The nanoindentation 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 in the solder bump

In general, formation and growth of intermetallic compounds (IMCs) play a major role in the reliability of the solder joint in electronics packaging and assembly. The formation of Cu-Sn or Ni-Sn IMCs have been observed at the interface of Sn-rich solders reacted with Cu or Ni substrates. In this study, a nanoindentation technique was employed to investigate nanohardness and reduced elastic moduli of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ IMCs in the solder joints. The Sn-3.5Ag and Sn-37Pb solder pastes were placed on a Cu/Ti/Si substrate and Ni foil then annealed at 240°C to fabricate solder joints. In Sn-3.5Ag joints, the magnitude of the hardness of the IMCs was in the order Ni₃Sn₄>Cu₆Sn₅>Cu₃Sn, and the elastic moduli of Cu₆Sn₅, Cu₃Sn, and Ni₃Sn₄ were 125 GPa, 136 GPa, and 142 GPa, respectively. In addition, the elastic modulus of the Cu₆Sn₅ IMC in the Sn-37Pb joint was similar to that for the bulk Cu₆Sn₅ specimen but less than that in the Sn-3.5Ag joint. This might be attributed to the strengthening effect of the dissolved Ag atoms in the Cu₆Sn₅ IMC to enhance the elastic modulus in the Sn-3.5Ag/Cu joint.     

Inhibiting AuSn<Subscript>4</Subscript> Formation by Controlling the Interfacial Reaction in Solder Joints

Au was used in an electronic package to protect the conductor from oxidation. However, Au dissolved into solders and reacted with the Sn-rich phase to form AuSn₄ during soldering. After aging, Au diffused from AuSn₄ toward the solder/metallization interface. If Ni₃Sn₄ formed at the soldering interface, a layer of AuSn₄ was redeposited on Ni₃Sn₄. In contrast, Au diffused into Cu₆Sn₅-based intermetallic compounds (IMCs) to produce either (Cu,Au)₆Sn₅ or (Cu,Ni,Au)₆Sn₅, while Cu₆Sn₅ or (Cu,Ni)₆Sn₅ was formed at the soldering interface. Gibbs free energy evaluation revealed that both (Ni,Au)₃Sn₄ and (Cu,Au)₆Sn₅ were more thermodynamically stable than AuSn₄. The maximum amount of Au diffused in Ni₃Sn₄ was 4.6 at.%, while the maximum dissolution of Au in Cu₆Sn₅ was 24.3 at.% at 150°C. Thus, dissolution of Au in Ni₃Sn₄ was limited, and residual Au rereacted with Sn to produce the layer-type AuSn₄. If Cu₆Sn₅ formed at the interface, most of the Au in AuSn₄ diffused into Cu₆Sn₅. Consequently, AuSn₄ formation could be inhibited by controlling formation of Cu₆Sn₅ in solder/under-bump metallization (UBM) assemblies.     

Growth of an intermetallic compound layer with Sn-3.5Ag-5Bi on Cu and Ni-P/Cu during aging treatment

Growth kinetics of intermetallic compound (IMC) layers formed between the Sn-3.5Ag-5Bi solder and the Cu and electroless Ni-P substrates were investigated at temperatures ranging from 70°C to 200°C for 0–60 days. With the solder joints between the Sn-Ag-Bi solder and Cu substrates, the IMC layer consisted of two phases: the Cu₆Sn₅ (η phase) adjacent to the solder and the Cu₃Sn (ε phase) adjacent to the Cu substrate. In the case of the electroless Ni-P substrate, the IMC formed at the interface was mainly Ni₃Sn₄, and a P-rich Ni (Ni₃P) layer was also observed as a by-product of the Ni-Sn reaction, which was between the Ni₃Sn₄ IMC and the electroless Ni-P deposit layer. With all the intermetallic layers, time exponent (n) was approximately 0.5, suggesting a diffusion-controlled mechanism over the temperature range studied. The interface between electroless Ni-P and Ni₃P was planar, and the time exponent for the Ni₃P layer growth was also 0.5. The Ni₃P layer thickness reached about 2.5 μm after 60 days of aging at 170°C. The activation energies for the growth of the total Cu-Sn compound layer (Cu₆Sn₅ + Cu₃Sn) and the Ni₃Sn₄ IMC were 88.6 kJ/mol and 52.85 kJ/mol, respectively.     

Effects of microstructural evolution and intermetallic layer growth on shear strength of ball-grid-array Sn-Cu solder joints

The shear strength of ball-grid-array (BGA) solder joints on Cu bond pads was studied for Sn-Cu solder containing 0, 1.5, and 2.5 wt.% Cu, focusing on the effect of the microstructural changes of the bulk solder and the growth of intermetallic (IMC) layers during soldering at 270°C and aging at 150°C. The Cu additions in Sn solder enhanced both the IMC layer growth and the solder/IMC interface roughness during soldering but had insignificant effects during aging. Rapid Cu dissolution from the pad during reflow soldering resulted in a fine dispersion of Cu₆Sn₅ particles throughout the bulk solder in as-soldered joints even for the case of pure Sn solder, giving rise to a precipitation hardening of the bulk solder. The increased strength of the bulk solder caused the fracture mode of as-soldered joints to shift from the bulk solder to the solder/IMC layer as the IMC layer grew over a critical thickness about 1.2 m for all solders. The bulk solder strength decreased rapidly as the fine Cu₆Sn₅ precipitates coarsened during aging. As a consequence, regardless of the IMC layer thickness and the Cu content of the solders, the shear strength of BGA solder joints degraded significantly after 1 day of aging at 150°C and the shear fracture of aged joints occurred in the bulk solder. This suggests that small additions of Cu in Sn-based solders have an insignificant effect on the shear strength of BGA solderjoints, especially during system use at high temperatures.     

Comparison of Sn2.8Ag20In and Sn10Bi10In solders for intermediate-step soldering

We chose Sn−2.8Ag−20In and Sn−10Bi−10In (numbers are in weight percentages unless specified otherwise) as Pb-free solder materials for intermediate-step soldering. We then investigated how the two solders reacted with the under bump metallurgy (UBM) of Au/Ni (Au: 1.5 μm and Ni: 3 μm) at 210°C, 220°C, 230°C, and 240°C for up to 4 min. All, of the Au UBM was dissolved into the solder matrix as soon as the interfacial reaction started. The reaction formed Au(In, Sn)₂ in the case of SnAgIn, and it formed Au(Sn, In)₄ and Au(In, Sn)₂ in the case of SnBiIn. The formation mechanism of the intermetallic phases is explained thermodynamically. The exposed Ni layer reacted with the solder and formed Ni₂₈Sn₅₅In₁₇ in case of SnAgIn, and formed Ni₃(Sn, In)₄ in case of SnBiIn, at the solder joint interface. Under the same soldering conditions, the Ni₃(Sn,In)₄ layer in the SnBiIn/UBM is thicker than the Ni₂₈Sn₅₅In₁₇ layer in the SnAgIn/UBM. Because of the thicker intermetallic compound layer, the SnBiIn solder joint has weaker shear strength than the SnAgIn solder joint.     

Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> Morphology Transition and Its Effect on Mechanical Properties of Eutectic Sn-Ag Solder Joints

The morphologies of Cu₆Sn₅ grains formed at the interface between Sn-3.5Ag (wt.% unless otherwise specified) and Cu substrates were studied in this work. Reflow experiments were performed for 60 s at peak temperatures of 513 K, 533 K, 543 K, and 553 K. Two morphologies of interfacial Cu₆Sn₅ grains were observed in wetting reactions: prism type, above 543 K, and scallop type, below 533 K. During aging, the two morphologies gradually transitioned to layer type. These three morphologies could be transformed into each other as long as the corresponding condition changed. The morphology transition of Cu₆Sn₅ in the wetting reaction was explained by the change in Jackson’s parameter with temperature. In addition, the effect of the Cu content in molten solder on interfacial Cu₆Sn₅ grains was examined. Significant differences in shear strength were observed for solder joints with different interfacial Cu₆Sn₅ morphologies in the case of a lower shear height. Joint strength is discussed in terms of the microstructure of the solder matrix and the morphology of interfacial Cu₆Sn₅ grains.     

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

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

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.     

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.     

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.     

Effect of phosphorus content on Cu/Ni-P/Sn-3.5Ag solder joint strength after multiple reflows

Electroless Ni-P layers with three different P contents (6.1 wt.%, 8.8 wt.%, and 12.3wt.%) were deposited on copper (Cu) substrates. Multilayered samples of Sn-3.5Ag/Ni-P/Cu stack were prepared and subjected to multiple reflows at 250°C. A tensile test was performed to investigate the effect of P content on the solder joint strength. The low P samples exhibited the highest joint strength after multiple reflows, while the strength of medium and high P samples decreased more rapidly. From interfacial analysis, the Ni₃Sn₄ intermetallic compound (IMC) formed at the interface of low P sample was found to be more stable, while the one of medium and high P samples spalled into the molten solder. The IMC spallation sped up the consumption of electroless Ni-P, leading to the large formation of Cu-Sn IMCs. Fractographic and microstructural analyses showed that the degradation in solder joint strength was due to the formation of layers of voids and growth of Cu-Sn IMCs between the solder and the Cu substrate.     

Reactions of Sn-3.5Ag-Based Solders Containing Zn and Al Additions on Cu and Ni(P) Substrates

In this study we consider the effect of separately adding 0.5 wt.% to 1.5 wt.% Zn or 0.5 wt.% to 2 wt.% Al to the eutectic Sn-3.5Ag lead-free solder alloy to limit intermetallic compound (IMC) growth between a limited volume of solder and the contact metallization. The resultant solder joint microstructure after reflow and high-temperature storage at 150°C for up to 1000 h was investigated. Experimental results confirmed that the addition of 1.0 wt.% to 1.5 wt.% Zn leads to the formation of Cu-Zn on the Cu substrate, followed by massive spalling of the Cu-Zn IMC from the Cu substrate. Growth of the Cu₆Sn₅ IMC layer is significantly suppressed. The addition of 0.5 wt.% Zn does not result in the formation of a Cu-Zn layer. On Ni substrates, the Zn segregates to the Ni₃Sn₄ IMC layer and suppresses its growth. The addition of Al to Sn-3.5Ag solder results in the formation of Al-Cu IMC particles in the solder matrix when reflowed on the Cu substrate, while on Ni substrates Al-Ni IMCs spall into the solder matrix. The formation of a continuous barrier layer in the presence of Al and Zn, as reported when using solder baths, is not observed because of the limited solder volumes used, which are more typical of reflow soldering.     

Effect of Ni layer thickness and soldering time on intermetallic compound formation at the interface between molten Sn-3.5Ag and Ni/Cu substrate

The binary eutectic Sn-3.5wt.%Ag alloy was soldered on the Ni/Cu plate at 250°C, the thickness of the Ni layer changing from 0 through 2 and 4 μm to infinity, and soldering time changing from 30 to 120 s at intervals of 30 s. The infinite thickness was equivalent to the bare Ni plate. The morphology, composition and phase identification of the intermetallic compound (IMC, hereafter) formed at the interface were examined. Depending on the initial Ni thickness, different IMC phases were observed at 30 s: Cu₆Sn₅ on bare Cu, metastable NiSn₃ + Ni₃Sn₄ on Ni(2 μm)/Cu, Ni₃Sn₄ on Ni(4 μm)/Cu, and Ni₃Sn + Ni₃Sn₄ on bare Ni. With increased soldering time, a Cu-Sn-based η-(Cu₆Sn₅)_1−xNi_x phase formed under the pre-formed Ni-Sn IMC layer both at 60 s in the Ni(2 μm)/Cu plate and at 90 s in the Ni(4 μm)/Cu plate. The two-layer IMC pattern remained thereafter. The wetting behavior of each joint was different and it may have resulted from the type of IMC formed on each plate. The thickness of the protective Ni layer over the Cu plate was found to be an important factor in determining the interfacial reaction and the wetting behavior.     

Interfacial reaction and failure mode analysis of the solder joints for flip-chip LED on ENIG and Cu-OSP surface finishes

The interfacial reactions and failure modes of the solder joints for flip-chip light emitting diode (LED) on electroless nickel/immersion gold (ENIG) and Cu with organic solderability preservatives (Cu-OSP) surface finishes were investigated in this study. The experimental results demonstrate that the interfacial reactions in the Au/Sn–Ag–Cu(SAC)/ENIG and Au/SAC/Cu systems are different but the failure mechanisms of the two types of solder joints are similar during the shear test. For the Au/SAC/ENIG system, the Au layer on the surface finish of the diodes dissolved into the molten solder and transformed into a continuous (Au, Ni)Sn₄ IMC layer at the diode/solder interface during reflow and the interfacial IMC at the solder/ENIG interface is dendritic Ni₃Sn₄ IMC grains which are surrounded by (Au, Ni)Sn₄. For the Au/SAC/Cu system, however, no IMC layers can be observed at the diode/solder interface. The interfacial IMC at the solder/Cu interface is (Cu, Au)₆Sn₅ and a Cu₃Sn IMC layer at the (Cu, Au)₆Sn₅/Cu interface. Tiny (Au, Cu)Sn₄ IMC grains distribute in the solder layer and surround the (Cu, Au)₆Sn₅ grains. For the two types of systems, the primary failure mode for the cathode is due to the broken of the Si-based insulation layer which led to a high residue stress and poor connection between the Si-based layer and the solder layer. Meanwhile, the failure of the solder joint for the anode is mainly because of the failure of the solder layer under the conductive via. The crack generally forms at this area and then propagated along the diode or the diode/solder interface.     

Elastic Moduli of (Ni,Cu)<Subscript>3</Subscript>Sn<Subscript>4</Subscript> Ternary Alloys from First-Principles Calculations

Based on first-principles calculations, the effect of Cu solubility on the elastic moduli of Ni₃Sn₄-based intermetallic compound (IMC) is investigated. It is found that the stiffness tensor of a (Ni,Cu)₃Sn₄ single crystal is anisotropic, and the presence of Cu in the crystal compound reduces the moduli of (Ni,Cu)₃Sn₄ due to reduced hybridization between Ni and Sn states. Furthermore, our results show that higher Cu concentration in the (Ni,Cu)₃Sn₄-based IMCs leads to thermodynamically less stable compounds. Based on the single-crystal results, the elastic properties of polycrystalline (Ni,Cu)₃Sn₄ are also obtained.     

Al/Ni : V/Ag metal stacks as rear‐side metallization for crystalline silicon solar cells

Rear sides of crystalline silicon solar cells are usually covered with aluminum on which it is difficult to solder. To ease soldering, we present a durability study for a Ni : V/Ag stack on evaporated Al as rear‐side metallization. We adapt this cost‐effective metallization stack from the microelectronic industry and investigate it as metallization for silicon solar cells. Here, a long‐term stability of the metallization and of the solder joint must be guaranteed for 25 years and is therefore evaluated in detail by thermal aging experiments. During this experiment, the mechanical stability of the solder joints is measured. The chemical stability and the intermetallic compound (IMC) growth within the solder joints are examined by secondary electron microscopy, backscattered electron imaging, and energy dispersive X‐ray analysis. Experiments with either a Sn–Ag‐coated copper tab or pure Sn–Ag solder show two different sorts of IMCs at the Ni : V/Solder interface. With the copper tab, a Cu–Ni–Sn compound, presumably (Cu_{1 ‐ x}Ni_x)₆Sn₅, grows at the Ni/solder interface, whereas in case of a pure Sn–Ag solder, a Ni–Sn compound grows, which is likely to be Ni₃Sn₄. Analysis of the reaction kinetics leads to activation energies of 77 and 42 kJ/mol, respectively, for a diffusion‐controlled IMC growth. By using temperature histograms of PV modules in the field, the necessary minimum Ni : V layer thickness is estimated: without a copper tab up to 1.6 µm Ni and with a copper tab less than 0.2 µm may be consumed by IMC formation during 25 years of lifetime. Copyright © 2012 John Wiley & Sons, Ltd.     

Formation and Growth of Intermetallic Compound Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> at Early Stages in Lead-Free Soldering

In this work, the early stages of the formation and growth of the intermetallic compound Cu₆Sn₅ during soldering reactions between a Cu substrate and liquid Sn are examined through phase-field simulations. The liquid Sn-based solder (L phase) and the copper substrate (α phase) are considered to be under metastable equilibrium conditions that eventually lead to nucleation of the Cu₆Sn₅ intermetallic compound (IMC) (η phase) at the solid/liquid interface. Nucleation is incorporated into the model through a classical treatment considering that individual nucleation events follow a Poisson distribution function. The driving forces for the nucleation and phase transformations are obtained by coupling the phase-field simulations to CALPHAD models. In the phase-field simulations, physical properties such as liquid surface as well as IMC interfacial energies are treated parametrically to probe the behavior of the system under various growth conditions. The simulations are compared with previous works and are shown to have good (qualitative) agreement with recent detailed studies on the early stages of the interaction between Cu and liquid Sn.     

Optimization of Pb-Free Solder Joint Reliability from a Metallurgical Perspective

To obtain the desired performance of Pb-free packages in mechanical tests, while the solder composition should be carefully selected, the influence of metals dissolved from the soldering pad or under bump metallization (UBM) should also be taken into account. Dissolved metals such as Cu can alter the intermetallic compound (IMC) formation, not only at the local interface but also on the other side of the joint. The high rate of interfacial cracking of Sn-Ag-Cu solder joints on Ni/Au-plated pads is attributed to the high stiffness of the solder and the dual IMC structure of (Cu,Ni)₆Sn₅ on Ni₃Sn₄ at the interface. Approaches to avoid this dual IMC structure at the interface are discussed. A rule for selecting the solder alloy composition and the pad surface materials on both sides of the joints is proposed for ball grid array (BGA) packages.     

Morphology and growth kinetics of intermetallic compounds in solid-state interfacial reaction of electroless Ni-P with Sn-based lead-free solders

A comparative study of solid/solid interfacial reactions of electroless Ni-P (15 at.% P) with lead-free solders, Sn-0.7Cu, Sn-3.5Ag, Sn-3.8Ag-0.7Cu, and pure Sn, was carried out by performing thermal aging at 150°C up to 1000 h. For pure Sn and Sn-3.5Ag solder, three distinctive layers, Ni₃Sn₄, SnNiP, and Ni₃P, were observed in between the solder and electroless Ni-P; while for Sn-0.7Cu and Sn-3.8Ag-0.7Cu solders, two distinctive layers, (CuNi)₆Sn₅ and Ni₃P, were observed. The differences in morphology and growth kinetics of the intermetallic compounds (IMCs) at the interfaces between electroless Ni-P and lead-free solders were investigated, as well as the growth kinetics of the P-enriched layers underneath the interfacial IMC layers. With increasing aging time, the coarsening of interfacial Ni₃Sn₄ IMC grains for pure Sn and Sn-3.5Ag solder was significantly greater than that of the interfacial (CuNi)₆Sn₅ IMC grains for Sn-0.7Cu and Sn-3.8Ag-0.7Cu solders. Furthermore, the Ni content in interfacial (CuNi)₆Sn₅ phase slightly increased during aging. A small addition of Cu (0.7 wt.%) resulted in differences in the type, morphology, and growth kinetics of interfacial IMCs. By comparing the metallurgical aspects and growth kinetics of the interfacial IMCs and the underneath P-enriched layers, the role of initial Cu and Ag in lead-free solders is better understood.     

NiSn4 Formation in As-Soldered Ni-Sn and ENIG-Sn Couples

Most research on Sn-Ni solder reactions has focused on the interfacial reactions with the substrate, whereas the microstructure which develops above the intermetallic layers has not been studied in detail. This paper shows that nonequilibrium NiSn₄ forms during solidification of the bulk solder in Sn-Ni and Sn-electroless nickel immersion gold (ENIG) solder reactions. With both substrates, the bulk solder solidified to contain Sn-NiSn₄ eutectic and primary Ni₃Sn₄ crystals, and the interfacial layers contained a Ni₃Sn₄ reaction layer on the Sn side. It is found that Cu, present from dissolution of Cu through cracks in the ENIG layer, promotes the formation of Sn-Ni₃Sn₄ eutectic. Thus, Sn-ENIG couples contained both Sn-NiSn₄ and Sn-Ni₃Sn₄ eutectic. It is further shown that NiSn₄ is not stable at soldering temperatures and that, during isothermal holding at 270°C to 220°C, NiSn₄ transforms into Ni₃Sn₄ and liquid or β-Sn.