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.     

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.     

Cross-Interaction Study of Cu/Sn/Pd and Ni/Sn/Pd Sandwich Solder Joint Structures

Cross-interactions between Cu/Sn/Pd and Ni/Sn/Pd sandwich structures were investigated in this work. For the Cu/Sn/Pd case, the growth behavior and morphology of the interfacial (Pd,Cu)Sn₄ compound layer was very similar to that of the single Pd/Sn interfacial reaction. This indicates that the growth of the (Pd,Cu)Sn₄ layer at the Sn/Pd interface would not be affected by the opposite Cu/Sn interfacial reaction. We can conclude that there is no cross-interaction effect between the two interfacial reactions in the Cu/Sn/Pd sandwich structure. For the Ni/Sn/Pd case, we observed that: (1) after 300 s of reflow time, the (Pd,Ni)Sn₄ compound heterogeneously nucleated on the Ni₃Sn₄ compound layer at the Sn/Ni interface; (2) the growth of the interfacial PdSn₄ compound layer was greatly suppressed by the formation of the (Pd,Ni)Sn₄ compound at the Sn/Ni interface. We believe that this suppression of PdSn₄ growth is caused by heterogeneous nucleation of the (Pd,Ni)Sn₄ compound in the Ni₃Sn₄ compound layer, which decreases the free energy of the entire sandwich reaction system. The difference in the chemical potential of Pd in the PdSn₄ phase at the Pd/Sn interface and in the (Pd,Ni)Sn₄ phase at the Sn/Ni interface is the driving force for the Pd atomic flux across the molten Sn. The diffusion of Ni into the ternary (Pd,Ni)Sn₄ compound layer controls the Pd atomic flux across the molten Sn and the growth of the ternary (Pd,Ni)Sn₄ compound at the Sn/Ni interface.     

Microstructure and shear strength evolution of Sn-Ag-Cu solder bumps during aging at different temperatures

Flip-chip devices with Sn-3.8Ag-0.7Cu solder on electroless Ni (EN) without immersion Au were studied after aging at different temperatures. The (Cu,Ni)₆Sn₅ intermetallics (IMCs) growth was volume diffusion controlled and Kirkendall voids were found in the Ni₃P layer even at the initial stage of high-temperature aging due to the faster diffusion of Ni in the Ni₃P layer via its column structure boundaries. The Ag₃Sn IMCs were distributed in the bulk solder, existing as plate- or lamella-like phases or as small particles around the β-Sn dendrites, and the (Cu,Ni)₆Sn₅ IMCs existed as facet-like phases. The plate- and lamella-like Ag₃Sn phases break up into small parts and these broken parts, together with small Ag₃Sn particles, coarsen into pebble-like phases during high-temperature aging. Shear tests showed that all the solder bumps fractured in the bulk solder. The shear strength of solder bumps decreases at the initial stage of aging at 150°C and 175°C, and the strength degradation during aging may be caused by the coarsening of small Ag₃Sn particles.     

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.     

Electromigration-Induced Interfacial Reactions in Cu/Sn/Electroless Ni-P Solder Interconnects

The effect of electromigration (EM) on the interfacial reaction in a line-type Cu/Sn/Ni-P/Al/Ni-P/Sn/Cu interconnect was investigated at 150°C under 5.0 × 10³ A/cm². When Cu atoms were under downwind diffusion, EM enhanced the cross-solder diffusion of Cu atoms to the opposite Ni-P/Sn (anode) interface compared with the aging case, resulting in the transformation of interfacial intermetallic compound (IMC) from Ni₃Sn₄ into (Cu,Ni)₆Sn₅. However, at the Sn/Cu (cathode) interface, the interfacial IMCs remained as Cu₆Sn₅ (containing less than 0.2 wt.% Ni) and Cu₃Sn. When Ni atoms were under downwind diffusion, only a very small quantity of Ni atoms diffused to the opposite Cu/Sn (anode) interface and the interfacial IMCs remained as Cu₆Sn₅ (containing less than 0.6 wt.% Ni) and Cu₃Sn. EM significantly accelerated the dissolution of Ni atoms from the Ni-P and the interfacial Ni₃Sn₄ compared with the aging case, resulting in fast growth of Ni₃P and Ni₂SnP, disappearance of interfacial Ni₃Sn₄, and congregation of large (Ni,Cu)₃Sn₄ particles in the Sn solder matrix. The growth kinetics of Ni₃P and Ni₂SnP were significantly accelerated after the interfacial Ni₃Sn₄ IMC completely dissolved into the solder, but still followed the t ^1/2 law.     

Study of Interfacial Reactions Between Sn(Cu) Solders and Ni-Co Alloy Layers

The interfacial reactions between electroplated Ni-yCo alloy layers and Sn(Cu) solders at 250°C are studied. For pure Co layers, CoSn₃ is the only interfacial compound phase formed at the Sn(Cu)/Co interfaces regardless of the Cu concentration. Also, the addition of Cu to Sn(Cu) solders has no obvious influence on the CoSn₃ compound growth at the Sn(Cu)/Co interfaces. For Ni-63Co layers, (Co,Ni,Cu)Sn₃ is the only interfacial compound phase formed at the Sn(Cu)/Ni-63Co interfaces. Unlike in the pure Co layer cases, the Cu additives in the Sn(Cu) solders clearly suppress the growth rate of the interfacial (Co,Ni,Cu)Sn₃ compound layer. For Ni-20Co layers, the interfacial compound formation at the Sn(Cu)/Ni-20Co interfaces depends on the Cu content in the Sn(Cu) solders and the reflow time. In the case of high Cu content in the Sn(Cu) solders (Sn-0.7Cu and Sn-1.2Cu), an additional needle-like interfacial (Ni _x ,Co _y ,Cu_1−x−y )₃Sn₄ phase forms above the continuous (Ni _x ,Cu _y ,Co_1−x−y )Sn₂ compound layer. The Ni content in the Ni-yCo layer can indeed reduce the interfacial compound formation at the Sn(Cu)/Ni-yCo interfaces. With pure Sn solders, the thickness of the compound layer monotonically decreases with the Ni content in the Ni-yCo layer. As for reactions with the Sn(Cu) solders, as the compound thickness decreases, the Ni content in the Ni-yCo layers increases.     

Interfacial reactions and shear strengths between Sn-Ag-based Pb-free solder balls and Au/EN/Cu metallization

The morphological and compositional evolutions of intermetallic compounds (IMCs) formed at three Pb-free solder/electroless Ni-P interface were investigated with respect to the solder compositions and reflow times. The three Pb-free solder alloys were Sn3.5Ag, Sn3.5Ag0.75Cu, and Sn3Ag6Bi2In (in wt.%). After reflow reaction, three distinctive layers, Ni₃Sn₄ (or Ni-Cu-Sn for Sn3.5Ag0.75Cu solder), NiSnP, and Ni₃P, were formed on the electroless Ni-P layer in all the solder alloys. For the Sn3.5Ag0.75Cu solder, with increasing reflow time, the interfacial intermetallics switched from (Cu,Ni)₆Sn₅ to (Cu,Ni)₆Sn₅+(Ni,Cu)₃Sn₄, and then to (Ni,Cu)₃Sn₄ IMCs. The degree of IMC spalling for the Sn3.5Ag0.75Cu solder joint was more than that of other solders. In the cases of the Sn3.5Ag and Sn3Ag6Bi2In solder joints, the growth rate of the Ni₃P layer was similar because these two type solder joints had a similar interfacial reaction. On the other hand, for the Sn3.5Ag0.75Cu solder, the thickness of the Ni₃P and Ni-Sn-P layers depended on the degree of IMC spalling. Also, the shear strength showed various characteristics depending on the solder alloys and reflow times. The fractures mainly occurred at the interfaces of Ni₃Sn₄/Ni-Sn-P and solder/Ni₃Sn₄.     

Cross-Interaction between Ni and Cu across Sn Layers with Different Thickness

The Ni/solder/Cu material sequence is one of the most common material sequences in the solder joints of electronic packages. In this study, the Ni/Sn/Cu ternary diffusion couples were used to investigate the solder volume effect on the cross-interaction between Ni and Cu. Experimentally, a pure Sn layer with the thickness of 100–400 μm was electroplated over Cu foils. A pure Ni layer (20 μm) was then deposited over the as-deposited Sn surface. The diffusion couples were aged at 160°C for different periods of time. With this technique, the diffusion couples were assembled without experiencing any high temperature process, such as reflow, which would have accelerated the interaction and caused difficulties in analysis. This study revealed that the cross-interaction could occur in as short as 30 min. A detailed atomic flux analysis showed that the Cu flux through the Sn layer was about 25–40 times higher than the Ni flux. Moreover, it was found that (Cu_1−x Ni _x )₆Sn₅ on the Ni side reduced the consumption rate of the Ni layer, and the cross-interaction also reduced the Cu₃Sn thickness on the Cu side.     

Interaction Between Ni and Cu Across 95Pb-5Sn High-Lead Layer

Ni/95Pb-5Sn/Cu ternary diffusion couples were used to investigate the cross-interaction between Ni and Cu across a layer of 95Pb-5Sn solder. High-lead solder layers with a thickness of 100 μm or 400 μm were electroplated over Cu foils. A pure Ni layer (20 μm) was then deposited over the as-deposited high-lead solder surface. The diffusion couples were then aged at 150°C to 250°C for different periods of time. With this technique, the diffusion couples were assembled without experiencing any high-temperature process such as reflow, which would have accelerated the interaction and caused difficulties in analysis. This study revealed that massive spalling also occurred during aging even though reflow was not used. The massive spalling began with the formation of microvoids. When the microvoids had congregated into large enough voids, intermetallic compounds (Cu₃Sn) started to spall from the interface. This spalling phenomenon occurred sooner with increasing temperature and decreasing solder volume.     

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.     

Interfacial reactions in the Sn-(Cu)/Ni,Sn-(Ni)/Cu,and Sn/(Cu,Ni) systems

Specimens with the Sn/Cu/Sn/Ni/Sn/Cu/Sn structure reacted at 200°C are prepared and examined. The Cu₆Sn₅ and Cu₃Sn phases are formed at the Sn/Cu interface, and the Cu₆Sn₅ and Ni₃Sn₄ phases at the Sn/Ni interface. The reaction path in the original Cu/Sn/Ni part of the specimen is Cu/Cu₃Sn/Cu₆Sn₅/Sn/Cu₆Sn₅/Ni₃Sn₄/Ni. The peculiar phenomenon of the Cu₆Sn₅ phase forming at both sides of the Sn phase is illustrated using the Sn-Cu-Ni phase diagram with a very wide compositional-homogeneity range of the Cu₆Sn₅ phase. Interfacial reactions at 240°C between pure Sn and (Cu,Ni) alloys of various compositions are determined. The Cu₆Sn₅ phase is formed when the NI content is less than 30 wt.%, and the Ni₃Sn₄ phase is formed when the Ni content is higher than 40 wt.%. When the Ni content is between 35 wt.% and 40 wt.%, both Cu₆Sn₅ and Ni₃Sn₄ phases are formed. It is also noticed that the formation of the Cu₃Sn phase at the Sn/(Cu,Ni) interface is suppressed with more than 1wt.%Ni addition in the substrate.     

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.     

Intermetallic compound formation and morphology evolution in the 95Pb5Sn flip-chip solder joint with Ti/Cu/Ni under bump metallization during reflow soldering

Intermetallic compound formation and morphology evolution in the 95Pb5Sn flip-chip solder joint with the Ti/Cu/Ni under bump metallization (UBM) during 350°C reflow for durations ranging from 50 sec to 1440 min were investigated. A thin intermetallic layer of only 0.4 μm thickness was formed at the 95Pb5Sn/UBM interface after reflow for 5 min. When the reflow was extended to 20 min, the intermetallic layer grew thicker and the phase identification revealed the intermetallic layer comprised two phases, (Ni,Cu)₃Sn₂ and (Ni,Cu)₃Sn₄. The detection of the Cu content in the intermetallic compounds indicated that the Cu atoms had diffused through the Ni layer and took part in the intermetallic compound formation. With increasing reflow time, the (Ni,Cu)₃Sn₄ phase grew at a faster rate than that of the (Ni,Cu)₃Sn₂ phase. Meanwhile, irregular growth of the (Ni,Cu)₃Sn₄ phase was observed and voids formed at the (Ni,Cu)₃Sn₂/Ni interface. After reflow for 60 min, the (Ni,Cu)₃Sn₂ phase disappeared and the (Ni,Cu)₃Sn₄ phase spalled off the NI layer in the form of a continuous layer. The gap between the (Ni,Cu)₃Sn₄ layer and the Ni layer was filled with lead. A possible mechanism for the growth, disappearance, and spalling of the intermetallic compounds at the 95Pb5Sn/UBM interface was proposed.     

Kinetics of Solid-State Reactive Diffusion in the (Pd-Ni)/Sn System

The growth of compounds during energization heating at the interconnection between a Sn-based solder and a multilayer Pd/Ni/Cu conductor may be inhibited by the alloying of Pd with Ni. To examine such influence of Ni on the compound growth, the kinetics of solid-state reactive diffusion in the (Pd-Ni)/Sn system was experimentally determined in the present study. Experiments were conducted using Sn/(Pd-Ni)/Sn diffusion couples with Ni mol fractions of y = 0.257, 0.505, and 0.746 which were prepared by a diffusion bonding technique. The diffusion couples were isothermally annealed in the temperature range of 433 K to 473 K for various times up to 771 h. During annealing, different compounds are formed as rather uniform layers at the interface in the diffusion couple. In all the annealed diffusion couples, (Pd,Ni)Sn₄ was observed clearly. Furthermore, (Pd,Ni)Sn₃ and (Pd,Ni)Sn₂ were recognized for y = 0.257, and Ni₃Sn₄ was discerned for y = 0.746. However, no other compounds except (Pd,Ni)Sn₄ were detected for y = 0.505. The total thickness of the compound layers is proportional to a power function of the annealing time. The exponent of the power function is rather close to 0.5 for y = 0.257 and 0.505 but smaller than 0.5 for y = 0.746. Thus, volume diffusion is the rate-controlling process of the compound growth for y = 0.257 and 0.505, but boundary diffusion contributes to the rate-controlling process for y = 0.746. At the experimental annealing times, the overall growth rate of the compound layers is insensitive to y at y < 0.5 but decreases monotonically with increasing value of y at y > 0.5. Consequently, the compound growth is actually decelerated by the addition of Ni into Pd with y > 0.5 in the multilayer Pd/Ni/Cu conductor.     

Interface Reaction Between Electroless Ni–Sn–P Metallization and Lead-Free Sn–3.5Ag Solder with Suppressed Ni3P Formation

The voids formed in the Ni₃P layer during reaction between Sn-based solders and electroless Ni–P metallization is an important cause of rapid degradation of solder joint reliability. In this study, to suppress formation of the Ni₃P phase, an electrolessly plated Ni–Sn–P alloy (6–7 wt.% P and 19–21 wt.% Sn) was developed to replace Ni–P. The interfacial microstructure of electroless Ni–Sn–P/Sn–3.5Ag solder joints was investigated after reflow and solid-state aging. For comparison, the interfacial reaction in electroless Ni–P/Sn–3.5Ag solder joints under the same reflow and aging conditions was studied. It was found that the Ni–Sn–P metallization is consumed much more slowly than the Ni–P metallization during soldering. After prolonged reaction, no Ni₃P or voids are observed under SEM at the Ni–Sn–P/Sn–3.5Ag interface. Two main intermetallic compounds, Ni₃Sn₄ and Ni₁₃Sn₈P₃, are formed during the soldering reaction. The reason for Ni₃P phase suppression and the overall mechanisms of reaction at the Ni–Sn–P/Sn–3.5Ag interface are discussed.     

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.     

IMC Growth at the Interface of Sn–2.0Ag–2.5Zn Solder Joints with Cu,Ni, and Ni–W Substrates

Growth of intermetallic compounds (IMC) at the interface of Sn–2.0Ag–2.5Zn solder joints with Cu, Ni, and Ni–W substrates have been investigated. For the Cu substrate, a Cu₅Zn₈ IMC layer with Ag₃Sn particles on top was observed at the interface; this acted as a barrier layer preventing further growth of Cu–Sn IMC. For the Ni substrate, a thin Ni₃Sn₄ film was observed between the solder and the Ni layer; the thickness of the film increased slowly and steadily with aging. For the Ni–W substrate, a thin Ni₃Sn₄ film was observed between the solder and Ni–W layer. During the aging process a thin layer of the Ni–W substrate was transformed into a bright layer, and the thickness of bright layer increased with aging.     

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 effect of isothermal aging on the thickness of intermetallic compound layer growth between low melting point solders and Ni-plated Cu substrate

The growth kinetics of the intermetallic compound (IMC) layer formed between two low melting point solders and electrolytic Ni/Cu substrate by solid-state isothermal aging were examined. The solders were 100 In and In-48Sn. A quantitative analysis of the IMC layer thickness as a function of aging time and aging temperature was performed. Experimental results showed that the IMCs, such as In₂₇Ni₁₀ and Ni₃(In, Sn)₄), were observed for different solders. Additionally, the growth rate of these IMCs increased with the aging temperature and time. The layer growth of the IMC in the couples of indium solder alloy/electrolytic Ni system satisfied the parabolic law at a given temperature range. As a whole, because the values of the time exponent (n) are approximately 0.5, the layer growth of the IMC was mainly controlled by the diffusion mechanism over the temperature range studied. The apparent activation energies for IMC growth were 60.03 kJ/mol for In₂₇Ni₁₀ and 72.84 kJ/mol for Ni₃(In, Sn)₄.