On the Asymmetric Growth Behavior of Intermetallic Compound Layers During Extended Reflow of Sn-Rich Alloy on Cu

When solder interconnects are fabricated, a Sn-based alloy is melted between two substrates with metallization layers, such as Cu or Ni. From the reaction between Sn and Cu, a Cu₆Sn₅ intermetallic compound (IMC) layer is formed at the solder/Cu interfaces. The morphology of the IMC layer greatly influences the mechanical behavior of the solder joint. Here, we report on the characterization of a novel, asymmetric growth behavior of IMC layers in Sn-3.9Ag-0.7Cu solder joints, based on gravity-induced spalling of the IMC.     

Effect of Aging Temperature on the Microstructure and Shear Strength of SAC0307-0.1Ni Lead-Free Solders in Copper Joints

This paper presents an investigation of the effects of aging temperature on the microstructure and shear strength of SAC0307-0.1Ni/Cu solder joints. Single-overlap shear solder joints were aged for 1 h at 80, 130, and 180°C. The microstructure of the interface between the solder and the Cu substrate contained phase of the intermetallic compounds (IMCs) (Cu,Ni)6Sn5 formed along the interface. The shape of scallop-like (Cu,Ni)6Sn5 IMCs changed to the long dendrite and grew larger at the interface of solder joints after increased aging temperature. In addition, a phase of particle-like Ag3Sn IMCs was formed in the solder matrix. The growth of the interfacial IMC layer in the solder joints increased with increasing the aging temperature. The thickness of this layer was controlled by diffusion mechanism. The shear strength of the as-reflowed solder joints was greater than that of the aged solder joints, and the shear strength of all the aged solder joints decreased with increasing the aging temperatures. Therefore, the aging temperature mainly affected the thickness of the interfacial layer of IMCs and the shear strength of the solder joints.     

Interfacial IMC Layer and Tensile Properties of Ni-Reinforced Cu/Sn–0.7Cu–0.05Ni/Cu Solder Joint: Effect of Aging Temperature

Thermal aging behavior on the intermetallic compounds (IMCs) layer and mechanical properties of Cu/Sn–0.7Cu/Cu and Cu/Sn–0.7Cu–0.05Ni/Cu joints has been investigated from aging temperature of 60–180 °C for 100 h. Layer thickness increases as aging temperature rose for both the joints. Mechanical properties deteriorates with increase in aging temperature. After aging at 180 °C, any signs of ductile fracture surface with a large amount of dimples are absent. Instead, an intergranular fracture surface is obtained for both the joints, indicating that the process transformes from ductile to brittle behavior. However, brittle Cu₃Sn layer is observed between Cu₆Sn₅ layer and Cu substrate for Cu/Sn–0.7Cu/Cu joint after aging at 60 °C, while (Cu, Ni)₃Sn IMC layer is detected until aged at 140 °C for Cu/Sn–0.7Cu–0.05Ni/Cu. Compared with Cu/Sn–0.7Cu/Cu joint, the interfacial morphology directly changes from scallop-shaped into layer-shaped structure with lower Gibbs free energy, and the layer thickness is obviously suppressed after addition of Ni particle. Excellent mechanical properties, including UTS, elongation, and hardness, are obtained for Cu/Sn–0.7Cu–0.05Ni/Cu because of the slight increase in layer thickness and dense layer-shaped interfacial morphology. Thermal aging reliability is enhanced for the Cu/Sn–0.7Cu–0.05Ni/Cu solder joint after doping with 0.05 wt% Ni particle.     

The growth of Cu-Sn intermetallics at a pretinned copper-solder interface

This article reports a comparative study of the formation and growth of intermetallic phases at the interface of Cu wetted with a thick solder joint or a thin, pretinned solder layer. The η phase (Cu₆Sn₅) forms when Cu is wet with eutectic solder at temperatures below 400 °C. The intermetallic layer is essentially unaffected by aging at 70 °C for as long as 13 weeks. On aging a eutectic joint at 170 °C, the η-phase intermetallic layer thickens and ε phase (Cu₃Sn) nucleates at the Cu/intermetallic interface and grows to a thickness comparable to that of the η phase, while a Pb-rich boundary layer forms in the solder. The aging behavior of a thin, pretinned eutectic layer is qualitatively different. At 170 °C, the Sn in the eutectic is rapidly consumed to form η-phase intermetallic, which converts to ε phase. The residual Pb withdraws into isolated islands, and the solderability of the surface deteriorates. When the pretinned layer is Pb-rich (95Pb-5Sn), the Sn in the layer is also rapidly converted into η phase, in the form of dendrites penetrating from the intermetallic at the Cu interface and discrete precipitates in the bulk. How ever, the development of the intermetallic largely ceases when the Sn is consumed; ε phase does not form, and the residual Pb remains as an essentially continuous layer, preserving the solderability of the sample. These observations are interpreted in light of the Cu-Sn and Pb-Sn phase diagrams, the temperature of initial wetting, and the relative diffusivities of Cu and Sn in the solder and intermetallic phases. A.J. SUNWOO, Formerly with the Lawrence Berkeley Laboratory, Berkeley, CA,     

Influence of reflow and thermal aging on the shear strength and fracture behavior of Sn-3.5Ag solder/Cu joints

The mechanical behavior of Sn-rich solder/Cu joints is highly sensitive to processing variables such as solder reflow time, cooling rate, and subsequent thermal aging. In this article, we focus on the lap shear behavior of Sn-3.5Ag/Cu joints as a function of solder yield strength and intermetallic thickness. Experimental results showed that the shear strength of the solder joints is primarily controlled by the mechanical properties of the solder, and not the intermetallic thickness. The thickness of intermetallic, however, controlled the fracture mode of the solder joints. At intermetallic thicknesses greater than 20 μm, brittle fracture between Cu₆Sn₅ and Cu₃Sn was the most common failure mechanism. Finite-element simulations were carried out to evaluate the effect of solder properties and of intermetallic thickness and morphology on lap shear behavior. The finite-element simulations corroborated the experimental findings, i.e., that increased solder strength results in increased joint strength. The simulations also showed that thicker intermetallics, especially of nodular morphology, yielded higher local plastic shear strain and work hardening rate.     

Effect of Rare-Earth (La,Ce, and Y) Additions on the Microstructure and Mechanical Behavior of Sn-3.9Ag-0.7Cu Solder Alloy

In this article, we report on the microstructure and mechanical properties of Ce- and Y-containing Sn-3.9Ag-0.7Cu solders. The microstructures of both as-processed solder and solder joints containing rare-earth (RE) elements (up to 0.5 wt pct) are more refined compared to conventional Sn-3.9Ag-0.7Cu, with decreases in secondary Sn dendrite size and spacing and a thinner Cu₆Sn₅ intermetallic layer at the Cu/solder interface. These results agree well with similar observations seen in La-containing solders reported previously. The monotonic shear behavior of reflowed Sn-3.9Ag-0.7Cu-X(Ce, Y)/Cu lap shear joints was studied as well as the creep behavior at 368 K (95 °C). The data were compared with results obtained for Sn-3.9Ag-0.7Cu and Sn-3.9Ag-0.7Cu-XLa alloys. All RE-containing alloys exhibited creep behavior similar to Sn-3.9Ag-0.7Cu. Alloys with Ce additions exhibited a small decrease in ultimate shear strength but higher elongations compared with Sn-Ag-Cu. Similar observations were seen in La-containing solders. The influence of the RE-containing intermetallics (CeSn₃ and YSn₃) that form in these alloys on the microstructural refinement, solidification behavior, and mechanical performance of these novel materials is discussed.     

Mechanical strength of thermally aged Sn-3.5Ag/Ni-P solder joints

This work presents an investigation on the influence of the solder/under bump metallization (UBM) interfacial reaction to the tensile strength and fracture behavior of Sn-3.5Ag/Ni-P solder joints under different thermal aging conditions. The tensile strength of Sn-3.5Ag/Ni-P solder joints decreases with aging temperature and duration. Four types of failure modes have been identified. The failure modes shift from the bulk solder failure mode in the as-soldered condition toward the interfacial failure modes. Kirkendall voids do not appear to affect the tensile strength of the joint. The volume change of Ni-P phase transformation during the thermal aging process generates high tensile stress inside the Ni-P layer; this stress causes mudflat cracks on the remaining Ni-P coating and also leads to its delamination from the underlying Ni substrate. In general, interfacial reaction and the subsequent growth of Ni₃Sn₄ intermetallic compound (IMC) layer during solid-state reaction are the main reasons for the decrease of tensile strength of the solder joints. The current study finds there is an empirical linear relation between the solder joint strength and the Ni₃Sn₄ intermetallic compound (IMC) thickness. Therefore, the IMC thickness may be used as an indication of the joint strength.     

Wettability of SnCuNi-xEu solders and mechanical properties of solder joints

Wettability balance method was used to investigate the wetting performance of Sn Cu Ni-x Eu on Cu substrate, and the mechanical properties and the fracture morphology were studied.The results indicated that the addition of Eu could enhance the properties of solder and solder joints, with the increase of Eu content, tendency of first increase and then decrease could be found in the wetting time, wetting force and the mechanical properties of Sn Cu Ni-x Eu, and the optimal content was 0.039%.For Sn Cu Ni-0.039 Eu solder joints, the optimum mechanical properties could be found, and the amplitude increased was 20%, with the observation of the fracture morphology, it was found that small dimples could be seen, the toughness fracture for Sn Cu Ni and mixture fracture for Sn Cu Ni-0.039 Eu could be demonstrated.And thermal fatigue behavior of Sn Cu Ni solder joints could be enhanced obviously with the 0.039%Eu addition.     

Intermetallic growth and mechanical behavior of low and high melting temperature solder alloys

The presence of an intermetallic is often an indication of good wetting in a solder joint. However, excessive intermetallic growth and the brittleness of the intermetallic layer may be detrimental to joint reliability. This study examined the growth and mechanical behavior of interfacial intermetallics between copper and six solder alloys commonly used in electronics assembly. The solder alloys tested were 60Sn-40Pb, 63Sn-37Pb, 95Sn-5Sb, 96.5Sn-3.5Ag, 50Pb-50In, 50Sn-50In, and 40In-40Sn-20Pb. The 50Sn-50In and 40In-40Sn-20Pb exhibited faster solid state growth of the intermetallic layer at 100 °C as compared to the near-eutectic Sn-Pb control solder. The 50In-50Pb had a slower growth rate, relative to 63Sn-37Pb, at the aging temperature of 170 °C due to slower reaction rate kinetics of indium with copper. The 96.5Sn-3.5Ag and 95Sn-5Sb had similar intermetallic growth rates at 170 °C and 205 °C, and the aging was comparable to that of the 63Sn-37Pb alloy. The 95Sn-5Sb solder/copper intermetallic had a faster growth rate of the Cu₃Sn layer than was observed in the Sn-Ag or Sn-Pb alloys. Modified fracture toughness and low load indentation tests were used to characterize the mechanical behavior of the intermetallics. The intermetallics were harder than both the base metal and the solder alloy. The fracture behavior of the joints in tension was dependent upon the strength of the solder alloy. Solders with low strengths failed in the solder by plastic deformation. The failure of solders with higher strengths was dependent upon intermetallic thickness. When the intermetallic was thin, fracture occurred in the solder or at the solder/ intermetallic interface. As the interfacial intermetallic thickened, the fracture path moved into the intermetallic layer.     

Effect of Lanthanum on Driving Force for Cu6Sn5 Growth and Improvement of Solder Joint Reliability

By means of adding low content of rare earth element La into Sn6(bPb40 solder alloy, the growth of Cu6Sn5 intermetallic compound at the interface of solder joint is hindered, and the thermal fatigue life of solder joint is increased by 2 times. The results of thermodynamic calculation based on diffusion kinetics show that, the driving force for Cu6Sn5 growth is lowered by adding small content of La in Sn60-Pb40 solder alloy. Meanwhile, there is aneffective local mole fraction range of La, in which, 0.18% is the limited value and 0.08% is the best value.     

Ultrasonic-Assisted Soldering of 5056 Aluminum Alloy Using Quasi-Melting Zn-Sn Alloy

To obtain sound butt-joints of 5056 aluminum alloy rods, ultrasonic-assisted soldering was conducted using Zn-18Sn and Zn-60Sn alloys. Each solder foil was inserted between rods of 5056 aluminum alloy. Ultrasonic vibration was propagated to faying surfaces at soldering temperatures below the liquidus temperature of the solder alloys, and then the samples were air cooled to room temperature. The optimum vibration time at the soldering temperature must be more than 2 seconds to have complete wetting and less than 4 seconds to avoid excessive dissolution of the 5056 alloy. The 5056 alloy joints soldered using quasi-melting. Zn-Sn alloys showed greater strength than the joints soldered at the temperatures over its respective liquidus temperature. As the soldering temperature was increased, the increased formation of the intermetallic compound Mg₂Sn or phases containing Mg generated by dissolution of 5056 into the solder layer decreased the joint strength. Ultrasonic-assisted soldering at an optimum temperature between solidus and liquidus of the Zn-Sn alloys is an important consideration for producing sound joints with sufficient strength.     

Effects of cerium on Sn-Ag-Cu alloys based on finite element simulation and experiments

Effect of small addition of rare earth on Sn-Ag-Cu solder was investigated by finite element method based on creep model of low stress and high stress and experiments respectively. It was found that addition of rare earths evidently improved the resistance to creep deformation of the solder, so that the reliability of Sn-Ag-Cu-Ce solder joint could be improved remarkably. Mechanical testing and microstructural analysis results showed that, mechanical properties of alloys bearing Ce were better than that of the original alloy, and the optimum content of Ce was about 0.03wt.%. After aging intermetallic compound between solder joint and Cu substrate was observed and analyzed by X-ray diffraction (XRD), scanning electron micrographs (SEM) and energy dispersive X-ray fluorescence spectrometer (EDX). Results showed that the thickness of intermetallic compound layer would became thinner when the addition of Ce was about 0.03wt.%, and the grains of intermetallic compound became finer, and the microstructure was more homogeneous than that in the original Sn-Ag-Cu/Cu interface.     

Brazing Inconel 625 Using Two Ni/(Fe)-Based Amorphous Filler Foils

For MBF-51 filler, the brazed joint consists of interfacial grain boundary borides, coarse Nb₆Ni₁₆Si₇, and Ni/Cr-rich matrix. In contrast, the VZ-2106 brazed joint is composed of interfacial Nb₆Ni₁₆Si₇ precipitates as well as grain boundary borides, coarse Nb₆Ni₁₆Si₇, and Ni/Cr/Fe-rich matrix. The maximum tensile strength of 443?MPa is obtained from the MBF-51 brazed specimen. The tensile strengths of VZ-2106 brazed joints are approximately 300?MPa. Both amorphous filler foils demonstrate potential in brazing IN-625 substrate.     

Interfacial Reactions,Microstructure, and Strength of Sn-8Zn-3Bi and Sn-9Zn-Al Solder on Cu and Au/Ni (P) Pads

The Sn-8Zn-3Bi and Sn-9Zn-Al Pb-free solders were used to mount passive components onto printed circuit boards (PCBs) with electroless Ni immersed Au (ENIG) finishing layers using a reflow soldering process. The component mounted boards were aged at 150 °C for 200 to 1100 hours. The interfacial reactions and microstructure of the interfaces between the solders and the pads were observed using scanning electron microscopy and energy-dispersive spectrometry (EDS). Both solder joints on the two pads had similar interfacial microstructures; i.e., a very thin γ ₂-AuZn₃ layer was formed at the interface of the solder and Ni-P layer. The γ ₂-AuZn₃ layer transformed to an ε-AuZn₈ intermetallic compounds (IMC) with a consistent thickness during aging. Zinc atoms redeposited onto the IMC layer increased with increasing aging time. After aging at 150 °C for various times, the shear strengths of the ENIG and organic solderability preservative (OSP) joints were evaluated. The shear strength of the Sn-8Zn-3Bi solder joint was better than that of the Sn-9Zn-Al solder joint. All of the solder joints deteriorated after aging; however, the degradations of the OSP solder joints were more evident than those of the ENIG solder joints.     

62Sn36Pb2Ag组装焊点长期贮存界面化合物生长动力学及寿命预测

Sn基合金焊接接头是电子产品不可或缺的关键部位,是实现电子元器件功能化的基础,电子整机失效往往由于焊点的损伤所导致,焊点的寿命预测对电子产品的可靠性研究具有重要意义.金属间化合物（IMC）厚度是衡量焊点质量的重要参数,以IMC层厚度为关键性能退化参数,以62Sn36Pb2Ag组装的小型方块平面封装（QFP）器件焊点为研究对象,采用扫描电子显微镜对在94、120和150°C三种温度贮存不同时间后的焊点微观形貌进行表征,测量了IMC层的厚度,基于阿伦尼乌斯方程建立了双侧界面金属间化合物生长动力学模型.并以其作为关键性能退化函数,通过对初始IMC厚度进行正态分布拟合获得失效密度函数,进而获得可靠度函数对焊点的长期贮存失效寿命进行了预测.研究结果有望对长期贮存焊点的寿命预测方式提供新的思路,为62Sn36Pb2Ag钎料的可靠应用提供试验和数据支撑.     

A Comparative Study on the Microstructure and Mechanical Properties of Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> and Cu<Subscript>3</Subscript>Sn Joints Formed by TLP Soldering With/Without the Assistance of Ultrasonic Waves

In this study, the microstructure and mechanical properties of Cu₆Sn₅ and Cu₃Sn intermetallic joints, formed by the transient liquid phase (TLP) soldering process with and without the assistance of ultrasonic waves (USWs), were compared. After the application of USWs in the TLP soldering process, Cu-Sn intermetallic compounds (IMCs) exhibited a novel noninterfacial growth pattern in the molten solder interlayer. The resulting Cu₆Sn₅ and Cu₃Sn joints consisted of refined equiaxed IMC grains with average sizes of 3 and 2.3 µm, respectively. The Cu₆Sn₅ grains in the ultrasonically soldered intermetallic joints demonstrated uniform mechanical properties with elastic modulus and hardness values of 123.0 and 5.98 GPa, respectively, while those of Cu₃Sn grains were 133.9 and 5.08 GPa, respectively. The shear strengths of ultrasonically soldered Cu₆Sn₅ and Cu₃Sn joints were measured to be 60 and 65 MPa, respectively, higher than that for reflow-soldered intermetallic joints. Ultrasonically soldered Cu₆Sn₅ and Cu₃Sn joints both exhibited a combination of transgranular and intergranular fractures during shear testing.     

Dissolution and interface reactions between the 95.5Sn-3.9Ag-0.6Cu, 99.3Sn-0.7Cu, and 63Sn-37Pb solders on silver base metal

Dissolution and intermetallic compound (IMC) layer development were examined for couples formed between 99.9 silver (Ag) and molten 95.5Sn-3.9Ag-0.6Cu (wt pct), 99.3Sn-0.7Cu, and 63Sn-37Pb solders, using a range of solder temperatures and exposure times. The interface reactions that controlled Ag dissolution were sensitive to the solder composition. The Ag₃Sn IMC layer thickness and interface microstructure as a whole exhibited nonmonotonic trends and were controlled primarily by the near-interface solder composition. The kinetics of IMC layer growth were weakly dependent upon the solder composition. The processes of Ag dissolution and IMC layer growth were independent of one another.     

Grain boundary sliding in the presence of grain boundary precipitates during transient creep

A constitutive rate equation for grain boundary sliding (GBS), in the presence of grain boundary precipitates, is developed. Langdon’s GBS model is modified by incorporating physically de-fined back stresses opposing dislocation glide and climb and by modifying the grain size de-pendence of creep rate. The rate equation accurately predicts the stress dependence of minimum creep rate and change in activation energy occurring as a result of changing the grain boundary precipitate distribution in complex Ni-base superalloys. The rate equation, along with the math-ematical formulations for internal stresses, is used to derive a transient creep model, where the transient is regarded as the combination of primary and secondary stages of creep in constant load creep tests. The transient creep model predicts that the transient creep strain is dependent on stress and independent of test temperature. It is predicted that a true steady-state creep will only be observed after an infinitely long time. However, tertiary creep mechanisms are expected to intervene and lead to an acceleration in creep rate long before the onset of a true steady state. The model accurately predicts the strain vs time relationships for transient creep in IN738LC Ni-base superalloy, containing different grain boundary carbide distributions, over a range of temperatures.