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.     

Electromigration in Sn-Bi Modified with Polyhedral Oligomeric Silsesquioxane

Electromigration (EM) behavior of eutectic Sn-Bi modified with cage-type polyhedral oligomeric silsesquioxane (POSS) trisilanol was investigated. A direct current (DC) was applied to solder joints newly designed for uniform current distribution throughout the joint. For this study, a current density of 10⁴ A/cm² was applied at 25°C and 50°C. The evolution of surface and interior microstructure due to current stressing was observed periodically using optical and scanning electron microscopy. The results revealed that the EM behavior was retarded significantly in solder joints with the addition of POSS trisilanol. Different from eutectic Sn-Bi solder joints, no continuous hillocks formed at the anode side and no cracks occurred at the cathode side in solder joints modified with POSS trisilanol even after 336 h of current stressing at 25°C. In addition, the accumulation of Bi/Sn phases at regions near the anode/cathode was also effectively limited. Joints stressed at 50°C also exhibited similar behavior. It is postulated that POSS trisilanol near the phase boundary provided significant restriction to the mass transport due to DC current stressing.     

Electromigration Reliability and Morphologies of Cu Pillar Flip-Chip Solder Joints with Cu Substrate Pad Metallization

The Cu pillar is a thick underbump metallurgy (UBM) structure developed to alleviate current crowding in a flip-chip solder joint under operating conditions. We present in this work an examination of the electromigration reliability and morphologies of Cu pillar flip-chip solder joints formed by joining Ti/Cu/Ni UBM with largely elongated ∼62 μm Cu onto Cu substrate pad metallization using the Sn-3Ag-0.5Cu solder alloy. Three test conditions that controlled average current densities in solder joints and ambient temperatures were considered: 10 kA/cm² at 150°C, 10 kA/cm² at 160°C, and 15 kA/cm² at 125°C. Electromigration reliability of this particular solder joint turns out to be greatly enhanced compared to a conventional solder joint with a thin-film-stack UBM. Cross-sectional examinations of solder joints upon failure indicate that cracks formed in (Cu,Ni)₆Sn₅ or Cu₆Sn₅ intermetallic compounds (IMCs) near the cathode side of the solder joint. Moreover, the ~52-μm-thick Sn-Ag-Cu solder after long-term current stressing has turned into a combination of ~80% Cu-Ni-Sn IMC and ~20% Sn-rich phases, which appeared in the form of large aggregates that in general were distributed on the cathode side of the solder joint.     

Electromigration behavior in Cu/Sn-8Zn-3Bi/Cu solder joint

Electromigration behavior in a one-dimensional Cu/Sn-8Zn-3Bi/Cu solder joint structure was investigated in ambient with a current density of 3.5 × 10⁴ A/cm² at 60 °C. Due to the compressive stress induced by volume expansion resulting from Cu-Zn intermetallic compound (IMC) growth, Cu₅Zn₈ IMC layers were squeezed out continuously along IMC/Cu interfaces at both the anode and the cathode with increasing the current stressing time, which was not only driven by the concentration gradient, but also accelerated by the electromigration. And a few voids propagated and formed at the anode and the cathode solder/IMC interfaces during electromigration. Additionally, Sn hillocks occurred in the bulk solder, and Sn hillocks formed at the anode side were larger than those at the cathode side.     

The effect of aging on microstructure,room temperature deformation,and fracture of Sn-Bi/Cu solder joints

The effects of isothermal aging on the microstructure and mechanical behavior of Sn-Bi/Cu solder joints are reported. Lap shear solder joints of eutectic Sn-Bi solder were aged for 3 to 30 days at 80°C and then loaded to failure in shear. Changes in the joint microstructure including interphase coarsening, intermetallic growth, and evolution of the intermetallic/solder interface are documented. The aging experiments reveal the segregation of the Bi-rich phase of the solder to the intermetallic/solder interface. The ultimate shear strength and ductility of the joints are reported at strain rates of 4.0 × 10⁻¹ to 4.0 × 10⁻⁵ S⁻¹ for 3 and 30 days aging. The strength of the joints decreases with strain rate for both aging conditions; the ductility is low and independent of strain rate for the joints aged three days and increases considerably with reduced strain rate for joints, aged 30 days. Fractographs and cross sections of the failed joints detail the effect of aging on the fracture mechanism.     

Electromigration studies of flip chip Sn95/Sb5 solder bumps on Cr/Cr-Cu/Cu under-bump metallization

The electromigration-induced failure of Sn95/Sb5 flip chip solder bumps was investigated. The failure of the joints was found at the cathode/chip side after current stressing with a density of 1×10⁴ A/cm² at 150°C for 13 sec. The growth of intermetallic compounds (IMCs) was observed at the anode side after current stressing. Voids were found near the current crowding area in the cathode/chip side, and the (Cu,Ni)₆Sn₅ IMC at the cathode/chip end was transformed into the Sn phase. The failure mechanism for Sn95/Sb5 flip chip solder joint is proposed in this paper.     

Kinetics of interface reaction in 40Sn-Bi/Cu and 40Sn-Bi-2Ag/Cu systems during aging in solid state

The characteristics of thickening kinetics of the intermetallic compounds (IMC) (Cu/sub 6/Sn/sub 5/+Cu/sub 3/Sn) during aging at 120/spl deg/C and 90/spl deg/C in solid state were studied by quantitative metallographic analysis for three solder alloy systems: 40Sn-Bi/Cu, 40Sn-Bi-2Ag/Cu and Sn-37Pb/Cu. The diffusion couples were prepared by hot immersion in the molten solder bath. The results showed that the rate of the IMC thickening increases in the order Sn-37Pb/Cu<40Sn-Bi/2-Ag/Cu<40Sn-Bi/Cu. The IMC thickening rate in the Sn-Bi eutectic system is one order of magnitude faster than that in Sn-Pb eutectic system. The time exponents are different from each other. The relationship between the increase of IMC and aging time follows a parabolic function in the Sn-Pb system, and a linear relation in the Sn-Bi system. Addition of Ag in Sn-Bi inhibits the IMC thickening process.     

Electromigration effect on intermetallic growth and Young’s modulus in SAC solder joint

Solid-state intermetallic compound (IMC) growth behavior plays and important role in solder joint reliability of electronic packaging assemblies. The directional impact of electromigration (EM) on the growth of interfacial IMCs in Ni/SAC/Ni, Cu/SAC/Ni single BGA ball solder joint, and fine pitch ball-grid-array (FPBGA) at the anode and cathode sides is reported in this study. When the solder joint was subjected to a current density of 5,000 A/cm² at 125°C or 150°C, IMC layer growth on the anode interface was faster than that on the cathode interface, and both were faster than isothermal aging due to the Joule heating effect. The EM affects the IMC growth rate, as well as the composition and mechanical properties. The Young’s modulus and hardness were measured by the nanoindentation continuous stiffness measurement (CSM) from planar IMC surfaces after EM exposure. Different values were observed at the anode and cathode. The energy-dispersive x-ray (EDX) line scan analysis was conducted at the interface from the cathode to anode to study the presence of species; Ni was found in the anode IMC at SAC/Cu in the Ni/SAC/Cu joint, but not detected when the current was reverse. Electron-probe microanalysis (EPMA) measurement on the Ni/SAC/Ni specimen also confirmed the polarized Ni and Cu distributions in cathode and anode IMCs, which were (Ni_0.57Cu_0.43)₃Sn₄ and (Cu_0.73Ni_0.27)₆Sn₅, respectively. Thus, the Young’s moduli of the IMC are 141 and 175 GPa, respectively.     

Cu5Zn8 Growth Reversion in Cu/Sn-8Zn-3Bi/Cu During Discontinuous Electromigration

A Cu/Sn-8Zn-3Bi/Cu structure was used to investigate the intermetallic compound (IMC) growth behavior during discontinuous electromigration under current density of 10⁴?A/cm² at 70°C. Cu₅Zn₈ IMC formed at both the anode and the cathode interfaces, and the thickness increased with the stressing time. With prolonging the current stressing time, a bulged Cu₅Zn₈ layer was squeezed out between the former Cu₅Zn₈ layer and Cu substrate in the samples to relax the excess compressive stress. Additionally, due to the back stress gradient built up by the Sn diffusion, the Zn atomic flux reacted with Cu to form Cu₅Zn₈ at the cathode side when the power was turned off. Finally, the total IMC thickness of the anode and the cathode under discontinuous current stressing showed a ??reversion?? in the 69?h and 310?h samples.     

The effect of intermetallic compound morphology on Cu diffusion in Sn-Ag and Sn-Pb solder bump on the Ni/Cu Under-bump metallization

The eutectic Sn-Ag solder alloy is one of the candidates for the Pb-free solder, and Sn-Pb solder alloys are still widely used in today’s electronic packages. In this tudy, the interfacial reaction in the eutectic Sn-Ag and Sn-Pb solder joints was investigated with an assembly of a solder/Ni/Cu/Ti/Si₃N₄/Si multilayer structures. In the Sn-3.5Ag solder joints reflowed at 260°C, only the (Ni_1−x,Cu_x)₃Sn₄ intermetallic compound (IMC) formed at the solder/Ni interface. For the Sn-37Pb solder reflowed at 225°C for one to ten cycles, only the (Ni_1−x,Cu_x)₃Sn₄ IMC formed between the solder and the Ni/Cu under-bump metallization (UBM). Nevertheless, the (Cu_1−y,Ni_y)₆Sn₅ IMC was observed in joints reflowed at 245°C after five cycles and at 265°C after three cycles. With the aid of microstructure evolution, quantitative analysis, and elemental distribution between the solder and Ni/Cu UBM, it was revealed that Cu content in the solder near the solder/IMC interface played an important role in the formation of the (Cu_1−y,Ni_y)₆Sn₅ IMC. In addition, the diffusion behavior of Cu in eutectic Sn-Ag and Sn-Pb solders with the Ni/Cu UBM were probed and discussed. The atomic flux of Cu diffused through Ni was evaluated by detailed quantitative analysis in an electron probe microanalyzer (EPMA). During reflow, the atomic flux of Cu was on the order of 10¹⁶−10¹⁷ atoms/cm²sec in both the eutectic Sn-Ag and Sn-Pb systems.     

Interfacial Reactions Between Pb-Free Solders and In/Ni/Cu Multilayer Substrates

This study investigates the interfacial reactions between Sn-3.0wt.% Ag-0.5wt.%Cu (SAC) and Sn-0.7wt.%Cu (SC) on In/Ni/Cu multilayer substrates using the solid–liquid interdiffusion bonding technique. Samples were reflowed first at 160°C, 180°C, and 200°C for various periods, and then aged at 100°C for 100 h to 500 h. The scalloped Cu₆Sn₅ phase was formed at the SAC/In/Ni/Cu and SC/In/Ni/Cu interfaces. When the reflowing temperatures were 160°C and 180°C, a ternary Ni-In-Sn intermetallic compound (IMC) was formed when the samples were further aged at 100°C. This ternary Ni-In-Sn IMC could be the binary Ni₃Sn₄ phase with extensive Cu and In solubilities, or the ternary Sn-In-Ni compound with Cu solubility, or even a quaternary compound. As the reflow temperature was increased to 200°C, only one Cu₆Sn₅ phase was formed at the solder/substrate interface with the heat treatment at 100°C for 500 h. Mechanical test results indicated that the formation of the Ni-In-Sn ternary IMC weakened the mechanical strength of the solder joints. Furthermore, the solid–liquid interdiffusion (SLID) technique in this work effectively reduced the reflow temperature.     

Interfacial segregation of Bi during current stressing of Sn-Bi/Cu solder interconnect

Effect of electromigration on the interfacial structure of solder interconnects was examined in a Sn-Bi/Cu interconnect system. At current densities above 10⁴ A/cm², Bi migrated along the direction of the electron flow in the solder alloy. A continuous Bi layer was found at the solder interface on the anode side, while a Sn-rich region formed at the cathode side of the electrical connection. The presence of the Bi layer inhibited further growth of Cu-Sn intermetallic phase at the interface by acting as a diffusion barrier to the reacting species.     

Interfacial behaviors of Sn-Pb,Sn-Ag-Cu Pb-free and mixed Sn-Ag-Cu/Sn-Pb solder joints during electromigration

SnPb-SnAgCu mixed solder joints with Sn-Pb soldering Sn-Ag-Cu Pb-free components are inevitably occurred in the high reliability applications. In this study, the interfacial behaviors in Sn-37Pb and Sn-3.0Ag-0.5Cu mixed solder joints was addressed and compared with Sn-37Pb solder joints and Sn-3.0Ag-0.5Cu solder joints with the influence from isothermal aging and electromigration. Considering the difference on the melting point between Sn-3.0Ag-0.5Cu and Sn-37Pb solder, two mixed solder joints: partial mixing and full mixing between Sn-Pb and Sn-Ag-Cu solders were reached with the peak reflowing temperature of 190 and 250 °C, respectively. During isothermal aging, the intermetallic compound (IMC) layer increased with aging time and its growth was diffusion controlled. There was also no obvious affect from the solder composition on IMC growth. After electromigration with the current density of 2.0 × 10³ A/cm², Sn-37Pb solder joints showed the shortest lifetime with the cracks observed at the cathode for the stressing time < 250 h. In Sn-3.0Ag-0.5Cu Pb-free solder joints, current stressing promoted the growth of IMC layer at the interfaces, but the growing rate of IMC at the anode interface was far faster than that at the cathode interface. Therefore, there existed an obvious polarity effect on IMC growth in Sn-Ag-Cu Pb-free solder joints. After Sn-37Pb was mixed with Sn-3.0Ag-0.5Cu Pb-free solder, whether the partial mixing or the full mixing between Sn-Pb and Sn-Ag-Cu can obviously depress both the crack formation at the cathode side and the IMC growth at the anode.     

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.     

Interfacial Reaction Effect on Electrical Reliability of Cu Pillar/Sn Bumps

Thermal annealing and electromigration (EM) tests were performed with Cu pillar/Sn bumps to understand the growth mechanism of intermetallic compounds (IMCs). Annealing tests were carried out at both 100°C and 150°C. At 150°C, EM tests were performed using a current density of 3.5 × 10⁴ A/cm². The electrical failure mechanism of the Cu pillar/Sn bumps was also investigated. Cu₃Sn formed and grew at the Cu pillar/Cu₆Sn₅ interface with increasing annealing and current-stressing times. The growth mechanism of the total (Cu₆Sn₅ + Cu₃Sn) IMC changed when the Sn phase in the Cu pillar/Sn bump was exhausted. The time required for complete consumption of the Sn phase was shorter during the EM test than in the annealing test. Both IMC growth and phase transition from Cu₆Sn₅ to Cu₃Sn had little impact on the electrical resistance of the whole interconnect system during current stressing. Electrical open failure in the Al interconnect near the chip-side Cu pillar edge implies that the Cu pillar/Sn bump has excellent electrical reliability compared with the conventional solder bump.     

Effects of Surface Finishes and Current Stressing on Interfacial Reaction Characteristics of Sn-3.0Ag-0.5Cu Solder Bumps

The effects of surface finishes on the in situ interfacial reaction characteristics of ball grid array (BGA) Sn-3.0Ag-0.5Cu lead-free solder bumps were investigated under annealing and electromigration (EM) test conditions of 130°C to 175°C with 5.0 × 10³ A/cm². During reflow and annealing, (Cu,Ni)₆Sn₅ intermetallic compound (IMC) formed at the interface of electroless nickel immersion gold (ENIG) finish. In the case of both immersion Sn and organic solderability preservative (OSP) finishes, Cu₆Sn₅ and Cu₃Sn IMCs formed. Overall, the IMC growth velocity of ENIG was much lower than that of the other finishes. The activation energies of total IMCs were found to be 0.52 eV for ENIG, 0.78 eV for immersion Sn, and 0.72 eV for OSP. The ENIG finish appeared to present an effective diffusion barrier between the Cu substrate and the solder, which leads to better EM reliability in comparison with Cu-based pad systems. The failure mechanisms were explored in detail via in situ EM tests.     

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.     

Effect of volume in interfacial reaction between eutectic Sn-3.5% Ag-0.5% Cu solder and Cu metallization in microelectronic packaging

The Sn-3.5Ag-0.5Cu (wt.%) is the most promising replacement for the eutectic tin-lead solder alloy. Here, an investigation has been carried out to compare the interfacial reactions of the Cu pad of a ball grid array (BGA) substrate with molten eutectic Sn-3.5% Ag-0.5% Cu solder having different volumes. Two different sizes of BGA solder balls were used: 760-μm and 500-μm diameter. Scanning electron microscopy (SEM) was used to measure the consumed thickness of the Cu and also the thickness of the intermetallic compound (IMC). The soldering reaction was carried out at 230°C, 240°C, and 250°C for 1 min, 5 min, 10 min, and 20 min. The Cu consumption was much higher for the Sn-Ag-Cu solder with higher volume. On the other hand, the mean thickness of the intermetallics for solder with smaller volume was thicker than that of the bigger solder balls. The Cu₃Sn compound was also observed at the interface between the Cu₆Sn₅ IMCs and Cu substrate for longer reflow for the both solder balls. Larger Cu₆Sn₅ IMCs were observed in the bulk of the solder with bigger volume. A simplistic theoretical approach is carried out to find out the amount of Cu₆Sn₅ IMCs in the bulk of the solder by measurement of the Cu consumption from the substrate and the thickness of the IMCs that form on the interface.     

elemental redistribution and interfacial reaction mechanism for the flip chip Sn-3.0Ag-(0.5 or 1.5)Cu solder bump with Al/Ni(V)/Cu under-Bump metallization during aging

In flip chip technology, Al/Ni(V)/Cu under-bump metallization (UBM) is currently applicable for Pb-free solder, and Sn−Ag−Cu solder is a promising candidate to replace the conventional Sn−Pb solder. In this study, Sn-3.0Ag-(0.5 or 1.5)Cu solder bumps with Al/Ni(V)/Cu UBM after assembly and aging at 150°C were employed to investigate the elemental redistribution, and reaction mechanism between solders and UBMs. During assembly, the Cu layer in the Sn-3.0Ag-0.5Cu joint was completely dissolved into solders, while Ni(V) layer was dissolved and reacted with solders to form (Cu_1−y,Ni_y)₆Sn₅ intermetallic compound (IMC). The (Cu_1−y,Ni_y)₆Sn₅ IMC gradually grew with the rate constant of 4.63 × 10⁻⁸ cm/sec^0.5 before 500 h aging had passed. After 500 h aging, the (Cu_1−y,Ni_y)₆Sn₅ IMC dissolved with aging time. In contrast, for the Sn-3.0Ag-1.5Cu joint, only fractions of Cu layer were dissolved during assembly, and the remaining Cu layer reacted with solders to form Cu₆Sn₅ IMC. It was revealed that Ni in the Ni(V) layer was incorporated into the Cu₆Sn₅ IMC through slow solid-state diffusion, with most of the Ni(V) layer preserved. During the period of 2,000 h aging, the growth rate constant of (Cu_1−y,Ni_y)₆Sn₅ IMC was down to 1.74 × 10⁻⁸ cm/sec^0.5 in, the Sn-3.0Ag-1.5Cu joints. On the basis of metallurgical interaction, IMC morphology evolution, growth behavior of IMC, and Sn−Ag−Cu ternary isotherm, the interfacial reaction mechanism between Sn-3.0Ag-(0.5 or 1.5)Cu solder bump and Al/Ni(V)/Cu UBM was discussed and proposed.     

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.