Sn Grain Research Articles

Correlation between microstructure and corrosion behaviour of SnBi-graphene oxide composite coatings

In the present study, microstructure-electrochemical property correlation in electrodeposited SnBi-GO composite coatings containing different amounts of graphene oxide has been investigated. The SnBi-GO coatings exhibited uniform and compact morphology for lower GO concentrations, whereas for higher GO concentrations the morphological defects appeared. X-ray diffraction analysis showed the presence of Sn rich and a Bi rich phase in the coatings. Crystallite size of Bi-rich phase decreased whereas for Sn-rich phase it remained nearly unaffected with increasing GO addition. Growth texture analysis showed that the incorporation of GO enhanced growth along the low index planes for the Sn-rich grains and along the high index planes for the Bi-rich grains. Corrosion properties of coatings examined by the potentiodynamic polarisation and impedance spectroscopy methods revealed that the corrosion rate of the coatings decreased with increase in the GO content till an “optimum” addition of GO, after which the corrosion rate significantly increased with further GO addition. Microstructural analysis conducted using the electron backscatter diffraction (EBSD) technique showed that the coating with “optimum” GO amount which yielded the minimum corrosion rate contained highest fraction of low energy low angle grain boundaries which was due to the lowering of Bi dissolution in the Sn grains. Initial decrease in the corrosion rate was attributed to coating compactness, growth of grains along low energy index planes and GO impermeability. Increase in the corrosion rate after the optimum was attributed to increase in the morphological defects and possible galvanic coupling between the GO and the metals in the coatings. The enhanced corrosion properties of SnBi-GO coatings make them a viable material for interconnections used in the electronic components.

Suppressed Growth of (Fe, Cr, Co, Ni, Cu)Sn2 Intermetallic Compound at Interface between Sn-3.0Ag-0.5Cu Solder and FeCoNiCrCu0.5 Substrate during Solid-state Aging

High-entropy alloys (HEAs) are promising materials for next-generation applications because of their mechanical properties, excellent high-temperature stability, and resistance against oxidation and corrosion. Although many researchers have investigated high-temperature HEA applications, few have considered low-temperature applications. Here we demonstrate an unprecedented intermetallic compound of (Fe, Cr, Co, Ni, Cu)Sn2 at the interface between Sn-3.0Ag-0.5Cu (SAC) solder and FeCoNiCrCu0.5 HEA substrate after reflow at 400 °C. Significantly suppressed growth of intermetallic compound without detachment from the substrate was observed during thermal aging at 150 °C for 150 h. Sn grains with an average grain size of at least 380 μm are observed. The results reveal a completely new application for the fields of Sn-Ag-Cu solder and HEA materials.

Creep Behaviors Along Characteristic Crystal Orientations of Sn and Sn-1.8Ag by Using Nanoindentation

The creep mechanisms of Sn and Sn-1.8Ag along specific orientations are investigated by the constant-strain-rate nanoindentation method. Due to the anisotropy of Sn, the mechanical behaviors could be very different along different crystal orientations. For microelectronic applications, Ag is often added to Sn to increase its strength. Data from creep test show that Ag addition increases the stress exponent by 3, which indicates that the rupture time could be extended by Ag addition. Moreover, the creep rate of Sn (100) grain is lower than that of Sn (001) grain in a low stress regime, namely, that [100] in Sn would have better creep resistance for usual applications. After indentation, transmission electron images of Sn samples show that the slip systems are $$ (1\bar{1}0) $$ $$ [11\bar{1}] $$ in (100) grain and (101) $$ [1\bar{1}\bar{1}] $$ in (001) grain. Lastly, Sn-1.8Ag has better performance along [100] in creep resistance due to greater hindrance of Ag atoms on dislocation motion and its critical threshold stress.

Formation and homogenisation of Sn[sbnd]Cu interconnects by self-propagated exothermic reactive bonding

We produced SnCu interconnects by self-propagated exothermic reactions using AlNi NanoFoil at ambient conditions, through the instantaneous localised heat across the interfaces between Sn electroplated Cu substrates. This technique presents a great potential for electronics integration with minimal thermal effects to the components. However, the metastable phases resulted from the non-equilibrium interfacial reactions and solidification were inevitable under a highly transient regime due to a drastic heating/cooling (over 107 K/s). In this study, Finite Element Analysis was performed to predict the temperature profiles across bonding interfaces, which were subsequently correlated with the formation and homogenisation of the bonded structures during the bonding and post-bonding ageing process. It has been revealed that, for nano-sized metastable phases, their formation, morphologies and distribution were primarily attributed to the convective mass transportation, liquid-solid inter-diffusion, and directional non-equilibrium solidification of Sn in molten zone of the bonding interfaces. The non-equilibrium phases initially formed in the SnCu interconnects can be homogenised towards the equilibrium status by accelerated ageing. This was achieved through the coalescing and subsequent growth of the original nano-sized metastable phases, as a result of the solid-diffusion of Cu and Ag atoms at intergranular boundary regions of Sn grains, AlNi NanoFoil/Sn. and Cu/Sn interfaces.

Effect of FeCoNiCrCu0.5 High-entropy-alloy Substrate on Sn Grain Size in Sn-3.0Ag-0.5Cu Solder

High-entropy alloys (HEAs) are well known for their excellent high-temperature stability, mechanical properties, and promising resistance against oxidation and corrosion. However, their low-temperature applications are rarely studied, particularly in electronic packaging. In this study, the interfacial reaction between a Sn-3.0Ag-0.5Cu solder and FeCoNiCrCu0.5 HEA substrate was investigated. (Cu0.76, Ni0.24)6Sn5 intermetallic compound was formed the substrate at the interface between the solder and the FeCoNiCrCu0.5 HEA substrate. The average Sn grain size on the HEA substrate was 246 μm, which was considerably larger than that on a pure Cu substrate. The effect of the substrate on Sn grain size is due to the free energy required for the heterogeneous nucleation of Sn on the FeCoNiCrCu0.5 substrate.

Modeling Effect of Grain Orientation on Degradation in Tin-Based Solder: I. Current-Driven Diffusion

In this and subsequent papers, we consider the effects of the microstructure, in particular grain orientations, and the associated anisotropy of Sn grains on electromigration-induced degradation in lead-free solder bumps. This paper investigates the effect of the anisotropy of Sn grains on current-driven self-diffusion and the resulting atomic flux divergence that is associated with material depletion at the cathode side of the bump and ultimately failure. Finite-element thermoelectric simulations of a section of a chip scale package and a refined submodel are carried out assuming single-crystal and bi-crystal solder bumps. Anisotropic material properties of $\beta $ -Sn are used in the simulations. By varying crystal orientation of the solder bumps, the effect of grain orientation on the current density, temperature, and atomic flux divergence is studied and discussed. In the future papers, we will consider the coupled mechanical response including stress-driven diffusion.

Improving thermal shock response of interfacial IMCs in Sn–Ag–Cu joints by using ultrathin-Ni/Pd/Au metallization in 3D-IC packages

The feasibility of novel ultrathin-Electroless Ni/Electroless Pd/Immersion Au (ultrathin-ENEPIG) metallization containing ultrathin electroless Ni layer for 3D-IC packaging application was evaluated by thermal shock test in this study. Via employing ultrathin-ENEPIG pad, the joints showed higher resistance against thermomechanical stress due to minor degradation in solder and the strength enhancement of interfacial IMC. In the as-fabricated joints, larger Sn grain formed in the ultrathin-ENEPIG joints in contrast to those in the conventional-ENEPIG one, which caused the distinct failure modes between two different systems under the test. Additionally, a dual-layer structure of high/low Ni–(Cu,Ni)6Sn5 and a single layer of low Ni–(Cu,Ni)6Sn5 could be observed in the conventional-ENEPIG and the ultrathin-ENEPIG joints, respectively, after testing. The dual layer (Cu,Ni)6Sn5 degraded the thermal shock performance of conventional-ENEPIG joints as a result of the weak bonding of interface between high/low Ni–(Cu,Ni)6Sn5 layers. However, through employment of ultrathin-ENEPIG substrates, the electroless Ni layer was completely exhausted and thus only one layer of low Ni–(Cu,Ni)6Sn5 intermetallic formed in the ultrathin-ENEPIG joints. The formation of dual-layer compound was suppressed and the fracture between layers was inhibited, leading to a stronger bonding at interface. Furthermore, the overall thickness of interfacial intermetallic compound was also reduced due to the suppression of high Ni–(Cu,Ni)6Sn5 layer. Influences of Ni thickness on the related mechanisms behind dual-layer intermetallic suppression and the Sn grain structure in ultrathin-ENEPIG joints were addressed and discussed in details.

Influence of Sn precursors on Ge1−xSnx growth using metal-organic chemical vapor deposition

We have investigated the metal-organic chemical vapor deposition (MOCVD) of Ge1−xSnx epitaxial layers using tetrakis-dimetyl-amino-tin (TDMASn), which is expected to be more easily decomposable than other Sn precursors such as tri-butyl-vinyl-tin (TBVSn). We found that the deposition rate of Sn atoms using TDMASn is higher than that using TBVSn at a growth temperature as low as 300 °C. Also, we examined the MOCVD growth of a high-Sn-content Ge1−xSnx layer with TDMASn at a low temperature below 300 °C, and we found that the Sn content saturated at about 5%. The volume of precipitated Sn grains on the surface increases with the partial pressure of Sn precursor, the reason why the substitutional Sn content in Ge1−xSnx is limited despite increasing the partial pressure of a Sn precursor would not be because of the deposition-rate limitation of Sn by surface chemical reaction but the precipitation of Sn in this MOCVD condition with TDMASn.

Effect of Sn grain orientation on growth of Cu-Sn intermetallic compounds during thermomigration in Cu-Sn2.3Ag-Ni microbumps

The effect of Sn orientation on thermomigration was investigated in Cu/Sn2.3Ag/Ni microbumps with diameter of 20 μm. Results showed that anisotropic diffusion of Cu in Sn grains affected both formation rates and locations of Cu6Sn5 intermetallic compounds (IMCs). Rapid growth of Cu6Sn5 IMCs at the cold-end interface was observed when the c-axis of Sn grains was parallel to the temperature gradient. In addition, diffusion of Cu atoms was blocked by a Sn grain with a high angle between the Sn c-axis and the thermal gradient, resulting in formation of Cu6Sn5 IMCs near the grain boundary inside the solder.

Effects of Sn grain size on intermetallic compounds formation in 5 µm diameter Cu/Sn pillar bumps

Uniform 5 µm diameter Cu/Sn micro-bump array has been fabricated by multilayer electrodeposition, and the effect of Sn grain orientation and size on Cu6Sn5 and Cu3Sn growth have been studied through FIB–SEM and electron backscatter diffraction. The solder layer in Ф5 µm Cu/matte Sn micro-bump we fabricated contains only four to five columnar Sn grains with no obvious preferred orientation and there is no significant difference in the growth of Cu6Sn5 in different oriented Sn grains. However, the Sn grain size has much more effect on intermetallic compound (IMC) growth. At 150 °C, the diffusion coefficient of Cu/bright-Sn system(~ 200 nm Sn) is calculated to be 13.09 × 10−17 m2 s−1, nearly three times that of Cu/matte–Sn system (2–5 µm Sn), about 4.51 × 10−17m2 s−1. Also, a simple model of boundary diffusion controlled IMC growth is proposed to explore the mechanism of grain size effect. Large Sn grain size will slow down the interfacial reaction in micro-bump due to low grain boundary density and large bulk diffusion distance.

Solid-State Diffusion of Bi in Sn: Effects of β-Sn Grain Orientation

Bismuth (Bi) is an alloying element being considered for inclusion in lead-free solder alloys to improve microstructure, properties, and reliability. It has been determined that aging a Bi-bearing alloy will preserve its strength, whereas the strength of SAC alloys is degraded. Examination of microstructure reveals that, over time, homogenization of Bi in the tin (Sn) matrix via solid-state diffusion occurs, which leads to stabilization of mechanical properties. In this study, the effect of Sn grain orientation on the diffusivity of Bi was analyzed. Sn was solidified slowly to produce coarse grains typical of grain morphologies in solder joints. Bi was subsequently sputtered onto the Sn, and samples underwent annealing at 125°C for 24 h. Electron probe microanalysis (EPMA) was utilized to collect compositional data, and diffusivities were extracted for several grain orientations. It was determined that the diffusivity of Bi in Sn has a low anisotropy ratio, with a difference in diffusivity of around an order of magnitude between Sn samples oriented with the concentration gradient perpendicular to the ‘c’ axis and those oriented parallel to the ‘c’ axis. As a result, low angle grain boundaries in the Sn samples can have a profound effect on the diffusivity and confound the orientation relationship.

Superior electromigration resistance of a microsized solder interconnection containing a single-Sn grain

With solder interconnections downsizing to the micrometer scale in integrated circuit packaging, the microstructural nonuniformity generated by the limited number of β-Sn grains is expected to have a significant influence on joint stability when subjected to electromigration. Using electron backscattered diffraction for direct observations, this study investigated the microstructural evolution of solder joints under current stressing. When a joint consists of multiple Sn grains, the grain boundaries that traverse the joint provide fast electromigration paths for Cu, causing severe intermetallic compound growth accompanied by grain rotation due to tensile stress, whereas significantly higher electromigration resistance is observed in a single-grain joint. This strong influence of the number of Sn grains indicates that, unlike the situation in large-sized joints, electromigration through grain boundaries is more significant than electromigration through a crystal lattice in joint degradation at the micrometer scale.

Interfacial IMC growth of SAC305/Cu joint with a novel dual-layer of Ni(P)/Cu plating during solid-state aging

A dual-layer barrier of electroless-plated Ni(P)/electroplated Cu was prepared as under-bump metallization (UBM) joined with Sn-3.0Ag-0.5Cu (SAC305) lead-free solder. The interfacial reaction and growth mechanisms of the intermetallic compounds (IMCs) formed between the SAC305 solder and the dual-layer UBM during reflow and isothermal aging at different temperatures were systematically investigated. It was found that only a single (Cu,Ni)6Sn5 IMC layer was observed between the SAC305 solder and electroless Ni(P) layer. At low aging temperature (150 °C), there was a very slow IMCs growth due to the protection of thin (Cu,Ni)6Sn5 layer, and the surface morphologies of interfacial (Cu,Ni)6Sn5 grains exhibited hexagonal-type rod-like. While during aging process of 180 °C temperature, most IMC morphologies of (Cu,Ni)6Sn5 grains exhibited scallop-like and only a small part of IMCs exhibited hexagonal-like. And these prism hexagonal-like IMCs gradually transformed into scallop-like morphologies with increasing aging time. When the aging temperature reached up to 210 °C, the IMCs had the fastest grow rate and almost all IMC morphologies of (Cu,Ni)6Sn5 grains exhibited scallop-like in every stage of reactions. Compared with the interfacial IMCs layer evolutions of the Sn-based solders with bare Cu or Ni(P)/Cu UBMs, the interfacial IMCs layer between the dual-layer of Ni(P)/Cu plating and solder possessed better thermal stability, indicating that the dual-layer of electroless-plated Ni(P)/electroplated Cu was a excellent diffusion barrier in the microelectronics packaging.

The effect of platinum contact metallization on Cu/Sn bonding

In this work, formation and evolution of microstructures in CuSn/Pt bonding were investigated after 320 °C reflow process as well as after high temperature storage test at 150 °C. Sputtered thin film platinum on silicon wafer and high purity platinum sheet were applied as contact metallizations for electroplated copper-tin based bonding metallurgy. As bonded microstructure showed PtSn4 intermetallic compound growth at the Pt/Sn interface, and both Cu6Sn5 and Cu3Sn phases formed at the Cu/Sn fiinterface. Both hexagonal and monoclinic Cu6Sn5 were found to coexist after 1000 h high temperature storage test. Platinum was discovered to dissolve into the Cu6Sn5 phase during soldering process and form (Cu, Pt)6Sn5 intermetallic compound exhibiting hexagonal allotropy. Meanwhile, under annealing, monoclinic Cu6Sn5 phase layer without platinum was observed to form between (Cu, Pt)6Sn5 grains and tin. Thermodynamic analysis was performed in order to reason the effects of Pt on the phase equilibria and phase stabilities. Results show that platinum has a significant impact on the stability of hexagonal Cu6Sn5.

Growth nature of in-situ Cu6Sn5-phase and their influence on creep and damping characteristics of Sn-Cu material under high-temperature and humidity

This paper describes the morphology and growth nature of in-situ Cu6Sn5 intermetallic compound (IMC) and their impacts on material properties of an environmental-friendly Sn-0. 7Cu (wt%) material when exposed to high-temperature (85 °C) and relative humidity (85%) environments. A detail microstructural characterization is carried-out by electron microscopy e.g., SEM, EBSD and TEM techniques. In as-cast Sn-Cu material, along with the fine matrix Sn grains, the in-situ Cu6Sn5 IMC in grain boundary and interior grain appear with a dimension of submicron size and more consistently spread in the matrix. Such fine and uniform dispersed IMC acted as a pinning effect that obstacle the dislocation movement and enhanced their hardness and creep performance. In contrast, after exposing at harsh environment, such in-situ IMC phase morphologies are changed and seemed to a coarse elongated-shaped IMC and reduced their aspect-ratio. These morphological changes negatively impact on the mechanical reliability of Sn-Cu material. A contrast between the as-cast specimen and specimen exposed to high-temperature and relative humidity presents that the electrical resistivity reduces to 12%, where their hardness degrades about 18.5%. However, it is worthy noted that the coarse IMC positively impacts on damping property of Sn-Cu material at given a strain and temperature.

The nucleation-controlled intermetallic grain refinement of Cu-Sn solid-liquid interdiffusion wafer bonding joints induced by addition of Ni particles

In this study, Ni particle was validated as an effective grain refiner for the Cu-Sn solid-liquid interdiffusion (SLID) wafer bonding joints, which induced a dispersive non-interfacial nucleation of intermetallic grains, effectively preventing the grain mergence phenomenon. With increasing the Ni particle addition, there were significant reductions in both the (Cu, Ni)6Sn5 grain size and the Cu3Sn layer thickness, 6 wt% addition of Ni particles even led a formation of the intermetallic joints only consisted of (Cu, Ni)6Sn5 grains with an average grain size of 1.3 μm. (Cu, Ni)6Sn5 grain refinement could notably enhanced the shear strength of the intermetallic joints.

IMC growth behavior along c-axis of Sn grain under current stressing

Due to the asymmetric structure of Sn unit cell, the electromigration behaviors of Sn based solder joints were significantly depended on the crystal orientation of Sn grain. The present investigation illustrated the effect of Sn grain c-axis on the intermetallic compounds (IMCs) growth under current stressing. The Cu particles reinforced Sn3.5Ag solder joint was selected to obtain various observed surfaces of the sample after 46 days current stressing. The front side, back side, right side and anode side of the sample represented different phenomena, which reflected and proved the importance of c-axis orientation on Cu atomic migration in three-dimension space. Besides, another composite solder joint was polished severe times on the observed surface which was full of IMCs. It showed that the IMCs were covered on the Sn-based solder, and there was crack at the cathode underneath the front surface after 38 days current stressing. The IMC growth behavior would be well understood by systematic study in this work.

Dependence of grain orientation in SABI333 solder joints on solidification temperature

Electron backscattered diffraction was employed to observe the orientations of as-solidified Sn3.0Ag3.0Bi3.0In (SABI333) joints. Unlike other Sn-based solder joints consisting of only one grain or of tricrystals, the SABI333 joints typically consisted of polycrystals. The polycrystals were found to originate from Sn grains with deflections (> 15°) from the [110] and [ $$1\bar {1}0$$ ] directions, i.e., the directions in which nucleation tends to occur, and the Bi and In atoms dissolved in the SnAg melt significantly enhanced the deflection. In addition, the orientations of the SABI333 solder joints were highly related to the solidification temperature. Higher solidification temperatures were associated with higher degrees of deviation. Additionally, two types of double tricrystals of Sn formed during solder joint solidification in the SABI333 joints. Although the characteristics of the twinning differed, both types had 80°–90° misorientations. One system featured four orientations of Sn grain twinning with a [100] or [010] axis, while the other one featured perpendicular tricrystals.

Electrochemical behavior of SnNi-graphene oxide composite coatings

It has been illustrated by researchers that graphene and graphene oxide can be embedded into metal matrix to produce highly corrosion resistant composite coatings. This paper investigates the effect of graphene oxide (GO) on microstructure and corrosion behavior of Sn-rich, SnNi-GO composite coatings. SnNi-GO composite coatings were electrodeposited on mild steel substrate by dispersing chemically synthesized GO into the electrolyte bath. Amount of GO in the composite coatings was varied by changing the concentration of GO in the electrolyte bath used for the electrodeposition. Morphological characterization of the coatings revealed the presence of rod shaped features in a flat matrix. Volume fraction of the rod-shaped features progressively increased with the addition of GO into the coatings. Structural characterization revealed the presence of Sn rich phase along with Ni3Sn4, Ni3Sn2 and Ni3Sn intermetallic phases in all the coatings. Crystallite size of the Sn-rich phase decreased significantly due to the addition of GO into the coatings. Corrosion behavior of the coatings was examined through potentiodynamic polarization and electrochemical impedance spectroscopy methods. A significant improvement in the corrosion resistance in terms of reduction in corrosion current and corrosion rate and increase in polarization resistance was noted in the case of SnNi coating containing GO. Corrosion resistance of the coatings increased with increase in the GO content. Microstructural characterization of the cross-section coating sample performed using transmission electron microscopy (TEM) technique revealed differences between the microstructures of coating containing different amounts of GO. All the coatings with GO contained almost pure Sn grains with Ni present at the grain boundaries. Size of the Sn sub-grains within the larger Sn grains, however, decreased with increase in the GO content in the coatings. Smaller Sn sub-grains and presence of Ni at the grains boundaries led to quicker formation of the corrosion products that are SnO and NiO with increasing GO content in the coatings eventually yielding the highest corrosion resistance value for the coating containing maximum amount of GO.

Refinement of the β-Sn Grains in Ni-Doped Sn-3.0Ag-0.5Cu Solder Joints with Cu-Based and Ni-Based Substrates

A fine-grain structure with random orientations of lead-free solder joints was successfully obtained in this study. The Sn-Ag-Cu solder alloys doped with minor Ni were reflowed with Ni-based or Cu-based substrates to fabricate the joints containing different Ni content. Adding 0.1 wt.% Ni into the solder effectively promoted the formation of fine Sn grains, and reflowing with Ni-based substrates further enhanced the effects of β-Sn grain refinement. The crystallographic characteristics and the microstructures were analyzed to identify the solidification mechanism of different types of microstructure in the joints. The phase precipitating order in the joint altered as the solder composition were modified by elemental doping and changing substrate, which significantly affected the efficiency of grain refinement and the final grain structure. The formation mechanism of fine β-Sn grains in the Ni-doped joint with a Ni-based substrate is attributable to the heterogeneous nucleation by Ni, whereas the Ni in the joint using ChouCu-based substrate is consumed to form an intermetallic compound and thus retard the effect of grain refining.