au_0.5Ni_0.5Sn₄ phase is a ternary AuSn₄-based compound with high Ni solubility. Due to the slight solubility and the fast diffusion of Au in the eutectic Pb−Sn at 150°C, the AuSn₄ intermetallics in the bulk solder can reconfigure to form a Au_xNi_1−xSn₄ compound at the interface where Ni is available. The Au_xNi_1−xSn₄ compound layer consists of nanocrystals arranged in a larger grainlike morphology. It appears that the inherent lattice strain of the Au_xNi_1−xSn₄ compound and the volume change due to its formation results in a nanocrystalline microstructure.     

Interfacial Reaction Between 95Pb-5Sn Solder Bump and 37Pb-63Sn Presolder in Flip-Chip Solder Joints

Interdiffusion and interfacial reaction of 95Pb-5Sn solder bumps and 37Pb-63Sn presolder in flip-chip solder joints during high-temperature storage were studied. Reaction temperatures included 100°C, 130°C, 150°C, and 175°C. It was found that Cu₆Sn₅ and Cu₃Sn formed on the board side and (Ni,Cu)₃Sn₄ formed on the chip side after 100 h of aging. After 2000 h of aging at 175°C, the Ni under-bump metallization (UBM) was exhausted. This caused the (Ni,Cu)₃Sn₄ layer at the chip-side interface to be gradually converted into (Cu_0.6Ni_0.4)₆Sn₅. It was also found that the consumption of the Ni UBM was faster than the case where eutectic Sn-Pb solder was used for the entire joint. Nevertheless, the consumption of the Cu on the substrate side was slower than the case where pure eutectic Sn-Pb solder was used for the entire joint.     

Intermetallic Reactions in Sn-0.4Co-0.7Cu Solder BGA Packages with an ENIG Surface Finish

As-cast Sn-0.4Co-0.7Cu solder contains both (Cu_0.98Co_0.02)₆Sn₅ and (Co_0.85Cu_0.15) Sn₃ intermetallic phases in the matrix. After reflowing, the Au thin film in the electroless Ni/immersion Au (ENIG) surface-finished Sn-0.4Co-0.7Cu solder ball grid array (BGA) packages dissolved rapidly into the solder matrix to form AuSn₄ intermetallics, and a thin layer of (Cu_0.57Ni_0.35Au_0.08)₆Sn₅ intermetallic compound appeared at the solder/pad interface, growing very slowly during aging at 100°C. Increasing the aging temperature to 150°C caused the formation of a new intermetallic layer, (Ni_0.79Cu_0.21)₃Sn₄, at the (Cu_0.57Ni_0.35Au_0.08)₆Sn₅/Ni interface. The reflowed Sn-0.4Co-0.7Cu BGA packages have a ball shear strength of 6.8 N, which decreases to about 5.7 N and 5.5 N after aging at 100°C and 150°C, respectively. The reflowed and aged solder joints fractured across the solder balls with ductile characteristics in ball shear tests.     

Morphological transition of interfacial Ni<Subscript>3</Subscript>Sn<Subscript>4</Subscript> grains at the Sn-3.5Ag/Ni joint

The transition in morphology of Ni₃Sn₄ grains that formed at the interface between liquid Sn-3.5Ag (numbers are in wt.% unless specified otherwise) solder and Ni substrate has been observed at 250–650°C. The morphological transition of Ni₃Sn₄ is due to the decrease of entropy of formation of the Ni₃Sn₄ phase and has been explained well by the change of Jackson’s parameter with temperature. According to the variation of solder joint strength with temperature, it decreased rapidly between 350°C and 450°C, where the thickness of the Ni₃Sn₄ intermetallic compound (IMC) layer was around 6.5 μm. However, the solder joint strength decreased slowly with an increase of soldering time without a significant drop, although the thickness of the IMC was larger than 6.5 μm. The notable drop of solder joint strength and the fracture mode transition with increase of soldering temperature appears to come from excessive lateral growth of IMC grains between 350°C and 450°C.     

Growth of an intermetallic compound layer with Sn-3.5Ag-5Bi on Cu and Ni-P/Cu during aging treatment

Growth kinetics of intermetallic compound (IMC) layers formed between the Sn-3.5Ag-5Bi solder and the Cu and electroless Ni-P substrates were investigated at temperatures ranging from 70°C to 200°C for 0–60 days. With the solder joints between the Sn-Ag-Bi solder and Cu substrates, the IMC layer consisted of two phases: the Cu₆Sn₅ (η phase) adjacent to the solder and the Cu₃Sn (ε phase) adjacent to the Cu substrate. In the case of the electroless Ni-P substrate, the IMC formed at the interface was mainly Ni₃Sn₄, and a P-rich Ni (Ni₃P) layer was also observed as a by-product of the Ni-Sn reaction, which was between the Ni₃Sn₄ IMC and the electroless Ni-P deposit layer. With all the intermetallic layers, time exponent (n) was approximately 0.5, suggesting a diffusion-controlled mechanism over the temperature range studied. The interface between electroless Ni-P and Ni₃P was planar, and the time exponent for the Ni₃P layer growth was also 0.5. The Ni₃P layer thickness reached about 2.5 μm after 60 days of aging at 170°C. The activation energies for the growth of the total Cu-Sn compound layer (Cu₆Sn₅ + Cu₃Sn) and the Ni₃Sn₄ IMC were 88.6 kJ/mol and 52.85 kJ/mol, respectively.     

Reflow and burn-in of a Sn-20In-0.8Cu ball grid array package with a Au/Ni/Cu pad

The intermetallic compounds formed after reflow and burn-in testing of a Sn-20In-0.8Cu solder ball grid array (BGA) package are investigated. Along with the formation of the Cu₆(Sn_0.78In_0.22)₅ precipitates (IM1) in the solder matrix, scallop-shaped intermetallic compounds (IM2) with a compositional mixture of Cu₆(Sn_0.87In_0.13)₅ and Ni₃(Sn_0.87In_0.13)₄ appear at the interfaces between the solder balls and Au/Ni/Cu pads. A significant number of intermetallic particles (IM3), with a composition of (Au_0.80Cu_0.20)(In_0.33Sn_0.67)₂, can also be found in the solder matrix. After aging at 115°C for 750 h, an additional intermetallic compound layer (IM4) with a composition of (Ni_0.91Cu_0.09)₃(Sn_0.77In_0.23)₂ is formed at the interface between IM2 and the Ni layer. The ball shear strength of the Sn-20In-0.8Cu BGA solder after reflow is 4.5 N and will rise to maximum values after aging at 75°C and 115°C for 100 h. With a further increase of the aging time at both temperatures, the joint strengths exhibit a tendency to decline linearly at about 1.7×10⁻³ N/h.     

Interfacial microstructure of Pb-free and Pb-Sn solder balls in the ball-grid array package

Ball-grid array (BGA) samples were aged at 155°C up to 45 days. The formation and the growth of the intermetallic phases at the solder joints were investigated. The alloy compositions of solder balls included Sn-3.5Ag-0.7Cu, Sn-1.0Ag-0.7Cu, and 63Sn-37Pb. The solder-ball pads were a copper substrate with an Au/Ni surface finish. Microstructural analysis was carried out by electron microprobe. The results show that a ternary phase, (Au,Ni)Sn₄, formed with Ni₃Sn₄ in the 63Sn-37Pb solder alloy and that a quaternary intermetallic phase, (Au,Ni)₂Cu₃Sn₅, formed in the Sn-Ag-Cu solder alloys. The formation mechanism of intermetallic phases was associated with the driving force for Au and Cu atoms to migrate toward the interface during aging.     

Intermetallic compounds formed during the reflow and aging of Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder ball grid array packages

After reflow of Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder balls on Au/Ni surface finishes in ball grid array (BGA) packages, scallop-shaped intermetallic compounds (Cu_0.70Ni_0.28Au_0.02)₆Sn₅ (IM1a) and (Cu_0.76Ni_0.24)₆(Sn_0.86In_0.14)₅ (IM1b), respectively, appear at the interfaces. Aging at 100°C and 150°C for Sn-3.8Ag-0.7Cu results in the formation of a new intermetallic phase (Cu_0.70Ni_0.14Au_0.16)₆Sn₅ (IM2a) ahead of the former IM1a intermetallics. The growth of the newly appeared intermetallic compound, IM2a, is governed by a parabolic relation with an increase in aging time, with a slight diminution of the former IM1a intermetallics. After prolonged aging at 150°C, the IM2a intermetallics partially spall off and float into the solder matrix. Throughout the aging of Sn-20In-2Ag-.5Cu solder joints at 75°C and 115°C, partial spalling of the IM1b interfacial intermetallics induces a very slow increase in thickness. During aging at 115°C for 700 h through 1,000 h, the spalled IM1b intermetallics in the solder matrix migrate back to the interfaces and join with the IM1b interfacial intermetallics to react with the Ni layers of the Au/Ni surface finishes, resulting in the formation and rapid growth of a new (Ni_0.85Cu_0.15)(Sn_0.71In_0.29)₂ intermetallic layer (IM2b). From ball shear tests, the strengths of the Sn-3.8Ag-0.7Cu and Sn-20In-2Ag-0.5Cu solder joints after reflow are ascertained to be 10.4 N and 5.4 N, respectively, which drop to lower values after aging. An erratum to this article is available at .     

Intermetallic reactions in Sn-3.5Ag solder ball grid array packages with Ag/Cu and Au/Ni/Cu pads

During the reflow process of Sn-3.5Ag solder ball grid array (BGA) packages with Ag/Cu and Au/Ni/Cu pads, Ag and Au thin films dissolve rapidly into the liquid solder, and the Cu and Ni layers react with the Sn-3.5Ag solder to form Cu₆Sn₅ and Ni₃Sn₄ intermetallic compounds at the solder/pad interfaces, respectively. The Cu₆Sn₅ intermetallic compounds also appear as clusters in the solder matrix of Ag surface-finished packages accompanied by Ag₃Sn dispersions. In the solder matrix of Au/Ni surface-finished specimens, Ag₃Sn and AuSn₄ intermetallics can be observed, and their coarsening coincides progressively with the aging process. The interfacial Cu₆Sn₅ and Ni₃Sn₄ intermetallic layers grow by a diffusion-controlled mechanism after aging at 100 and 150°C. Ball shear strengths of the reflowed Sn-3.5Ag packages with both surface finishes are similar, displaying the same degradation tendencies as a result of the aging effect.     

Fast Concurrent Growth of Ni3Sn4 and Voids During Solid-State Reaction Between Sn-Rich Solder and Ni Substrates

To simulate the growth of Ni₃Sn₄ phase layers in Sn-based solder joints with Ni substrates during solid-state aging, Sn/(Cu_1−x Ni _x )₆Sn₅/Ni and Sn/Ni diffusion couples were aged isothermally at 180°C and 200°C, and the growth kinetics of the (Ni,Cu)₃Sn₄ and Ni₃Sn₄ layers in the respective couples were monitored during the isothermal aging. Once the (Ni,Cu)₃Sn₄ layer was formed at the (Cu,Ni)₆Sn₅/Ni interface, it grew unexpectedly fast with concurrent growth of voids formed in the Sn layer during prolonged aging at both temperatures. The results obtained from the various types of diffusion couples revealed that the voids formed in the Sn layer were Kirkendall voids, due to the (Ni,Cu)₃Sn₄ layer growing predominantly at the (Ni,Cu)₃Sn₄/Ni interface by fast diffusion of Sn across the (Ni,Cu)₃Sn₄ layer. It is proposed that the accelerated growth of the (Ni,Cu)₃Sn₄ and Ni₃Sn₄ layers after the formation of voids in the Sn layer is due to the relaxation of vacancy oversaturation and the enhanced annihilation rate of incoming vacancies in the presence of the voids in the Sn layer.     

Electromigration in a Sn-3 wt.%Ag-0.5 wt.%Cu-3 wt.%Bi Solder Stripe Between Two Cu Electrodes Under Current Stressing

The electromigration behavior of a Sn-3 wt.%Ag-0.5 wt.%Cu-3 wt.%Bi solder stripe between two Cu electrodes under current stressing at various densities has been investigated for a current stressing time of 72 h and a temperature of 120°C. After current stressing at a density of 1.0 × 10⁴ A/cm², the solder matrix exhibited a slight microstructural change as well as formation of a distributed Cu₆Sn₅ phase near the anode-side solder/Cu interface. Upon increasing the current density to 3.9 × 10⁴ A/cm² and 5.0 × 10⁴ A/cm², a high density of distributed Cu₆Sn₅ phase was formed across the entire solder stripe, resulting in pronounced microstructural change of the solder. Hillocks were also formed near the anode-side interface due to accumulation of a Sn-rich phase, a Bi-rich phase, and a distributed Cu₆Sn₅ phase, while voids were formed in the solder matrix and at the opposite cathode side. The mechanisms of formation of the distributed Cu₆Sn₅ phase and migration of Bi and Sn are discussed.     

Electromigration in Line-Type Cu/Sn-Bi/Cu Solder Joints

In this study, the different electromigration (EM) behaviors of eutectic Sn-Bi solder in the solid and molten states were clarified using line-type Cu/Sn-Bi/Cu solder joints. When the eutectic Sn-Bi solder was in the solid state during the EM test, a Bi-rich layer formed at the anode side while a Sn-rich band formed at the cathode side, and the intermetallic compound (IMC) at the cathode side was thicker than that at the anode side. The growth of the Bi-rich layer exhibited a linear dependence on the time of stressing. While the actual temperature of the solder joint increased to 140°C and the solder was in a molten state or partially molten state, two separate Bi-rich layers formed at the anode side and a great many Cu₆Sn₅ IMC precipitates formed between the two Bi-rich layers. Also, the IMC layer at the cathode side was thinner than that at the anode side. With a current-crowding-reduced structure, the products of diffusivity and effective charge number of Bi in the eutectic Cu/Sn-Bi/Cu solder joints stressed with current density of 5 × 10³ A/cm² at 35°C, 55°C, and 75°C were calculated.     

Mechanical properties and intermetallic compound formation at the Sn/Ni and Sn-0.7wt.%Cu/Ni joints

The Ni/Sn/Ni and Ni/Sn-0.7wt.%Cu/Ni couples are reacted at 200°C for various lengths of time. The tensile strengths of these annealed specimens are determined at room temperature. In addition, the interfacial reactions and fracture surfaces of the specimens are examined as well. These properties are important for the evaluation of the usage of Sn-0.7wt.%Cu lead-free solders, which has been not available in the literature. Only the Ni₃Sn₄ phase is formed at the Sn/Ni interface, but both the Cu₆Sn₅ and Ni₃Sn₄ phases are formed at the Sn-0.7wt.%Cu/Ni interface. The thickness of the intermetallic compound layers grows, while the joint strength decreases with longer reaction time. With a 1-h reaction at 200°C, the fracture surface is in the solder matrix for both of the two kinds of couples. Shifting toward the compound layer with longer reaction time, the fracture surface is in the Ni₃Sn₄ layer in the Sn/Ni couple and is at the interface between the Cu₆Sn₅ and Ni₃Sn₄ in the Sn-0.7wt.%Cu/Ni after reacting at 200°C for 240 h.     

Inhibiting the formation of (Au1−xNix)Sn4 and reducing the consumption of Ni metallization in solder joints

The effects of adding a small amount of Cu into eutectic PbSn solder on the interfacial reaction between the solder and the Au/Ni/Cu metallization were studied. Solder balls of two different compositions, 37Pb-63Sn (wt.%) and 36.8Pb-62.7Sn-0.5Cu, were used. The Au layer (1 ± 0.2 μm) and Ni layer (7 ± 1 μm) in the Au/Ni/Cu metallization were deposited by electroplating. After reflow, the solder joints were aged at 160°C for times ranging from 0 h to 2,000 h. For solder joints without Cu added (37Pb-63Sn), a thick layer of (Au_1−xNi_x)Sn₄ was deposited over the Ni₃Sn₄ layer after the aging. This thick layer of (Au_1−xNi_x)Sn₄ can severely weaken the solder joints. However, the addition of 0.5wt.%Cu (36.8Pb-62.7Sn-0.5Cu) completely inhibited the deposition of the (Au_1−xNi_x)Sn₄ layer. Only a layer of (Cu_1-p-qAu_pNi_q)₆Sn₅ formed at the interface of the Cu-doped solder joints. Moreover, it was discovered that the formation of (Cu_1-p-qAu_pNi_q)₆Sn₅ significantly reduced the consumption rate of the Ni layer. This reduction in Ni consumption suggests that a thinner Ni layer can be used in Cu-doped solder joints. Rationalizations for these effects are presented in this paper.     

The Isothermal Section of the Sn-Sb-Ni Ternary System at 270°C

Phase equilibria of the Sn-Sb-Ni ternary system at 270°C are experimentally determined in this work. Experimental results reveal both the Sn₃Sb₂ and β-SnSb phases, and thus the phase relationship near the Sn-rich corner differs from that in the 250°C isothermal section. Instead of a liquid + Ni₃Sn₄ + β-SnSb three-phase region, liquid + Ni₃Sn₄ + Sn₃Sb₂, Ni₃Sn₄ + Sn₃Sb₂ + τ, and Sn₃Sb₂ + β-SnSb + τ three-phase regions are found near the Sn-rich corner. A ternary phase (τ) is found, with stoichiometry of Ni(Sn_1?x ,Sb _x )₃, x = 0.5 to 0.78. Additionally, the Ni₃Sn₂ and NiSb phases form a continuous solution. The Ni₃Sn and Ni₃Sb phases have different structures, and thus they do not form a continuous phase region. The Ni₃Sb phase has an extended Sn solubility of 14.9 at.%, while the maximum Sb solubility in the Ni₃Sn phase is 3.2 at.%.     

Microstructure and shear strength evolution of Sn-Ag-Cu solder bumps during aging at different temperatures

Flip-chip devices with Sn-3.8Ag-0.7Cu solder on electroless Ni (EN) without immersion Au were studied after aging at different temperatures. The (Cu,Ni)₆Sn₅ intermetallics (IMCs) growth was volume diffusion controlled and Kirkendall voids were found in the Ni₃P layer even at the initial stage of high-temperature aging due to the faster diffusion of Ni in the Ni₃P layer via its column structure boundaries. The Ag₃Sn IMCs were distributed in the bulk solder, existing as plate- or lamella-like phases or as small particles around the β-Sn dendrites, and the (Cu,Ni)₆Sn₅ IMCs existed as facet-like phases. The plate- and lamella-like Ag₃Sn phases break up into small parts and these broken parts, together with small Ag₃Sn particles, coarsen into pebble-like phases during high-temperature aging. Shear tests showed that all the solder bumps fractured in the bulk solder. The shear strength of solder bumps decreases at the initial stage of aging at 150°C and 175°C, and the strength degradation during aging may be caused by the coarsening of small Ag₃Sn particles.     

Kinetics of interfacial reaction between eutectic Sn-Pb solder and Cu/Ni/Pd metallizations

The interfacial microstructure and the kinetics of interfacial reaction between the eutectic Sn-Pb solder and electroplated Ni/Pd on Cu substrate (Cu/Ni/NiPd/Ni/Pd) were studied both in the liquid- and solid-state of the solder. The liquidstate reaction was carried out at 200°C, 225°C, and 250°C for 30 s, 60 s, 150 s, and 300 s at each temperature. The solid-state aging was carried out at 125°C for up to 43 days. The interfacial microstructure was characterized by imaging and energy dispersive x-ray analysis in scanning electron microscope. Depending on the thickness of the Pd-layer, both PdSn₄ and PdSn₃ phases were observed near the solder-substrate interface. These results were correlated with the initial thickness of the Pd-layer and the diffusion path in the calculated Pd-Pb-Sn isothermal sections. For the aforementioned isothermal reactions, only one Ni-bearing intermetallic (Ni₃Sn₄) was observed at the solder-substrate interface. The presence of Ni₃Sn₄ intermetallic was consistent with the expected diffusion path based on the calculated Ni-Pb-Sn isothermal sections. Selective etching of solder revealed that PdSn₄ and PdSn₃ had a faceted rod morphology, and Ni₃Sn₄ had a faceted scallop morphology which gave rise to rugged Ni₃Sn₄-solder interface. Segregation of Pb on the facets of PdSn₄ and PdSn₃ was also observed. The growth kinetics of the Ni₃Sn₄ intermetallic layer at the solder-substrate interface was analyzed using an Arrhenius-type of equation. In the thickness regime of 0.16 to 1.2 μm, the growth kinetics yielded a time exponent n=3.1, an apparent activation energy of 9260 J/mol both in the liquid- and solid-state of the solder, and a temperature dependent pre-exponential factor. The latter was attributed to the presence of one or more phases ahead of the growing layer.     

Intermetallic reactions in a Sn-20In-2.8Ag solder ball-grid-array package with Au/Ni/Cu pads

The intermetallic compounds formed during the reflow and aging of Sn-20In-2.8Ag ball-grid-array (BGA) packages are investigated. After reflow, a large number of cubic-shaped AuIn₂ intermetallics accompanied by Ag₂In precipitates appear in the solder matrix, while a Ni(Sn_0.72Ni_0.28)₂ intermetallic layer is formed at the solder/pad interface. With further aging at 100°C, many voids can be observed in the solder matrix and at the solder/pad interface. The continuous distribution of voids at the interface of specimens after prolonged aging at 100°C causes their bonding strength to decrease from 5.03 N (as reflowed) to about 3.50 N. Aging at 150°C induces many column-shaped (Cu_0.74Ni_0.26)₆(Sn_0.92In_0.08)₅ intermetallic compounds to grow rapidly and expand from the solder/pad interface into the solder matrix. The high microhardness of these intermetallic columns causes the bonding strength of the Sn-20In-2.8Ag BGA solder joints to increase to 5.68 N after aging at 150°C for 500 h.     

Interfacial reaction between 42Sn-58Bi solder and electroless Ni-P/immersion Au under bump metallurgy during aging

The interfacial reaction between 42Sn-58Bi solder (in wt.% unless specified otherwise) and electroless Ni-P/immersion Au was investigated before and after thermal aging, with a focus on the formation and growth of an intermetallic compound layer, consumption of under bump metallurgy (UBM), and bump shear strength. The immersion Au layer with thicknesses of 0 μm (bare Ni), 0.1 μm, and 1 μm was plated on a 5-μm-thick layer of electroless Ni-P (with 14–15 at.% P). The 42Sn-58Bi solder balls were then fabricated on three different UBM structures by using screen printing and pre-reflow. A Ni₃Sn₄ layer formed at the joint interface after the pre-reflow for all three UBM structures. On aging at 125°C, a quaternary phase, identified as Sn₇₇Ni₁₅Bi₆Au₂, was observed above the Ni₃Sn₄ layer in the UBM structures that contain Au. The thick Sn₇₇Ni₁₅Bi₆Au₂ layer degraded the integrity of the solder joint, and the shear strength of the solder bump was about 40% less than the nonaged joints.