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.     

Intermetallic compound layer formation between copper and hot-dipped 100In, 50In-50Sn, 100Sn,and 63Sn-37Pb coatings

The growth kinetics of intermetallic compound layers formed between four hot-dipped solder coatings and copper by solid state, thermal aging were examined. The solders were l00Sn, 50In-50Sn, 100In, and 63Sn-37Pb (wt.%); the substrate material was oxygen-free, high conductivity Cu. The total intermetallic layer of the 100Sn/Cu system exhibited a combination of parabolic growth at lower aging temperatures and t^0.42 growth at the higher temperatures. The combined apparent activation energy was 66 kJ/mol. These results are compared to the total layer growth observed with the 63Sn-37Pb/Cu system which showed parabolic kinetics at similar temperatures and an apparent activation energy of 45 kJ/mol. Both 100Sn and 63Sn-37Pb diffusion couples showed a composite intermetallic layer comprised of Cu₃Sn and Cu₆Sn₅. The intermetallic compound layer formed between In and Cu changed from a CuIn₂ stoichiometry at short annealing times to a Cu₅₇In₄₃ composition at longer periods. The growth kinetics were parabolic with an apparent activation energy of 20 kJ/mol. The intermetallic layer growth of the 50In-50Sn/Cu system exhibited extreme variations in the layer thicknesses which prohibited a quantitative assessment of the growth kinetics. The layer was comprised of two compounds: Cu₂₆Sn₁₃In₈ which was the dominant phase and a thin layer of Cu₁₇Sn₉In24 adjacent to the 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.     

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.     

Thin-film reactions during diffusion soldering of Cu/Ti/Si and Au/Cu/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> with Sn interlayers

The multilayer thin-film systems of Cu/Ti/Si and Au/Cu/Al₂O₃ were diffusion-soldered at temperatures between 250°C and 400°C by inserting a Sn thin-film interlayer. Experimental results showed that a double layer of intermetallic compounds (IMCs) η-(Cu_0.99Au_0.01)₆Sn₅/δ-(Au_0.87Cu_0.13)Sn was formed at the interface. Kinetics analyses revealed that the growth of intermetallics was diffusion-controlled. The activation energies as calculated from Arrhenius plots of the growth rate constants for (Cu_0.99Au_0.01)₆Sn₅ and (Au_0.87Cu_0.13)Sn are 16.9 kJ/mol and 53.7 kJ/mol, respectively. Finally, a satisfactory tensile strength of 132 kg/cm² could be attained under the bonding condition of 300°C for 20 min.     

Multiphase Field Simulations of Intermetallic Compound Growth During Lead-Free Soldering

A multiphase field simulation of the morphological evolution of the intermetallic compounds (IMCs) formed during the reaction between liquid Sn-based solder and a copper substrate is presented. The Cu-substrate, Cu₃Sn (ε phase), and Cu₆Sn₅ (η phase) layers (or grains), as well as the Sn-liquid phase are considered. Interfaces are defined by the presence of more than one phase at a given point of the computational domain. The interface structure is determined by assuming that all components of coexisting phases within the interfacial region have equal chemical potentials, which are in turn calculated from CALPHAD thermodynamic databases. Fast grain boundary (GB) diffusion in the η IMC layer, η grain coarsening and growth with growth/shrinkage of ε grains, and dissolution of Cu from the substrate will be discussed and compared with previous works. The simulation will address the kinetics of the IMC growth during soldering and the influence of fast GB diffusion on the concurrent coarsening of Cu₃Sn and Cu₆Sn₅ grains. The kinetic behavior of the substrate/IMC/solder system as a function of model parameters will also be presented.     

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.     

Textured growth of Cu/Sn intermetallic compounds

The growth of Cu-Sn intermetallic compounds (IMCs) at the molten Pb-Sn solder/Cu interface was studied over a range of temperatures and for a range of solder compositions. Strong peaks of and planes of η-phase (Cu₆Sn₅) were detected by x-ray diffraction when the Sn content was high. In the low Sn solder (27Sn-73Pb), the η-phase peaks were absent at the two high temperatures, but the (2 12 0) peak of the ε-phase (Cu₃Sn) was prominent. A texture was detected in both layers in and (002) pole figures constructed for the η phase and ε phase, respectively. The growth directions were identified to be 〈101〉 and 〈102〉 for the η phase and 〈102〉 and 〈031〉 for the ε phase, normal to the Cu surface. The growth direction does not change with the morphology and the thickness of the IMC layer. The morphology of the η layer varied gradually from a cellular film with a rugged interface to a dense film with a scalloped interface as the Pb content, temperature, and reaction time increased. The ε layer was always dense and nearly planar.     

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.     

Thickening kinetics of interfacial Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> and Cu<Subscript>3</Subscript>Sn layers during reaction of liquid tin with solid copper

Thickening behavior of interfacial η (Cu₆Sn₅) phase and ɛ (Cu₃Sn) phase intermetallic layers was investigated in liquid tin/solid copper reaction couples over reaction times from 30 sec to over 4,000 min and temperatures from 250°C to 325°C. A scanning electron microscope (SEM) was used to quantify the interfacial microstructure at each processing condition. The η developed with a scalloped morphology, while the ɛ always grew as a somewhat undulated planar layer in phase with the η. The thickness of each phase was quantitatively evaluated from SEM micrographs using imaging software. Thickening kinetics of the ɛ and η compounds were modeled using time- and temperature-dependent empirical power-law equations. From the model, values for the kinetic exponent, rate constant, and activation energy were established for each intermetallic layer. Measured values for the kinetic exponents and activation energies suggest that thickening of the η is controlled by a grain-boundary diffusion mechanism, and growth of the ɛ occurs by solid-state diffusion, probably grain-boundary diffusion.     

Intermetallic compounds formed during the interfacial reactions between liquid In-49Sn solder and Ni substrates

The interfacial reactions between liquid In-49Sn solder and Ni substrates at temperatures ranging from 150°C to 450°C for 15 min to 240 min have been investigated. The intermetallic compounds formed at the In-49Sn/Ni interfaces are identified to be a ternary Ni₃₃In₂₀Sn₄₇ phase using electron-probe microanalysis (EPMA) and x-ray diffraction (XRD) analyses. These interfacial intermetallics grow with increasing reaction time by a diffusion-controlled mechanism. The activation energy calculated from the Arrhenius plot of reaction constants is 56.57 kJ/mol.     

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.     

Reflow and burn-in of a Sn-20In-0.8Cu ball grid array package with a Au/Ni/Cu pad

The intermetallic compounds formed after reflow and burn-in testing of a Sn-20In-0.8Cu solder ball grid array (BGA) package are investigated. Along with the formation of the Cu₆(Sn_0.78In_0.22)₅ precipitates (IM1) in the solder matrix, scallop-shaped intermetallic compounds (IM2) with a compositional mixture of Cu₆(Sn_0.87In_0.13)₅ and Ni₃(Sn_0.87In_0.13)₄ appear at the interfaces between the solder balls and Au/Ni/Cu pads. A significant number of intermetallic particles (IM3), with a composition of (Au_0.80Cu_0.20)(In_0.33Sn_0.67)₂, can also be found in the solder matrix. After aging at 115°C for 750 h, an additional intermetallic compound layer (IM4) with a composition of (Ni_0.91Cu_0.09)₃(Sn_0.77In_0.23)₂ is formed at the interface between IM2 and the Ni layer. The ball shear strength of the Sn-20In-0.8Cu BGA solder after reflow is 4.5 N and will rise to maximum values after aging at 75°C and 115°C for 100 h. With a further increase of the aging time at both temperatures, the joint strengths exhibit a tendency to decline linearly at about 1.7×10⁻³ N/h.     

Solid-state intermetallic compound layer growth between copper and 95.5Sn-3.9Ag-0.6Cu solder

Long-term, solid-state intermetallic compound (IMC) layer growth was examined in 95.5Sn-3.9Ag-0.6Cu (wt.%)/copper (Cu) couples. Aging temperatures and times ranged from 70°C to 205°C and from 1 day to 400 days, respectively. The IMC layer thicknesses and compositions were compared to those investigated in 96.5Sn-3.5Ag/Cu, 95.5Sn-0.5Ag-4.0Cu/Cu, and 100Sn/Cu couples. The nominal Cu₃Sn and Cu₆Sn₅ stoichiometries were observed. The Cu₃Sn layer accounted for 0.4–0.6 of the total IMC layer thickness. The 95.5Sn-3.9Ag-0.6Cu/Cu couples exhibited porosity development at the Cu₃Sn/Cu interface and in the Cu₃Sn layer as well as localized “plumes” of accelerated Cu₃Sn growth into the Cu substrate when aged at 205°C and t>150 days. An excess of 3–5at.%Cu in the near-interface solder field likely contributed to IMC layer growth. The growth kinetics of the IMC layer in 95.5Sn-3.9Ag-0.6Cu/Cu couples were described by the equation x=x_o+Atⁿexp [−ΔH/RT]. The time exponents, n, were 0.56±0.06, 0.54±0.07, and 0.58±0.07 for the Cu₃Sn layer, the Cu₆Sn₅, and the total layer, respectively, indicating a diffusion-based mechanism. The apparent-activation energies (ΔH) were Cu₃Sn layer: 50±6 kJ/mol; Cu₆Sn₅ layer: 44±4 kJ/mol; and total layer: 50±4 kJ/mol, which suggested a fast-diffusion path along grain boundaries. The kinetics of Cu₃Sn growth were sensitive to the Pb-free solder composition while those of Cu₆Sn₅ layer growth were not so.     

Selective formation of intermetallic compounds in Sn-20In-0.8Cu ball grid array solder joints with Au/Ni surface finishes

After Sn-20In-0.8Cu solder balls are reflowed on a ball grid array (BGA) substrate (substrate A) with an Au/Ni surface finish, scallop-shaped intermetallic compounds with a composition of 0.83[Cu₆(Sn_0.87In_0.13)₅] + 0.17[Ni₃(Sn_0.87In_0.13)₄] are formed at the solder/pad interface. The distribution of the intermetallics is not altered by gravity or by multiple reflows of the solder joints. As another substrate (substrate B) is further attached onto the primary reflowed BGA assembly to form a sandwich structure subjected to subsequent multiple reflows, the Cu₆(Sn_0.87In_0.13)₅ interfacial intermetallic scallops remain still on the side of substrate A while many Au(In_0.91Sn_0.09)₂ intermetallics of cubic shape appear near the solder/Ni interface on the side of substrate B. When the Sn-20In-0.8Cu solder balls are assembled simultaneously in between two substrates (A and B), Au(In_0.91Sn_0.09)₂ intermetallic cubes of equal proportion are observed to form on both sides of the assembly. In summarizing the results, it is proposed that the diffusion of Cu atoms in the Sn-20In-0.8Cu solder toward the Ni layers after Au thin-film dissolution on Au/Ni surface finishes led to the formation of Cu₆(Sn_0.87Zn_0.17)₅ intermetallic compounds, which prevailed over the gravitational effect so that no intermetallic sedimentation in the liquid solder would occur. The appearance of Au(In_0.91Sn_0.09)₂ at the Ni/Sn-20In-0.8Cu interfaces was hindered by the preferential formation of Cu₆(Sn_0.87Zn_0.17)₅ until the Cu atoms in the Sn-20In-0.8Cu solder matrix were consumed to a lower content via the attachment of a second substrate to the assembly.     

Experimental determination and thermodynamic calculation of the phase equilibria in the Cu-In-Sn system

The phase equilibria of the Cu-In-Sn system were investigated by means of the diffusion couple method, differential scanning calorimetry (DSC) and metallography. The isothermal sections at 110–900 C, as well as vertical sections at 10wt.%Cu–70wt.%Cu were determined. It was found that there are large solubilities of In in the ε(Cu₃Sn), δ(Cu₄₁Sn₁₁), and η phases in the Cu-Sn system, and large solubilities of Sn in the γ, η, and δ(Cu₇In₃) phases in the Cu-In system. The η phase was found to continuously form from the Cu-In side to the Cu-Sn side, and a ternary compound (Cu₂In₃Sn) was found to exist at 110 C. Thermodynamic assessment of the Cu-In-Sn system was also carried out based on experimental data of activity and phase equilibria using the CALPHAD method, in which the Gibbs energies of the liquid, fcc and bcc phases are described by the subregular solution model and that of compounds, including two ternary compounds, are represented by the sublattice model. The thermodynamic parameters for describing the phase equilibria were optimized, and agreement between the calculated and experimental results was obtained.     

Influence of initial morphology and thickness of Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> and Cu<Subscript>3</Subscript>Sn intermetallics on growth and evolution during thermal aging of Sn-Ag solder/Cu joints

Intermetallic-layer formation and growth in Pb-free solder joints, during solder reflow or subsequent aging, has a significant effect on the thermal and mechanical behavior of solder joints. In this study, the influence of initial intermetallic morphology on growth rate, and kinetics were examined in a Sn-3.5Ag solder reflowed on Cu. The initial morphology of the intermetallic was tailered by cooling in water, air, or furnace conditions. Solder aging was conducted at 100°C, 140°C, and 175°C and aged for 0–1,000 h. Cooling rate, aging temperature, and aging time played an important role on microstructure evolution and growth kinetics of Cu₆Sn₅ (η) and Cu₃Sn (ɛ) intermetallic layers. Prior to aging, faster cooling rates resulted in a relatively planar Cu₆Sn₅ layer, while a nodular Cu₆Sn₅ morphology was present for slower cooling. Intermetallic-growth rate measurements after aging at various times, indicated a mixed growth mechanism of grain-boundary and bulk diffusion. These mechanisms are discussed in terms of the initial intermetallic thickness and morphology controlled by cooling rate, diffusion kinetics, and the competition between Cu₆Sn₅ and Cu₃Sn growth.     

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.     

Electric current effects upon the Sn/Cu and Sn/Ni interfacial reactions

Electromigration-induced failures in integrated circuits have been intensively studied recently; however, electromigration effects upon interfacial reactions have not been addressed. These electromigration effects in the Sn/Cu and Sn/Ni systems were investigated in this study by analyzing their reaction couples annealed at 200°C with and without the passage of electric current. The intermetallics formed were ε-(Cu₃Sn) and η-(Cu₆Sn₅) phases in the Sn/Cu couples and Ni₃Sn₄ phase in the Sn/Ni couples. The same intermetallics were formed in the two types of couples with and without the passage of electric current. The thickness of the reaction layers was about the same in the two types of couples of the Sn/Cu system. In the Sn/Ni system, the growth of the intermetallic compound was enhanced when the flow direction of electrons and that of diffusion of Sn were the same. But the effect became inhibiting if the directions of these two were opposite. Theoretical calculation indicated that in the Sn/Ni system, the electromigration effect was significant and was 28% of the chemical potential effect for the Sn element flux when the Ni₃Sn₄ layer was 10 μm thick. For the Sn and Cu fluxes in the Sn/Cu reaction couples, similar calculations showed that the electromigration effects were only 2 and 4% of the chemical potential effects, respectively. These calculated results were in good agreement with the experimental observations that in the Sn/Cu system the electric current effects were insignificant upon the interfacial reactions.