The Al/Ni interfacial reactions under the influence of electric current

Electromigration has been observed and quite extensively investigated in the compositionally homogeneous conducting lines in the integrated circuit devices; however, the effect of electric current upon the interfacial reactions has not been discussed. This study investigated the effect of electric current upon the chemically driven interfacial reactions in the Al/Ni system. Al/Ni reaction couples annealed at 400°C with and without the passing of electric current were examined. Two intensities of electric currents, 5×10² A/cm² and 10³ A/cm², were used in this study. Same intermetallics, Al₃Ni and Al₃Ni₂, were formed at the interfaces; however, the thickness of the reaction layer in the reaction couples with the passing of electric current was much larger than those without electric current. This novel phenomenon has never been reported in the literature, and the understanding of its mechanism needs further investigation.     

Interfacial reactions between Ni substrate and the component Bi in solders

The reactions between Ni and liquid Bi at 300, 360, 420, and 480°C were studied. Bismuth is an important element in many electronic solders, while Ni is used in many printed circuit board surface finishes. It was found that the only intermetallic compound formed was NiBi₃. The other intermetallic compound NiBi, which is therm odynamically stable at these temperatures, did not form. Reaction at 300°C produced a thick reaction zone, which is a two-phase mixture of NiBi₃ needles dispersed in Bi matrix. The thickness of the reaction zone increased rapidly with reaction time, reaching 400 μm after 360 min. Reactions at 360 and 420°C produced very thin reaction zones, and the major interaction was the dissolution of Ni into liquid Bi. Reaction at 480°C produced extremely thin reaction zone, and the dissolution of Ni into liquid Bi was very fast and was the major interaction. It is proposed that the formation of the reaction zone is controlled by two factors: the solubility limit and the diffusivity of Ni in liquid Bi. Small diffusivity and small solubility limit, i.e., lower temperature, tend to favor the formation of a thick reaction zone. In addition to the NiBi₃ formed within the reaction zone, NiBi₃ also formed outside the reaction zone in the form of long needles with hexagonal cross section. The dissolution rate of Ni into Bi is comparable to that of Ni into Sn at the same temperature, and is much slower than the dissolution rates for Au, Ag, Cu, and Pd into Sn.     

Reactive wetting between molten Sn-Bi and Ni substrate

Wetting and interfacial reactions occurred when molten Sn-Bi alloys were in contact with a Ni substrate. Wetting properties of Sn-Bi alloys on the Ni substrate and their interfacial reactions were examined, and the effects of interfacial reactions on wetting properties were discussed. Couples made of various molten Sn-Bi alloys and Ni substrates were reacted at 300°C. It was found that, when the Bi content was greater than 98 wt.%, the intermetallic phase formed at the interface was NiBi₃ phase. The Ni₃Sn₄ phase was found in the other Sn-Bi/Ni couples that had Bi contents varied from 92 wt.% to 97.5 wt.%. An isothermal section at 300°C for the ternary Sn-Bi-Ni system was proposed to illustrate the reaction paths of Sn-Bi/Ni couples. Wetting properties, including wetting time and wetting force of molten Sn-Bi alloys, were determined by using a wetting balance. Surface tensions at 300°C were calculated based on the experimental measurements. For molten Sn-Bi alloys with Bi contents varying from 92 wt.% to 99 wt.%, their surface tensions were all about 0.34 N/m. On the basis of theoretical analysis and experimental observations, the surface tensions of molten metals, determined by using the wetting balance, are not significantly affected if the interfacial reaction is not excessive. However, the wetting time determined by using the wetting balance was altered by the reaction that occurred and the compound formed at the interface.     

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.     

Mechanical properties and intermetallic compound formation at the Sn/Ni and Sn-0.7wt.%Cu/Ni joints

The Ni/Sn/Ni and Ni/Sn-0.7wt.%Cu/Ni couples are reacted at 200°C for various lengths of time. The tensile strengths of these annealed specimens are determined at room temperature. In addition, the interfacial reactions and fracture surfaces of the specimens are examined as well. These properties are important for the evaluation of the usage of Sn-0.7wt.%Cu lead-free solders, which has been not available in the literature. Only the Ni₃Sn₄ phase is formed at the Sn/Ni interface, but both the Cu₆Sn₅ and Ni₃Sn₄ phases are formed at the Sn-0.7wt.%Cu/Ni interface. The thickness of the intermetallic compound layers grows, while the joint strength decreases with longer reaction time. With a 1-h reaction at 200°C, the fracture surface is in the solder matrix for both of the two kinds of couples. Shifting toward the compound layer with longer reaction time, the fracture surface is in the Ni₃Sn₄ layer in the Sn/Ni couple and is at the interface between the Cu₆Sn₅ and Ni₃Sn₄ in the Sn-0.7wt.%Cu/Ni after reacting at 200°C for 240 h.     

Interfacial reactions in In-Sn/Ni couples and phase equilibria of the In-Sn-Ni system

The In-Sn-Ni alloys of various compositions were prepared and annealed at 160°C and 240°C. No ternary compounds were found; however, most of the binary compounds had extensive ternary solubility. There was a continuous solid solution between the Ni₃Sn phase and Ni₃In phase. The Sn-In/Ni couples, made of Sn-In alloys with various compositions, were reacted at 160°C and 240°C and formed only one compound for all the Sn-In alloys/Ni couples reacted up to 8 h. At 240°C, Ni₂₈In₇₂ phase formed in the couples made with pure indium, In-10at.%Sn and In-11at.%Sn alloys, while Ni₃Sn₄ phase formed in the couples made of alloys with compositions varied from pure Sn to In-12at.%Sn. At 160°C, except in the In/Ni couple, Ni₃Sn₄ formed by interfacial reaction.     

Peltier Effect on Sn/Co Interfacial Reactions

It is observed that the interfacial reactions in Sn/Co couples are different at the anode and cathode sides as a result of temperature differences caused by the Peltier effect. The Sn/Co interfacial reactions were examined at 180°C with the passage of an electric current of 5000 A/cm². The reaction phase was CoSn₃. The reaction layer at the Co/Sn anode interface in which the electrons moved from Co to Sn was thicker than that at the Sn/Co cathode interface, but this phenomenon could not be reasonably accounted for using the electromigration effect. The temperature of Sn at the Co/Sn anode interface was 4.5°C higher than that at the Sn/Co cathode interface with the passage of 5000 A/cm² electric current at 180°C. Temperature differences were determined with a carefully designed cathode–anode switching experiment using thermocouples, and the results were confirmed with thermal infrared microscope measurements and calculated results based on heat transfer models.     

Temperature-Dependent Phase Segregation in Cu/42Sn-58Bi/Cu Reaction Couples under High Current Density

The effects of current stressing at 10⁴ A/cm² on Cu/42Sn-58Bi/Cu reaction couples with a one-dimensional structure at 23°C, 50°C, and 114°C were investigated. The microstructural evolution during electromigration was examined using scanning electron microscopy. The temperature dependence of the coarsening of the Bi-rich phase, the dominant migrating entity, and hillock/whisker formation in eutectic Sn-Bi were investigated under high current density. During current stressing at 10⁴ A/cm², the average size of the Bi-rich phase remained the same at 23°C, increased at 50°C, and shrank at 140°C. Bi accumulated near the anode side at both high (50°C, 140°C) and low temperature (23°C). At high temperatures, both Sn and Bi diffused towards the anode side, but Bi moved ahead of Sn during current stressing. However, at low temperatures, Sn reversed its direction of migration to the cathode side. Pure Bi hillocks/whiskers and a mixed structure of Sn and Bi hillocks were extruded as a consequence of compressive stress from electromigration- induced mass flow towards the anode side.     

Intermetallic reactions in reflowed and aged Sn-9Zn solder ball grid array packages with Au/Ni/Cu and Ag/Cu pads

During the reflowing of Sn-9Zn solder ball grid array (BGA) packages with Au/Ni/Cu and Ag/Cu pads, the surface-finished Au and Ag film dissolved rapidly and reacted with the Sn-9Zn solder to form a γ₃-AuZn₄/γ-Au₇Zn₁₈ intermetallic double layer and ε-AgZn₆ intermetallic scallops, respectively. The growth of γ₃-AuZn₄ is prompted by further aging at 100°C through the reaction of γ-Au₇Zn₁₈ with the Zn atoms dissolved from the Zn-rich precipitates embedded in the β-Sn matrix of Sn-9Zn solder BGA with Au/Ni/Cu pads. No intermetallic compounds can be observed at the solder/pad interface of the Sn-9Zn BGA specimens aged at 100°C. However, after aging at 150°C, a Ni₄Zn₂₁ intermetallic layer is formed at the interface between Sn-9Zn solder and Ni/Cu pads. Aging the immersion Ag packages at 100°C and 150°C caused a γ-Cu₅Zn₈ intermetallic layer to appear between ε-AgZn₆ intermetallics and the Cu pad. The scallop-shaped ε-AgZn₆ intermetallics were found to detach from the γ-Cu₅Zn₈ layer and float into the solder ball. Accompanied with the intermetallic reactions during the aging process of reflowed Sn-9Zn solder BGA packages with Au/Ni/Cu and Ag/Cu pads, their ball shear strengths degrade from 8.6 N and 4.8 N to about 7.2 N and 2.9 N, respectively.     

Electric current effects on Sn/Ag interfacial reactions

The effect of electric current on the Sn/Ag interfacial reaction was studied at 140°C and 200°C, by examining the growth of phase (ε-Ag₃Sn) in the Sn/Ag reaction couples with a constant current density. Only at 140°C was the growth of phase affected by the passage of electric current. The growth rate was enhanced when diffusion of Sn and electron flow were in the same direction, and retarded when they were in the opposite direction. It was found that the diffusion coefficient of Sn through Ag₃Sn was 3.37 μm²/h and the apparent effective charge for Sn in Ag₃Sn was −90, at 140°C.     

Long-term aging study on the solid-state reaction between 58Bi42Sn solder and Ni substrate

The reaction between Ni and eutectic BiSn solder at 85°C, 100°C, 120°C, and 135°C was studied. Reaction times ranging from 25 h to 3600 h were used. Only Ni₃Sn₄ was detected as a result of the reaction. None of the other Ni-Sn intermetallic compounds and none of the Ni-Bi intermetallic compounds were observed. The growth of Ni₃Sn₄ followed diffusion-controlled kinetics and was very slow, with the layer thickness reaching only 16 μm after 3600 h of aging at 135°C. The eutectic BiSn microstructure coarsened very quickly. Substantial coarsening can be observed at 135°C for only 200 h of aging. In addition, fine Bi-rich particles within the Sn-rich phase of the solder were found. The amount of these fine Bi-rich particles increased with the aging temperature. It is believed that the formation of these fine Bi-rich particles is due to the fact that the Sn-rich phase can dissolve substantial amounts of Bi. It was also found that, as aging time increased, the region immediately adjacent to the Ni₃Sn₄ layer was preferentially occupied by the Bi-rich phase. This is because Sn in that region had reacted with Ni to form Ni₃Sn₄, leaving a nearly continuous Bi-rich phase above the Ni₃Sn₄. Since Bi-rich alloys tend to be brittle, a nearly continuous Bi-rich phase might weaken the strength of a solder joint. The Ni₃Sn₄ grain size increased gradually from the Ni/Ni₃Sn₄ interface to the Ni₃Sn₄/BiSn interface, which is probably an Ostwald ripening phenomenon.     

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.     

Investigation of interfacial reaction between Sn-Ag eutectic solder and Au/Ni/Cu/Ti thin film metallization

This paper reports the formation of intermetallic compounds in Au/Ni/Cu/Ti under-bump-metallization (UBM) structure reacted with Ag-Sn eutectic solder. In this study, UBM is prepared by evaporating Au(500 Å)/Ni(1000 Å)/Cu(7500 Å) /Ti (700 Å) thin films on top of Si substrates. It is then reacted with Ag-Sn eutectic solder at 260 C for various times to induce different stages of the interfacial reaction. Microstructural examination of the interface, using both chemical and crystallographic analysis, indicates that two types of intermetallic compounds are formed during the interfacial reaction. The first phase, formed at the intial stage of the reaction, is predominantly Ni₃Sn₄. At longer times the Ni₃Sn₄ phase transforms into (Cu, Ni)₆Sn₆, probably induced by interdiffusion of Cu and Ni. At this stage, the underlying Cu layer also reacts with Sn and forms the same phase, (Cu,Ni)₆Sn₅. As a result, the fully reacted interface is found to consist of two intermetallic layers with the same phase but different morphologies.     

Interfacial Reactions in Sn-Ag/Co Couples

Sn-Ag alloys are important solders, and Co and Co alloys are investigated as barrier layers. Interfacial reactions in Sn-Ag/Co couples were examined in this study for Ag contents of 1.0 wt.%, 2.0 wt.%, and 3.5 wt.% and reaction temperatures of 250°C, 200°C, and 150°C. Only CoSn₃ formed in Sn-Ag/Co couples reacted at 250°C, but both CoSn₃ and Ag₃Sn formed in couples reacted at 200°C and 150°C. The reaction layer was 100 μm thick in Sn-3.5 wt.%Ag/Co couples reacted at 200°C for 110 h. The reaction rates were lower if Ag was added, but remained very fast compared with those for Ni and Ni-based substrates.     

Au/Ge/Ni ohmic contacts to n-Type InP

The relationship between the electrical properties and microstructure for annealed Au/Ge/Ni contacts to n-type InP, with an initial doping level of 10¹⁷ cm^-3, have been studied. Metal layers were deposited by electron beam evaporation in the following sequence: 25 nm Ni, 50 nm Ge, and 40 nm Au. Annealing was done in a nitrogen atmosphere at 250-400‡C. The onset of ohmic behavior at 325‡C corresponded to the decomposition of a ternary Ni-In-P phase at the InP surface and the subsequent formation of Ni₂P plus Au₁₀In₃, producing a lower barrier height at the InP interface. This reaction was driven by the inward diffusion of Au and outward diffusion of In. Further annealing, up to 400‡C, resulted in a decrease in contact resistance, which corresponded to the formation of NiP and Au₉ln₄ from Ni₂P and Au₁₀In₃,respectively, with some Ge doping of InP also likely. A minimum contact resistance of 10^-7 Ω-cm² was achieved with a 10 s anneal at 400‡C.     

Electromigration-Induced Failure of Ni/Cu Bilayer Bond Pads Joined with Sn(Cu) Solders

The effect of electromigration (EM) on Sn(Cu)/Ni/Cu solder joint interfaces under current stressing of 10⁴ A/cm² at 160°C was studied. In the pure Sn/Ni/Cu case, the interfacial compound layer was mainly the Cu₆Sn₅ compound phase, which suffered serious EM-induced dissolution, eventually resulting in serious Cu-pad consumption. In the Sn-0.7Cu case, a (Cu,Ni)₆Sn₅ interfacial compound layer formed at the joint interface, which showed a strong resistance to EM-induced dissolution. Thus, there was no serious consumption of the Cu pad under current stressing. In the Sn-3.0Cu case, we believe that the␣massive Cu₆Sn₅ phase in the solder matrix eased possible EM-induced dissolution at the interfacial compound layer due to current stressing.     

Effect of Cross-Interaction between Ni and Cu on Growth Kinetics of Intermetallic Compounds in Ni/Sn/Cu Diffusion Couples during Aging

The solid-state, cross-interaction between the Ni layer on the component side and the Cu pad on the printed circuit board (PCB) side in ball grid array (BGA) solder joints was investigated by employing Ni(15 μm)/Sn(65 μm)/Cu ternary diffusion couples. The ternary diffusion couples were prepared by sequentially electroplating Sn and Ni on a Cu foil and were aged isothermally at 150, 180, and 200°C. The growth of the intermetallic compound (IMC) layer on the Ni side was coupled with that on the Cu side by the mass flux across the Sn layer that was caused by the difference in the Ni content between the (Cu_1−x Ni _x )₆Sn₅ layer on the Ni side and the (Cu_1−y Ni _y )₆Sn₅ layer on the Cu side. As the consequence of the coupling, the growth rate of the (Cu_1−x Ni _x )₆ Sn₅ layer on the Ni side was rapidly accelerated by decreasing Sn layer thickness and increasing aging temperature. Owing to the cross-interaction with the top Ni layer, the growth rate of the (Cu_1−y Ni _y )₆Sn₅ layer on the Cu side was accelerated at 150°C and 180°C but was retarded at 200°C, while the growth rate of the Cu₃Sn layer was always retarded. The growth kinetic model proposed in an attempt to interpret the experimental results was able to reproduce qualitatively all of the important experimental observations pertaining to the growth of the IMC layers in the Ni/Sn/Cu diffusion couple.     

Co Solubility in Sn and Interfacial Reactions in Sn-Co/Ni Couples

It has been reported that minute Co additions to Sn-based solders are very effective for reducing undercooling, probably due to low Co solubility in Sn. In this study, Co solubility in molten Sn was determined experimentally. According to results of metallographic analysis, Co solubility in molten Sn is as low as 0.04 wt.% at 250°C. Interfacial reactions in Sn-Co/Ni couples at 250°C were examined for Co contents from 0.01 wt.% to 0.4 wt.%. The Ni₃Sn₄ phase was the only interfacial reaction phase in almost the entire Sn-0.01 wt.%Co/Ni couple. For Sn-Co/Ni couples with a Co content higher than 0.01 wt.%, a thin, continuous Ni₃Sn₄ layer and a discontinuous decahedron (Ni,Co)Sn₄ phase were formed in the initial stage of reaction. The reaction products evolved with time. With longer reaction time, the Sn content in the decahedron (Ni,Co)Sn₄ phase decreased, and the (Ni,Co)Sn₄ phase transformed into the (Ni,Co)Sn₂ phase and cleaved into a sheet, which then detached from the interface, after which Ni₃Sn₄ began to grow significantly with longer reaction times.     

The Sn/Ni-7Wt.%V interfacial reactions

Ni-7wt.%V(8at.%V) is an important under bump metallization material, and Sn is the primary element in most solders. This study examines the Sn/Ni-8at.%V interfacial reactions at 160°C, 200°C, 250°C, 300°C, 325°C, 350°C, and 400°C. Unlike the interfacial reactions in the Sn/Ni couples, a ternary T phase and the binary Ni₃Sn₄ phase are formed at 160°C. The vanadium solubility in the Ni₃Sn₄ phase is only 0.2 at.%, while the T phase contains 13.9at.%V. Similar results are found in the couples at 200°C, and the reaction paths are Sn/Ni₃Sn₄/T/Ni-V. The reaction paths are liquid/T/Ni₃Sn₄/Ni-V at 250°C and 300°C and are liquid/Ni₃Sn₄/Ni-V at 350°C and 400°C. Because the reaction products and the reaction rates in the Sn/Ni-8at.%V and Sn/Ni couples are different, reliabilities of the electronic products with the Ni-8at.%V barrier layer should not be assessed based only on the results of the Sn/Ni couples.     

A comparative study of the kinetics of interfacial reaction between eutectic solders and Cu/Ni/Pd metallization

A comparative study of the kinetics of interfacial reaction between the eutectic solders (Sn-3.5Ag, Sn-57Bi, and Sn-38Pb) and electroplated Ni/Pd on Cu substrate (Cu/Ni/NiPd/Ni/Pd) was performed. The interfacial microstructure was characterized by imaging and energy dispersive x-ray analysis in scanning electron microscope (SEM). For a Pd-layer thickness of less than 75 nm, the presence or the absence of Pd-bearing intermetallic was found to be dependent on the reaction temperature. In the case of Sn-3.5Ag solder, we did not observe any Pd-bearing intermetallic after reaction even at 230°C. In the case of Sn-57Bi solder the PdSn₄ intermetallic was observed after reaction at 150°C and 180°C, while in the case of Sn-38Pb solder the PdSn₄ intermetallic was observed after reaction only at 200°C. The PdSn₄ grains were always dispersed in the bulk solder within about 10 μm from the solder/substrate interface. At higher reaction temperatures, there was no Pd-bearing intermetallic due to increased solubility in the liquid solder. The presence or absence of Pd-bearing intermetallic was correlated with the diffusion path in the calculated Pd-Sn-X (X=Ag, Bi, Pb) isothermal sections. In the presence of unconsumed Ni, only Ni₃Sn₄ intermetallic was observed at the solder-substrate interface by SEM. The presence of Ni₃Sn₄ intermetallic was consistent with the expected diffusion path based on the calculated Ni-Sn-X (X=Ag, Bi, Pb) isothermal sections. Selective etching of solders revealed that Ni₃Sn₄ had a faceted scallop morphology. Both the radial growth and the thickening kinetics of Ni₃Sn₄ intermetallic were studied. In the thickness regime of 0.14 μm to 1.2 μm, the growth kinetics always yielded a time exponent n >3 for liquid-state reaction. The temporal law for coarsening also yielded time exponent m >3. The apparent activation energies for thickening were: 16936J/mol for the Sn-3.5Ag solder, 17804 J/mol for the Sn-57Bi solder, and 25749 J/mol for the Sn-38Pb solder during liquid-state reaction. The corresponding activation energies for coarsening were very similar. However, an apparent activation energy of 37599 J/mol was obtained for the growth of Ni₃Sn₄ intermetallic layer during solid-state aging of the Sn-57Bi/substrate diffusion couples. The kinetic parameters associated with thickening and radial growth were discussed in terms of current theories.