Interfacial Reaction of Sn and Cu-<Emphasis Type="Italic">x</Emphasis>Zn Substrates After Reflow and Thermal Aging

Zn additions to Cu under bump metallurgy (UBM) in solder joints were the subject of this study. An alternative design was implemented to fabricate pure Sn as the solder and Cu-xZn (x = 15 wt.% and 30 wt.%) as the UBM to form the reaction couple. As the Zn content increased from 15 wt.% to 30 wt.% in the Sn/Cu-Zn system, growth of both Cu₃Sn and Cu₆Sn₅ was suppressed. In addition, no Kirkendall voids were observed at the interface in either Sn/Cu-Zn couple during heat treatment. After 40-day aging, different multilayered phases of [Cu₆Sn₅/Cu₃Sn/Cu(Zn)] and [Cu₆Sn₅/Cu(Zn,Sn)/CuZn] formed at the interface of [Sn/Cu-15Zn] and [Sn/Cu-30Zn] couples, respectively. The growth mechanism of intermetallic compounds (IMCs) during aging is discussed on the basis of the composition variation in the joint assembly with the aid of electron-microscopic characterization and the Sn-Cu-Zn ternary phase diagram. According to these analyses of interfacial morphology and IMC formation in the Sn/Cu-Zn system, Cu-Zn is a potential UBM for retarding Cu pad consumption in solder joints.     

Interfacial Reactions of Sn-3.5Ag-<Emphasis Type="Italic">x</Emphasis>Zn Solders and Cu Substrate During Liquid-State Aging

The effects of Zn (1 wt.%, 3 wt.%, and 7 wt.%) additions to Sn-3.5Ag solder and various reaction times on the interfacial reactions between Sn-3.5Ag-xZn solders and Cu substrates a during liquid-state aging were investigated in this study. The composition and morphological evolution of interfacial intermetallic compounds (IMCs) changed significantly with the Zn concentration and reaction time. For the Sn-3.5Ag-1Zn/Cu couple, CuZn and Cu₆Sn₅ phases formed at the interface. With increasing aging time, the Cu₆Sn₅ IMC layer grew thicker, while the CuZn IMC layer drifted into the solder and decomposed gradually. Cu₅Zn₈ and Ag₅Zn₈ phases formed at the interfaces of Sn-3.5Ag-3Zn/Cu and Sn-3.5Ag-7Zn/Cu couples. With increasing reaction time, the Cu₅Zn₈ layer grew and Cu atoms diffused from the substrate to the solder, which transformed the Ag₅Zn₈ to (Cu,Ag)₅Zn₈. The Cu₆Sn₅ layer that formed between the Cu₅Zn₈ layer and Cu was much thinner at the Sn-3.5Ag-7Zn/Cu interface than at the Sn-3.5Ag-3Zn/Cu interface. Additionally, we measured the thickness of interfacial IMC layers and found that 3 wt.% Zn addition to the solder was the most effective for suppressing IMC growth at the interfaces.     

Interfacial Reactions and Joint Strengths of Sn-<Emphasis Type="Italic">x</Emphasis>Zn Solders with Immersion Ag UBM

The solder joint microstructures of immersion Ag with Sn-xZn (x = 0 wt.%, 1 wt.%, 5 wt.%, and 9 wt.%) solders were analyzed and correlated with their drop impact reliability. Addition of 1 wt.% Zn to Sn did not change the interface microstructure and was only marginally effective. In comparison, the addition of 5 wt.% or 9 wt.% Zn formed layers of AgZn₃/Ag₅Zn₈ at the solder joint interface, which increased drop reliability significantly. Under extensive aging, Ag-Zn intermetallic compounds (IMCs) transformed into Cu₅Zn₈ and Ag₃Sn, and the drop impact resistance at the solder joints deteriorated up to a point. The beneficial role of Zn on immersion Ag pads was ascribed to the formation of Ag-Zn IMC layers, which were fairly resistant to the drop impact, and to the suppression of the brittle Cu₆Sn₅ phase at the joint interface.     

Mechanical Properties and Electrochemical Corrosion Behavior of Al/Sn-9Zn-<Emphasis Type="Italic">x</Emphasis>Ag/Cu Joints

The effect of Ag content on the wetting behavior of Sn-9Zn-xAg on aluminum and copper substrates during soldering, as well as the mechanical properties and electrochemical corrosion behavior of Al/Sn-9Zn-xAg/Cu solder joints, were investigated in the present work. Tiny Zn and coarsened dendritic AgZn₃ regions were distributed in the Sn matrix in the bulk Sn-9Zn-xAg solders, and the amount of Zn decreased while that of AgZn₃ increased with increasing Ag content. The wettability of Sn-9Zn-1.5Ag solder on Cu substrate was better than those of the other Sn-9Zn-xAg solders but worse than that of Sn-9Zn solder. The wettability of Sn-9Zn-1.5Ag on the Al substrate was also better than those of the other Sn-9Zn-xAg solders, and even better than that of Sn-9Zn solder. The Al/Sn-9Zn/Cu joint had the highest shear strength, and the shear strength of the Al/Sn-9Zn-xAg/Cu (x = 0 wt.% to 3 wt.%) joints gradually decreased with increasing Ag content. The corrosion resistance of the Sn-9Zn-xAg solders in Al/Sn-9Zn-xAg/Cu joints in 5% NaCl solution was improved compared with that of Sn-9Zn. The corrosion potential of Sn-9Zn-xAg solders continuously increased with increasing Ag content from 0 wt.% to 2 wt.% but then decreased for Sn-9Zn-3Ag. The addition of Ag resulted in the formation of the AgZn₃ phase and in a reduction of the amount of the eutectic Zn phase in the solder matrix; therefore, the corrosion resistance of the Al/Sn-9Zn-xAg/Cu joints was improved.     

Interfacial Reactions between Cu<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ni<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Alloy Underbump Metallurgy and Sn-Ag-<Emphasis Type="Italic">z</Emphasis>Cu Solders

A reaction study of Cu _x Ni _y alloy (x = 0.2–0.95) under bump metallization (UBM) with Sn-Ag-zCu solder (z = 0–0.7) was conducted. Formation and separation of intermetallic compounds (IMCs), effect of Cu addition to the Cu _x Ni _y alloy and the solders, and compatibility of reaction products with currently available phase diagrams are extensively investigated. The increase of Cu content both in the Cu _x Ni _y alloy and in the solder promoted IMC growth and Cu _x Ni _y consumption; though, with regard to solder composition, the reverse trend was true of the solder reactions in the literature. The liquid + Cu₆Sn₅ area in the Sn-rich corner needs to be larger compared to the currently available Cu-Ni-Sn ternary phase diagram, and the maximum simultaneous soluble point of Ni and Cu in Sn needs also to be moved to the Ni-Sn side (e.g., Sn-0.6Cu-0.3Ni).     

Effect of Aluminum Concentration on the Interfacial Reactions of Sn-3.0Ag-<Emphasis Type="Italic">x</Emphasis>Al Solders with Copper and ENIG Metallizations

Aluminum was added into Sn-3.0Ag (wt.%) solder to investigate the effect of aluminum concentration on the interfacial reaction of Sn-3.0Ag-xAl solders with copper or electroless nickel immersion gold (ENIG) metallizations. Four different Sn-3.0Ag-xAl solders (x = 0 wt.%, 0.1 wt.%, 0.5 wt.%, and 1.0 wt.%) were used for comparison. It was found that the composition, morphology, and thickness of interfacial reaction products were strongly dependent on aluminum concentration. At low aluminum concentration (0.1 wt.%), the typical Cu₆Sn₅ layer was formed at the interface. When the aluminum concentration was 0.5 wt.%, a continuous CuAl₂ layer spalled off from the interfacial Cu-Sn intermetallic compound (IMC) layer. Only a planar CuAl₂ layer was observed at the interface when the aluminum concentration was increased to 1.0 wt.%. In Sn-Ag-Al/ENIG reactions, Ni₃Sn₄ was formed and spallation occurred near the interface in the Sn-3.0Ag and Sn-3.0Ag-0.1Al solder joints. When the aluminum concentration was higher than 0.1 wt.%, a thin planar AuAl compound formed at the interface. There was no P-rich phase formation that retarded the spalling phenomenon. The aluminum additive in Sn-Ag solder inhibited the growth of IMCs in the reaction with copper or ENIG metallizations, which was favorable for the reliability of solder joints.     

Reaction Mechanism and Mechanical Properties of the Flip-Chip Sn-3.0Ag-0.5Cu Solder Bump with Cu/Ni-<Emphasis Type="Italic">x</Emphasis>Cu/Ti Underbump Metallization After Various Reflows

Ni underbump metallization (UBM) has been widely used as the diffusion barrier between solder and Cu pads. To retard the fast dissolution rate of Ni UBM, Cu was added into Ni thin films. The Ni-Cu UBM can provide extra Cu to the solders to maintain the Cu₆Sn₅ intermetallic compound (IMC) at the interface, which can thus significantly decrease the Ni dissolution rate. In this study, the Cu content of the sputtered Cu/Ni-xCu/Ti UBM was varied from 0 wt.% to 20 wt.%. Sn-3Ag-0.5Cu solder was reflowed with Cu/Ni-Cu/Ti UBM one, three, and five times. Reflow and cooling conditions altered the morphology of the IMCs formed at the interface. The amount of (Cu,Ni)₆Sn₅ increased with increasing Cu content in the Ni-Cu film. The Cu concentration of the intermetallic compound was strongly dependent on the composition of the Ni-Cu films. The results of this study suggest that Cu-rich Ni-xCu UBM can be used to suppress interfacial spalling and improve shear strength and pull strength of solder joints.     

Interfacial reaction between multicomponent lead-free solders and Ag,Cu, Ni,and Pd substrates

The interfacial reaction between two prototype multicomponent lead-free solders, Sn-3.4Ag-1Bi-0.7Cu-4In and Sn-3.4Ag-3Bi-0.7Cu-4In (mass%), and Ag, Cu, Ni, and Pd substrates are studied at 250°C and 150°C. The microstructural characterization of the solder bumps is carried out by scanning electron microscopy (SEM) coupled with energy dispersive x-ray analysis. Ambient temperature, isotropic elastic properties (bulk, shear, and Young’s moduli and Poisson’s ratio) of these solders along with eutectic Sn-Ag, Sn-Bi, and Sn-Zn solders are measured. The isotropic elastic moduli of multicomponent solders are very similar to the eutectic Sn-Ag solder. The measured solubility of the base metal in liquid solders at 250°C agrees very well with the solubility limits reported in assessed Sn-X (X=Ag, Cu, Ni, Pd) phase diagrams. The measured contact angles were generally less than 15° on Cu and Pd substrates, while they were between 25° and 30° on Ag and Ni substrates. The observed intermediate phases in Ag/solder couples were Ag₃Sn after reflow at 250°C and Ag₃Sn and ζ (Ag-Sn) after solid-state aging at 150°C. In Cu/solder and Ni/solder couples, the interfacial phases were Cu₆Sn₅ and (Cu,Ni)₆Sn₅, respectively. In Pd/solder couples, only PdSn₄ after 60-sec reflow, while both PdSn₄ and PdSn₃ after 300-sec reflow, were observed.     

Interfacial microstructure and bonding strength of Sn-3Ag-0.5Cu and Sn-3Ag-0.5Cu-0.5Ce-xZn solder BGA packages with immersion Ag surface finish

Although it has been verified that tin whiskers can be prevented by the addition of 0.5 wt.% Zn into a Sn-3Ag-0.5Cu-0.5Ce solder, no detailed studies have been conducted on interfacial reactions and mechanical properties of Sn-3Ag-0.5Cu-0.5Ce-xZn solder joints with an immersion Ag surface finish. The intermetallic compounds formed during the reflow and aging of Sn-3Ag-0.5Cu and Sn-3Ag-0.5Cu-0.5Ce-xZn solder ball grid array (BGA) packages were investigated. Because more heterogeneous nucleation sites, provided by CeSn₃ intermetallics and Zn atoms, formed in the Sn-3Ag-0.5Cu-0.5Ce-xZn solder matrix, and Cu and Zn have a stronger affinity than Cu and Sn, the Cu-Sn intermetallics growth in Sn-3Ag-0.5Cu-0.5Ce-xZn solder joints with Ag/Cu pads was suppressed. The 0.2% Zn addition for inhibiting rapid whisker growth in RE-doped Sn-Ag-Cu solder joints is more appropriate than 0.5 wt.% additions, as excess Zn addition causes poor oxidation resistance and inferior bonding strength.     

Effects of Minor Ni Doping on Microstructural Variations and Interfacial Reactions in Cu/Sn-3.0Ag-0.5Cu-<Emphasis Type="Italic">x</Emphasis>Ni/Au/Ni Sandwich Structures

The effects of Ni doping on microstructural variations and interfacial reactions in Cu/Sn-3.0Ag-0.5Cu-xNi/Au/Ni sandwich structures were investigated. The sandwich structures, i.e., Cu/Sn-3.0Ag-0.5Cu/Au/Ni and Cu/Sn-3.0Ag-0.5Cu-0.1Ni/Au/Ni (wt.%), were reflowed and isothermally aged at 150°C for 500 h. The behavior of Ni and Cu migration in the solders before and after aging was investigated using field-emission electron probe microanalysis (FE-EPMA), and the microstructure evolution of the solders with Ni doping was investigated. It was observed that Ni migrated to the board Cu-side, while Cu tended to migrate toward the Ni/Au package side, and two different types of (Cu,Ni)₆Sn₅ intermetallic compounds (IMCs), one with 19.8 at.% to 23.4 at.% Ni and the other with 1.3 at.% to 6.4 at.% Ni content, were found. Regarding interfacial reactions, it was identified that the local Ni and Cu concentrations affected the formation of (Cu,Ni)₆Sn₅. Redistribution of Ni and Cu was correlated with the formation mechanism of interfacial (Cu,Ni)₆Sn₅.     

The Shear Strength and Fracture Behavior of Sn-Ag-<Emphasis Type="Italic">x</Emphasis>Sb Solder Joints with Au/Ni-P/Cu UBM

This study investigates the effects of Sb addition on the shear strength and fracture behavior of Sn-Ag-based solders with Au/Ni-P/Cu underbump metallization (UBM) substrates. Sn-3Ag-xSb ternary alloy solder joints were prepared by adding 0 wt.% to 10 wt.% Sb to a Sn-3.5Ag alloy and joining them with Au/Ni-P/Cu UBM substrates. The solder joints were isothermally stored at 150°C for up to 625 h to study their microstructure and interfacial reaction with the UBM. Single-lap shear tests were conducted to evaluate the mechanical properties, thermal resistance, and failure behavior. The results show that UBM effectively suppressed intermetallic compound (IMC) formation and growth during isothermal storage. The Sb addition helped to refine the Ag₃Sn compounds, further improving the shear strength and thermal resistance of the solders. The fracture behavior evolved from solder mode toward the mixed mode and finally to the IMC mode with increasing added Sb and isothermal storage time. However, SnSb compounds were found in the solder with 10 wt.% Sb; they may cause mechanical degradation of the solder after long-term isothermal storage.     

Investigation of Dissolution Behavior of Metallic Substrates and Intermetallic Compound in Molten Lead-free Solders

This study investigates the dissolution behavior of the metallic substrates Cu and Ag and the intermetallic compound (IMC)-Ag₃Sn in molten Sn, Sn-3.0Ag-0.5Cu, Sn-58Bi and Sn-9Zn (in wt.%) at 300, 270 and 240°C. The dissolution rates of both Cu and Ag in molten solder follow the order Sn > Sn-3.0Ag-0.5Cu >Sn-58Bi > Sn-9Zn. Planar Cu₃Sn and scalloped Cu₆Sn₅ phases in Cu/solders and the scalloped Ag₃Sn phase in Ag/solders are observed at the metallic substrate/solder interface. The dissolution mechanism is controlled by grain boundary diffusion. The planar Cu₅Zn₈ layer formed in the Sn-9Zn/Cu systems. AgZn₃, Ag5Zn₈ and AgZn phases are found in the Sn-9Zn/Ag system and the dissolution mechanism is controlled by lattice diffusion. Massive Ag₃Sn phases dissolved into the solders and formed during solidification processes in the Ag₃Sn/Sn or Sn-3.0Ag-0.5Cu systems. AgZn₃ and Ag₅Zn₈ phases are formed at the Sn-9Zn/Ag₃Sn interface. Zn atoms diffuse through Ag-Zn IMCs to form (Ag, Zn)Sn₄ and Sn-rich regions between Ag₅Zn₈ and Ag₃Sn.     

Interfacial Reactions and Microstructures of Sn-0.7Cu-<Emphasis Type="Italic">x</Emphasis>Zn Solders with Ni-P UBM During Thermal Aging

The effects of the addition of Zn to Sn-0.7Cu solders are investigated. The study is focused on the interfacial reactions, microstructures, and mechanical properties after reaction with Ni-P under bump metallurgies (UBMs). The Zn contents in Sn-0.7Cu-xZn are varied as 0.2, 0.4, and 0.8 (in wt.% unless otherwise specified). In the reaction with Ni-P UBM during thermal aging at 150°C for 1000 h, (Cu,Ni)₆Sn₅ intermetallic compounds (IMCs) are formed at the Sn-0.7Cu/UBM interface, whereas Zn is incorporated into IMCs to form (Cu,Ni,Zn)₆Sn₅ in the Zn-doped solders. As the Zn content increases, the interfacial IMC thickness is reduced. A total reduction of about 40% in IMC thickness was observed for the 0.8% Zn-doped Sn-Cu. The same IMC particles are also observed in the matrix of each solder. In Sn-0.7Cu, (Cu,Ni)₆Sn₅ particles are coarsened during aging, while (Cu,Ni,Zn)₆Sn₅ particles in the Zn-added solders are less coarsened and remain much smaller than (Cu,Ni)₆Sn₅. The growth rate of (Cu,Ni)₆Sn₅ during thermal aging is significantly suppressed by the addition of Zn. Consequently, after reaction with Ni-P UBM, the Zn-doped solders exhibit a thermally stable microstructure as measured by hardness and shear strength.     

Effects of Minor Additions of Zn on Interfacial Reactions of Sn-Ag-Cu and Sn-Cu Solders with Various Cu Substrates during Thermal Aging

The effects of Zn additions to Sn-0.7Cu and Sn-3.8Ag-0.7Cu (all in wt.% unless specified otherwise) Pb-free solders on the interfacial reactions with Cu substrates were investigated. The study was focused on the intermetallic compound (IMC) growth, Cu consumption and void formation as a function of thermal aging and solder composition. Four different kinds of Cu substrates (high-purity Cu, oxygen-free Cu, vacuum-sputtered Cu, and electroplated Cu) were prepared to compare their interfacial reaction behaviors with Zn-added solders. Thermal aging was performed at 150°C for up to 1000 h to accelerate the interfacial reactions between solders and Cu substrates. Growth of IMCs (Cu₆Sn₅ and Cu₃Sn) in Zn-added solders was slower than those without Zn additions. The growth of the Cu₃Sn phase, in particular, was drastically reduced in the Zn-added solders on all four Cu substrates. On an electroplated Cu substrate, numerous voids were observed in the Cu₃Sn phase for Sn-Cu and Sn-Ag-Cu solders aged at 150°C for 1000 h. However, these voids were largely eliminated in the Zn-added solders. On the other three Cu substrates, the conditions which produce a high density of voids were not found after aging both solders with and without Zn. The Cu consumption with Zn-added solders was also significantly lower. The beneficial effects of Zn additions on interfacial reaction behaviors are reported, and the corresponding mechanisms in suppressing void formation and Cu consumption due to Zn additions will be discussed.     

Formation of Intermetallic Compounds Between Liquid Sn and Various CuNi<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Metallizations

Interfacial reactions between liquid Sn and various Cu-Ni alloy metallizations as well as the subsequent phase transformations during the cooling were investigated with an emphasis on the microstructures of the reaction zones. It was found that the extent of the microstructurally complex reaction layer (during reflow at 240°C) does not depend linearly on the Ni content of the alloy metallization. On the contrary, when Cu is alloyed with Ni, the rate of thickness change of the total reaction layer first increases and reaches a maximum at a composition of about 10 at.% Ni. The reaction layer is composed of a relatively uniform continuous (Cu,Ni)₆Sn₅ reaction layer (a uniphase layer) next to the NiCu metallizations and is followed by the two-phase solidification structures between the single-phase layer and Sn matrix. The thickness of the two-phase layer, where the intermetallic tubes and fibers have grown from the continuous interfacial (Cu,Ni)₆Sn₅ layer, varies with the Ni-to-Cu ratio of the alloy metallization. In order to explain the formation mechanism of the reaction layers and their observed kinetics, the phase equilibria in the Sn-rich side of the SnCuNi system at 240°C were evaluated thermodynamically utilizing the available data, and the results of the Sn/Cu _x Ni_1−x diffusion couple experiments. With the help of the assessed data, one can also evaluate the minimum Cu content of Sn-(Ag)-Cu solder, at which (Ni,Cu)₃Sn₄ transforms into (Cu,Ni)₆Sn₅, as a function of temperature and the composition of the liquid solders.     

Effect of microelements addition on the interfacial reaction between Sn-Ag-Cu solders and the Cu substrate

Three kinds of Sn-Ag-based lead-free solders, Sn-3.5Ag-0.7Cu, Sn-3.5Ag-0.5Cu-0.07Ni-0.01Ge, and Sn-3.5Ag-0.07Ni (in wt.%), were selected to explore the effect of microelements (Ni and Ge) on the interfacial reaction between the solder and the Cu substrate. The thickness of the interfacial intermetallics formed with the Sn-3.5Ag-0.5Cu-0.07Ni-0.01Ge and Sn-3.5Ag-0.07Ni solders is several times that of the Sn-3.5Ag-0.7Cu solder. The added microelements converted the feature of interfacial intermetallics from pebble shape to worm shape. However, the results of x-ray diffraction (XRD) analysis suggest that the interfacial intermetallics formed with both solders have the same crystal structure. The results of energy dispersive spectroscopy (EDS) analysis show that the major interfacial intermetallic formed with the Sn-3.5Ag-0.7Cu solder is Cu₆Sn₅, while it is (Cu_x,Ni_1−x)₆Sn₅ with Sn-3.5Ag-0.5Cu-0.07Ni-0.01Ge. Ni influences the interfacial intermetallics and plays the influential role on the difference of interfacial reaction rate between liquid solder and solid Cu and the morphology of interfacial intermetallics. Additionally, the growth kinetics of the interfacial intermetallic compounds (IMCs) formed in the systems of Cu/Sn-3.5Ag-0.7Cu and Cu/Sn-3.5Ag-0.07Ni at high-temperature storage was also explored.     

Effects of Ni Additions on the Growth of Cu<Subscript>3</Subscript>Sn in High-Lead Solders

In reactions between solders and Cu, additions of minor alloying elements, such as Fe, Co or Ni, to solders often reduce the Cu₃Sn growth rate. Nevertheless, the mechanism for this effect remains unresolved. To provide more experimental observations that are essential for uncovering this mechanism, growth of Cu₃Sn in the reaction between Cu and high-lead solders with or without Ni additions has been studied. The solders used for this study were 10Sn-90Pb and 5Sn-95Pb doped with 0 wt.%, 0.03 wt.%, 0.06 wt.%, 0.1 wt.% or 0.2 wt.% Ni. Reaction conditions included one reflow at 350°C for 2 min and solid-state aging at 160°C for up to 2000 h. The effect of Ni on the growth of Cu₃Sn is discussed in detail based on the experimental results.     

Interfacial Reaction and Die Attach Properties of Zn-Sn High-Temperature Solders

Interfacial reaction and die attach properties of Zn-xSn (x = 20 wt.%, 30 wt.%, and 40 wt.%) solders on an aluminum nitride–direct bonded copper substrate were investigated. At the interface with Si die coated with Au/TiN thin layers, the TiN layer did not react with the solder and worked as a good protective layer. At the interface with Cu, CuZn₅, and Cu₅Zn₈ IMC layers were formed, the thicknesses of which can be controlled by joining conditions such as peak temperature and holding time. During multiple reflow treatments at 260°C, the die attach structure was quite stable. The shear strength of the Cu/solder/Cu joint with Zn-Sn solder was about 30 MPa to 34 MPa, which was higher than that of Pb-5Sn solder (26 MPa). The thermal conductivity of Zn-Sn alloys of 100 W/m K to 106 W/m K was sufficiently high and superior to those of Au-20Sn (59 W/m K) and Pb-5Sn (35 W/m K).     

Microstructure and Creep Deformation of Sn-Ag-Cu-Bi/Cu Solder Joints

Sn-Ag-Cu solder is one of the candidate alternatives to Sn-Pb-based solders. In order to improve its performance, different materials have been added to Sn-Ag-Cu-based solders. Several studies on Sn-Ag-Cu-based solders with Bi additions have shown Sn-Ag-Cu-Bi to be a class of solders with good wetting behavior and good performance that show great promise for use in the electronics assembly and packaging industry. To investigate the mechanical reliability of the Sn-Ag-Cu-Bi solders further, single-lap shear creep characteristics have been studied in this work. Dog-bone-type solder joint specimens were formed using five types of solder alloys, Sn-3.0Ag-0.5Cu and Sn-3.0 Ag-0.5Cu-xBi (x = 1 wt.% to 4 wt.%) with Cu substrates, and creep tests were performed at temperatures of 120°C and 150°C under stresses of 5 MPa to 10 MPa. Results indicate that the rupture times for Sn-3.0Ag-0.5Cu-xBi solder joints up to 4 wt.% of Bi are longer than the rupture time for Sn-3.0Ag-0.5Cu. Stress exponents ranged from 3 to 7 for temperatures of 150°C and 120°C with stresses under 10 MPa. Microstructural analyses using scanning electron microscopy (SEM) were performed and related to the creep behavior of the solder joints.     

Reaction of Liquid Sn-Ag-Cu-Ce Solders with Solid Copper

Small amounts of the rare-earth element Ce were added to the Sn-rich lead-free eutectic solders Sn-3.5Ag-0.7Cu, Sn-0.7Cu, and Sn-3.5Ag to improve their properties. The microstructures of the solders without Ce and with different amounts (0.1 wt.%, 0.2 wt.%, and 0.5 wt.%) of Ce were compared. The microstructure of the solders became finer with increasing Ce content. Deviation from this rule was observed for the Sn-Ag-Cu solder with 0.2 wt.% Ce, and for the Sn-0.7Cu eutectic alloy, which showed the finest microstructure without Ce. The melting temperatures of the solders were not affected. The morphology of intermetallic compounds (IMC) formed at the interface between the liquid solders and a Cu substrate at temperatures about 40°C above the melting point of the solder for dipping times from 2 s to 256 s was studied for the basic solder and for solder with 0.5 wt.% Ce addition. The morphology of the Cu₆Sn₅ IMC layer developed at the interface between the solders and the substrate exhibited the typical scallop-type shape without significant difference between solders with and without Ce for the shortest dipping time. Addition of Ce decreased the thickness of the Cu₆Sn₅ IMC layer only at the Cu/Sn-Ag-Cu solder interface for the 2-s dipping. A different morphology of the IMC layer was observed for the 256-s dipping time: The layers were less continuous and exhibited a broken relief. Massive scallops were not observed. For longer dipping times, Cu₃Sn IMC layers located near the Cu substrate were also observed.