The microstructure characterization of ultrasmall eutectic Bi-Sn solder bumps on Au/Cu/Ti and Au/Ni/Ti under-bump metallization

The microstructure of the ultrasmall eutectic Bi-Sn solder bumps on Au/Cu/Ti and Au/Ni/Ti under-bump metallizations (UBMs) was investigated as a function of cooling rate. The ultrasmall eutectic Bi-Sn solder bump, about 50 μm in diameter, was fabricated by using the lift-off method and reflowed at various cooling rates using the rapid thermal annealing system. The microstructure of the solder bump was observed using a backscattered electron (BSE) image and the intermetallic compound was identified using energy dispersive spectroscopy (EDS) and an x-ray diffractometer (XRD). The Bi facet was found at the surface of the ultrasmall Bi-Sn solder bumps on the Au/Cu/Ti UBM in almost all specimens, and the interior microstructure of the bumps was changed with the solidification rate. The faceted and polygonal intermetallic compound was found in the case of the Bi-Sn solder bump on the Au (0.1 μm)/Ni/Ti UBM, and it was confirmed to be the (Au_1−x−yBi_xNi_y)Sn₂ phase by XRD. The intermetallic compounds grown form the Au (0.1 μm)/Ni/Ti UBM interface, and they interrupted the growth of Bi and Sn phases throughout the solder bump. The ultrasmall eutectic Bi-Sn solder bumps on the Au (0.025 μm)/Ni/Ti UBM showed similar microstructures to those on the Au/Cu/Ti UBM.     

A study on interfacial reactions between electroless Ni-P under bump metallization and 95.5Sn-4.0Ag-0.5Cu alloy

Even though electroless Ni-P and Sn-Ag-Cu solders are widely used materials in flip-chip bumping technologies, interfacial reactions of the ternary Cu-Ni-Sn system are not well understood. The growth of intermetallic compounds (IMCs) at the under bump metallization (UBM)/solder interface can affect solder-joint reliability, so analysis of IMC phases and understanding their growth kinetics are important. In this study, interfacial reactions between electroless Ni-P UBM and the 95.5Sn-4.0Ag-0.5Cu alloy were investigated, focusing on identification of IMC phases and IMC growth kinetics at various reflowing and aging temperatures and times. The stable ternary IMC initially formed at the interface after reflowing was the (Cu,Ni)₆Sn₅ phase. However, during aging, the (Cu,Ni)₆Sn₅ phase slowly changed into the quaternary IMC composed of Cu, Ni, Sn, and a small amount of Au. The Au atoms in the quaternary IMC originated from immersion Au plated on electroless Ni-P UBM. During further reflowing or aging, the (Ni,Cu)₃Sn₄ IMC started forming because of the limited Cu content in the solder. Morphology, composition, and crystal structure of each IMC were identified using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Small amounts of Cu in the solder affect the types of IMC phases and the amount of the IMC. The activation energies of (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄ IMCs were used to estimate the growth kinetics of IMCs. The growth of IMCs formed in aging was very slow and temperature-dependent compared to IMCs formed in reflow because of the higher activation energies of IMCs in aging. Comparing activation energies of each IMC, growth mechanism of IMCs at electroless Ni-P/SnAgCu solder interface will be discussed.     

Interfacial reaction and microstructural evolution for electroplated Ni and electroless Ni in the under bump metallurgy with 42Sn58Bi solder during annealing

In the present study, several under bump metallization (UBM) schemes using either electroplated Ni or electroless Ni (EN) as the solderable layer are investigated. The EN and electroplated Ni are first deposited on Cu/Al₂O₃ substrates, followed by electroplating of thin gold coatings. Joints of 42Sn-58Bi/Au/EN/Cu/Al₂O₃ and 42Sn-58Bi/Au/Ni/Cu/Al₂O₃ are annealed at 145 C and 185CC for 30–180 minutes to investigate the interfacial reaction between the solder and metallized substrates. For 42Sn-58Bi/Au/Ni-5.5wt.%P/Cu/Al₂O₃, 42Sn-58Bi/Au/Ni-12.1wt.%P/Cu/Al₂O₃, and 42Sn-58Bi/Au/Ni/CU/Al₂O₃ joints annealed at 145 C, only Ni₃Sn₄ intermetallic compound (IMC) formed at the solder/EN interace. When annealed at an elevated temperature of 185 C, plate-like Ni₃Sn₄ IMC forms at the solder/Ni-5.5wt.%P interface, while a trace of (Ni, Cu)₃Sn₄ IMC is observed at the solder/Ni-12.1wt.%P interface and within the solder region. For the electroplated Ni-based multi-metallization substrate, the Ni₃Sn₄ IMC is present at the solder/Ni interface during annealing at 185 C for a short period of time. In the 42Sn-58Bi/Au/EN/Cu/Al₂O₃ joint, the EN spalls off the EN layer and migrates into the solder region when annealed at 185 C. The interface of the solder/electroplating Ni becomes saw-toothed as the annealing temperature is raised to 185 C. In addition, an enrichment of phosphorus is observed at the interface of the Ni-Sn IMC and EN.     

Optimal phosphorous content selection for the soldering reaction of Ni-P under bump metallization with Sn-Ag-Cu solder

Nickel plating has been used as the under bump metallization (UBM) in the microelectronics industry. The electroplated Ni-P UBM with different phosphorous contents (7 wt.%, 10 wt.%, and 13 wt.%) was used to evaluate the interfacial reaction between Ni-P UBM and Sn-3Ag-0.5Cu solder paste during multiple reflow. (Cu,Ni)₆Sn₅ intermetallic compounds (IMC) formed in the SnAgCu solder/Ni-P UBM interface after the first reflow. For three times reflow, (Ni,Cu)₃Sn₄ IMC formed, while (Cu,Ni)₆Sn₅ IMC spalled into the solder matrix. With further increasing cycles of reflow, the Ni-Sn-P layer formed between (Ni,Cu)₃Sn₄ IMC and Ni-P UBM for Ni-10wt.%P and Ni-13wt.%P UBM. However, almost no Ni-Sn-P layer was revealed for the Ni-7wt.%P UBM even after ten cycles of reflow. In consideration of the wettability of Ni-P UBM, the interfacial reaction of SnAgCu/Ni-P, and dissolution of Ni-P UBM, the optimal phosphorous selection in Ni-P UBM was proposed and also discussed.     

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.     

Morphology and kinetic study of the interfacial reaction between the Sn-3.5Ag solder and electroless Ni-P metallization

This work summarizes the interfacial reaction between lead-free solder Sn-3.5Ag and electrolessly plated Ni-P metallization in terms of morphology and growth kinetics of the intermetallic compounds (IMC). Comparison with pure Ni metallization is made in order to clarify the role of P in the solder reaction. During reflow, the IMCs formed with the Ni-P under-bump metallization (UBM) exist in chunky crystal blocks and small crystal agglomerates, while the ones with the sputtered Ni UBM exhibit uniformly scallop grains with faceted surfaces. The IMC thickness increases with reflow time following approximately a t^1/3 power law for both systems. The IMC growth rate is higher with the Ni-P UBM than the Ni UBM. The thickness of the Ni₃Sn₄ layer increases linearly with the square root of thermal aging time, indicating that the growth of the IMCs is a diffusion-controlled process. The activation energy for Ni₃Sn₄ growth in solid-state reaction is found to be 110 kJ/mol and 91 kJ/mol for the Ni-P and sputtered Ni UBMs, respectively. Kirkendall voids are detected inside the Ni₃P layer in the Sn-3.5Ag/Ni-P system. No such voids are found in the Sn-3.5Ag/Ni system.     

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.     

A comparative study of the kinetics of interfacial reaction between eutectic solders and Cu/Ni/Pd metallization

A comparative study of the kinetics of interfacial reaction between the eutectic solders (Sn-3.5Ag, Sn-57Bi, and Sn-38Pb) and electroplated Ni/Pd on Cu substrate (Cu/Ni/NiPd/Ni/Pd) was performed. The interfacial microstructure was characterized by imaging and energy dispersive x-ray analysis in scanning electron microscope (SEM). For a Pd-layer thickness of less than 75 nm, the presence or the absence of Pd-bearing intermetallic was found to be dependent on the reaction temperature. In the case of Sn-3.5Ag solder, we did not observe any Pd-bearing intermetallic after reaction even at 230°C. In the case of Sn-57Bi solder the PdSn₄ intermetallic was observed after reaction at 150°C and 180°C, while in the case of Sn-38Pb solder the PdSn₄ intermetallic was observed after reaction only at 200°C. The PdSn₄ grains were always dispersed in the bulk solder within about 10 μm from the solder/substrate interface. At higher reaction temperatures, there was no Pd-bearing intermetallic due to increased solubility in the liquid solder. The presence or absence of Pd-bearing intermetallic was correlated with the diffusion path in the calculated Pd-Sn-X (X=Ag, Bi, Pb) isothermal sections. In the presence of unconsumed Ni, only Ni₃Sn₄ intermetallic was observed at the solder-substrate interface by SEM. The presence of Ni₃Sn₄ intermetallic was consistent with the expected diffusion path based on the calculated Ni-Sn-X (X=Ag, Bi, Pb) isothermal sections. Selective etching of solders revealed that Ni₃Sn₄ had a faceted scallop morphology. Both the radial growth and the thickening kinetics of Ni₃Sn₄ intermetallic were studied. In the thickness regime of 0.14 μm to 1.2 μm, the growth kinetics always yielded a time exponent n >3 for liquid-state reaction. The temporal law for coarsening also yielded time exponent m >3. The apparent activation energies for thickening were: 16936J/mol for the Sn-3.5Ag solder, 17804 J/mol for the Sn-57Bi solder, and 25749 J/mol for the Sn-38Pb solder during liquid-state reaction. The corresponding activation energies for coarsening were very similar. However, an apparent activation energy of 37599 J/mol was obtained for the growth of Ni₃Sn₄ intermetallic layer during solid-state aging of the Sn-57Bi/substrate diffusion couples. The kinetic parameters associated with thickening and radial growth were discussed in terms of current theories.     

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.     

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.     

Reflow soldering and isothermal solid-state aging of Sn-Ag eutectic solder on Au/Ni surface finish

The reaction between the eutectic Sn-3.5Ag solder and the Au/Ni surface finish during reflow as well as during isothermal aging was studied. The Au layer was electroplated and had a thickness of the one μm. The peak reflow temperature was fixed at 250 C while the reflow time was varied between 10 sec and one h. Samples that went through 90 sec reflow time were then subjected to 160 C isothermal aging for up to 875 h. It was found that during reflow the Au layer reacted very quickly with the solder to form AuSn₄. One μm of Au layer was consumed in less than 10 sec. As the aging time increased, AuSn₄ grains began to separate themselves from the Ni layer at the roots of the grains and started to fall into the solder. When, the reflow time reached 30 sec, all the Au intermetallic head left the interface, and Ni₃Sn₄ started, to form at the interface. The Ni₃Sn₄ growth rate followed linear kinetics initially (<240 sec), but the growth rate slowed down afterward. During the isothermal aging, only a small amount of (Au_xNi_1-x)Sn₄ resettled back to the interface, and a continuous (Au_0.45Ni_0.55)Sn₄ layer did not form at the interface, unlike the case for the Sn-37Pb solder. This is an important advantage for Sn-3.5 Ag over Sn-37Pb because a continuous (Au_0.45Ni_0.55)Sn₄ layer inevitably will weaken a solder joint. Our observation indicated that many (Au_xNi_1-x)Sn₄ particles were trapped by the Ag₃Sn particles, and were hindered from resettling back to the interface.     

Comparison of Sn2.8Ag20In and Sn10Bi10In solders for intermediate-step soldering

We chose Sn−2.8Ag−20In and Sn−10Bi−10In (numbers are in weight percentages unless specified otherwise) as Pb-free solder materials for intermediate-step soldering. We then investigated how the two solders reacted with the under bump metallurgy (UBM) of Au/Ni (Au: 1.5 μm and Ni: 3 μm) at 210°C, 220°C, 230°C, and 240°C for up to 4 min. All, of the Au UBM was dissolved into the solder matrix as soon as the interfacial reaction started. The reaction formed Au(In, Sn)₂ in the case of SnAgIn, and it formed Au(Sn, In)₄ and Au(In, Sn)₂ in the case of SnBiIn. The formation mechanism of the intermetallic phases is explained thermodynamically. The exposed Ni layer reacted with the solder and formed Ni₂₈Sn₅₅In₁₇ in case of SnAgIn, and formed Ni₃(Sn, In)₄ in case of SnBiIn, at the solder joint interface. Under the same soldering conditions, the Ni₃(Sn,In)₄ layer in the SnBiIn/UBM is thicker than the Ni₂₈Sn₅₅In₁₇ layer in the SnAgIn/UBM. Because of the thicker intermetallic compound layer, the SnBiIn solder joint has weaker shear strength than the SnAgIn solder 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.     

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.     

Morphological and Microstructural Evolution of Phosphorous-Rich Layer in SnAgCu/Ni-P UBM Solder Joint

Interfacial morphologies and microstructure of Sn-3Ag-0.5Cu/Ni-P under bump metallization (UBM) with various phosphorous contents were investigated by transmission electron microscope (TEM) and field emission electron probe microanalyzer (FE-EPMA). It was revealed that as the Ni-Sn-P compound was formed between the solder matrix and Ni-P UBM, the conventionally so-called phosphorous-rich (P-rich) layer was transformed to a series of layer compounds, including Ni₃P, Ni₁₂P₅ and Ni₂P. The relationship between Ni-Sn-P formation and evolution of P-rich layers was probed by electron microscopic characterization with the aid of the phase diagram of Ni-P. On the basis of the TEM micrograph, the selected area diffraction (SAD) pattern, and the FE-EPMA results, the detailed phase evolution of P-rich layers in the SnAgCu/Ni-P joint was revealed and proposed.     

Compound formation for electroplated Ni and electroless Ni in the under-bump metallurgy with Sn-58Bi solder during aging

The Ni-based under-bump metallurgies (UBMs) are of interest because they have a slower reaction rate with Sn-rich solders compared to Cu-based UBMs. In this study, several UBM schemes using Ni as the diffusion barrier are investigated. Joints of Sn-58Bi/Au/electroless nickel (EN)/Cu/Al₂O₃ and Sn-58Bi/Au/electroplated nickel/Cu/Al₂O₃ were aged at 110°C and 130°C for 1–25 days to study the interfacial reaction and microstructural evolution. The Sn-Bi solder reacts with the Ni-based multimetallization and forms the ternary Sn-Ni-Bi intermetallic compound (IMC) during aging at 110°C. Compositions of ternary IMC were (78–80)at.%Sn-(12–16)at.%Ni-(5–8)at.%Bi in joints of Sn-58Bi/Au/Ni-5.5wt.%P/Cu, Sn-58Bi/Au/Ni-12wt.%P/Cu, and Sn-58Bi/Au/Ni/Cu. Elevated aging at 130°C accelerates the IMC growth rate and results in the formation of (Ni,Cu)₃Sn₄ and (Cu,Ni)₆Sn₅ adjacent to the ternary Sn-Ni-Bi IMC for the Sn-58Bi/Au/Ni-12wt.%P/Cu and Sn-58Bi/Au/Ni/Cu joints, respectively. The Cu content in the (Cu,Ni)₆Sn₅ IMC is six times that in (Ni,Cu)₃Sn₄. Electroplated Ni fails to prevent Cu diffusion toward the Ni/solder interface as compared to EN-based joints. Cracks are observed in the Sn-58Bi/Au/Ni-5.5wt.%P/Cu/Al₂O₃ joint aged at 130°C for 25 days. It is more favorable to employ Ni-12wt.%P for the Sn-58Bi/Au/EN/Cu joint. Electroless nickel, with the higher P content of 12 wt.%, is a more effective diffusion barrier during aging. In addition, P enrichment occurs near the interface of the EN/solder, and the degree of P enrichment is enhanced with aging time. The Au(Sn,Bi)₄, with pyramidal and cubic shape, is observed in the Sn-58Bi/Au/Ni/Cu/Al₂O₃ joint.     

Inhibiting growth of the Au0.5Ni0.5Sn4 intermetallic layer in Pb-Sn solder joints reflowed on Au/Ni metallization

Au/Ni metallization has become increasingly common in microelectronic packaging when Cu pads are joined with Pb-Sn solder. Recent work has shown that a ternary compound with stoichiometry Au_0.5Ni_0.5Sn₄ redeposits onto the interface during aging, compromising the strength of the joint. In the present work the growth of the Au_0.5Ni_0.5Sn₄ layer is documented and methods for inhibiting its growth were investigated. It was determined that multiple reflows, both with and without additional aging, can substantially limit the thickness of the ternary layer.     

Cross-interaction of under-bump metallurgy and surface finish in flip-chip solder joints

The cross-interaction of the under-bump metallurgy (UBM)/solder interface and the solder/surface-finish interface in flip-chip solder joints was investigated. In this study, the UBM on the chip side was a single layer of Cu (8.5 μm), and the surface finish on the substrate side was a 0.2-μm Au layer over 5-μm Ni. It was shown that, after two reflows, the Ni layer of the surface finish had been covered with (Cu_1−xNi_x)₆Sn₅. This shows that the effect of cross-interaction of the two interfaces is important even during the reflow stage. During subsequent solid-state aging at 115°C, 135°C, and 155°C, the formation of (Cu_1−xNi_x)₆Sn₅ over the Ni layer was found to have the effect of reducing the Ni consumption rate. At the same time, the Cu consumption rate of the UBM was accelerated. The results of this study show that the selection of the UBM and the surface finish has to be considered together because the cross-interaction of the two interfaces plays an important role.     

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.     

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.