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.     

Kinetic analysis of the interfacial reactions in Ni/Sn/Cu sandwich structures

The mutual interaction between Sn/Ni and Sn/Cu interfacial reactions in a Ni/Sn/Cu sandwich sample has been studied. The major interfacial reaction product on the Cu side was Cu₆Sn₅, while on the Ni side, a ternary (Cu,Ni)₆Sn₅ compound layer was formed. We found that the growth kinetics of the interfacial compound layers on both sides reached a steady state in the late reflow stage. The interfacial compound layer on the Cu side retained a constant thickness. On the other hand, the interfacial compound layer on the Ni side grew at a relatively fast rate, which was found to be linear with time. Our results indicate that the growth of the ternary (Cu,Ni)₆Sn₅ compound layer was controlled by the Cu dissolution flux at the solder/Cu₆Sn₅ compound interface. The dissolution constant of the Cu₆Sn₅ compound into the molten Sn was determined to be 0.13 μm/s. Institute of Materials Science and Engineering, National Central University.     

Cross-interaction between Au and Cu in Au/Sn/Cu ternary diffusion couples

Both Au and Cu are so-called fast diffusers in Sn, and can diffuse very long distances in Sn in a relatively short time. In this study, the cross-interaction between Au and Cu across a layer of Sn was investigated through the use of the Au/Sn/Cu ternary diffusion couples. A 7-μm Au layer and a 100-μm Sn layer were electroplated over Cu foils to produce the Au/Sn/Cu diffusion couples. Aging at 200°C revealed that cross-interaction could occur in as short a time as 10 min. Evidence of this cross-interaction included the formation of (Cu_1−xAu_x)₆Sn₅ on the Au side of the diffusion couples as well as on the Cu side. The reaction products on the Au side included the Au-Sn binary compounds. Between the Au-Sn compounds and the Sn was (Cu_1−xAu_x)₆Sn₅. The reaction products on the Cu side initially was only (Cu_1−xAu_x)₆Sn₅, but a layer of Aufree Cu₃Sn eventually formed between (Cu_1−xAu_x)₆Sn₅ and Cu. A detailed atomic flux analysis showed that the Cu flux through the Sn layer was about 2–3 times higher than the Au flux at any moment. The results of this study show that the cross-interaction of Au and Cu in solders is extremely rapid, and cannot be ignored in those solder joints that have both elements present.     

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 Reaction of Sn and Cu-<Emphasis Type="Italic">x</Emphasis>Zn Substrates After Reflow and Thermal Aging

Zn additions to Cu under bump metallurgy (UBM) in solder joints were the subject of this study. An alternative design was implemented to fabricate pure Sn as the solder and Cu-xZn (x = 15 wt.% and 30 wt.%) as the UBM to form the reaction couple. As the Zn content increased from 15 wt.% to 30 wt.% in the Sn/Cu-Zn system, growth of both Cu₃Sn and Cu₆Sn₅ was suppressed. In addition, no Kirkendall voids were observed at the interface in either Sn/Cu-Zn couple during heat treatment. After 40-day aging, different multilayered phases of [Cu₆Sn₅/Cu₃Sn/Cu(Zn)] and [Cu₆Sn₅/Cu(Zn,Sn)/CuZn] formed at the interface of [Sn/Cu-15Zn] and [Sn/Cu-30Zn] couples, respectively. The growth mechanism of intermetallic compounds (IMCs) during aging is discussed on the basis of the composition variation in the joint assembly with the aid of electron-microscopic characterization and the Sn-Cu-Zn ternary phase diagram. According to these analyses of interfacial morphology and IMC formation in the Sn/Cu-Zn system, Cu-Zn is a potential UBM for retarding Cu pad consumption in solder joints.     

Solid-State Reactions between Cu(Ni) Alloys and Sn

Solid-state interfacial reactions between Sn and Cu(Ni) alloys have been investigated at the temperature of 125°C. The following results were obtained. Firstly, the addition of 0.1 at.% Ni to Cu decreased the total thickness of the intermetallic compound (IMC) layer to about half of that observed in the␣binary Cu/Sn diffusion couple; the Ni addition decreased especially the thickness of Cu₃Sn. Secondly, the addition of 1 to 2.5 at.% Ni to Cu further decreased the thickness of Cu₃Sn, increased that of Cu₆Sn₅ (compared to that in the binary Cu/Sn couple) and produced significant amount of voids at the Cu/Cu₃Sn interface. Thirdly, the addition of 5 at.% Ni to Cu increased the total thickness of the IMC layer to about two times that observed in the binary Cu/Sn diffusion couple and made the Cu₃Sn disappear. Fourthly, in contrast to the previous case, the addition of 10 at.% Ni to Cu decreased the total IMC (Cu₆Sn₅) thickness again close to that of the Cu/Sn couple. With this Ni content no voids were detected. The results are rationalized with the help of␣the thermodynamics of the Sn-Cu-Ni system as well as with kinetic considerations.     

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.     

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.     

Fast Concurrent Growth of Ni3Sn4 and Voids During Solid-State Reaction Between Sn-Rich Solder and Ni Substrates

To simulate the growth of Ni₃Sn₄ phase layers in Sn-based solder joints with Ni substrates during solid-state aging, Sn/(Cu_1−x Ni _x )₆Sn₅/Ni and Sn/Ni diffusion couples were aged isothermally at 180°C and 200°C, and the growth kinetics of the (Ni,Cu)₃Sn₄ and Ni₃Sn₄ layers in the respective couples were monitored during the isothermal aging. Once the (Ni,Cu)₃Sn₄ layer was formed at the (Cu,Ni)₆Sn₅/Ni interface, it grew unexpectedly fast with concurrent growth of voids formed in the Sn layer during prolonged aging at both temperatures. The results obtained from the various types of diffusion couples revealed that the voids formed in the Sn layer were Kirkendall voids, due to the (Ni,Cu)₃Sn₄ layer growing predominantly at the (Ni,Cu)₃Sn₄/Ni interface by fast diffusion of Sn across the (Ni,Cu)₃Sn₄ layer. It is proposed that the accelerated growth of the (Ni,Cu)₃Sn₄ and Ni₃Sn₄ layers after the formation of voids in the Sn layer is due to the relaxation of vacancy oversaturation and the enhanced annihilation rate of incoming vacancies in the presence of the voids in the Sn layer.     

Interfacial Reactions of Sn-3.5Ag-<Emphasis Type="Italic">x</Emphasis>Zn Solders and Cu Substrate During Liquid-State Aging

The effects of Zn (1 wt.%, 3 wt.%, and 7 wt.%) additions to Sn-3.5Ag solder and various reaction times on the interfacial reactions between Sn-3.5Ag-xZn solders and Cu substrates a during liquid-state aging were investigated in this study. The composition and morphological evolution of interfacial intermetallic compounds (IMCs) changed significantly with the Zn concentration and reaction time. For the Sn-3.5Ag-1Zn/Cu couple, CuZn and Cu₆Sn₅ phases formed at the interface. With increasing aging time, the Cu₆Sn₅ IMC layer grew thicker, while the CuZn IMC layer drifted into the solder and decomposed gradually. Cu₅Zn₈ and Ag₅Zn₈ phases formed at the interfaces of Sn-3.5Ag-3Zn/Cu and Sn-3.5Ag-7Zn/Cu couples. With increasing reaction time, the Cu₅Zn₈ layer grew and Cu atoms diffused from the substrate to the solder, which transformed the Ag₅Zn₈ to (Cu,Ag)₅Zn₈. The Cu₆Sn₅ layer that formed between the Cu₅Zn₈ layer and Cu was much thinner at the Sn-3.5Ag-7Zn/Cu interface than at the Sn-3.5Ag-3Zn/Cu interface. Additionally, we measured the thickness of interfacial IMC layers and found that 3 wt.% Zn addition to the solder was the most effective for suppressing IMC growth at the interfaces.     

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.     

Investigation of the Growth of Intermetallic Compounds Between Cu Pillars and Solder Caps

In flip chip applications, Cu pillars with solder caps are regarded as next-generation electronic interconnection technology, because of high input/output density. However, because of diffusion and reaction of Sn and Cu during the high-temperature reflow process, intermetallic compounds (IMC) are formed, and grow, at the interface between the cap and the pillar. Understanding the growth behavior of interfacial IMC is critical in the design of solder interconnections, because excessive growth of IMC can reduce the reliability of connections. In this study, the growth of IMC during thermal cycling, an accelerated method of testing the service environment of electronic devices, was studied by use of focused ion beam–scanning electron microscopy. Under alternating high and low-temperature extremes, growth of Cu₆Sn₅ (η-phase) and Cu₃Sn (ε-phase) IMC was imaged and measured as a function of the number of cycles. The total IMC layer grew significantly thicker but became more uniform during thermal cycling. The Cu₃Sn layer was initially thinner than the Cu₆Sn₅ layer but outgrew the Cu₆Sn₅ layer after 1000 cycles. It was found that, with limited Cu and Sn diffusion, consumption of Cu₆Sn₅ for growth of the Cu₃Sn layer can result in a thinner Cu₆Sn₅ layer after thermal cycling.     

TEM Observations of the Growth of Intermetallic Compounds at the SnBi/Cu Interface

The microstructure of the eutectic SnBi/Cu interface was investigated by transmission electron microscopy to study the growth mechanisms of the intermetallic compounds (IMCs). Although the growth kinetics of the total IMC layer were similar, the individual Cu₃Sn layer grew faster on polycrystalline Cu than on single-crystal substrates. It was found that, on polycrystalline Cu, newly formed Cu₃Sn grains with a smaller grain size nucleated and grew at both the Cu/Cu₃Sn and Cu₃Sn/Cu₆Sn₅ interfaces during reflow and solid-state aging. The consumption of Cu₆Sn₅ to form Cu₃Sn was faster at the Cu₃Sn/Cu₆Sn₅ interface. While on single-crystal Cu new Cu₃Sn grains nucleated only at the Cu/Cu₃Sn interface, the directional growth of the initial columnar Cu₃Sn controlled the advance of the Cu₃Sn/Cu₆Sn₅ interface.     

Interfacial Reactions of Sn-9Zn-<Emphasis Type="Italic">x</Emphasis>Cu (<Emphasis Type="Italic">x</Emphasis> = 1, 4, 7, 10) Solders with Ni Substrates

This study investigates the effects of various reaction times and Cu contents on the interfacial reactions between Sn-9Zn-xCu alloys and Ni substrates. After aging at 255°C for 1 h to 3 h, the Ni₅Zn₂₁ and Cu₅Zn₈ phases formed at the interface of Sn-9Zn/Ni and Sn-9Zn-1wt.%Cu/Ni couples, respectively. The (Ni,Zn)₃Sn₄ phase was found in the Sn-9Zn-4wt.%Cu/Ni couple, and the (Cu,Ni)₆Sn₅ and Cu₆Sn₅ phases formed, respectively, in the Sn-9Zn-7wt.%Cu/Ni and Sn-9Zn-10wt.%Cu/Ni couples. As the reaction time was increased from 5 h to 24 h, the (Cu₅Zn₈ + Ni₅Zn₂₁) phases replaced the Cu₅Zn₈ phase to form in the Sn-9Zn-1wt.%Cu/Ni couple; the (Ni,Zn)₃Sn₄ phase formed in the Sn-9Zn-4wt.%Cu/Ni couple, and (CuZn + Cu₆Sn₅) formed in the Sn-9Zn-10wt.%Cu alloys. Experimental results indicate that intermetallic compound (IMC) formation in Sn-9Zn-xCu/Ni couples changes dramatically with reaction time and Cu content. The Sn-Zn-Ni, Sn-Cu-Ni, and Sn-Zn-Cu ternary isothermal sections greatly help us to understand the IMC evolutions in the Sn-9Zn-xCu/Ni couples.     

Interfacial Microstructure and Growth Kinetics of Intermetallic Compound Layers in Sn-4 wt.%Ag/Cu-X (X = Zn,Ag, Sn) Couples

In the current study, the interfacial microstructures of Sn-Ag/Cu-X alloy (X = Ag, Sn or Zn) couples were investigated. The experimental results confirm that addition of Ag or Zn can effectively suppress the growth of the Cu₃Sn layer, while addition of Sn accelerates the growth of the Cu₃Sn layer. Meanwhile, the formation of voids is effectively suppressed by alloying the Cu substrate. The disappearance of voids and the absence of the Cu₃Sn layer were well explained in terms of the phase diagram and the diffusion flux: the Cu₃Sn phase is a nonequilibrium phase based on the Sn-Cu-Zn ternary phase diagram, since a high-Zn region is formed at the Cu₆Sn₅/Cu-Zn alloy interface; in addition, the high Sn diffusion flux in the Cu₆Sn₅ can suppress the growth of Cu₃Sn and the formation of voids.     

Nanotribological Characteristics of Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript>, Cu<Subscript>3</Subscript>Sn,and Ni<Subscript>3</Subscript>Sn<Subscript>4</Subscript> Intermetallic Compounds

Nanotribological characteristics, including the coefficient of friction, wear coefficient, and wear resistance, of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ intermetallic compounds developed by the annealing of Sn–Cu or Sn–Ni diffusion couples were investigated in this work. The scratch test conditions combined a constant normal load of 10 mN, 20 mN, or 30 mN and a scratch rate of 0.1 μm/s, 1 μm/s, or 10 μm/s. Experimental results indicated that, as the normal load increases, the pile-up grows taller and the scratch deepens, leading to a greater coefficient of friction and wear coefficient, and reduced wear resistance. Moreover, the scratch rate does not have a significant effect on the nanotribological characteristics except for those of Cu₆Sn₅ and Cu₃Sn under a normal load of 10 mN. Though the hardness of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ is similar, Ni₃Sn₄ appears to be more prone to wear damage.     

Temperature Effect on Intermetallic Compound Growth Kinetics of Cu Pillar/Sn Bumps

The in situ intermetallic compound (IMC) growth in Cu pillar/Sn bumps was investigated by isothermal annealing at 120°C, 150°C, and 180°C using an in situ scanning electron microscope. Only the Cu₆Sn₅ phase formed at the interface between the Cu pillar and Sn during the reflow process. The Cu₃Sn phase formed and grew at the interfaces between the Cu pillar and Cu₆Sn₅ with increased annealing time. Total (Cu₆Sn₅ + Cu₃Sn) IMC thickness increased linearly with the square root of annealing time. The growth slopes of total IMC decreased after 240 h at 150°C and 60 h at 180°C, due to the fact that the Cu₆Sn₅ phase transforms to the Cu₃Sn phase when all of the remaining Sn phase in the Cu pillar bump is completely exhausted. The complete consumption time of the Sn phase at 180°C was shorter than that at 150°C. The apparent activation energy for total IMC growth was determined to be 0.57 eV.     

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.     

Isothermal solidification of Cu/Sn diffusion couples to form thin-solder joints

Isothermal solidification of conventional Cu/Sn diffusional couples was performed to form thin (30 μm) joints consisting of Cu-Sn intermetallics. During initial stages of isothermal solidification, both Cu₆Sn₅ and Cu₃Sn phases grow, even though the former is the dominant. After consumption of all available Sn, the Cu₃Sn phase grows reactively at the expense of Cu and Cu₆Sn₅. Finally, we obtain solder joints that consist of only Cu₃Sn. Indentation fracture-toughness measurements show that Cu₃Sn is superior to Cu₆Sn₅. Furthermore, indentations of Cu₃Sn exhibit the presence of shear bands, which are not observed in Cu₆Sn₅, implying that the former is more ductile than the latter. Ductile intermetallic-based joints formed by isothermal solidification are promising candidates to form thin (as thin as 5–10 μm or less) solder joints, as they are thermally and thermodynamically stable compared to conventional solder joints. Excess copper in the interconnect provides ductility to the interconnect.     

A Study on the Thermal Reliability of Cu/SnAg Double-Bump Flip-Chip Assemblies on Organic Substrates

The Cu/SnAg double-bump structure is a promising candidate for fine-pitch flip-chip applications. In this study, the interfacial reactions of Cu (60 μm)/SnAg (20 μm) double-bump flip chip assemblies with a 100 μm pitch were investigated. Two types of thermal treatments, multiple reflows and thermal aging, were performed to evaluate the thermal reliability of Cu/SnAg flip-chip assemblies on organic printed circuit boards (PCBs). After these thermal treatments, the resulting intermetallic compounds (IMCs) were identified with scanning electron microscopy (SEM), and the contact resistance was measured using a daisy-chain and a four-point Kelvin structure. Several types of intermetallic compounds form at the Cu column/SnAg solder interface and the SnAg solder/Ni pad interface. In the case of flip-chip samples reflowed at 250°C and 280°C, Cu₆Sn₅ and (Cu, Ni)₆Sn₅ IMCs were found at the Cu/SnAg and SnAg/Ni interfaces, respectively. In addition, an abnormal Ag₃Sn phase was detected inside the SnAg solder. However, no changes were found in the electrical contact resistance in spite of severe IMC formation in the SnAg solder after five reflows. In thermally aged flip-chip samples, Cu₆Sn₅ and Cu₃Sn IMCs were found at the Cu/SnAg interface, and (Cu, Ni)₆Sn₅ IMCs were found at the SnAg/Ni interface. However, Ag₃Sn IMCs were not observed, even for longer aging times and higher temperatures. The growth of Cu₃Sn IMCs at the Cu/SnAg interface was found to lead to the formation of Kirkendall voids inside the Cu₃Sn IMCs and linked voids within the Cu₃Sn/Cu column interfaces. These voids became more evident when the aging time and temperature increased. The contact resistance was found to be nearly unchanged after 2000 h at 125°C, but increases slightly at 150°C, and a number of Cu/SnAg joints failed after 2000 h. This failure was caused by a reduction in the contact area due to the formation of Kirkendall and linked voids at the Cu column/Cu₃Sn IMC interface.