Microstructure Of Sn Research Articles

Viscoplastic Creep Response and Microstructure of As-Fabricated Microscale Sn-3.0Ag-0.5Cu Solder Interconnects

The viscoplastic behavior of as-fabricated, undamaged, microscale Sn-3.0 Ag-0.5Cu (SAC305) Pb-free solder is investigated and compared with that of eutectic Sn-37Pb solder and near-eutectic Sn-3.8Ag-0.7Cu (SAC387) solder from prior studies. Creep measurements of microscale SAC305 solder shear specimens show significant piece-to-piece variability under identical loading. Orientation imaging microscopy reveals that these specimens contain only a few, highly anisotropic Sn grains across the entire joint. For the studied loads, the coarse-grained Sn microstructure has a more significant impact on the scatter in primary creep compared to that in the secondary creep. The observed lack of statistical homogeneity (microstructure) and joint-dependent mechanical behavior of microscale SAC305 joints are consistent with those observed for functional microelectronics interconnects. Compared with SAC305 joints, microscale Sn-37Pb shear specimens exhibit more homogenous behavior and microstructure with a large number of small Sn (and Pb) grains. Creep damage in the Pb-free joint is predominantly concentrated at highly misoriented Sn grain boundaries. The coarse-grained Sn microstructure recrystallizes into new grains with high misorientation angles under creep loading. In spite of the observed joint-dependent behavior, as-fabricated SAC305 is significantly more creep resistant than Sn-37Pb solder and slightly less creep resistant than near-eutectic SAC387 solder. Average model constants for primary and secondary creep of SAC305 are presented. Since the viscoplastic measurements are averaged over a wide range of grain configurations, the creep model constants represent the effective continuum behavior in an average sense. The average secondary creep behavior suggests that the dominant creep mechanism is dislocation climb assisted by dislocation pipe diffusion.

Influence of temperature-induced liquid–liquid structure transition on directional solidification microstructure of Sn–Bi40wt-% alloy

Prior investigation suggested that a temperature-induced liquid–liquid structure transition (TI-LLST) could occur in Sn–Bi40wt-% alloy around 879°C. Based on the results of TI-LLST, the Sn–Bi40wt-% alloys are melted and held at the temperature above and below TI-LLST, respectively, and then solidified from the same temperature. The aim of this work was to explore the influence of TI-LLST on directional solidification microstructure by using the Bridgman method. The results show that dendritic arm spacing decreases and the microstructure is refined markedly for the samples solidified from the melt which experienced the TI-LLST.

Electromigration and Thermomigration in Pb-Free Flip-Chip Solder Joints

Pb-free solders have replaced Pb-containing SnPb solders in the electronic packaging industry due to environmental concerns. Both electromigration (EM) and thermomigration (TM) have serious reliability issues for fine-pitch Pb-free solder bumps in the flip-chip technology used in consumer electronic products. We review the unique features of EM and TM in flip-chip solder bumps, emphasizing the effects of current crowding and Joule heating. In addition, the challenges to a better understanding of EM and TM in Pb-free solders are discussed. For example, the anisotropic nature of Sn microstructure in Pb-free solders can enhance the dissolution rates of Ni and Cu in solders driven by EM and TM.

Effect of praseodymium on the microstructure and properties of Sn3.8Ag0.7Cu solder

Effects of Pr addition on wettability, microstructure of Sn3.8Ag0.7Cu solder were studied, the mechanical properties of solder joints were investigated and the fracture morphologies were also analyzed in this paper. The results indicate that adding appropriate amount of Pr can evidently improve the wettability of solder, and it is also found that Pr can refine the β-Sn dendrites and reduce the intermetallic compounds growth inside the solder due to the fine PrSn3 particles formed in the solder which can act as heterogeneous nucleation sites. Moreover, the joints soldered with the SnAgCuPr solders possess sound mechanical properties which may result from the finer microstructure improved by the Pr.

Fluid flow and microstructure formation in a rotating magnetic field during the directional solidification process

Abstract The influence of the fluid flow driven by rotating magnetic field (RMF) on the solidification microstructures of Sn–Bi alloys was investigated. It is found that, for the Sn–50 wt.% Bi hypoeutectic alloy, the primary phase (Sn) shows a morphology transition from coarse dendrite to equiaxed dendrite, moreover, the grains are fractured and refined with the application of RMF. As for the Sn–65 wt.% Bi hypereutectic alloy, the volume fraction gradient of the primary phases (Bi) at different positions reduces with the increase of magnetic intensity, and the macrosegregation is eliminated effectively. The theoretical calculation indicates that the tangential rate of the fluid flow has a maximum value at 0.55 r 0 , which is three magnitudes larger than the radial component. Furthermore, the microstructure formation mechanisms under the RMF condition were analyzed on basis of the calculation results and solidification theories.

Wetting properties and interfacial microstructures of Sn–Zn– xGa solders on Cu substrate

The wetting properties and interfacial microstructures of Sn–9Zn– xGa lead-free solders with Cu substrate were investigated. The wetting property is improved remarkably with the increase of Ga content in the Sn–9Zn lead-free solder. The lower surface tension, which results from the decrease of the oxidation of the Zn atoms owing to the formation of the Ga-rich protective film covered on the liquid solder, is the key reason for the better wettability. During soldering, the Cu 5Zn 8 compounds layer form at the interface of Sn–9Zn/Cu and the IMCs formed at the solder/Cu surface become much thicker when the Ga content is from 0.1 wt.% to 3 wt.%. However, neither Cu–Sn compounds nor Ga-rich phases are observed at the solder/Cu surface.

Formation mechanism of Sn-patch between SnAgCu solder and Ti/Ni(V)/Cu under bump metallization

An Sn-patch formed in Ni(V)-based under bump metallization during reflow and aging. To elucidate the evolution of the Sn-patch, the detailed compositions and microstructure in Sn–Ag–Cu and Ti/Ni(V)/Cu joints were analyzed by a field emission electron probe microanalyzer (EPMA) and transmission electron microscope (TEM), respectively. There existed a concentration redistribution in the Sn-patch, and its microstructure also varied with aging. The Sn-patch consisted of crystalline Ni and an amorphous Sn-rich phase after reflow, whereas V2Sn3 formed with amorphous an Sn-rich phase during aging. A possible formation mechanism of the Sn-patch was proposed.

Experimental investigation of the effect of solidification processing parameters on the rod spacings in the Sn–1.2 wt.% Cu alloy

Sn–1.2 wt.% Cu alloy was prepared in a graphite crucible under the vacuum atmosphere. The samples were directionally solidified upwards under argon atmosphere with different temperature gradients ( G = 2.69–8.88 K/mm) at a constant growth rate ( V = 6.80 μm/s) and with different growth rates ( V = 2.78–136.36 μm/s) at a constant temperature gradient ( G = 2.69 K/mm) by using a Bridgman-type directional solidification apparatus. The microstructure of Sn–1.2 wt.% Cu alloy seems to be rod eutectic structure. The rod spacings ( λ) were measured from both transverse and longitudinal sections of the samples. The influence of the growth rate ( V) and temperature gradient ( G) on the rod spacings ( λ) and undercoolings (Δ T) was analysed. The values of λ 2 V, λ 2 G, Δ Tλ, Δ TV −0.5 and Δ TG 0.5 were determined by using the Jackson–Hunt eutectic theory. The results obtained in the present work have been compared with the similar experimental results obtained in the previous works for binary alloys.

Microstructure and Orientation Evolution of the Sn Phase as a Function of Position in Ball Grid Arrays in Sn-Ag-Cu Solder Joints

The microstructure evolution of Sn-Ag-Cu solder joints during aging and thermal cycling is studied, with a focus on the Sn grain orientation in plastic ball grid array (PBGA) packages. Thermally cycled PBGA packages with a full array of 196 solder joints were examined after being subjected to various preconditions. Each PBGA package was polished to obtain plan-view crosssections of each solder joint. Solder joints were characterized using both polarized optical microscopy and orientation imaging microscopy (OIM). The observations reveal that the distribution of single and multigrain Sn microstructure as a function of position in the package is dependent on the sample’s preconditions and thermal cycle history. Based on distribution maps from polarized optical microscopy observation, thermal aging has a relatively small impact on the overall fraction of single-grained solder joints. Thermal cycling, however, can cause many single-grained joints to transform into multigrained solder joints. The dependence of the grain structure distribution on different preconditions and evolution of the grain structures during thermal cycling are discussed.

Effects of composition and cooling rate on the microstructure of Sn–3.7Ag–0.9Zn–Bi solders

The effects of Bi addition, of less than 3 wt.%, and applied cooling rate on the solidified microstructure of the eutectic Sn–3.7Ag–0.9Zn (weight percent, hereafter) solder were investigated. As observed by microstructural analysis, the increase of Bi content favors the separation of the β-Sn and AgZn intermetallic compounds (IMCs) in the eutectic Sn–Ag–Zn solder. And there are some Bi precipitates formed along with the primary β-Sn dendrites as the concentration of Bi exceeds 2%. As the applied cooling rate increases, the microstructure of the Sn–3.7Ag–0.9Zn–Bi solder is refined, and the segregation of Bi is restrained. By increasing the amount of Bi, the microhardness of the solder increases.

Effects of trace amount addition of rare earth on properties and microstructure of Sn–Ag–Cu alloys

Ternary lead free solder alloys Sn–Ag–Cu were considered as the promising alternatives to conventional SnPb alloys comparing with other solders. In the present work, effects of trace amounts of rare earth Ce on the wettability, mechanical properties and microstructure of Sn–Ag–Cu solder have been investigated by means of scanning electron microscopy and energy dispersive X-ray analysis systematically. The results indicate that adding trace amount of rare earth Ce can remarkably improve the wettability, mechanical strength of Sn–Ag–Cu solder joint at different temperature, especially when the content of rare earth Ce is at about 0.03%, the tensile strength will be 110% times or more than that of the lead free solder joint without rare earth Ce addition. Moreover, it was observed that the trace amount of rare earth Ce in Sn–Ag–Cu solder may refine the joint matrix microstructure, modify the Cu6Sn5 intermetallic phase at the copper substrate/solder interface, and the intermetallic compound layer thickness was reduced significantly. In addition, since rare earth Ce possesses a higher affinity to Sn in the alloy, adding of rare earth Ce can also lead to the delayed formation and growth of the intermetallic compounds of Ag3Sn and Cu6Sn5 in the alloy.

Effect of Cyclic Damage on the Constitutive Behavior & Microstructure of Sn3.0Ag0.5Cu (SAC305) Solder

AbstractThis study examines the effect of cyclic damage on the constitutive response and microstructural evolution of SAC305 solder. Cyclic damage is induced through isothermal, mechanical cycling tests at high strain rate and room temperature, using modified lap shear microscale specimens (180μm wide solder joint). The properties of interest are elastic, plastic, yield, and viscoplastic material constitutive behavior. In the current study, creep strain accumulation is accommodated when determining the constants, unlike those reported in prior studies [1]. Insights into the evolution of the measured properties are provided by correlating previously reported microstructural grain evolution of microscale SAC305 solder as a function of cyclic damage [2, 3].The hysteresis response and the elastic, plastic and yield measurements from the initial cycles show significant piece-to-piece variability (similar to prior virgin state viscoplastic measurements [3]). The scatter arises since as-reflowed SAC solder joints at length scales of 200μm consist of only a few anisotropic Sn grains that make the joint mechanically inhomogeneous. However, when subject to mechanical cycling fatigue at room temperature these joints undergo grain homogenization due to recrystallization, which is a possible explanation to the drop in scatter with progressing damage. The observed grain evolution is similar to that seen in solder joints under life-cycle loading.The elastic-plastic response and yield strength of SAC305 solder do not show significant contribution from creep deformations at the chosen load levels. The properties degrade with increasing accumulated cyclic damage, at a rate that is proportional to the severity of the cyclic load. The yield stress measurements suggest that SAC305 obeys an independent hardening rule, rather than isotropic or kinematic hardening. The performance of a continuum damage mechanics based model from prior studies in representing the measured degradation in elastic, plastic and yield properties is discussed [4,5].Comparison of the creep behavior of cycled SAC305 specimens (at 50% load drop) with that of uncycled specimens shows that the effective creep compliance and effective secondary creep strain rate increase significantly. As a point of comparison, the creep resistance of cycled SAC305 specimens is even lower than that of as-reflowed Sn37Pb specimens. Similar changes are seen in the stress relaxation behavior. Challenges and limitations of the current studies are included.

Influence of Cu addition on intermetallic compound formation and microstructure of Sn–3Ag–1˙5Sb–xCu solder joints

This study investigates the influence of 0–1˙5 wt-%Cu addition on the microstructure and the intermetallic compound (IMC) formation of the as soldered Sn–3Ag–1˙5Sb–xCu (wt-%) solders and following thermal storage at 150°C for 0, 25, 200 and 600 h, with the intention of identifying the optimum Cu addition for industrial applications. The experimental results show that the melting point of Sn–3Ag–1˙5Sb–xCu solder decreases with Cu addition. For Cu additions of 1˙0 wt-% or higher, an IMC of Cu6Sn5 particles is dispersed throughout the matrix, resulting in a dispersion strengthening effect, and its size increases with the levels of Cu addition increasing. The coarsened long strip like Cu6Sn5 with a length of more than 100 μm growing from the upper interface of IMC layer into the solder matrix is observed in the solder with 1˙5 wt-%Cu addition after thermal storage. Cu6Sn5 grains in the IMC layer develop the ripening grains with a more hexagonal or polygonal shape and smooth edged flat surfaces instead of scallop shape. Additionally, the microhardness of each solder increases with Cu addition and decreases with increasing time of thermal storage at 150°C.

Creep properties of Sn–Sb based lead-free solder alloys

Full implementation of the new generation of lead-free solders requires detailed knowledge and understanding of their mechanical behavior. This paper reports on structure, thermal and tensile creep properties of Sn–5 wt.%Sb, Sn–5 wt.%Sb–3.5 wt.%Ag, and Sn–5 wt.%Sb–1.5 wt.%Au lead-free solder alloys. The results show that the microstructure of Sn–5Sb alloy is characterized by the presence of cubed intermetallic compound (IMC) of SbSn particles (<5 μm) within β-Sn matrix. The two ternary alloys exhibit additional constituent phases of IMCs Ag 3 Sn for Sn–5Sb–3.5Ag and AuSn 4 for Sn–5Sb–1.5Au alloys. Attention has been paid to the role of IMCs on creep behavior. The tensile creep tests were performed within the temperature range 25–130 °C at constant applied stresses. Activation energy ( Q ) and stress exponent ( n ) were determined to clarify the deformation mechanism. This study revealed that the solder alloy Sn–5Sb–1.5Au have potential to gave a good combination of higher creep resistance and rupture time, lower melting temperature and higher fusion heat compared with the other two alloys.

The shearing behavior and microstructure of Sn–4Ag–0.5Cu solder joints on a Ni–P–carbon nanotubes composite coating

Abstract This study employed an electroless Ni–P–carbon nanotubes (Ni–P–CNTs) composite coating as a pad finish for electronic packaging. It was aimed at investigating the effect of CNTs on the mechanical behavior and microstructure of ball grid array (BGA) solder joints after multiple reflows. Electroless Ni–P and electroless Ni–P–CNTs composite coatings with the same P-content were prepared for comparison. It was found that the CNTs in the coating increased the brittleness of solder joints and weakened their shear strength. After shearing tests, more brittle fractures occurred in the intermetallic compound (IMC) layer in the Sn–4Ag–0.5Cu/Ni–P–CNTs (SAC/Ni–P–CNTs) solder joints. According to the scanning electron microscope (SEM) observation, a Ni3Sn4 IMC layer and a P-rich layer were formed in the solder joints on both coatings after multiple reflows. The IMC layers in the SAC/Ni–P solder joints were found to be compact with chunky-shaped grains, whilst the IMC layers in the SAC/Ni–P–CNTs solder joints were porous with needle-shaped grains. Due to the high stability of the CNTs in the metal matrix, the CNTs dispersed in the Ni–P coating acted as a retardant in the reaction of the Ni with the solder. Therefore, the growth rates of both the Ni3Sn4 IMC layer and the P richer layer in the SAC/Ni–P–CNTs solder joints were slower than those in the SAC/Ni–P solder joints.

Spalling behavior of interfacial intermetallic compounds in Pb-free solder joints subjected to temperature cycling loading

Morphological evolution of interfacial intermetallic compounds (IMC) subjected to thermal cycling loading was studied numerically and experimentally. In this study, the microstructure of Sn3.5Ag0.7Cu solder joint and its interfacial IMCs were investigated using Tape Ball Grid Array assemblies. Needle-type and hemispherical-type morphologies of interfacial IMCs were formed after reflow. During temperature cycling, the IMC grains ripened into large hemispherical-type and spheroidal-type morphologies and spalled into the solder joint. Finite element analysis showed that the spalling behavior was promoted by the geometrical changes of the IMC structure and the induced cyclic shear stresses during temperature cycling loading. For needle-type grains, the stress was concentrated at the tip of the IMCs, while the maximum shear stress was redistributed to the roots of the spheroidal-type grains.

Effect of Al content on the formation of intermetallic compounds in Sn–Ag–Zn lead-free solder

The effect of addition of Al, up to 1 wt.%, on the formation of intermetallic compounds in the microstructure of Sn–3.7%Ag–0.9%Zn lead-free solder was investigated. The typical microstructure of Sn–Ag–Zn solder is composed of the β-Sn phase and mixed granules which contain the intermetallic compounds (IMCs) of Ag3Sn and AgZn. After alloying with 0.5 wt.% Al, the microstructure of the explored solder evolves into a mixture of the bulk Agl IMCs and β-Sn phase, and most of the bulk Ag2Al IMCs distribute on and around the grain boundaries. The addition of 1 wt.% Al into the Sn–Ag–Zn solder brings many granules and bars of the Ag2Al IMCs, while the amount of the bulk Ag2Al IMCs decrease greatly. The above observation suggests that the bulk Ag2Al IMCs is replaced by the granule-like Ag2Al IMCs appearing along the grain boundaries. Since the grain size of the solder alloyed with 0.5%Al is relatively small as compared to the one alloyed with 1 wt.% Al, the growth of the Ag2Al IMCs was prompted through the feasible diffusion channels along the grain boundaries. Thus, the addition of Al plays an important role on the morphology of the Ag2Al IMCs in the final microstructure of the explored Sn–Ag–Zn solder.

Interfacial intermetallic phases and nanoeutectic in rapidly quenched Sn–Ag–Cu on Au under bump metallization

The microstructure of Sn 96.4Ag 2.8Cu 0.8 (at.%) solder alloy when rapidly quenched onto an Au surface finish has been found to range from nanometer to micrometer-sized features. This striking microstructure contained a spectrum of phases from the Au–Sn phase diagram, while Ag and Cu did not appear to participate in the solidification process. Within the solder contact, a eutectic with 40 nm interlamellar spacing was observed, along with the presence of layered intermetallics. Observations of these phases and their orientation offer better understanding of the solidification of Pb-free solders onto Au.

Effects of aging treatment on mechanical properties and microstructure of Sn–8.5Zn–0.5Ag–0.01Al–0.1Ga Solder

The aging effect on mechanical properties of Sn–8.5Zn–0.5Ag–0.01Al–0.1Ga solder was investigated. The specimens were treated at four different temperatures (−10 °C, room temperature, 80, 150 °C) for five different durations (0, 75, 150, 300, 1000 h). The corresponding microstructural variation was also investigated. The results showed that the microstructure of the solder did not change prominently when aging at −10 °C, although the tensile strength slightly increases. On the other hand, the solder underwent age-softening at room temperature, 80 and 150 °C. It appears that the age-softening of the solder was associated with the coarsening of Sn–Zn eutectic structure and grain growth of Sn matrix of the solder. The tensile strengths of the specimens aged at 150 °C were greater than those at 80 °C. This observation was explained by the solid solution hardening of greater Zn dissolution at higher temperature.

Shear strength and interfacial microstructure of Sn–Ag– xNi/Cu single shear lap solder joints

This study investigates composite lead-free solders fabricated by adding between 0.5 and 3 wt% of Ni particles in situ to Sn–3.5 wt%Ag lead-free solder. The single lap shear strength, fracture behavior and microstructural evolution characteristics of the as-reflowed specimens are examined and compared with those of specimens thermally aged at 150 °C for various aging times. In general, it is found that the single lap shear strength of the joints increases with increasing Ni addition in the as-reflowed condition, but decreases with increasing storage time in the aged specimens. For Ni additions of 0.5 and 1 wt%, the specimens fracture in the solder near the intermetallic compound (IMC) layer/solder interface, which suggests that the solder matrix has a lower strength than the IMC layer. The presence of elongated dimple-like structures on the fracture surfaces of these specimens is indicative of a ductile failure mode. For Ni additions of more than 1 wt%, the specimens fracture with brittle characteristics at the solder/IMC interface, which indicates that an increased Ni addition increases the strength of the solder matrix beyond that of the interfacial layer.