Microstructural evolution of a lead-free solder alloy Sn-Bi-Ag-Cu prepared by mechanical alloying during thermal shock and aging

In a previous study, a lead-free solder, Sn-6Bi-2Ag-0.5Cu, was developed by mechanical alloying. The alloy shows great potential as a lead-free solder system. In the present work, the microstructural evolution during thermal shock and aging was examined. In the as-soldered joints small bismuth (1 μm to 2 μm) and Ag₃Sn (1 μm) particles were finely dispersed in a nearly pure tin matrix with a small amount of η-Cu₆Sn₅ phase in the bulk of solder. During thermal shock and aging microstructural evolution occurred with Cu-Sn intermetallic compound (IMC) layer growth at interface, bismuth phase coarsening and Ag₃Sn phase coarsening. The microstructure of the solder appeared to be stable at high temperature. The shear strength of the present solder joint is higher than that of Sn-37Pb and Sn-3.5Ag solders. Shear failure occurred Cu-Sn IMC layer-solder interface and in the bulk of solder.     

Interfacial microstructure between Sn-3Ag-xBi alloy and Cu substrate with or without electrolytic Ni plating

The microstructure of the interfacial phase of Sn-3Ag-xBi alloy on a Cu substrate with or without electrolytic Ni plating was evaluated. Bismuth additions into Sn-Ag alloys do not affect interfacial phase formations. Without plating, η-Cu₆Sn₅/ε-Cu₃Sn interfacial phases developed as reaction products in the as-soldered condition. The η-phase Cu₆Sn₅ with a hexagonal close-packed structure grows about 1-μm scallops. The ε-phase Cu₃Sn with an orthorhombic structure forms with small 100-nm grains between η-Cu₆Sn₅ and Cu. For Ni plating, a Ni₃Sn₄ layer of monoclinic structure formed as the primary reaction product, and a thin η-Ni₃Sn₂ layer of hexagonal close-packed structure forms between the Ni₃Sn₄ and Ni layer. In the Ni layer, Ni-Sn compound particles of nanosize distribute by Sn diffusion into Ni. On the total thickness of interfacial reaction layers, Sn-3Ag-6Bi joints are thicker by about 0.9 μm for the joint without Ni plating and 0.18 μm for the joint with Ni plating than Sn-3Ag joints, respectively. The thickening of interfacial reaction layers can affect the mechanical properties of strength and fatigue resistance.     

The effect of intermetallic compound morphology on Cu diffusion in Sn-Ag and Sn-Pb solder bump on the Ni/Cu Under-bump metallization

The eutectic Sn-Ag solder alloy is one of the candidates for the Pb-free solder, and Sn-Pb solder alloys are still widely used in today’s electronic packages. In this tudy, the interfacial reaction in the eutectic Sn-Ag and Sn-Pb solder joints was investigated with an assembly of a solder/Ni/Cu/Ti/Si₃N₄/Si multilayer structures. In the Sn-3.5Ag solder joints reflowed at 260°C, only the (Ni_1−x,Cu_x)₃Sn₄ intermetallic compound (IMC) formed at the solder/Ni interface. For the Sn-37Pb solder reflowed at 225°C for one to ten cycles, only the (Ni_1−x,Cu_x)₃Sn₄ IMC formed between the solder and the Ni/Cu under-bump metallization (UBM). Nevertheless, the (Cu_1−y,Ni_y)₆Sn₅ IMC was observed in joints reflowed at 245°C after five cycles and at 265°C after three cycles. With the aid of microstructure evolution, quantitative analysis, and elemental distribution between the solder and Ni/Cu UBM, it was revealed that Cu content in the solder near the solder/IMC interface played an important role in the formation of the (Cu_1−y,Ni_y)₆Sn₅ IMC. In addition, the diffusion behavior of Cu in eutectic Sn-Ag and Sn-Pb solders with the Ni/Cu UBM were probed and discussed. The atomic flux of Cu diffused through Ni was evaluated by detailed quantitative analysis in an electron probe microanalyzer (EPMA). During reflow, the atomic flux of Cu was on the order of 10¹⁶−10¹⁷ atoms/cm²sec in both the eutectic Sn-Ag and Sn-Pb systems.     

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.     

Effects of Co Alloying and Size on Solidification and Interfacial Reactions in Sn-57 wt.%Bi-(Co)/Cu Couples

Solidification and interfacial reactions in Sn-57 wt.%Bi-(Co)/Cu couples are investigated. The addition of 0.05 wt.% Co and 0.5 wt.% Co and changes between 2 g and 20 mg solder sizes have no significant effects on the undercooling of Sn-57 wt.%Bi solders. Both η-Cu₆Sn₅ and ε-Cu₃Sn phases are formed in Sn-57 wt.%Bi/Cu couples reacted at 80°C, 100°C, and 100°C, whereas only η-Cu₆Sn₅ is formed at 160°C. The formation of ε-Cu₃Sn is suppressed and only η-Cu₆Sn₅ is found in the couple with 0.05 wt.% Co and 0.5 wt.% Co addition in Sn-57 wt.%Bi solder. Both the growth rate of η-Cu₆Sn₅ and the dissolution rate of the Cu substrate increase with Co addition. The morphology of η-Cu₆Sn₅ is also altered with Co addition, becoming a porous structure with solder trapped in the voids.     

Growth of intermetallic compounds in the Sn-9Zn/Cu joint

We have studied the microstructure of the Sn-9Zn/Cu joint in soldering at temperatures ranging from 230°C to 270°C to understand the growth of the mechanism of intermetallic compound (IMC) formation. At the interface between the Sn-9Zn solder and Cu, the results show a scallop-type ε-CuZn₄ and a layer-type γ-Cu₅Zn₈, which grow at the interface between the Sn-9Zn solder and Cu. The activation energy of scallop-type ε-CuZn₄ is 31 kJ/mol, and the growth is controlled by ripening. The activation energy of layer-type γ-Cu₅Zn₈ is 26 kJ/mol, and the growth is controlled by the diffusion of Cu and Zn. Furthermore, in the molten Sn-9Zn solder, the results show η-CuZn grains formed in the molten Sn-9Zn solder at 230°C. When the soldering temperature increases to 250°C and 270°C, the phase of IMCs is ε-CuZn₄.     

Effects of microstructural evolution and intermetallic layer growth on shear strength of ball-grid-array Sn-Cu solder joints

The shear strength of ball-grid-array (BGA) solder joints on Cu bond pads was studied for Sn-Cu solder containing 0, 1.5, and 2.5 wt.% Cu, focusing on the effect of the microstructural changes of the bulk solder and the growth of intermetallic (IMC) layers during soldering at 270°C and aging at 150°C. The Cu additions in Sn solder enhanced both the IMC layer growth and the solder/IMC interface roughness during soldering but had insignificant effects during aging. Rapid Cu dissolution from the pad during reflow soldering resulted in a fine dispersion of Cu₆Sn₅ particles throughout the bulk solder in as-soldered joints even for the case of pure Sn solder, giving rise to a precipitation hardening of the bulk solder. The increased strength of the bulk solder caused the fracture mode of as-soldered joints to shift from the bulk solder to the solder/IMC layer as the IMC layer grew over a critical thickness about 1.2 m for all solders. The bulk solder strength decreased rapidly as the fine Cu₆Sn₅ precipitates coarsened during aging. As a consequence, regardless of the IMC layer thickness and the Cu content of the solders, the shear strength of BGA solder joints degraded significantly after 1 day of aging at 150°C and the shear fracture of aged joints occurred in the bulk solder. This suggests that small additions of Cu in Sn-based solders have an insignificant effect on the shear strength of BGA solderjoints, especially during system use at high temperatures.     

Effect of cooling rate on growth of the intermetallic compound and fracture mode of near-eutectic Sn-Ag-Cu/Cu pad: Before and after aging

Several near-eutectic solders of (1) Sn-3.5Ag, (2) Sn-3.0Ag-0.7Cu, (3) Sn-3.0Ag-1.5Cu, (4) Sn-3.7Ag-0.9Cu, and (5) Sn-6.0Ag-0.5Cu (in wt.% unless specified otherwise) were cooled at different rates after reflow soldering on the Cu pad above 250°C for 60 sec. Three different media of cooling were used to control cooling rates: fast water quenching, medium cooling on an aluminum block, and slow cooling in furnace. Both the solder composition and cooling rate after reflow have a significant effect on the intermetallic compound (IMC) thickness (mainly Cu₆Sn₅). Under fixed cooling condition, alloys (1), (3), and (5) revealed larger IMC thicknesses than that of alloys (2) and (4). Slow cooling produced an IMC buildup of thicker than 10 μm, while medium and fast cooling produced a thickness of thinner than 5 μm. The inverse relationship between IMC thickness and shear strength was confirmed. All the fast- and medium-cooled joints revealed a ductile mode (fracture surface was composed of the β-Sn phase), while the slow-cooled joints were fractured in a brittle mode (fracture surface was composed of Cu₆Sn₅ and Cu₃Sn phases). The effect of isothermal aging at 130°C on the growth of the IMC, shear strength, and fracture mode is also reported.     

Intermetallic formation in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with immersion Ag surface finish

The intermetallic compounds formed in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Ag/Cu pads are investigated. After reflow, scallop-shaped η-Cu₆Sn₅ and continuous planar η-(cu_0.9Ni_0.1)₆Sn₅ intermetallics appear at the interfaces of the Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder joints, respectively. In the case of the Sn3Ag0.5Cu specimens, an additional ε-Cu₃Sn intermetallic layer is formed at the interface between the η-Cu₆Sn₅ and Cu pads after aging at 150°C, while the same type of intermetallic formation is inhibited in the Sn3Ag0.5Cu0.06Ni0.01Ge packages. In addition, the coarsening of Ag₃Sn precipitates also abates in the solder matrix of the Sn3Ag0.5Cu0.06Ni0.01Ge packages, which results in a slightly higher ball shear strength for the specimens.     

Microstructure Changes and Physical Properties of the Intermetallic Compounds Formed at the Interface Between Sn-Cu Solders and a Cu Substrate Due to a Minor Addition of Ni

The influence of Cu content and added Ni on the morphology of the intermetallic compound (IMC) layer formed at the interface between liquid Sn-Cu-based solders and a Cu substrate and on the strength of simulated solder joints was investigated. The reaction of a Sn-0.7Cu alloy with the substrate led to the formation of a thin layer of Cu₆Sn₅ (η-phase) with typical scallop morphology that did not grow with longer reaction times. Higher Cu content such as in the Sn-1.4Cu alloy led to extensive growth with increased reaction time; at long reaction times, Cu₃Sn (ε-phase) was observed at the interface between the Cu substrate and the Cu₆Sn₅ layer. A small nickel addition to the Sn-0.7Cu alloy significantly changed the IMC morphology, accelerated its growth kinetics, prevented formation of the Cu₃Sn layer, and reduced the rate of substrate dissolution.     

Effect of thermal cycling on the growth of intermetallic compounds at the Sn-Zn-Bi-In-P lead-free solder/Cu interface

The low-temperature Sn-9Zn-1.5Bi-0.5In-0.01P lead-free solder alloy is used to investigate the intermetallic compounds (IMCs) formed between solder and Cu substrates during thermal cycling. Metallographic observation, scanning electron microscopy, transmission electron microscopy, and electron diffraction analysis are used to study the IMCs. The γ-Cu₅Zn₈ IMC is found at the Sn-9Zn-1.5Bi-0.5In-0.01P/Cu interface. The IMC grows slowly during thermal cycling. The fatigue life of the Sn-9Zn-1.5Bi-0.5In-0.01P solder joint is longer than that of Pb-Sn eutectic solder joint because the IMC thickness of the latter is much greater than that of the former. Thermodynamic and diffusivity calculations can explain the formation of γ-Cu₅Zn₈ instead of Cu-Sn IMCs. The growth of IMC layer is caused by the diffusion of Cu and Zn elements. The diffusion coefficient of Zn in the Cu₅Zn₈ layer is determined to be 1.10×10⁻¹² cm²/sec. A Zn-rich layer is found at the interface, which can prevent the formation of the more brittle Cu-Sn IMCs, slow down the growth of the IMC layer, and consequently enhance the fatigue life of the solder joint.     

Reliability investigation and interfacial reaction of ball-grid-array packages using the lead-free Sn-Cu solder

The interfacial reactions between two Sn-Cu (Sn-0.7Cu and Sn-3Cu, wt.%) ball-grid-array (BGA) solders and the Au/Ni/Cu substrate by solid-state isothermal aging were examined at temperatures between 70°C and 170°C for 0 to 100 days. For the Sn-0.7Cu solder, a (Cu,Ni)₆Sn₅ layer was observed in the samples aged at 70–150°C. After isothermal aging at 170°C for 50 days, the solder/Ni interface exhibited a duplex structure of (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄. For the Sn-3Cu solder, only the (Cu,Ni)₆Sn₅ layer was formed in all aged samples. Compared to these two Sn-Cu solders, the Cu content in the (Cu,Ni)₆Sn₅ layer formed at the interface increased with the Cu concentration in the Sn-xCu solders. And, the shear strength was measured to evaluate the effect of the interfacial reactions on the mechanical reliability as a function of aging conditions. The shear strength significantly decreased after aging for 1 day and then remained nearly unchanged by further prolonged aging. In all the samples, the fracture always occurred in the bulk solder. Also, we studied the electrical property of Cu/Sn-3Cu/Cu BGA packages with the number of reflows. The electrical resistivity increased with the number of reflows because of an increase of intermetallic compound (IMC) thickness.     

Electromigration Reliability and Morphologies of Cu Pillar Flip-Chip Solder Joints with Cu Substrate Pad Metallization

The Cu pillar is a thick underbump metallurgy (UBM) structure developed to alleviate current crowding in a flip-chip solder joint under operating conditions. We present in this work an examination of the electromigration reliability and morphologies of Cu pillar flip-chip solder joints formed by joining Ti/Cu/Ni UBM with largely elongated ∼62 μm Cu onto Cu substrate pad metallization using the Sn-3Ag-0.5Cu solder alloy. Three test conditions that controlled average current densities in solder joints and ambient temperatures were considered: 10 kA/cm² at 150°C, 10 kA/cm² at 160°C, and 15 kA/cm² at 125°C. Electromigration reliability of this particular solder joint turns out to be greatly enhanced compared to a conventional solder joint with a thin-film-stack UBM. Cross-sectional examinations of solder joints upon failure indicate that cracks formed in (Cu,Ni)₆Sn₅ or Cu₆Sn₅ intermetallic compounds (IMCs) near the cathode side of the solder joint. Moreover, the ~52-μm-thick Sn-Ag-Cu solder after long-term current stressing has turned into a combination of ~80% Cu-Ni-Sn IMC and ~20% Sn-rich phases, which appeared in the form of large aggregates that in general were distributed on the cathode side of the solder joint.     

Intermetallic growth kinetics for Sn-Ag, Sn-Cu, and Sn-Ag-Cu lead-free solders on Cu, Ni, and Fe-42Ni substrates

Soldering with the lead-free tin-base alloys requires substantially higher temperatures (∼235–250°C) than those (213–223°C) required for the current tin-lead solders, and the rates for intermetallic compound (IMC) growth and substrate dissolution are known to be significantly greater for these alloys. In this study, the IMC growth kinetics for Sn-3.7Ag, Sn-0.7Cu, and Sn-3.8Ag-0.7Cu solders on Cu substrates and for Sn-3.8Ag-0.7Cu solder with three different substrates (Cu, Ni, and Fe-42Ni) are investigated. For all three solders on Cu, a thick scalloped layer of η phase (Cu₆Sn₅) and a thin layer of ε phase (Cu₃Sn) were observed to form, with the growth of the layers being fastest for the Sn-3.8Ag-0.7Cu alloy and slowest for the Sn-3.7Ag alloy. For the Sn-3.8Ag-0.7Cu solder on Ni, only a relatively uniform thick layer of η phase (Cu,Ni)₆Sn₅ growing faster than that on the Cu substrate was found to form. IMC growth in both cases appears to be controlled by grain-boundary diffusion through the IMC layer. For the Fe-42Ni substrate with the Sn-3.8Ag-0.7Cu, only a very thin layer of (Fe,Ni)Sn₂ was observed to develop.     

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.     

Mechanisms for interfacial reactions between liquid Sn-3.5Ag solders and cu substrates

Intermetallic compounds formed during the soldering reactions between Sn-3.5Ag and Cu at temperatures ranging from 250°C to 375°C are investigated. The results indicate that scallop-shaped η-Cu₆(Sn_0.933 Ag_0.007)₅ intermetallics grow from the Sn-3.5Ag/Cu interface toward the solder matrix accompanied by Cu dissolution. Following prolonged or higher temperature reactions, ɛ-Cu₃ (Sn_0.996 Ag_0.004) intermetallic layers appear behind the Cu₆(Sn_0.933 Ag_0.007)₅ scallops. The growth of these interfacial intermetallics is governed by a kinetic relation: ΔX=tⁿ, where the n values for η and ɛ intermetallics are 0.75 and 0.96, respectively. The mechanisms for such nonparabolic growth of interfacial intermetallics during the liquid/solid reactions between Sn-3.5Ag solders and Cu substrates are probed.     

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.     

Interfacial reaction and joint reliability of fine-pitch flip-chip solder bump using stencil printing method

This study was focused on the formation and reliability evaluation of solder joints with different diameters and pitches for flip chip applications. We investigated the interfacial reaction and shear strength between two different solders (Sn-37Pb and Sn-3.0Ag-0.5Cu, in wt.%) and ENIG (Electroless Nickel Immersion Gold) UBM (Under Bump Metallurgy) during multiple reflow. Firstly, we formed the flip chip solder bumps on the Ti/Cu/ENIG metallized Si wafer using a stencil printing method. After reflow, the average solder bump diameters were about 130, 160 and 190 μm, respectively. After multiple reflows, Ni₃Sn₄ intermetallic compound (IMC) layer formed at the Sn-37Pb solder/ENIG UBM interface. On the other hand, in the case of Sn-3.0Ag-0.5Cu solder, (Cu,Ni)₆Sn₅ and (Ni,Cu)₃Sn₄ IMCs were formed at the interface. The shear force of the Pb-free Sn-3.0Ag-0.5Cu flip chip solder bump was higher than that of the conventional Sn-37Pb flip chip solder bump.     

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 reactions in Sn-8Zn-20In solder ball grid array packages with Au/Ni/Cu and Ag/Cu pads

During the reflow process of Sn-8Zn-20In solder joints in the ball grid array (BGA) packages with Au/Ni/Cu and Ag/Cu pads, the Au and Ag thin films react with liquid solder to form γ₃-AuZn₄/γ-Au₇Zn₁₈ and ε-AgZn₆ intermetallics, respectively. The γ₃/γ intermetallic layer is prone to floating away from the solder/Ni interface, and the appearance of any interfacial intermetallics cannot be observed in the Au/Ni surface finished Sn-8Zn-20In packages during further aging treatments at 75°C and 115°C. In contrast, ε-CuZn₅/γ-Cu₅Zn₈ intermetallics are formed at the aged Sn-8Zn-20In/Cu interface of the immersion Ag BGA packages. Bonding strengths of 3.8N and 4.0N are found in the reflowed Sn-8Zn-20In solder joints with Au/Ni/Cu and Ag/Cu pads, respectively. Aging at 75°C and 115°C gives slight increases of ball shear strength for both cases.