Interfacial reactions in Ag-Sn/Cu couples

Interfacial reactions between Sn-3.5 wt.%Ag, Sn-25 wt.%Ag, and Sn-74 wt.%Ag alloys with Cu substrate at 240°C and 450°C have been studied here by examining the reaction couples. It is found that Sn is the fastest diffusion species among the three elements during the reaction, while Ag is the slowest. The reaction path is liquid/η/ɛ₁/Cu for the Sn-3.5 wt.%Ag/Cu couples reacted at 240°C. The paths are liquid/ɛ₁/δ/Cu, liquid/ɛ₁/δ/Cu, ɛ₂/ζ/ɛ₁/δ/Cu, for the Sn-3.5 wt.%Ag/Cu, Sn-25 wt.%Ag/Cu, and Sn-74 wt.%Ag/Cu couples at 450°C, respectively. These reaction paths are in agreement with the isothermal sections of the Ag-Sn-Cu ternary system at 240°C and 450°C. The isothermal sections are proposed based on the limited ternary phase equilibria data and the phase diagrams of its three constituent binary systems, Ag-Sn, Cu-Sn, and Ag-Cu.     

Interdiffusion analysis of the soldering reactions in Sn-3.5Ag/Cu couples

Extensive microstructural and kinetic studies on the formation and growth of the intermetallics of Sn-rich solder/Cu couples have been reported. However, experimental data on the interdiffusion mechanisms during soldering reactions are limited and in conflict. The interdiffusion processes for soldering of Sn-3.5Ag alloy/Cu couples were investigated by using the Cr-evaporated surface as a reference line. At the beginning of soldering, Cu was observed to outdiffuse to the molten Sn−3.5Ag alloy until saturation, and the Sn−Ag solder dissolved with Cu collapsed below the reference line. As a result, the scallop-shaped Cu₆Sn₅ intermetallic compound was formed at the newly-formed Sn−Ag−Cu solder/Cu interface below the original Cu surface. When the soldered joint was reflowed at the lower temperature to suppress the Cu dissolution, the Cu₆Sn₅/Cu interface moved into the Cu substrate. Therefore, Sn is the dominant diffusing species for the intermetallic formation during the soldering process, although the extensive Cu dissolution occurs at the early stage of soldering.     

Comparative study of interfacial reactions of Sn-Ag-Cu and Sn-Ag solders on Cu pads during reflow soldering

The interfacial reaction in soldering is a crucial subject for the solder-joint integrity and reliability in electronic packaging technology. However, electronic industries are moving toward lead-free alloys because of environmental concerns. This drive has highlighted the fact that the industry has not yet arrived at a decision for lead-free solders. Among the lead-free alloys, Sn-3.5Ag and Sn-3.5Ag-0.5Cu are the two potential candidates. Here, detailed microstructural studies were carried out to compare the interfacial reaction of Sn-3.5Ag and Sn-3.5Ag-0.5Cu solder with a ball grid array (BGA) Cu substrate for different reflow times. The Cu dissolution from the substrate was observed for different soldering temperatures ranging from 230°C to 250°C, and the dissolution was found to increase with time and temperature. Dissolution of Cu in the Sn-3.5Ag solder is so fast that, at 240°C, 12 μm of the Cu substrate is fully consumed within 5 min. Much less dissolution is observed for the Sn-3.5Ag-0.5Cu solder. In respect to such high dissolution, there is no significant difference observed in the intermetallic compound (IMC) thickness at the interface for both solder alloys. A simplistic theoretical approach is carried out to find out the amount of Cu₆Sn₅ IMCs in the bulk of the solder by the measurement of the Cu consumption from the substrate and the thickness of the IMCs that form on the interface.     

Ambient Temperature Ultrasonic Bonding of Si-Dice Using Sn-3.5wt.%Ag

Ultrasonic bonding of Si-dice to type FR-4 printed circuit boards (PCB) with Sn-3.5wt.%Ag solder at ambient temperature was investigated. The under-bump metallization (UBM) on the Si-dice comprised Cu/Ni/Al from top to bottom with thicknesses of 0.4 μm, 0.4 μm, and 0.3 μm, respectively. The pads on the PCBs consisted of Au/Ni/Cu with thicknesses of 0.05/5/18 μm, sequentially from top to bottom. Solder was supplied as Sn-3.5wt.%Ag foil rolled to 100 μm thickness, and inserted in the joints. The ultrasonic bonding time was varied from 0.5 s to 3.0 s, and the ultrasonic power was 1400 W. The experimental results showed that reliable joints could be produced between the Si-dice and the PCBs with Sn-3.5wt.%Ag solder. The joint breaking force of “Si-die/solder/FR-4” increased with bonding times up to 2.5 s with a maximum value of 65 N. A bonding time of 3.0 s proved to be excessive, and resulted in cracks along the intermetallic compound between the UBM and solder, which caused a decrease in the bond strength. The intermetallic compound produced by ultrasonic bonding between the UBM and solder was confirmed to be (Cu, Ni)₆Sn₅. An erratum to this article can be found at     

Kinetics of intermetallic formation at Sn-37Pb/Cu interface during reflow soldering

The kinetics of the intermetallic layer formation at Sn-37wt.%Pb solder/Cu pad interface during reflow soldering were studied. The growth kinetics were analyzed theoretically by assuming that the mass flux of Cu through channels between scalloplike grains primarily contributes to the growth. Rate-controlling steps considered for the mass flux were the Cu dissolution from the bottom of the channels, diffusion through the channel, and the formation reaction of the intermetallic layer. These results indicated that a transition in the growth rate observed around 120–150 sec of reflow time may be associated with transition of the rate-controlling step from the Cu dissolution to the Cu diffusion through the channel.     

Adhesion strength of the Sn-9Zn-xAg/Cu interface

The adhesion strength of the Sn-9Zn-xAg/Cu interface was studied. The strength increased from 3.34±0.68 MPa to 7.79±0.57 MPa and from 4.75±1.04 MPa to 10.70±0.75 MPa for the Sn-9Zn-1.5Ag/Cu and Sn-9Zn-2.5Ag/Cu interfaces, respectively, as soldered at 250°C for the soldering time from 10 sec to 30 sec. However, the strength decreased from 8.11±0.72 MPa to 5.61±0.36 MPa for the Sn-9Zn-3.5Ag/Cu interface at 250°C for 10–30 sec. Both prolonging soldering time and raising temperature are beneficial for the adhesionstrength enhancement of the Sn-9Zn-1.5Ag/Cu and Sn-9Zn-2.5Ag/Cu interfaces because of the increment of wettability, but it is detrimental to the Sn-9Zn-3.5Ag/Cu interface because of microvoid formation.     

Microstructural modifications and properties of Sn-Ag-Cu solder joints induced by alloying

Slow cooling (1–3°C/sec) of Sn-Ag-Cu and Sn-Ag-Cu-X (X = Fe, Co) solder-joint specimens, made by hand soldering, simulated reflow in a surface-mount assembly to achieve similar as-solidified joint microstructures for realistic shear-strength testing, using Sn-3.5Ag (wt.%) as a baseline. Consistent with predictions from a recent Sn-Ag-Cu ternary phase-diagram study, either Sn dendrites, Ag₃Sn primary phase, or Cu₆Sn₅ primary phase were formed during solidification of joint samples made from the selected near-eutectic Sn-Ag-Cu alloys. Minor substitution of Co for Cu in Sn-3.7Ag-0.9Cu refined the joint-matrix microstructure by an apparent catalysis effect on the Cu₆Sn₅ phase, whereas Fe substitution promoted extreme refinement of the Sn-dendritic phase. Ambient-temperature shear strength was reduced by Sn dendrites in the joint microstructure, especially coarse dendrites in solute poor Sn-Ag-Cu, e.g., Sn-3.0Ag-0.5Cu, while Sn-3.7Ag-0.9Cu with Co and Fe additions have increased shear strength. At elevated (150°C) temperature, no significant difference exists between the maximum shear-strength values of all of the alloys studied.     

Reactions of lead-free solders with CuNi metallizations

We have done experimental research on the dissolution rate and intermetallic growth on Cu, Ni, and CuNi-alloy substrates as a function of time and Cu/Ni ratio of the substrate. Reactions that occur when CuNi metallizations are soldered with lead-free solders were investigated. The experiments were performed using Sn-3.5Ag and Sn-3.8Ag-0.7Cu solders and different CuNi alloys. To determine the rate of dissolution of the substrate material into the solder, CuNi foils of different concentrations were immersed in Sn-3.5Ag and Sn-3.8Ag-0.7Cu solder baths for soldering times ranging from 15 sec to 5 min at 250°C. In addition, reflows of solder balls were made on top of bulk substrates to study the reaction when there is a practically infinite amount of CuNi available compared to the amount of solder. Thin film experiments were also done, where Ni containing under bump metallizations (UBMs) were fabricated and reflowed with eutectic SnAg solder balls. The nickel slows down the dissolution of the UBM into the solder and the formation of intermetallics during reflow compared to Cu metallizations. The solder/UBM interfaces were analyzed with SEM to find out how Ni concentration affects the reaction, and how much Ni is needed to obtain a sufficiently slow reaction rate.     

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

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

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.     

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.     

Effect of cooling rate on microstructure and shear strength of pure Sn,Sn-0.7Cu,Sn-3.5Ag,and Sn-37Pb solders

The microstructure and shear strength characteristics of pure Sn and the eutectic compositions of Sn-37Pb, Sn-0.7Cu, and Sn-3.5Ag prepared under identical reflow conditions but subjected to two different cooling conditions were evaluated at room temperature. For the four solders, the ultimate shear strength increased with increasing strain rate from 10⁻⁵ s⁻¹ to 10⁻¹ s⁻¹. Decreasing the cooling rate tended to decrease the ultimate shear strength for both the Sn-0.7Cu and Sn-3.5Ag solders. The effects of work hardening resulting from increased strain rate were more prevalent in quench-cooled (QC) samples.     

Effect of Strain Rate and Temperature on the Tensile Properties of Tin-Based Lead-Free Solder Alloys

The tensile properties of Sn-3Ag-0.5Cu, Sn-3.5Ag, and Sn-0.7Cu lead-free solders were investigated on small-scale specimens and compared with those of Sn-37Pb eutectic solder at various strain rates from 1 × 10⁻⁴ s⁻¹ to 1 × 10⁻² s⁻¹ and over a wide temperature range from 25°C to 150°C. The tests were under true strain-rate-controlled conditions. The ductility of each lead-free solder is relatively constant while that for Sn-Pb eutectic solder strongly depends on strain rate and temperature. The strain rate sensitivity index m for lead-free solders is relatively stable and showed little dependence on temperature, whereas the values of m for Sn-37Pb increased linearly with increasing temperature.     

Wave soldering with a low melting point Bi-Sn Alloy: Effects of soldering temperatures and circuit board finishes

Wave soldering with low solid fluxes at temperatures as low as 175°C on test boards with a Cu/Imidazole surface finish has been shown to be feasible using a Pb-free, Bi-45%Sn-0.33%Ag solder that melts at temperatures of ∼140∼145°C. Other surface finishes such as Pd/Ni, Au/Ni, and Bi exhibit unacceptable soldering at temperatures below 210°C. Intermediate in performance are Sn surface finishes, which exhibit acceptable soldering at 190°C, but not at 175°C. Acceptable joints wave soldered on Cu/Im finishes passed class I/II inspection criterion and exhibited pull strengths in excess of the ultimate strength of the component leads.     

Interfacial reactions in the Sn-Ag/Au couples

Ag-Sn alloys are one of the most promising lead-free solders. Their reactions with Au substrates have been examined by using the reaction couple technique. Sn-3.5wt.%Ag/Au and Sn-25wt.%Ag/Au couples have been prepared and reacted at 120, 150, 180 and 200 C for various lengths of time. Three phases, δ-AuSn, ɛ₂-AuSn₂, and η-AuSn₄, are found in all the couples. The thickness of the reaction layers inccreases with higher temperatures and longer reaction time, and their growth rates are described by using the parabolic law. Arrhenius equation is used to describe the temperature dependence of the growth rates. The activation energy of the growth of the intermetallic layers in both kinds of the reaction couples is similar and is determined to be 76.74 KJ/mole. Based on the reaction path knowledge and interfacial morphology, it is concluded that Sn is the fastest diffusion species in the couples.     

Effect of Ni layer thickness and soldering time on intermetallic compound formation at the interface between molten Sn-3.5Ag and Ni/Cu substrate

The binary eutectic Sn-3.5wt.%Ag alloy was soldered on the Ni/Cu plate at 250°C, the thickness of the Ni layer changing from 0 through 2 and 4 μm to infinity, and soldering time changing from 30 to 120 s at intervals of 30 s. The infinite thickness was equivalent to the bare Ni plate. The morphology, composition and phase identification of the intermetallic compound (IMC, hereafter) formed at the interface were examined. Depending on the initial Ni thickness, different IMC phases were observed at 30 s: Cu₆Sn₅ on bare Cu, metastable NiSn₃ + Ni₃Sn₄ on Ni(2 μm)/Cu, Ni₃Sn₄ on Ni(4 μm)/Cu, and Ni₃Sn + Ni₃Sn₄ on bare Ni. With increased soldering time, a Cu-Sn-based η-(Cu₆Sn₅)_1−xNi_x phase formed under the pre-formed Ni-Sn IMC layer both at 60 s in the Ni(2 μm)/Cu plate and at 90 s in the Ni(4 μm)/Cu plate. The two-layer IMC pattern remained thereafter. The wetting behavior of each joint was different and it may have resulted from the type of IMC formed on each plate. The thickness of the protective Ni layer over the Cu plate was found to be an important factor in determining the interfacial reaction and the wetting behavior.     

Interfacial reaction between Sn-0.7Cu (-Ni) solder and Cu substrate

The interfacial reaction between Sn-0.7mass%Cu-(Ni) solders and a Cu substrate was investigated to reveal the effect of the addition of Ni to Sn-Cu solder on the formation of intermetallic compounds (IMCs). Sn-0.7Cu-xNi solders (x=0, 0.05, 0.1, 0.2 mass%) were prepared. For the reflow process, specimens were heated in a radiation furnace at 523 K for 60 sec, 300 sec, and 720 sec to estimate the interfacial reaction between the molten solder and Cu substrate. Then, for the aging process, some specimens were heat-treated in an oil bath at 423 K for 168 h and 504 h. The cross sections of soldered specimens were observed to measure the dissolution thickness of the Cu substrate and the thickness of the IMC and to investigate the microstructures of IMC. The results showed that, just after the reflow process, the dissolution thickness of the Cu substrate increased with the increase of Ni content in the Sn-0.7Cu-xNi solder and the thickness of the IMC between the solder and Cu substrate was the minimum in the Sn-0.7Cu-0.05Ni solder. After the aging process, the IMC grew with the increase of aging time. In the case of 0.05% Ni, the IMC thickness was the thinnest regardless of aging time. It is clear that 0.05% Ni addition to Sn-0.7Cu solder very effectively inhibits the formation and growth of the IMC between solder and Cu substrate. Electron probe microanalysis of the IMC showed that the IMC layer in the Sn-0.7Cu-Ni solder contained Ni, and the IMC was expressed as (Cu_1−y ,Ni _y )₆Sn₅.     

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.     

The Crystal Orientation of β-Sn Grains in Sn-Ag and Sn-Cu Solders Affected by Their Interfacial Reactions with Cu and Ni(P) Under Bump Metallurgy

Recently, it has been reported that the crystal orientation and grain size of the β-Sn phase in Sn-rich solders have profound effects on the reliabilities of Pb-free solder joints, such as thermo-mechanical fatigue and electromigration. Additionally, it is also known that the microstructure of the Sn-rich solders is strongly affected by their alloy composition. In this study the grain size and orientation of the β-Sn phase were investigated in terms of their alloy composition and interfacial reactions with two different under bump metallurgies (UBMs), Cu and Ni(P). Solder balls (380 μm in diameter) of pure Sn, Sn-0.5Cu, Sn-0.5Ag, and Sn-1.8Ag (in weight percent) were reflowed on Cu and Ni(P) UBMs. After the reflow at 250°C for 120 s, the microstructure of the solder joints was analyzed by cross-polarization light microscopy and electron backscatter diffraction. For the compositional analysis of solder joints, electron probe micro-analysis was used and thermodynamics calculations were also performed. During reflow on Cu or Ni(P) UBM, Cu and Ni atoms were dissolved quickly and were saturated to their solubility limits in the solders, causing changes in composition and β-Sn grain orientation.     

Effects of Cu, Ag and Sb on the creep-rupture strength of lead-free solder alloys

The materials used in the present research are pure Sn metal and Sn-0.5% Cu, Sn-3.5%Ag, Sn-0.3%Sb, and Sn-3.5%Ag-0.5%Cu alloys. Effects of Cu, Ag and Sb on the creep-rupture strength of lead-free solder alloys have been investigated. Creep tests are performed at the stress and temperature range of 3 to 12 MPa and 378 to 403 K, respectively. A 3.5% addition of Ag had the largest contribution to the creep-rupture strength of Sn metal among the single addition of 0.5%Cu, 3.5%Ag, and 0.3%Sb. The combined addition of 3.5%Ag and 0.5%Cu makes the largest creep-rupture strength. The effects of these elements on the microstructure of the lead-free alloys are also investigated with optical microscope (OM) and transmission electron microscope (TEM) observations.