Cross-Interaction of Interfacial Reactions in Ni (Au/Ni/Cu)-SnAg-Cu Solder Joints during Reflow Soldering and Thermal Aging

Interfacial reactions in Ni-SnAg-Cu and Au/Ni/Cu-SnAg-Cu solder joints were investigated to understand the coupling effect between different pads during soldering and thermal aging processes. Scanning electron microscopy (SEM) was used to characterize the microstructures and phases. The element distributions in the joints were identified using the x-ray mapping technique. The thickness variation of intermetallic compounds (IMCs) with aging time was also measured. The results showed that interfacial reactions were not only affected by the compositions of solders and the local metallizations but the remote pads as well. The Au surface finish had an effect on the growth of IMCs at the interfaces. No redeposition of (Au, Ni)Sn₄ was found in the Au/Ni/Cu-SnAg-Cu solder joint. The effect of Cu on the formation of IMCs and redeposition of (Au, Ni)Sn₄ was also discussed. An erratum to this article can be found at     

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.     

Suppressing Growth of the Cu<Subscript>5</Subscript>Zn<Subscript>8</Subscript> Intermetallic Layer in Sn-Zn-Ag-Al-Ga/Cu Solder Joints

Cu₅Zn₈ normally forms between Sn-Zn solder and Cu metallization. In this study, the growth of the intermetallic compound (IMC) layer is slowed by increasing the silver content in the Sn-8.5Zn-Ag-Al-Ga/Cu system. Experimental results showed that the total thickness of the IMC layers formed with 1.5 wt.% silver content was about half that without silver. The reduction might be due to the formation of the intermetallic compound (Ag,Cu)-Zn at the interface in addition to silver.     

Effect of Aluminum Concentration on the Interfacial Reactions of Sn-3.0Ag-<Emphasis Type="Italic">x</Emphasis>Al Solders with Copper and ENIG Metallizations

Aluminum was added into Sn-3.0Ag (wt.%) solder to investigate the effect of aluminum concentration on the interfacial reaction of Sn-3.0Ag-xAl solders with copper or electroless nickel immersion gold (ENIG) metallizations. Four different Sn-3.0Ag-xAl solders (x = 0 wt.%, 0.1 wt.%, 0.5 wt.%, and 1.0 wt.%) were used for comparison. It was found that the composition, morphology, and thickness of interfacial reaction products were strongly dependent on aluminum concentration. At low aluminum concentration (0.1 wt.%), the typical Cu₆Sn₅ layer was formed at the interface. When the aluminum concentration was 0.5 wt.%, a continuous CuAl₂ layer spalled off from the interfacial Cu-Sn intermetallic compound (IMC) layer. Only a planar CuAl₂ layer was observed at the interface when the aluminum concentration was increased to 1.0 wt.%. In Sn-Ag-Al/ENIG reactions, Ni₃Sn₄ was formed and spallation occurred near the interface in the Sn-3.0Ag and Sn-3.0Ag-0.1Al solder joints. When the aluminum concentration was higher than 0.1 wt.%, a thin planar AuAl compound formed at the interface. There was no P-rich phase formation that retarded the spalling phenomenon. The aluminum additive in Sn-Ag solder inhibited the growth of IMCs in the reaction with copper or ENIG metallizations, which was favorable for the reliability of solder joints.     

Effect of Interfacial Reaction on the Tensile Strength of Sn-3.5Ag/Ni-P and Sn-37Pb/Ni-P Solder Joints

This work investigates the effect of interfacial reaction on the mechanical strength of two types of solder joints, Sn-3.5Ag/Ni-P and Sn-37Pb/Ni-P. The tensile strength and fracture behavior of the joints under different thermal aging conditions have been studied. It is observed that the tensile strength decreases with increasing aging temperature and duration. Associated with the tensile strength decrease is the transition of failure modes from within the bulk solder in the as-soldered condition toward failures at the interface between the solder and the intermetallic compounds (IMCs). For the same aging treatment, the strength of the Sn-3.5Ag/Ni-P joint degrades faster than that of Sn-37Pb/Ni-P. The difference between the two types of joints can be explained by the difference in their interfacial reaction and growth kinetics. An empirical relation is established between the solder joint strength and the Ni₃Sn₄ intermetallic compound thickness.     

Effects of Electromigration on Interfacial Reactions in the Ni/Sn-Zn/Cu Solder Interconnect

Electromigration in the Ni/Sn-Zn/Cu solder interconnect was studied with an average current density of 3.51 × 10⁴ A/cm² for 168.5 h at 150°C. When the electrons flowed from the Ni side to the Cu side, uniform layers of Ni₅Zn₂₁ and Cu₅Zn₈ were formed at the Ni/Sn-Zn and Cu/Sn-Zn interfaces. However, upon reversing the current direction, where electron flow was from the Cu side to the Ni side, a thicker Cu₆Sn₅ phase replaced the Ni₅Zn₂₁ phase at the Ni/Sn-Zn interface, whereas at the Cu/Sn-Zn interface, a thicker β-CuZn phase replaced the Cu₅Zn₈ phase.     

陶瓷封装倒装焊器件Sn-Pb微焊点的电迁移行为

高密度陶瓷封装倒装焊器件的焊点尺寸已降低至100μm以下,焊点电流密度达到10~4 A/cm~2以上,由此引发的电迁移失效成为不可忽视的问题。以陶瓷封装菊花链倒装焊器件为研究对象,开展了Sn10Pb90、Sn63Pb37焊点热电环境可靠性评估试验,通过电连接检测及扫描电子显微镜(SEM)等方法对焊点互连情况进行分析。结果表明,Sn63Pb37焊点阴极侧金属间化合物(IMC)增长明显,表现出明显的极化现象,IMC厚度的平方与通电时间呈线性关系。通电时间达到576 h后Sn63Pb37焊点阴极侧产生微裂纹,而Sn10Pb90焊点在通电576 h后仍未出现异常,表现出优异的电迁移可靠性。研究结果对于直径100μm微焊点的陶瓷封装倒装焊器件的应用具有重要的意义。     

Formation of bulk Ag<Subscript>3</Subscript>Sn intermetallic compounds in Sn-Ag lead-free solders in solidification

A series of Sn-Ag solders were prepared by arc melting and their phase evolution was investigated as a function of cooling rates. It was found that bulk Ag₃Sn intermetallic compounds (IMCs) separated out only in the slowly cooled Sn-4.0Ag solder. This would be explained by the strong kinetic undercooling, arising from the rapid cooling conditions, which leads to the actual eutectic point shifts in the direction of higher Ag concentration. Thus, the eutectic and hypereutectic alloys experience a metastable hypoeutectic solidification route instead. All formed fractions of bulk Ag₃Sn IMCs in solders, measured by quantitative microstructural analysis and thermal analysis, are larger than those predicted by the equilibrium phase diagram. The reasoning for this could be attributed to fine Ag₃Sn phases, which cling to the primary Ag₃Sn crystal during the eutectic reaction for their matching crystalline orientation relationship. Furthermore, the fraction of bulk Ag₃Sn IMCs increases gradually with increasing the cooling rate in the slowly cooled Sn-4.0Ag alloy, which fits with the prediction of eutectic solidification theory: the increase of cooling rate would decrease the surface energy of fine Ag₃Sn particles and primary Ag₃Sn crystal, and make fine Ag₃Sn particles cling to primary Ag₃Sn crystal easily to form bulk Ag₃Sn IMCs.     

Effect of Electromigration on the Mechanical Performance of Sn-3.5Ag Solder Joints with Ni and Ni-P Metallizations

The effect of moderate electric current density (1 × 10³ to 3 × 10³ A/cm²) on the mechanical properties of Ni-P/Sn-3.5Ag/Ni-P and Ni/Sn-3.5Ag/Ni solder joints was investigated using a microtensile test. Thermal aging was carried out at 160°C for 100 h while the current was passed. The interfacial microstructure and intermetallic compound (IMC) growth were analyzed. It was found that, at these levels of current density, there were no observable voids or hillocks. Samples aged at 160°C without current stressing failed mostly inside the bulk solder with significant prior plastic deformation. The passage of current was found to cause brittle failure of the solder joints and this tendency for brittle failure increased with increasing current density. Fractographic analysis showed that, in most of the electrically stressed samples, fracture occurred at the interface region between the solder and the joining metals. The critical current density that caused brittle fracture was about 2 × 10³ A/cm². Once brittle fracture occurred, the tensile toughness, defined as the energy per unit fractured area, was usually lower than ~5 kJ/m², compared with the case of ductile fracture where this value was typically greater than ~9 kJ/m². When comparing the two types of joint, the brittle failure was found to be more severe with the Ni than with the Ni-P joint. This work also found that the passage of electric current affects the IMC growth rate more significantly in the Ni than in the Ni-P joint. In the case of the Ni joint, the Ni₃Sn₄ IMC at the anode side was appreciably thicker than that formed at the cathode side. However, in the case of electroless Ni-P metallization, this difference was much smaller.     

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 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.     

Interfacial Reaction between Zn-Al-Based High-Temperature Solders and Ni Substrate

The Zn-Al(-Cu) eutectic alloys (melting point 381°C) are candidates for use as Pb-free high-temperature solders as a substitute for Pb-based solders, which are suitable for severe working environments such as the engine room of hybrid vehicles equipped with an inverter system as well as a heat engine. In this study, the interfacial reaction between Zn-Al(-Cu) alloys and the Ni substrate during soldering, aging, and thermal cycling was investigated. Semiconductor chips and Ni substrates were soldered with Zn-Al(-Cu) alloys at various temperatures under a nitrogen atmosphere. The soldered assemblies were then heat-treated at 200°C and 300°C to examine the microstructural evolution at the soldered interface. The effect of severe thermal cycles between −40°C and 250°C in air on the microstructure and fracture behavior at the solder joint was investigated. Even after a 1000-cycle test, the thickness of the Al₃Ni₂ layer formed at the interface between the Zn-Al-based solder and the Ni substrate, which is responsible for the damage of the soldered assemblies, was quite small.     

Electrochemical Migration Characteristics of Eutectic Sn-Pb Solder Alloy in NaCl and Na<Subscript>2</Subscript>SO<Subscript>4</Subscript> Solutions

Anodic dissolution and the electrochemical migration characteristics of eutectic Sn-Pb solder alloy in deaerated 0.001% NaCl and Na₂SO₄ solutions were investigated using anodic polarization and water drop tests. Anodic polarization results revealed that a Pb-rich phase was preferentially ionized in deaerated 0.001% NaCl solution and an Sn-rich phase was predominantly ionized in deaerated 0.001% Na₂SO₄ solution, which coincides well with the composition of the dendrites formed during water drop tests. X-ray diffraction and photoelectron spectroscopy results showed that the surface oxide film formed on pure Sn in deaerated 0.001% NaCl solution is more stable than that formed on pure Sn in deaerated 0.001% Na₂SO₄ solution. The surface oxide film formed on pure Pb in deaerated 0.001% Na₂SO₄ solution is more stable than that formed on pure Sn in deaerated 0.001% NaCl solution. Therefore, the quality of the surface film of eutectic Sn-Pb solder in a chemical environment seems to be critical not only for corrosion resistance, but also for electrochemical migration resistance.     

Enhanced Wettability of Oxidized Copper with Lead-Free Solder by Ar-H<Subscript>2</Subscript> Plasmas for Flip-Chip Bumping

Enhanced solder wettability (SW) of oxidized-Cu (OC) with 96.5Sn-3Ag-0.5Cu lead-free solder (LFS) by Ar-H₂ plasmas was investigated. The SW of OC was significantly improved from 0% wetting of Cu oxidized in air at 260°C for 1 h to 100% wetting of OC modified by Ar-H₂ plasmas for 10 min. The SW of Cu was found to be highly dependent on the surface characteristics of Cu. By decreasing the total surface energy (TSE), decreasing the polar surface energy (PSE), and increasing the dispersive surface energy (DSE) on the surfaces of OC modified by Ar-H₂ plasmas, the SW with LFS improves. X-ray photoelectron spectroscopy (XPS) indicates that Ar-H₂ plasma treatment is used to remove the copper oxides CuO and Cu₂O from the OC surfaces. The ratio of the total amount of Cu₂O to CuO was found to be a good indication of how the copper oxides CuO and Cu₂O affect the PSE, DSE, and SW of Cu.     

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

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

Analysis of the redeposition of AuSn<Subscript>4</Subscript> on Ni/Au contact pads when using SnPbAg,SnAg, and SnAgCu solders

Interfacial reactions between SnPbAg, SnAg, and SnAgCu solders and Ni/Au surface finish on printed wiring board and especially the redeposition of AuSn₄ intermetallic compound have been investigated. The following major results were obtained. The first phase to form during soldering in the (SnPbAg)/Ni/Au and the (SnAg)/Ni/Au systems was Ni₃Sn₄. During the subsequent solid-state annealing, the redeposition of AuSn₄ as (Au,Ni)Sn₄ occurred in both systems. This was explained with the help of the concept of local equilibrium and the corresponding ternary phase diagrams. It was concluded that the stabilizing effect of Ni on the (Au,Ni)Sn₄ provided the driving force for the redeposition. Contrarily, when the solder alloy contained some Cu, the first intermetallic to form was (Cu,Ni,Au)₆Sn₅ and no redeposition of AuSn₄ was observed. Thus, a very small addition of Cu to the Sn-rich solder alloys changed the behavior of the interconnection system completely. This behavior was explained thermodynamically by using Cu-Ni-Sn and Au-Cu-Sn ternary phase diagrams. The growth kinetics of the interfacial reaction products in the three systems was observed to be somewhat different. The reasons for the observed differences are also discussed.     

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.     

On the Nature of the Interface between Ag<Subscript>3</Subscript>Sn Intermetallics and Sn in Sn-3.5Ag Solder Alloys

We report on the nature of the orientation of Ag₃Sn and the Ag₃Sn/Sn interface in Sn-3.5Ag solder. Orientation imaging microscopy (OIM) and transmission electron microscopy (TEM) were used to characterize the orientation and nature of the interface, respectively. OIM and TEM showed that Sn-3.5Ag containing spherical Ag₃Sn particles does not have a preferred orientation with respect to the Sn matrix. However, needle-like Ag₃Sn formed during slower cooling appeared to have a preferred orientation within individual Sn colonies. The interface between Sn and Ag₃Sn appeared to be incoherent, as confirmed by high-resolution TEM analysis.     

Effect of Ionization Characteristics on Electrochemical Migration Lifetimes of Sn-3.0Ag-0.5Cu Solder in NaCl and Na<Subscript>2</Subscript>SO<Subscript>4</Subscript> Solutions

Anodic dissolution characteristics and electrochemical migration (ECM) behavior of Sn-3.0Ag-0.5Cu solder in NaCl and Na₂SO₄ solutions were investigated using anodic polarization tests and water drop tests (WDT). The ECM lifetime of Sn-3.0Ag-0.5Cu solder in NaCl solution (42.4 s) was longer than that in Na₂SO₄ solution (34.8 s). The pitting potential of Sn-3.0Ag-0.5Cu solder in NaCl solution (135 mV, SCE) was higher than that in Na₂SO₄ solution (−367 mV, SCE). The passivity film (SnO₂) formed on Sn-3.0Ag-0.5Cu solder during WDTs in NaCl solution was thicker than that formed in Na₂SO₄ solution. Therefore, the longer ECM lifetime of Sn-3.0Ag-0.5Cu solder in NaCl solution than in Na₂SO₄ solution can be attributed to the higher pitting potential in the NaCl solution, which is ascribed to the formation of a thicker passivity film (SnO₂) in the former. It was confirmed that microelements such as Ag and Cu do not take part in ECM because they form chemically stable intermetallic compounds with Sn. We believe that Sn is the only element that contributes to ECM, and dissolution of Sn at the anode is possibly the rate-determining step of ECM of Sn-3.0Ag-0.5Cu solder.     

Electron backscattered diffraction and energy dispersive X-ray spectroscopy study of the phase NiSn<Subscript>4</Subscript>

Electron backscattered diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDS) have been performed on a plate-shaped phase formed through the reaction of Sn and Ni. The phase is formed during extended thermal cycling tests on ceramic capacitors having electroplated tin end terminations. The morphology is identical to that of a phase labeled NiSn₃ by J. Haimovich. The phase is shown to have a stoichiometry, NiSn₄, and a crystal structure isomorphous to PdSn₄, PtSn₄, and AuSn₄ (Aba2, #41, oC20). The structure can also be described with the higher symmetry structure (Ccca, #68, oC20).