Interfacial Reactions Between Sn-Zn Alloys and Au Substrate

The interfacial reactions of Sn-xZn/Au couples aged at 160°C were investigated. When the Zn content was 3?wt.%, binary Au-Sn intermetallic compounds (IMCs) and ternary Au-Sn-Zn phase were formed at the interface. Both binary Au-Sn and Au-Zn and ternary Au-Sn-Zn IMCs were formed at the Sn-5Zn/Au interface. When the Zn content was 7?wt.%, Sn-xZn/Au couples were completely transformed into an Au-Zn system. Based on ln?d?=?nln?t?+?ln?k, where d is IMC thickness, t is aging time, and n is the growth exponent, the n value of Sn-xZn/Au (x?<?5?wt.%) couples was between 0.25 and 0.33. Sn-xZn/Au (x?=?7?wt.% to 15?wt.%) couples also had similar results when the aging times were 144?h. The n value of the Sn-50Zn/Au couple was 0.5, and the reaction mechanism was diffusion controlled. The n value for the Sn-90Zn/Au couple was 0.19. The results indicated that adding Zn to Sn-Zn alloys would change the reaction system from the Au-Sn system into the Au-Zn system.     

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.     

Interfacial Reactions Between Sn-Zn Alloys and Ni Substrates

Interfacial reactions between Sn-Zn alloys and Ni substrates after annealing at 463 K have been studied. Only one intermetallic compound (IMC), γ-Ni₅Zn₂₁, was observed at the Sn-Zn/Ni interfaces, although other IMC(s) in the Ni-Sn and Ni-Zn system can stably exist at this temperature. Taking into account nucleation along with diffusion kinetics, formation of the reaction product has been reasonably modeled. In addition, growth kinetics for the reaction layer has been quantified. The results indicate that the growth rate of the γ-Ni₅Zn₂₁ layer increases with Zn content, and that the layer growth obeys a parabolic law with annealing time, implying a diffusion-controlled mechanism.     

Study of the Effects of Zn Content on the Interfacial Reactions Between Sn-Zn Solders and Ni Substrates at 250°C

The interfacial reactions of Ni with Sn-Zn alloys with 1 wt.% to 9 wt.% Zn at 250°C were examined. The Zn content greatly affected the intermetallic compounds formed and microstructural evolution. A continuous Ni₅Zn₂₁ layer was formed for the Sn-Zn/Ni couples with a Zn content higher than 5 wt.%. A stable reaction layer existed at the interface and grew thicker with time. When decreasing to 3 wt.% Zn, two thin reaction layers of Ni₅Zn₂₁ and (Ni,Zn)₃Sn₄ were simultaneously observed initially, and then an extremely large faceted Ni₅Zn₂₁ phase was formed near the boundary between the Ni₅Zn₂₁ layer and the solder. Furthermore, when the Zn content was lower than 2 wt.%, the dominant phase changed to (Ni,Zn)₃Sn₄. The Zn concentration of the solder gradually decreased with reaction, and thus the interfacial stability was reduced. Subsequently, a large amount of (Ni,Zn)₃Sn₄ grains were dispersed into the molten solder, and finally the reaction product at the interface changed to Ni₃Sn₄.     

Sn-Zn系无铅焊料研究和亟待解决的问题

文章介绍了无铅焊料研究的趋势及Sn-Zn系焊料研究取得的最新进展,分析了Sn-Zn系焊料与Sn-Ag系焊料研究的差距。最后,文章还总结了Sn-Zn系焊料研究面临的困难,找出了存在于基础研究和应用研究领域的若干问题,指出Sn-Zn系焊料的研究还需要更加深入。     

Microstructure and Sn Crystal Orientation Evolution in Sn-3.5Ag Lead-Free Solders in High-Temperature Packaging Applications

Understanding the reliability of eutectic Sn-3.5Ag lead-free solders in high-temperature packaging applications is of significant interest in power electronics for the next-generation electric grid. Large-area (2.5 mm × 2.5 mm) Sn-3.5Ag solder joints between silicon dies and direct bonded copper substrates were thermally cycled between 5°C and 200°C. Sn crystal orientation and microstructure evolution during thermal cycling were characterized by electron backscatter diffraction in the scanning electron microscope. Comparisons were made between the observed initial texture and microstructure and its evolution during thermal cycling. Gradual lattice rotation and grain boundary misorientation evolution observed due to thermal cycling suggested a continuous recrystallization mechanism. Recrystallization behavior was correlated with dislocation slip activities.     

Microstructure Evolution and the Constitutive Relations of High-Temperature Solders

This study characterized the temperature-dependent constitutive parameters (yield strength, ultimate tensile strength, elastic modulus, strain hardening exponent) from the mechanical behavior of five high-temperature solders, 95Sn-5Sb, 95Pb-5In, 90Pb-10Sn, 92.5Pb-5Sn-2.5Ag, and 93Pb-3Sn-2In-2Ag, chosen such that T _m > 518 K. To model appropriately their mechanical responses under high-temperature thermal cycling, where the temperatures exceed 473 K, the material’s parameters must be determined as a function of temperature. Uni-axial tensile tests were, therefore, carried out between 298 K and 473 K to determine the constitutive behavior of each solder. 95Sn-5Sb exhibited the highest strength over the temperature range tested except near 473 K. Pb-based alloys with a higher degree of solid solution (>5%) showed greater strengthening than those primarily strengthened by coarse precipitates. Additionally, microstructure changes in 90Pb-10Sn and 95Sn-5Sb were shown to be responsible for unexpected mechanical behavior at elevated temperatures.     

Interfacial Reaction Between Cu Substrates and Zn-Al Base High-Temperature Pb-Free Solders

Chemical reactions between Cu substrates and Zn-Al high-temperature solder alloys, Zn-4Al and Zn-4Al-1Cu (mass%), at temperatures ranging from 420°C to 530°C were experimentally investigated by a scanning electron microscope using backscattered electrons (SEM-BSE) and an electron probe microanalyzer (EPMA). Intermediate phases (IMPs), β(A2) or β′(B2), γ(D8₂), and ε(A3) phases formed and grew during the soldering and aging treatments. The consumption rate of the IMP for Cu substrates is described by the square root of t in both the alloys, while the additional Cu in the molten Zn-Al alloy slightly suppresses the consumption of Cu substrates. The growth of IMPs during soldering treatment is controlled by the volume diffusion of constituent elements, and its activation energy increases in the order of Q _ε < Q _γ < Q _β. In view of the aging process, the growth of IMPs is considered to be controlled by the volume diffusion. In particular, the layer thickness of γ rapidly grows over 200°C, although the thickness of the β layer grows very slowly.     

Influence of Phosphorus Content on the Interfacial Microstructure Between Sn–3.5Ag Solder and Electroless Ni–P Metallization on Cu Substrate

Under bump metallization (UBM), which usually consists of a few thin metallic layers, provides good solderable surface while protecting the underlying metallization from reacting with solder. Electroless nickel (Ni-P) with a thin layer of immersion gold has been considered as one of the promising candidates for under bump and substrate metallizations. However, the presence of P in electroless Ni-P causes more complicated interfacial reactions with solder than pure Ni. The amount of P in the Ni-P layer affects the soldering reaction in terms of microstructure and reaction kinetics. In this paper, influence of P content on the interfacial microstructure between Sn-3.5Ag solder and electroless Ni-P metallization on Cu substrate has been investigated. Electroless Ni-P layers of three different P contents (6.1, 8.8, and 12.3 wt.%) with the same thickness were plated on Cu substrate. Multilayered samples with Sn-3.5Ag/Ni-P/Cu stack were then prepared and subjected to multiple reflows. Various types of interfacial compounds (IFCs) such as Ni₃Sn₄, Ni₃P, Ni-Sn-P, Cu-Sn, and Ni-Cu-Sn formed depending upon the number of reflows. Ni₃Sn₄ intermetallic compound that formed in the low P sample was found to be more stable, whereas, Ni₃Sn₄ that formed in the medium and high P samples mostly spalled off into the molten solder during reflow. The Ni ₃Sn₄ spallation was found responsible for thicker Cu-Sn and Ni-Cu-Sn intermetallics in the medium and high P samples as compared to that of low P sample. Explanation for the observed interfacial microstructure is proposed in the paper in detail     

Microstructure Evolution of SnAgCuEr Lead-free Solders Under High Temperature Aging

In this work, we have systematically investigated the evolution of microstructure and of intermetallic compounds (IMCs), in particular, for lead-free SnAgCuEr solders during isothermal aging tests. The effect of trace amounts of the rare earth element Er on this process has also been studied. The results indicate that diffusion and reassembly occur in the solder matrix during the aging process, and the major influence of the rare earth element Er is concentrated on the nucleation sites. ErSn₃ IMCs formed from the molten solder provide heterogeneous nucleation sites for the IMCs in the soldering and aging process. Subsequently, the Cu-Sn IMCs produced during soldering and Ag-Sn IMCs precipitated during the aging process have uniform size and evenly distribute in the solder matrix, and the refinement effect has been achieved in Er-containing solder joints. In addition, some cracks can be seen in Er-free solder joints, and the cracks may nucleate and propagate in the structure along the compound/solder boundaries after long aging times.     

SnAgCuY钎料高温时效过程的显微组织演化

研究了无铅钎料合金Sn3.8Ag0.7Cu高温时效过程中显微组织,特别是金属间化合物(IMC)的演化规律,以及稀土Y的添加对其产生的影响。结果表明:在高温时效过程中合金内部组元发生扩散与重组,伴随着共晶组织的逐渐溶解,新的IMC在组织内部呈球形弥散析出。结晶初期形成的具有规则形状的较粗大的IMC逐渐发生解体,树枝状富Sn相逐渐取代共晶组织成为受腐蚀的对象。随着时效时间的延长,合金内部各组元的成分也在不断发生变化。     

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.     

Correlations Between the Microstructure and Fatigue Life of Near-Eutectic Sn-Ag-Cu Pb-Free Solders

Relationships between the microstructure of near-eutectic Sn-Ag-Cu Pb-free solder joints and room-temperature fatigue lifetimes were studied. Correlations between the lifetimes of single Sn grained, SAC205 solder joints with the orientation of the Sn grain, and with differences in Ag₃Sn and Cu₆Sn₅ precipitate microstructures were sought. Correlations between the number of Sn grains and fatigue life were observed. Surprisingly, it was found that Ag₃Sn precipitates were highly segregated from Cu₆Sn₅ precipitates on a length scale of approximately 20 μm. Furthermore, large (factor of two) variations of the Sn dendrite arm size were observed within given samples. Such variations in values of dendrite arm size within a single sample were much larger than observed variations of this parameter between individual samples. Few significant differences were observed in the average size of precipitates in different samples. Although effects of average precipitate microstructure on lifetimes were not clearly delineated, one sample showed an anomalously high number of the smallest size (30 nm to 50 nm) Ag₃Sn precipitates, and this sample also exhibited a much longer lifetime than all the other samples. Thus, some evidence was presented that samples of particular orientations and precipitate microstructures can exhibit anomalous fatigue lifetimes.     

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 Between Molten Sn-Bi Based Solders and Electroless Ni-P Coatings for Liquid Solder Interconnects

This paper reports on the interfacial reactions and lifetime of electroless Ni-P coatings in contact with molten Sn-Bi based solders. A layer of approximately 4 $mu{hbox{m}}$ thick electroless Ni-P in contact with the molten Sn-58Bi solder began to fail at 48 h at temperatures between 200 $^{circ}{hbox{C}}$ and 240 $^{circ}{hbox{C}}$ . Elemental additions to modify the solder, included 1–2wt.% of Al, Cr, Si, Zn, Ag, Au, Ru, Ti, Pt, Nb, and Cu. Of these, only Cu modified the interfacial intermetallic compound growth from ${hbox{Ni}}_{3}{hbox{Sn}}_{4}$ to $({hbox{Cu,Ni}})_{6}{hbox{Sn}}_{5}$ , resulting in significantly decreased consumption rates of the Ni-P substrate in contact with the molten solder and increasing the lifetime of the Ni-P layer to between 430 and 716 h. Micro cracks were observed in all but the thinnest Ni-P layers, allowing the solder to penetrate.      

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.     

Effect of Zn Addition on Interfacial Reactions Between Sn-4Ag Solder and Ag Substrates

In this study, the effect of Zn (Zn = 1 wt.%, 3 wt.%, and 7 wt.%) additions to Sn-4Ag solder reacting with Ag substrates was investigated under solid-state and liquid-state conditions. The composition and microstructure of the intermetallic compounds (IMCs) significantly changed due to the introduction of different Zn contents. In the case of Sn-4Ag solder with 1 wt.% Zn, a continuous Ag-Sn IMC layer formed on the Ag substrates; discontinuous Ag-Zn layers and Sn-rich regions formed on the Ag substrates under liquid-state conditions when the Sn-4Ag solders contained 3 wt.% and 7 wt.% Zn. If 3 wt.% Zn was added to Sn-4Ag solder, the Ag-Sn IMC would be transformed into a Ag-Zn IMC with increasing aging time. Rough interfaces between the IMCs and the Ag substrates were observed in Sn-4Ag-7Zn/Ag joints after reflowing at 260°C for 15 min; however, the interfaces between the IMCs and the Ag substrates became smooth for Sn-4Ag-1Zn/Ag and Sn-4Ag-3Zn/Ag joints. The nonparabolic growth mechanism of IMCs was probed in the Sn-4Ag-3Zn/Ag joints during liquid-state reaction, and can be attributed to the detachment of IMCs. On the other hand, the effect of gravity was also taken into account to explain the formation of IMCs at the three different interfaces (bottom, top, and vertical) during the reflow procedure.     

Size and Substrate Effects upon Undercooling of Pb-Free Solders

The degrees of undercooling of various Pb-free solders are determined using differential scanning calorimetry. The effects of size, composition, and substrate upon undercooling are examined. Ni is the most effective element among Cu, Ni, and Ag in reducing the undercooling of Sn solders, both as an alloying addition and as a substrate. The degrees of undercooling and their variations are more significant for smaller-sized solders, but the relative orders of undercooling of various solders remain the same. It is concluded that the primary factors controlling undercooling are the primary solidification phase and the substrate. Different compositions of melts could have different primary solidifications, resulting in different degrees of undercooling. When the primary solidification phase and the substrates are the same, the degrees of undercooling could be different if the compositions of the melts are different. However, this compositional effect is not very significant.