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 reaction and solder joint reliability of Pb-free solders in lead frame chip scale packages (LF-CSP)

Chip scale packages (CSP) have essential solder joint quality problems, and a board level reliability is a key issue in design and development of the CSP type packages. There has been an effort to eliminate Pb from solder due to its toxicology. To evaluate the various solder balls in CSP package applications, Pb-free Sn-Ag-X (X=In, Cu, Bi) and Sn-9Zn-1Bi-5In solder balls were characterized by melting behavior, phases, interfacial reaction, and solder joint reliability. For studying joint strength between solders and under bump metallurgy (UBM) systems, various UBMs were prepared by electroplating and electroless plating. After T/C (temperature cycle) test, Sn3.5Ag8.5In solder was partially corroded and its shape was distorted. This phenomenon was observed in a Sn3Ag10In 1Cu solder system, too. Their fractured surface, microstructure of solder joint interface, and of bulk solder ball were examined and analyzed by optical microscopy, SEM and EDX. To simulate the real surface mounting condition and evaluate the solder joint reliability on board level, Daisy chain test samples using LF-CSP packages were prepared with various Pb-free solders, then a temperature cycle test (−65∼ 150°C) was performed. All tested Pb-free solders showed better board level solder joint reliability than Sn-36Pb-2Ag. Sn-3.5Ag-0.7Cu and Sn-9Zn-1Bi-5In solders showed 35%, 100% superior solder joint reliability than Sn-36Pb-2Ag solder ball, respectively.     

Soldering reactions between In49Sn and Ag thick films

The interfacial reactions between In49Sn solders and Ag thick films at temperatures ranging from 200°C to 350°C have been studied. The intermetallic compound formed at the Ag/In49Sn interface is Ag₂In enveloped in a thin layer of AgIn₂. Through the measurement of the thickness decrease of Ag thick films, it has been determined that the reaction kinetics of Ag₂In has a linear relation to reaction time. Morphology observations indicated that the linear reaction of Ag₂In was caused by the floating of Ag₂In into the In49Sn solder as a result of the In49Sn solder penetrating into the porous Ag thick film. A sound joint can be obtained when a sufficient thickness of the Ag thick film (over 19.5 μm) reacts with the In49Sn solder. In this case, the tensile tested specimens fracture in the In49Sn matrix.     

Influence of Lanthanum Additions on the Microstructure and Microhardness of Sn-3.5Ag Solder

This study evaluates the effects of different amounts of lanthanum (La) additions on the microstructure and microhardness of Sn-3.5Ag solders. Sn-3.5Ag-xLa ternary solders were prepared by adding 0 wt.% to 1.0 wt.% La to Sn-3.5Ag alloy. Copper substrates were then dipped in the molten solders and these samples aged at 150°C for up to 625 h. The microstructure and microhardness of the as-solidified solder and the aged solder/copper samples were investigated. The Sn-3.5Ag-xLa solders comprised β-Sn, Ag₃Sn, and LaSn₃ phases, and their microstructure was refined by La additions. As-cast, the addition of La increased the microhardness of the Sn-Ag solder due to the refining effect of Ag₃Sn particles and increased formation of LaSn₃ compounds. As the aging time was increased, the microhardness of the solders decreased and the Ag₃Sn compounds coarsened. However, the coarsening of Ag₃Sn compounds was retarded by La, and the size and amount of LaSn₃ compounds did not change perceptibly with aging time. Therefore, La additions can improve the microhardness and thermal resistance of solder joints.     

Formation of a Metastable Phase at the Interface Between Sn and Ag–Pd Substrates During Liquid-State Reaction

Ag–Pd alloys are widely used as thick-film conductors and are potential alternatives to the expensive Au bump. In this work, because Sn is the primary element in solders, we investigated Sn/Ag–Pd interfacial reactions at 250°C as a means of assessing the reliability and evaluating reflow reactions at joints between solder and Ag–Pd conductor contacts, and in the Ag bump combined with the solder cap. The experimental results showed that Sn/Ag–Pd interfacial reactions at 250°C are different from those of Sn/Ag and Sn/Pd. A metastable Sn–Ag–Pd ternary phase is formed when the amount of Pd added is 20–40 at.%. Because, in commercial applications, at least 20 wt.% Pd (~20 at.% Pd) is used in Ag–Pd alloys to eliminate the silver-migration phenomenon, assessment of the reliability of Ag bumps and the soldered joints of Ag–Pd thick film hybrid circuits must be based on Sn/Ag–Pd interfacial reactions, not those of Sn/Pd and Sn/Ag.     

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 of Sn-Ag-Cu solders modified by minor Zn alloying addition

The near-ternary eutectic Sn-Ag-Cu alloys have been identified as leading Pb-free solder candidates to replace Pb-bearing solders in microelectronic applications. However, recent investigations on the processing behavior and solder joints reliability assessment have revealed several potential reliability risk factors associated with the alloy system. The formation of large Ag₃Sn plates in Sn-Ag-Cu joints, especially when solidified in a relatively slow cooling rate, is one issue of concern. The implications of large Ag₃Sn plates on solder joint performance and several methods to control them have been discussed in previous studies. The minor Zn addition was found to be effective in reducing the amount of undercooling required for tin solidification and thereby to suppress the formation of large Ag₃Sn plates. The Zn addition also caused the changes in the bulk microstructure as well as the interfacial reaction. In this paper, an in-depth characterization of the interfacial reaction of Zn-added Sn-Ag-Cu solders on Cu and Au/Ni(P) surface finishes is reported. The effects of a Zn addition on modification of the interfacial IMCs and their growth kinetics are also discussed.     

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.     

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).     

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.     

Intermetallic Compound Formation and Evolution in Solid-State Sn/Immersion-Ag/Cu Trilayer Interfacial Reactions on a Flexible Polymer Board

Immersion Ag is a promising candidate Pb-free surface finish on printed circuit boards (PCBs). For flexible PCB and optoelectronic packaging, solid-state bonding rather than reflow is commonly used to join the chips to the PCB with Sn-based solders, after which the immersion Ag layer remains at the joint interface and participates in the interfacial reactions at the solder joints. Solder joint samples composed of a Sn/Ag/Cu trilayer on flexible PCBs were prepared to study the interfacial reactions at 150°C and 200°C. Three phases, Ag₃Sn, Cu₆Sn₅, and Cu₃Sn, were sequentially formed at the interface. Remarkable change of the morphology of the Ag₃Sn phase was observed during thermal aging. The thickness of the immersion Ag layer was found to have significant effects on the growth rates of the Cu₆Sn₅ and Cu₃Sn phases and the void formation in the Cu₃Sn phase.     

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.     

Generation of Tin(II) Oxide Crystals on Lead-Free Solder Joints in Deionized Water

The effect of the anode and cathode on the electrochemical corrosion behavior of lead-free Sn-Ag-Cu and Sn-Ag-Cu-Bi solder joints in deionized water was investigated. Corrosion studies indicate that SnO crystals were generated on the surfaces of all lead-free solder joints. The constituents of the lead-free solder alloys, such as Ag, Cu, and Bi, did not affect the corrosion reaction significantly. In contrast to lead-free solders, PbO _x was formed on the surface of the traditional 63Sn-37Pb solder joint in deionized water. A cathode, such as Au or Cu, was necessary for the electrochemical corrosion reaction of solders to occur. The corrosion reaction rate decreased with reduction of the cathode area. The formation mechanism of SnO crystals was essentially a galvanic cell reaction. The anodic reaction of Sn in the lead-free solder joints occurred through solvation by water molecules to form hydrated cations. In the cathodic reaction, oxygen dissolved in the deionized water captures electrons and is deoxidized to hydroxyl at the Au or Cu cathode. By diffusion, the anodic reaction product Sn²⁺ and the cathodic reaction product OH⁻ meet to form Sn(OH)₂, some of which can dehydrate to form more stable SnO·xH₂O crystals on the surface of the solder joints. In addition, thermodynamic analysis confirms that the Sn corrosion reaction could occur spontaneously.     

Effect of small Sn–3.5Ag–0.5Cu additions on the structure and properties of Sn–9Zn solder in ball grid array packages

Sn–9Zn with various additions of Sn–3.5Ag–0.5Cu powder was prepared by mechanically dispersing different weight percentages (1, 3, 5 and 7) of Sn–Ag–Cu powder into Sn–9Zn solder paste. In the Sn–Zn solder, scallop-shaped AuZn₃ intermetallic compound was found at the interfaces. On the other hand, in the Sn–3.5Ag–0.5Cu content solders, an additional ε-AgZn₃ intermetallic compound layer was found to be well adhered on the top surface of the AuZn₃ layer and the ε-AgZn₃ layer thickness increased with the number of reflow cycles. In addition, fine spherical-shaped ε-AgZn₃ intermetallic compound particles as well as an acicular-shaped Zn-rich phase was clearly observed in the β-Sn matrix. On increasing the Sn–Ag–Cu content, the shear load was increased from 1.80 to 2.03 kg after one reflow cycle. In the Sn–3.5Ag–0.5Cu content solders, the fracture surfaces exhibited typical ductile behavior with very rough dimpled surfaces while the fracture surface in the Sn–Zn solder gave fractures with a brittle appearance. In the fracture surface of the Sn–3.5Ag–0.5Cu content solders, some dimples were clearly observed associated with the formation of spherical-shaped ε-AgZn₃ intermetallic compound particles.     

Wetting reaction of Sn-Ag based solder systems on Cu substrates plated with Au and/or Pd layer

The wetting behavior of SnAg based Pb-free solders on Cu and Cu substrates plated with Au, Pd, and Au/Pd thin films have been studied. The wetting angle and kinetics of interfacial reaction were measured. The Au-plated substrates exhibit better wetting than the Pd-plated substrates. In the case of SnAg on Pd-plated Cu, SEM observation revealed that the solder cap was surrounded by an innerring of Cu−Sn compound and an outer ring of Pd−Sn compound. This implies that the molten SnAg solder had removed the Pd and wetted the Cu directly in the equilibrium state. The effects of pre-doping Cu in the SnAg solder on wetting behavior were also investigated. We found that wettability decreases with increasing Cu content in the solder. We also observed that the SnAgCu solders have a lower Cu consumption rate than the SnAg solder.     

Processing and aging characteristics of eutectic Sn-3.5Ag solder reinforced with mechanically incorporated Ni particles

Composite solders offer improved properties compared to non-composite solders. Ni reinforced composite solder was prepared by mechanically dispersing 15 vol.% of Ni particles into eutectic Sn-3.5Ag solder paste. The average size of the Ni particle reinforcements was approximately 5 microns. The morphology, size and distribution of the reinforcing phase were characterized metallographically. Solid-state isothermal aging study was performed on small realistic size solder joints to study the formation and growth of the intermetallic (IM) layers at Ni reinforcement/solder and Cu substrate/solder interfaces. Effects of reflow on microstructure and solderability, were studied using Cu substrates. Regarding solderability, the wetting angle of multiple reflowed Ni reinforced composite solder was compared to the solder matrix alloy, eutectic Sn-3.5Ag. General findings of this study revealed that Ni particle reinforced composite solder has comparable wetting characteristics to eutectic Sn-3.5Ag solder. Significant IM layers growth was observed in the Ni composite solder joint under isothermal aging at 150 C. Microstructural evolution was insignificant when aging temperature was lower than 100 C. Multiple reflow did not significantly change the microstructure in Ni composite solder joint.     

Formation of bulk Ag<Subscript>3</Subscript>Sn intermetallic compounds in Sn-Ag lead-free solders in solidification

A series of Sn-Ag solders were prepared by arc melting and their phase evolution was investigated as a function of cooling rates. It was found that bulk Ag₃Sn intermetallic compounds (IMCs) separated out only in the slowly cooled Sn-4.0Ag solder. This would be explained by the strong kinetic undercooling, arising from the rapid cooling conditions, which leads to the actual eutectic point shifts in the direction of higher Ag concentration. Thus, the eutectic and hypereutectic alloys experience a metastable hypoeutectic solidification route instead. All formed fractions of bulk Ag₃Sn IMCs in solders, measured by quantitative microstructural analysis and thermal analysis, are larger than those predicted by the equilibrium phase diagram. The reasoning for this could be attributed to fine Ag₃Sn phases, which cling to the primary Ag₃Sn crystal during the eutectic reaction for their matching crystalline orientation relationship. Furthermore, the fraction of bulk Ag₃Sn IMCs increases gradually with increasing the cooling rate in the slowly cooled Sn-4.0Ag alloy, which fits with the prediction of eutectic solidification theory: the increase of cooling rate would decrease the surface energy of fine Ag₃Sn particles and primary Ag₃Sn crystal, and make fine Ag₃Sn particles cling to primary Ag₃Sn crystal easily to form bulk Ag₃Sn IMCs.     

A Study on the Physical Properties and Interfacial Reactions with Cu Substrate of Rapidly Solidified Sn-3.5Ag Lead-Free Solder

A rapidly solidified Sn-3.5Ag eutectic alloy produced by the melt-spinning technique was used as a sample in this research to investigate the microstructure, thermal properties, solder wettability, and inhibitory effect of Ag₃Sn on Cu₆Sn₅ intermetallic compound (IMC). In addition, an as-cast Sn-3.5Ag solder was prepared as a reference. Rapidly solidified and as-cast Sn-3.5Ag alloys of the same size were soldered at 250°C for 1 s to observe their instant melting characteristics and for 3 s with different cooling methods to study the inhibitory effect of Ag₃Sn on Cu₆Sn₅ IMC. Experimental techniques such as scanning electron microscopy, differential scanning calorimetry, and energy-dispersive spectrometry were used to observe and analyze the results of the study. It was found that rapidly solidified Sn-3.5Ag solder has more uniform microstructure, better wettability, and higher melting rate as compared with the as-cast material; Ag₃Sn nanoparticles that formed in the rapidly solidified Sn-3.5Ag solder inhibited the growth of Cu₆Sn₅ IMC during aging significantly much strongly than in the as-cast material because their number in the rapidly solidified Sn-3.5Ag solder was greater than in the as-cast material with the same soldering process before aging. Among the various alternative lead-free solders, this study focused on comparison between rapidly solidified and as-cast solder alloys, with the former being observed to have better properties.     

Intermetallic reactions in Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Au/Ni surface finishes

The intermetallic compounds (IMCs) formed during the reflow and aging of Sn3Ag0.5Cu and Sn3Ag0.5Cu0.06Ni0.01Ge solder BGA packages with Au/Ni surface finishes were investigated. After reflow, the thickness of (Cu, Ni, Au)₆Sn₅ interfacial IMCs in Sn3Ag0.5Cu0.06Ni0.01Ge was similar to that in the Sn3Ag0.5Cu specimen. The interiors of the solder balls in both packages contained Ag₃Sn precipitates and brick-shaped AuSn₄ IMCs. After aging at 150°C, the growth thickness of the interfacial (Ni, Cu, Au)₃Sn₄ intermetallic layers and the consumption of the Ni surface-finished layer on Cu the pads in Sn3Ag0.5Cu0.06Ni0.01Ge solder joints were both slightly less than those in Sn3Ag0.5Cu. In addition, a coarsening phenomenon for AuSn₄ IMCs could be observed in the solder matrix of Sn3Ag0.5Cu, yet this phenomenon did not occur in the case of Sn3Ag0.5Cu0.06Ni0.01Ge. Ball shear tests revealed that the reflowed Sn3Ag0.5Cu0.06Ni0.01Ge packages possessed bonding strengths similar to those of the Sn3Ag0.5Cu. However, aging treatment caused the ball shear strength in the Sn3Ag0.5Cu packages to degrade more than that in the Sn3Ag0.5Cu0.06Ni0.01Ge packages.     

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

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