Morphological transition of interfacial Ni<Subscript>3</Subscript>Sn<Subscript>4</Subscript> grains at the Sn-3.5Ag/Ni joint

The transition in morphology of Ni₃Sn₄ grains that formed at the interface between liquid Sn-3.5Ag (numbers are in wt.% unless specified otherwise) solder and Ni substrate has been observed at 250–650°C. The morphological transition of Ni₃Sn₄ is due to the decrease of entropy of formation of the Ni₃Sn₄ phase and has been explained well by the change of Jackson’s parameter with temperature. According to the variation of solder joint strength with temperature, it decreased rapidly between 350°C and 450°C, where the thickness of the Ni₃Sn₄ intermetallic compound (IMC) layer was around 6.5 μm. However, the solder joint strength decreased slowly with an increase of soldering time without a significant drop, although the thickness of the IMC was larger than 6.5 μm. The notable drop of solder joint strength and the fracture mode transition with increase of soldering temperature appears to come from excessive lateral growth of IMC grains between 350°C and 450°C.     

Influence of Palladium Thickness on the Soldering Reactions Between Sn-3Ag-0.5Cu and Au/Pd(P)/Ni(P) Surface Finish

This study provides a comparison of the influence of Pd(P) thickness on reactions during soldering with the Sn-3Ag-0.5Cu alloy. Soldering was carried out in an infrared-enhanced conventional reflow oven, and a multiple reflow test method (up to ten cycles) was performed. With increasing Pd(P) thickness, the (Cu,Ni)₆Sn₅ grew more slowly at the solder/Ni(P) interface, while the Ni₂SnP/Ni₃P bilayer became predominant after the first reflow. These three intermetallics, i.e., (Cu,Ni)₆Sn₅, Ni₂SnP, and Ni₃P, gradually coarsened as the number of reflow cycles increased. Furthermore, an additional (Ni,Cu)₃Sn₄ layer appeared between (Cu,Ni)₆Sn₅ and Ni₂SnP, especially for the case of a thicker Pd(P) layer (0.2 μm). The attachment of the (Ni,Cu)₃Sn₄ to the Ni₂SnP, however, was fairly poor, and a series of microcracks formed along the (Ni,Cu)₃Sn₄/Ni₂SnP interface. To quantify the mechanical response of the interfacial microstructures, shear testing was conducted at two different shear speeds (0.0007 m/s and 2 m/s). The results indicated that the interfacial strength and the Pd(P) thickness were strongly correlated.     

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.     

Aging study on interfacial microstructure and solder-ball shear strength of a wafer-level chip-size package with Au/Ni metallization on a Cu pad

Eutectic solder balls (63Sn-37Pb) joined to Cu pads with an Au/Ni metallization have been widely used in wafer-level chip-size package (WLCSP) technology for providing electrical and mechanical interconnections between components. However, some reliability issues must be addressed regarding the intermetallic compounds (IMCs). The formation of a brittle IMC layer between the solder/Cu pad interface impacts considerably upon the solder-ball shear strength. In addition, it will degrade the long-term operating reliability of the WLCSP. This study investigates, by means of experiments, the growth of the IMC layer under isothermal aging for the eutectic Sn-Pb solder reflowed on a Cu pad with an Au/Ni metallization. Forming the Cu pad with an Au/Ni metallization was achieved by a simple semiconductor-manufacturing process. The effects of the intermetallic layer on solder-ball shear strength were examined for various parameters, including the thickness of the Au layer, solder-ball size, and the diameter of the Cu pad. Experimental results indicate that two IMC layers, Au_0.5Ni_0.5Sn₄ and Ni₃Sn₄, form at the solder/Cu pad interface after aging. The Au_0.5Ni_0.5Sn₄ intermetallic layer dominates the total thickness of the IMC layer and grows with aging time while the solder-ball shear strength decreases after aging. The degradation of the solder-ball shear strength was found to be caused mainly by the formation of the Au_0.5Ni_0.5Sn₄ layer. The experimental results established that a thinner Au layer on Cu pad can effectively control the degradation of solder-ball shear strength, and this is especially true for smaller ball sizes.     

Optimization of the Ni(P) Thickness for an Ultrathin Ni(P)-Based Surface Finish in Soldering Applications

The effects of the Ni(P) thickness δ _Ni(P) on the interfacial reaction between an Sn-3Ag-0.5Cu solder and an Au/Pd(P)/Ni(P)/Cu pad (thickness: 0.05/0.05/0.1–0.3/20 μm) and the resulting mechanical properties were investigated using scanning electron microscopy equipped with an electron backscatter diffraction system, a focused ion beam system, electron probe microanalysis, and high-speed ball shear (HSBS) testing. Regardless of δ _Ni(P), all of the Au/Pd(P)/Ni(P) surface finishes examined were completely exhausted in one reflow, exposing the Cu pad underneath the solder. Cu₆Sn₅ dissolved with various Ni contents, termed (Cu,Ni)₆Sn₅, was the dominant intermetallic compound (IMC) species at the solder/Cu interface. Additionally, Ni₂SnP and Ni₃P IMCs might form with the (Cu,Ni)₆Sn₅ in the thick Ni(P) case, i.e., δ _Ni(P) = 0.3 μm, and the two IMCs (Ni₂SnP and Ni₃P) were gradually eliminated from the interface after multiple reflows. A mass balance analysis indicated that the growth of the Ni-containing IMCs, rather than the dissolution of the metallization pad, played a key role in the Ni(P) exhaustion. The HSBS test results indicated that the mechanical strength of the solder joints was also δ _Ni(P) dependent. The combined results of the interfacial reaction and the mechanical evaluation provided the optimal δ _Ni(P) value for soldering applications.     

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.     

Influence of Ni concentration and Ni3Sn4 nanoparticles on morphology of Sn-Ag-Ni solders by mechanical alloying

The mechanical alloying (MA) process was employed as an alternative method to produce the lead-free solder pastes of Sn-3.5Ag-xNi (x=0.1, 0.5, 1.0, 1.5, and 2.0) in this study. When the Ni concentration was low (x=0.1, 0.5), MA particles agglomerated to a flat ingot with particle sizes >100 μm. For higher Ni concentration (x=1.0, 1.5, and 2.0), MA particles turned into fragments with particle sizes <100 μm. The particle size of the solders appeared to be dependent on the Ni concentration. To reduce the particle size of SnAgNi alloys with low Ni concentration, Ni₃Sn₄ nanoparticles were doped into Sn and Ag powders to derive a Ni₃Sn₄-doped solder. For the Ni₃Sn₄-doped solder, the particle size was smaller than that doped by the pure Ni. The distinction of milling mechanism between Ni₃Sn₄-doped solder and the pure Ni-doped solder by MA process was probed and discussed. In addition, differential scanning calorimetry (DSC) results ensured its feasibility in applying the solder material in the reflow process. Wettability tests between solders and Cu substrate also revealed that the wetting angles for Ni₃Sn₄-doped solder with low Ni concentration (0.1 and 0.5 wt.%) were smaller than those for pure Ni-doped solder. The wetting angles on both Cu substrate and electroplated Ni metallization for SnAgNi solders were also comparable with commercial Sn-3.5Ag and Sn-3.0Ag-0.5Cu solders. Favorable wettability of the as-derived solder in this study was clearly demonstrated.     

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.     

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.     

Interfacial reaction between Sn-Bi alloy and Ni substrate

Interfacial reactions between Sn-Bi alloys of different compositions and Ni substrates at 423 K for different durations were investigated. Only one interfacial phase, Ni₃Sn₄, was detected despite the existence of several other intermetallic compounds (IMCs) in Ni-Sn and Ni-Bi binary systems. This observation (only Ni₃Sn₄ was formed at the interface) was explained as a combination of the driving force for formation of the IMC and diffusion of Ni. The change of Ni₃Sn₄ layer thickness as a function of annealing time, which obeys a parabolic rule, was further confirmed. The thickness of Ni₃Sn₄ was also found to decrease with increasing Bi content in the Sn-Bi alloy.     

Interfacial reaction and microstructural evolution for electroplated Ni and electroless Ni in the under bump metallurgy with 42Sn58Bi solder during annealing

In the present study, several under bump metallization (UBM) schemes using either electroplated Ni or electroless Ni (EN) as the solderable layer are investigated. The EN and electroplated Ni are first deposited on Cu/Al₂O₃ substrates, followed by electroplating of thin gold coatings. Joints of 42Sn-58Bi/Au/EN/Cu/Al₂O₃ and 42Sn-58Bi/Au/Ni/Cu/Al₂O₃ are annealed at 145 C and 185CC for 30–180 minutes to investigate the interfacial reaction between the solder and metallized substrates. For 42Sn-58Bi/Au/Ni-5.5wt.%P/Cu/Al₂O₃, 42Sn-58Bi/Au/Ni-12.1wt.%P/Cu/Al₂O₃, and 42Sn-58Bi/Au/Ni/CU/Al₂O₃ joints annealed at 145 C, only Ni₃Sn₄ intermetallic compound (IMC) formed at the solder/EN interace. When annealed at an elevated temperature of 185 C, plate-like Ni₃Sn₄ IMC forms at the solder/Ni-5.5wt.%P interface, while a trace of (Ni, Cu)₃Sn₄ IMC is observed at the solder/Ni-12.1wt.%P interface and within the solder region. For the electroplated Ni-based multi-metallization substrate, the Ni₃Sn₄ IMC is present at the solder/Ni interface during annealing at 185 C for a short period of time. In the 42Sn-58Bi/Au/EN/Cu/Al₂O₃ joint, the EN spalls off the EN layer and migrates into the solder region when annealed at 185 C. The interface of the solder/electroplating Ni becomes saw-toothed as the annealing temperature is raised to 185 C. In addition, an enrichment of phosphorus is observed at the interface of the Ni-Sn IMC and EN.     

Effects of Pd(P) Thickness on the Microstructural Evolution Between Sn-3Ag-0.5Cu and Ni(P)/Pd(P)/Au Surface Finish During the Reflow Process

The microstructural evolution between Sn-3Ag-0.5Cu (SAC305) solder and Ni(P)/Pd(P)/Au finish during the reflow process was investigated for various Pd(P) thicknesses (0?μm to 0.6?μm). The reflow process was carried out in a belt-conveying reflow oven with peak temperature of 260°C. In the early stages of the reflow process, the Pd(P) layer either dissolved or spalled in the form of (Pd,Ni)Sn₄ into the molten solder, leaving behind an Ni₂SnP/Ni₃P bilayer on the Ni(P) layer. From the dissolution of the spalled (Pd,Ni)Sn₄ particles during the reflow process, the solubility of Pd in the molten SAC305 solder in the reflow process was estimated to be 0.18?wt.% to 0.25?wt.%. Regardless of the ratio of solder volume to pad opening size, the Ni₂SnP layer that formed in the early stage of reflow had a significant influence on the subsequent formation and growth of (Cu,Ni)₆Sn₅ at the solder interface. As the Ni₂SnP layer became thicker with increasing Pd(P) thickness, the formation of (Cu,Ni)₆Sn₅ became increasingly sluggish and occurred only at locations where the Ni₂SnP layer was locally thin or discontinuous, leading to a discontinuous morphology of (Cu,Ni)₆Sn₅. This was attributed to the Ni₂SnP layer that became an increasingly effective barrier to Ni diffusion with increasing thickness. Based on the experimental results, this study suggests detailed mechanisms underlying the effects of the Pd(P) thickness on the morphology and growth of the (Cu,Ni)₆Sn₅ formed during the reflow process.     

Kinetics of interfacial reaction between eutectic Sn-Pb solder and Cu/Ni/Pd metallizations

The interfacial microstructure and the kinetics of interfacial reaction between the eutectic Sn-Pb solder and electroplated Ni/Pd on Cu substrate (Cu/Ni/NiPd/Ni/Pd) were studied both in the liquid- and solid-state of the solder. The liquidstate reaction was carried out at 200°C, 225°C, and 250°C for 30 s, 60 s, 150 s, and 300 s at each temperature. The solid-state aging was carried out at 125°C for up to 43 days. The interfacial microstructure was characterized by imaging and energy dispersive x-ray analysis in scanning electron microscope. Depending on the thickness of the Pd-layer, both PdSn₄ and PdSn₃ phases were observed near the solder-substrate interface. These results were correlated with the initial thickness of the Pd-layer and the diffusion path in the calculated Pd-Pb-Sn isothermal sections. For the aforementioned isothermal reactions, only one Ni-bearing intermetallic (Ni₃Sn₄) was observed at the solder-substrate interface. The presence of Ni₃Sn₄ intermetallic was consistent with the expected diffusion path based on the calculated Ni-Pb-Sn isothermal sections. Selective etching of solder revealed that PdSn₄ and PdSn₃ had a faceted rod morphology, and Ni₃Sn₄ had a faceted scallop morphology which gave rise to rugged Ni₃Sn₄-solder interface. Segregation of Pb on the facets of PdSn₄ and PdSn₃ was also observed. The growth kinetics of the Ni₃Sn₄ intermetallic layer at the solder-substrate interface was analyzed using an Arrhenius-type of equation. In the thickness regime of 0.16 to 1.2 μm, the growth kinetics yielded a time exponent n=3.1, an apparent activation energy of 9260 J/mol both in the liquid- and solid-state of the solder, and a temperature dependent pre-exponential factor. The latter was attributed to the presence of one or more phases ahead of the growing layer.     

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.     

Metallurgical reaction of the Sn-3.5Ag solder and Sn-37Pb solder with Ni/Cu under-bump metallization in a flip-chip package

Several international legislations recently banned the use of Pb because of environmental concerns. The eutectic Sn-Ag solder is one of the promising candidates to replace the conventional Sn-Pb solder primarily because of its excellent mechanical properties. In this study, interfacial reaction of the eutectic Sn-Ag and Sn-Pb solders with Ni/Cu under-bump metallization (UBM) was investigated with a joint assembly of solder/Ni/Cu/Ti/Si₃N₄/Si multilayer structures. After reflows, only one (Ni,Cu)₃Sn₄ intermetallic compound (IMC) with faceted and particlelike grain feature was found between the solder and Ni. The thickness and grain size of the IMC increased with reflow times. Another (Cu,Ni)₆Sn₅ IMC with a rod-type grain formed on (Ni,Cu)₃Sn₄ in the interface between the Sn-Pb solder and the Ni/Cu UBM after more than three reflow times. The thickness of the (Ni,Cu)₃Sn₄ layer formed in the Sn-Pb system remained almost identical despite the numbers of reflow; however, the amounts of (Cu,Ni)₆Sn₅ IMC increased with reflow times. Correlations between the IMC morphologies, Cu diffusion behavior, and IMC transformation in these two solder systems will be investigated with respect to the microstructural evolution between the solders and the Ni/Cu UBM. The morphologies and grain-size distributions of the (Ni,Cu)₃Sn₄ IMC formed in the initial stage of reflow are crucial for the subsequent phase transformation of the other IMC.     

Diffusion behavior of Cu in Cu/electroless Ni and Cu/electroless Ni/Sn-37Pb solder joints in flip chip technology

For Cu pads used as under bump metallization (UBM) in flip chip technology, the diffusion behavior of Cu in the metallization layer is an important issue. In this study, isothermal interdiffusion experiments were performed at 240°C for different times with solid-solid and liquid-solid diffusion couples assembled in Cu/electroless-Ni (Ni-10 wt.% P) and Cu/electroless Ni (Ni-10 wt.% P)/ Sn-37Pb joints. The diffusion structure and concentration profiles were examined by scanning electron microscopy and electron microprobe analysis. The interdiffusion fluxes of Cu, Ni and P were calculated from the concentration profiles with the aid of Matano plane evaluation. The values of J_Cu, J_Ni, and J_P decreased with increasing annealing time. The average effective interdiffusion coefficients on the order of 10⁻¹⁴ cm²/s were also evaluated within the diffusion zone. The amounts of Cu dissolved in the intermetallic compounds (IMCs) Ni₃Sn₄ and Ni₃P that precipitate after annealing the Cu/electroless Ni/Sn-37Pb joints were about 0.25 at.% and 0.5 at.%, respectively. For the short period of annealing, it appears that the presence of electroless Ni (EN) with the Sn-Pb soldering reaction assisted the diffusion of Cu through the EN layer.     

Inhibiting the formation of (Au1−xNix)Sn4 and reducing the consumption of Ni metallization in solder joints

The effects of adding a small amount of Cu into eutectic PbSn solder on the interfacial reaction between the solder and the Au/Ni/Cu metallization were studied. Solder balls of two different compositions, 37Pb-63Sn (wt.%) and 36.8Pb-62.7Sn-0.5Cu, were used. The Au layer (1 ± 0.2 μm) and Ni layer (7 ± 1 μm) in the Au/Ni/Cu metallization were deposited by electroplating. After reflow, the solder joints were aged at 160°C for times ranging from 0 h to 2,000 h. For solder joints without Cu added (37Pb-63Sn), a thick layer of (Au_1−xNi_x)Sn₄ was deposited over the Ni₃Sn₄ layer after the aging. This thick layer of (Au_1−xNi_x)Sn₄ can severely weaken the solder joints. However, the addition of 0.5wt.%Cu (36.8Pb-62.7Sn-0.5Cu) completely inhibited the deposition of the (Au_1−xNi_x)Sn₄ layer. Only a layer of (Cu_1-p-qAu_pNi_q)₆Sn₅ formed at the interface of the Cu-doped solder joints. Moreover, it was discovered that the formation of (Cu_1-p-qAu_pNi_q)₆Sn₅ significantly reduced the consumption rate of the Ni layer. This reduction in Ni consumption suggests that a thinner Ni layer can be used in Cu-doped solder joints. Rationalizations for these effects are presented in this paper.     

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.     

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.     

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.