Sn Layer Thickness Research Articles

Solid-state interfacial reaction of Sn-Ag solders with polycrystalline Cu and (0 0 1) Cu substrate

• For sn-Ag solder, the growth of Cu 3 Sn on (0 0 1)Cu is faster than on polycrystalline Cu in aging. • Ag increases the difference in Cu 3 Sn thickness on (0 0 1)Cu and polycrystalline Cu in aging. • Ag is beneficial to maintain the preferred orientation of Cu 6 Sn 5 in aging. The growth of Cu 3 Sn is associated with the formation of Kirkendall voids and is important to the reliability of solder joints. In this paper, the effect of Ag on the growth behavior of Cu 3 Sn on (0 0 1) Cu and polycrystalline Cu during aging was compared. Besides, the orientation evolution of Cu 6 Sn 5 on (0 0 1) Cu during aging was also investigated. Results suggested the growth rate of Cu 3 Sn on (0 0 1) Cu is higher than that of polycrystalline Cu during solid-state aging and the difference in the Cu 3 Sn layer thickness increases with the increases of Ag content in the solder. Moreover, the addition of Ag is beneficial to maintaining the preferred orientation of Cu 6 Sn 5 on (0 0 1) Cu during solid-state aging. The results have great help to deepen the understanding of the difference in growth behavior of intermetallic compounds on (0 0 1) Cu and polycrystalline Cu during aging.

Cryogenic Temperature Growth of Sn Thin Films on Ferromagnetic Co(0001)

AbstractTopological electronic materials hold great promise for revolutionizing spintronics, owing to their topological protected, spin‐polarized conduction edge or surface state. One of the key bottlenecks for the practical use of common binary and ternary topological insulator materials is the large defect concentration that leads to a high background carrier concentration. Elemental tin in its α‐phase is a room temperature topological semimetal, which is intrinsically less prone to defect‐related shortcomings. Recently, the growth of ultrathin α‐Sn films on ferromagnetic Co surfaces has been achieved; however, thicker films are needed to reach the 3D topological Dirac semimetallic state. Here, the growth of α‐Sn films on Co at cryogenic temperatures was explored. Very low‐temperature growth holds the promise of suppressing undesired phases, alloying across the interfaces, as well as the formation of Sn pillars or hillocks. Nevertheless, the critical Sn layer thickness of ≈3 atomic layers, above which the film partially transforms into the undesired b‐phase, remains the same as for room‐temperature growth. From ferromagnetic resonance studies, and supported by electron microscopy, it can be concluded that for cryogenic Sn layer growth, the interface between Sn and Co remains sharp and the magnetic properties of the Co layer stay intact.

A large unidirectional magnetoresistance in Fe–Sn heterostructure devices

Unidirectional magnetoresistance (UMR) is an attractive magnetic-field sensing technique as it enables us to detect the in-plane direction of the external magnetic field with a single element. However, the UMR amplitude is typically several orders of magnitude smaller than those of other directional magnetoresistances exhibited in ferromagnetic thin films, hindering sensing applications using the UMR. For a directional magnetic sensor using the UMR, an in-depth understanding of the mechanisms of the UMR and its enhancement are highly desirable. In this study, the structural dependence of the UMR in Fe–Sn heterostructure devices is investigated. We find a weak dependence of the UMR on interface configuration in the oxide cap/Fe–Sn heterostructures on various oxide substrates. In contrast, the normalized UMR amplitude is enhanced by a factor of 27 with increasing Fe–Sn layer thickness from 4.0 to 100 nm. These results suggest that the magnetothermal effect dominates the large UMR in the Fe–Sn heterostructure devices.

Surface Quality Improvement of 3D Microstructures Fabricated by Micro-EDM with a Composite 3D Microelectrode.

Three-dimensional (3D) microelectrodes used for processing 3D microstructures in micro-electrical discharge machining (micro-EDM) can be readily prepared by laminated object manufacturing (LOM). However, the microelectrode surface always appears with steps due to the theoretical error of LOM, significantly reducing the surface quality of 3D microstructures machined by micro-EDM with the microelectrode. To address the problem above, this paper proposes a filling method to fabricate a composite 3D microelectrode and applies it in micro-EDM for processing 3D microstructures without steps. The effect of bonding temperature and Sn film thickness on the steps is investigated in detail. Meanwhile, the distribution of Cu and Sn elements in the matrix and the steps is analyzed by the energy dispersive X-ray spectrometer. Experimental results show that when the Sn layer thickness on the interface is 8 μm, 15 h after heat preservation under 950 °C, the composite 3D microelectrodes without the steps on the surface were successfully fabricated, while Sn and Cu elements were evenly distributed in the microelectrodes. Finally, the composite 3D microelectrodes were applied in micro-EDM. Furthermore, 3D microstructures without steps on the surface were obtained. This study verifies the feasibility of machining 3D microstructures without steps by micro-EDM with a composite 3D microelectrode fabricated via the proposed method.

Effect of layer thickness on structural, morphological and superconducting properties of Nb3Sn films fabricated by multilayer sequential sputtering

Superconducting Nb3Sn films can be synthesized by controlling the atomic concentration of Sn. Multilayer sequential sputtering of Nb and Sn thin films followed by high temperature annealing is considered as a method to fabricate Nb3Sn films, where the Sn composition of the deposited films can be controlled by the thickness of alternating Nb and Sn layers. We report on the structural, morphological and superconducting properties of Nb3Sn films fabricated by multilayer sequential sputtering of Nb and Sn films on sapphire substrates followed by annealing at 950 °C for 3 h. We have investigated the effect of Nb and Sn layer thickness and Nb:Sn ratio on the properties of the Nb3Sn films. The crystal structure, surface morphology, surface topography, and film composition were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and energy dispersive X-ray spectroscopy (EDS). The results showed Sn loss from the surface due to evaporation during annealing. Superconducting Nb3Sn films of critical temperature up to 17.93 K were fabricated.

Effect of Cu Substrate Roughness and Sn Layer Thickness on Whisker Development from Sn Thin-Films.

The effect of copper substrate roughness and tin layer thickness were investigated on whisker development in the case of Sn thin-films. Sn was vacuum-evaporated onto both unpolished and mechanically polished Cu substrates with 1 µm and 2 μm average layer thicknesses. The samples were stored in room conditions for 60 days. The considerable stress—developed by the rapid intermetallic layer formation—resulted in intensive whisker formation, even in some days after the layer deposition. The developed whiskers and the layer structure underneath them were investigated with both scanning electron microscopy and ion microscopy. The Sn thin-film deposited onto unpolished Cu substrate produced less but longer whiskers than that deposited onto polished Cu substrate. This phenomenon might be explained by the dependence of IML formation on the surface roughness of substrates. The formation of IML wedges is more likely on rougher Cu substrates than on polished ones. Furthermore, it was found that with the decrease of layer thickness, the development of nodule type whiskers increases due to the easier diffusion of other atoms into the whisker bodies.

Fabrication of Highly Reliable Joint Based on Cu/Ni/Sn Double-Layer Powder for High Temperature Application

Abstract A highly reliable three-dimensional network structure joint was fabricated based on Cu/Ni/Sn powder with double-layer coatings and transient liquid phase bonding (TLPB) technology for high temperature application. The Cu/Ni/Sn joint is characterized by Cu metal particles embedded in the matrix of (Cu,Ni)6Sn5/Ni3Sn4 intermetallic compounds (IMCs), with a low void ratio, and can be reflowed at low temperatures (<260°C), but it can reliably work at a high temperature up to 415°C. Cu/Ni/Sn double-layer powders with different Sn layer and Ni layer thickness were was fabricated and compressed as preform used for TLPB joint bonding. The microstructure and phase composition evolution for Cu/Sn and Cu/Ni/Sn systems during reflow and aging were comparatively studied. Two kinds of interfacial structure designs were made, and corresponding interfacial microscopic morphology was analyzed and compared under once and twice reflow soldering processes. The results indicated that the Sn-coating layer was completely consumed to form (Cu,Ni)6Sn5/Ni3Sn4 IMCs, and the Cu/Ni/Sn joint had a lower void ratio and a higher shear strength than those of Cu/Sn. The mechanism of the Ni-coating layer inhibiting phase transformation was studied. The high reliable three-dimensional network structure joint based on Cu/Ni/Sn double-layer powder was fabricated for high temperature application.

Fabrication of highly reliable joint based on Cu@Ni@Sn double-layer powder for high temperature application

Abstract A highly reliable three-dimensional network structure joint was fabricated based on Cu@Ni@Sn core-shell powder and transient liquid phase bonding (TLPB) technology for high temperature application. Cu@Ni@Sn joint is characterized by Cu metal particles embedded in the matrix of (Cu,Ni)6Sn5/Ni3Sn4 intermetallics (IMCs), low level of voiding is achieved, they can be reflowed at a low temperatures (<260°C), but reliably working at high temperature up to 415°C. Cu@Ni@Sn double-layer microparticles with different Sn layer and Ni layer thickness were fabricated and compressed as preform used for TLPB joint bonding, the microstructure and phase composition evolution for Cu@Sn and Cu@Ni@Sn system were comparatively studied during reflowing and aging process. Different kinds of interfacial structure designs were made, interfacial microscopic morphology was analyzed and compared under once and twice reflowing soldering process. Results indicated that Sn coating layer was completely consumed to form (Cu,Ni)6Sn5/Ni3Sn4 IMCs, Cu@Ni@Sn bondline have lower void rate and higher shear strength than that of Cu@Sn. The mechanism of Ni coating layer inhibit Cu atom diffusing towards Cu6Sn5 to form Cu3Sn was studied. The high reliable three-dimensional network structure joint based on Cu@Ni@Sn double-layer powder was fabricated for high temperature application.

STUDY OF Co/Sn MULTILAYER SYSTEM WITH VARIOUS TIN LAYER THICKNESS AND REFLOW TEMPERATURES

Co–Sn–Co multi-layer films with various Sn-layer thicknesses have been deposited by using the electrodeposition technique. The deposition was performed at room temperature on gold coated silicon substrates. The thickness of Sn layer was kept [Formula: see text]4.5, 2.5, 1.5 and 1[Formula: see text][Formula: see text]m, while the thickness of each Co layer was fixed to [Formula: see text]0.75[Formula: see text][Formula: see text]m. The Sn layer was sandwiched between two Co layers. The total thickness of the films was [Formula: see text]6, 4, 3, 2.5[Formula: see text][Formula: see text]m, respectively. The intended ratio of Sn was about 25 at.% while Co was about 75 at.%. The separation inside the Sn-layer has been observed at higher Sn-layer thicknesses with increasing reflow temperature. It may be due to the combined effect of thermal stress arising during cooling from elevated reflow temperatures and solidification shrinkage in the thick Sn layer due to the formation of IMC CoSn2. However, 1[Formula: see text][Formula: see text]m has been found to be a critical thickness, which remains intact when sandwiched between two cobalt layers. The structure of these multi-layer films was studied as a function of temperature. It has been observed from X-ray diffraction (XRD) that the as-deposited films exhibit strong peaks belonging to elemental Co and Sn. At temperatures of 270–290∘C, CoSn2 begins to appear and grow at the interfaces and in the middle of Sn layer with the decrease in elemental Sn. However, sufficient amount of Co is still present in pure form as is evident by the XRD and field emission scanning electron microscopy images at all temperatures. This study confirms that only one reaction product, viz. CoSn2, formed in as-deposited during reflow at temperatures 270–290∘C for a fixed time of 10[Formula: see text]min, although several other stable IMCs, e.g. Co3Sn2, CoSn, CoSn2, CoSn3 exist in the Co–Sn system at 250∘C according to phase diagram. It is not uncommon that all the thermodynamically stable IMCs do not form in the system due to kinetic reasons. Accordingly, the formation of IMCs through interfacial reaction has been discussed in this paper.

The influence of the crystallographic structure of the intermetallic grains on tin whisker growth

In this paper, the relationship between the crystallographic structure of Cu-Sn intermetallic grains and Sn whisker growth was investigated. In order to prevent the influence of the elements in the alloy composition and the effect of the soldering process on the formation of the intermetallic layer, 99.99% pure Sn was vacuum evaporated onto Cu substrates. The Sn layer thickness was sub-micron region (∼400 nm in average) to reach considerable and rapid compressive stress on the tin layer originated by the intermetallic formation. The samples were stored at room temperature for 1 month. Different types of whiskers (nodule and filament) and the layer structure underneath were studied with a scanning ion microscopy and transmission electron microscopy. It was found that not only the thickness of the intermetallic layer and shape of the intermetallic grains affects the whisker growth but the crystallographic structure of the intermetallic grains as well. The susceptibility of the Sn layer to whisker development is higher in those regions where the intermetallic layer is composed of monocrystalline grains instead of those regions, where it is composed of polycrystalline grains. This effect can be explained by the higher compressive stress generated by the monocrystalline intermetallics compared to the polycrystalline ones.

Inhibition of Anodic Dissolution of Steel by Sn2+ in Perchloric Acid Solution

The inhibition effect of Sn2+ on the anodic dissolution of Fe-0.05C steel was investigated electrochemically in a solution of 0.2 M HClO4. Anodic currents were suppressed in the presence of Sn2+, and the suppression behavior depended on the concentration of Sn2+ and the electrode potentials. The experimental condition for the current suppression is explained by the theory of underpotential deposition (UPD). X-ray photoelectron spectroscopy (XPS) depth analysis revealed the deposition of metallic Sn on the steel, while inductively coupled plasma-mass spectrometry (ICP-MS) analysis demonstrated that the Sn layer thickness was monolayer-level. This strongly suggests that the anodic dissolution of iron was suppressed by the UPD of Sn. The proposed mechanism for corrosion inhibition by Sn-UPD also explains the superior corrosion resistance of Sn-bearing steel under atmospheric environments.

Pronounced effects of Zn additions on Cu-Sn microjoints for chip-stacking applications

The strong effects of Zn addition on the reaction between Sn-based solders and Cu substrate are widely known. Nevertheless, all of the previous studies are limited to the situation in which the solder volume was relatively large, and the effects of Zn on the much smaller microjoints remain unclear. In this work, the effect of Zn addition on the reaction in microjoints is investigated. Sandwich structures of Cu/Sn/Cu and Cu/SnxZn/Cu (x = 0.4, 0.7, and 1 wt%) are prepared by thermal compression bonding. The thickness of Sn or SnxZn layer is controlled at 10 μm. It is found that Zn addition significantly retards the growth of Cu3Sn, and the growth of Cu6Sn5 is also slowed down. The results of this study suggest that it is possible to avoid the total conversion of Sn into brittle intermetallics during the lifetime of microjoints by the addition of a proper amount of Zn.

Impact of thickness on the structural properties of high tin content GeSn layers

We have grown various thicknesses of GeSn layers in a 200mm industrial Reduced Pressure – Chemical Vapor Deposition cluster tool using digermane (Ge2H6) and tin tetrachloride (SnCl4). The growth pressure (100Torr) and the F(Ge2H6)/F(SnCl4) mass-flow ratio were kept constant, and incorporation of tin in the range of 10–15% was achieved with a reduction in temperature: 325°C for 10% to 301°C for 15% of Sn. The layers were grown on 2.5µm thick Ge Strain Relaxed Buffers, themselves on Si(001) substrates. We used X-ray Diffraction, Atomic Force Microscopy, Raman spectroscopy and Scanning Electron Microscopy to measure the Sn concentration, the strain state, the surface roughness and thickness as a function of growth duration. A dramatic degradation of the film was seen when the Sn concentration and layer thickness were too high resulting in rough/milky surfaces and significant Sn segregation.

Thermodynamic reassessment of the Au-Pt-Sn system and microstructural evolution of the (AuSn)eut-Pt interconnection

The Calphad method has been employed to reassess the thermodynamic description of the Au-Pt-Sn ternary system by remodeling the ternary compound and the solubility of the third elements in relevant binary compounds. A set of consistent parameters was obtained with reproducing most of the experimental results. The microstructures of as-soldered and aged Au-20 wt%Sn|Pt reaction couples were characterized by using scanning electron microscopy with energy-dispersive X-ray microanalysis (EDS), as well as scanning transmission electron microscopy with EDS. During soldering, the (Au1-x,Ptx)Sn (0 ≤ x ≤ 0.5) intermetallic compound layer was formed at solder/Pt interface and evolved with prolonged bonding time. When the as-soldered samples were subsequently annealed at 150 °C for up to 4600 h, only one interfacial intermetallic compound (Au1-x,Ptx)Sn was observed. The average thickness of the (Au1-x,Ptx)Sn layer was reached ∼ 3.3 μm already after 100 h of aging and remained practically constant with the progress of the aging process. When samples were isothermally aged for 6848 h, in addition to the thick (Au1-x,Ptx)Sn interfacial layer, a thin (no more than 100 nm) PtSn layer was observed at the interface of (Au1-x,Ptx)Sn/Pt. These results indicate that the Au-20 wt%Sn|Pt interconnections are thermally stabile at 150 °C. The interfacial reaction mechanism of Au-20 wt%Sn|Pt at 150 °C has been proposed by combining experimental results with the basic thermodynamic considerations. Moreover, the microstructure of Au-20 wt%Sn|Pt was compared with those of Au-20 wt%Sn|Ni and Au-20 wt%Sn|Cu. The advantages and disadvantages of Au-20 wt%Sn solder on Ni, Cu and Pt contact metallizations were discussed.

An ambient temperature ultrasonic bonding technology based on Cu micro-cone arrays for 3D packaging

An ambient temperature ultrasonic bonding based on Cu micro-cone arrays (MCAs) plated with thin Sn for potential application in 3D packaging is introduced. The interfacial morphology is investigated in joints bonded by this method. As the thickness of Sn layer in bonding interface was about 300nm, the best bonding quality was obtained under the condition of low pressure of 8Mpa and short time of 1s, meanwhile, the voids of interfaces vanished. Under the effect of ultrasound, the Cu MCAs embedded into each other effectively and a stable physical occlusion was formed. The thin Sn transformed into Cu3Sn in the bonding interface due to quick atom diffusion. The bonding materials of two sides are the same, which makes the preparation of bonding process easier. Without solder balls, ultrasonic bonding joints with smaller size can be directly achieved using Cu MCAs plated with a thin Sn layer.

Effects of Sn thickness on morphology and evolution of Ni3Sn4 grains formed between molten Sn and Ni substrate

The morphology and growth behavior of Ni3Sn4 grains at the Sn/Ni interface during soldering were systematically investigated. Sn/Ni reaction samples were examined in three electroplated Sn thicknesses, 5 μm, 10 μm and 100 μm. In soldering at 280 °C, the formed Ni3Sn4 grains exhibit two kinds of morphologies, needle-like grains and larger chunk-type grains. Experimental observations reveal that the amount of the chunk-type grains increased as the Sn layer thickness was decreased, which is attributed to the Ni content in molten Sn. Moreover, needle-like grains display a hexagonal columnar structure with (100) and (110) facets. With the reaction proceeding, the needle-like grains coarsen with cube root of reaction time. The faceted chunk-type grains are transformed from the boomerang-shaped grains, consisting of two inverted triangular plates sharing an edge. The crystal plane orientations of the chunk-type Ni3Sn4 grains were also analyzed based on its crystalline structure. Additionally, the Sn/Ni reactions at 250 °C indicate that the morphology of the chunk-type grains is also affected by the reaction temperature.

The Effects of Nickel Underlayer and Solder Dipping as Tin Whisker Mitigations in Tin Surface Finishes

Driven by environmental concerns and the enforcement of Restriction of Hazardous Substances Directive (RoHS) to ban the use of lead in electronics, the global electronics industry has migrated toward lead-free electronics. However, the adoption of lead-free tin (Sn) surface finish is known to form whiskers. These whiskers grow spontaneously from the Sn finish layer as a stress-relief over time causing device failures. In the present research, whisker growth is investigated via immersion Sn finishes on Cu substrate. The effects of Sn layer thickness, addition of Ni under-layer and solder dipping on whisker growth are investigated by storing the samples under ambient temperature for up to 24 weeks. The effects of external stresses were also studied using bending test. The results showed that whisker length on immersion Sn increases with time for all the samples either with or without Ni under-layer. Thicker Sn coating showed more whisker growth compared with thinner Sn coating. The longest whisker length of 23μm was observed for Sn coating with 2μm thickness. The addition of Ni as under-layer was found to be more effective in mitigating the whisker growth by extending the incubation time for whisker formation. Compared to immersion Sn, solder dipping in pure Sn showed no whisker growth. However, alloying Sn with 0.4%wtCu resulted in whisker growth indicating the role of Cu in promoting whiskers formation.

Evolution of microstructure and stress of and associated whisker growth on Sn layers sputter-deposited on Cu substrates

After sputter deposition of Sn (layer thickness of 350 nm) on Cu substrates and during subsequent aging at room temperature, Cu and Sn reacted to form the intermetallic phase Cu6Sn5 in the Sn layer at the Cu/Sn interface, which led within a few hours of aging to the development of a compressive stress parallel to the Cu/Sn interface in the Sn layer. One day after aging at room-temperature whisker formation occurred on the surface of the Sn layer. It was shown that whisker growth is associated with long-range Sn diffusion parallel to the Cu/Sn interface. Sn layers of the same thickness sputter deposited on pure Si substrates exhibited throughout the same aging time at room temperature a tensile stress parallel to the Cu/Sn interface (no intermetallic phase formation took place) and whisker formation did not occur. The interrelationship of intermetallic compound formation, stress development, and whisker growth is discussed.

Room temperature interfacial reactions in electrodeposited Au/Sn couples

Abstract Two Au/Sn couples electrodeposited on metallized Si wafers, a Au/Sn couple (Au deposited first) and a Sn/Au couple (Sn deposited first), were employed to study the room temperature interfacial reactions between Au and Sn. The electroplating sequence has a significant effect on the phases and the microstructures of the reaction regions. AuSn 4 and AuSn are formed in the as-deposited Au/Sn couples, while Au 5 Sn and AuSn are formed in the as-deposited Sn/Au couples. Upon ageing, AuSn 2 formation depends on the Sn (Au) content (i.e. the Sn (Au) layer thickness) in the Au/Sn (Sn/Au) couples. In aged Au/Sn couples, with 1.5 μm of Au and 3 μm of Sn, AuSn, AuSn 2 and AuSn 4 are sequentially distributed in the reaction region with fine Kirkendall voids at the AuSn/Au interface. In the aged Sn/Au couples, with 15 μm of Sn and 1.5 μm of Au, AuSn is distributed on either side of the original Sn/Au interface, while Au 5 Sn only exists on the Au side. The presence of a thin interfacial SnO 2 film significantly affects phase formation and reaction kinetics.

Effect of Cross-Interaction between Ni and Cu on Growth Kinetics of Intermetallic Compounds in Ni/Sn/Cu Diffusion Couples during Aging

The solid-state, cross-interaction between the Ni layer on the component side and the Cu pad on the printed circuit board (PCB) side in ball grid array (BGA) solder joints was investigated by employing Ni(15 μm)/Sn(65 μm)/Cu ternary diffusion couples. The ternary diffusion couples were prepared by sequentially electroplating Sn and Ni on a Cu foil and were aged isothermally at 150, 180, and 200°C. The growth of the intermetallic compound (IMC) layer on the Ni side was coupled with that on the Cu side by the mass flux across the Sn layer that was caused by the difference in the Ni content between the (Cu1−x Ni x )6Sn5 layer on the Ni side and the (Cu1−y Ni y )6Sn5 layer on the Cu side. As the consequence of the coupling, the growth rate of the (Cu1−x Ni x )6 Sn5 layer on the Ni side was rapidly accelerated by decreasing Sn layer thickness and increasing aging temperature. Owing to the cross-interaction with the top Ni layer, the growth rate of the (Cu1−y Ni y )6Sn5 layer on the Cu side was accelerated at 150°C and 180°C but was retarded at 200°C, while the growth rate of the Cu3Sn layer was always retarded. The growth kinetic model proposed in an attempt to interpret the experimental results was able to reproduce qualitatively all of the important experimental observations pertaining to the growth of the IMC layers in the Ni/Sn/Cu diffusion couple.