Microstructure and mechanical properties of Sn–Zn–Bi–Cr lead-free solder

In this paper, the effect of trace addition of Cr on the mechanical properties and reliability on Sn–8Zn–3Bi solder alloys was investigated. It has been demonstrated that the microstructure of solder alloys was refined after doping traces of Cr. The elongation reaches up to 40.63% after doping 0.1% Cr; and the fracture mechanism converts from quasi-cleavage fracture into ductile fracture. With aging time of 0, 4, 9 and 16 days, mechanical property of Sn–8Zn–3Bi–0.3Cr alloy was improved slightly. It was found that the Sn–Zn–Cr phase was increased and Zn in alloy was consumed after aging, so that the amount of primary Zn phase was reduced and microstructure was improved.     

Microstructure and elevated-temperature shear strength of Zn–4Al–3Mg–xSn high-temperature lead-free solders

The microstructure and shear strength of the high-temperature Zn–4Al–3Mg, Zn–4Al–3Mg–7Sn, and Zn–4Al–3Mg–13Sn solder alloys were investigated in the temperature range of 25–200 °C. The results revealed that the shear yield stress (SYS) and ultimate shear strength (USS) of all three alloys decrease with increasing test temperature. The ternary base alloy showed higher strength levels up to 145 °C, above which all alloys behave similarly. The superiority of the ternary alloy is ascribed to the higher volume fraction of the fine α−η eutectic and eutectoid structures and the hard MgZn₂ and Mg₂Zn₁₁ particles. Introduction of Sn into the base alloy, however, resulted in substantial decrease in the strength, due to the presence of the soft Sn that reduces the volume fraction of the eutectic structure and the hard second phase particles. Despite the weakening effects of Sn, the strength of quaternary alloys is still higher than those of the Zn–Sn and Pb–Sn high-temperature solders.     

Interfacial reactions of Sn–58Bi and Sn–0.7Cu lead-free solders with Alloy 42 substrate

Eutectic Sn–58 wt%Bi (SB) and Sn–0.7 wt%Cu (SC) lead-free solders reacting with Alloy 42 substrates were investigated at 240, 270, 300, and 330 °C for various reaction times. The FeSn₂ phase was the only phase formed at the solder/Alloy 42 interface for all couples. However, this FeSn₂ phase had two different microstructures. The intermetallic compound (IMC) morphology near the solder/Alloy 42 interface was dense with a tiny microstructure after reflowing at 240 °C. Massive spalling FeSn₂ phases with large and platy grain were observed in the solder matrix. When the reflow temperature was increased the IMC morphology assumed bulk or cylindrical shape with no spalling IMCs observed at the solder/Alloy 42 interface. The IMC thickness increased with increasing reaction temperature and time. Both SB/Alloy 42 and SC/Alloy 42 couples presented a diffusion controlled growth mechanism in. The activation energies for SB/Alloy 42 and SC/Alloy 42 couples were 86.2 and 103.4 kJ/mol, respectively.     

Formation and growth of interfacial intermetallic layers of Sn–8Zn–3Bi–0.3Cr on Cu,Ni and Ni–W substrates

In this paper, the formation and growth of intermetallic compounds (IMCs) of Sn–8Zn–3Bi–0.3Cr solder on Cu, Ni and Ni–W substrates have been investigated. For the Cu substrate, only Cu₅Zn₈ intermetallic compound was observed. For the Ni substrate, a Ni₅Zn₂₁ film formed at the interface due to the fast reaction between Ni and Zn. For the Ni–W substrate, a thin Ni₅Zn₂₁ film appeared between the solder and Ni–W layer, whose thickness decreases with the increase of W content. A bright layer was also found to form below the Ni₅Zn₂₁ layer as aging time extended, which is caused by the diffusion of Zn into Ni–W layer.     

Inhomogeneous deformation and microstructure evolution of Sn–Ag-based solder interconnects during thermal cycling and shear testing

Orientation imaging microscopy was adopted to characterize the microstructural changes in Sn–Ag-based solder interconnects during thermal cycling and shear testing. The deformation and microstructure evolution of Sn–Ag-based solder interconnects are inhomogeneous, depending on the orientations of β-Sn grains in the as-solidified microstructure. Recovery or recrystallization can take place even under pure shear stress at room temperature, and it tends to occur at high-angle grain boundaries in multi-grained solder interconnects, while it localizes in near-interface region in solder interconnects with only one grain inside. During thermal cycling, the hardness of recrystallized microstructure decreased significantly due to the segregation of Ag₃Sn IMC particles towards the newly-formed recrystallized boundaries, increasing the ease of localized deformation in this weakened microstructure. As a consequence, cracks were propagated intergranularly in the recrystallized microstructure.     

Microstructure and morphology of interfacial intermetallic compound CoSn3 in Sn–Pb/Co–P solder joints

A systematical microscopic analysis on structure, morphology, and growth of CoSn₃ intermetallic compound (IMC) that formed at the interface between Sn–Pb alloy and Co–P films was carried out using scanning electron microscopy with back-scattered electron imaging and high-resolution transmission electron microscopy with energy dispersive spectrometry. CoSn₃ IMC with two kinds of morphology was found out after Sn–Pb alloy reacted with Co–P films with different microstructures. One kind of CoSn₃ with stacking fault was distributed densely on nanocrystalline and amorphous Co–P films, and the other kind of CoSn₃ without stacking fault existed sparsely on the Co–P film with nanocrystalline/amorphous mixed structure. The stacking fault was caused by the fast growth of CoSn₃ for the cases of Co-7 at.% P and Co-23 at.% P. Co-12 at.% P film with a nanocrystalline/amorphous mixed microstructure had the best diffusion-barrier property among Co–P films with different compositions, because the diffusion of Sn into Co–P was the least. Our study shed light on diffusion-barrier performance of Co-based metallization.     

Impact strength of Sn–3.0Ag–0.5Cu solder bumps during isothermal aging

In order to investigate the fracture behavior of Sn–3.0Ag–0.5Cu solder bump, solder balls with the diameter of 0.76 mm were soldered on Cu pad in this study, then high speed impact test and static shear test of solder bumps were carried out to measure the joint strength of the soldering interface. The effect of isothermal aging on joint strength as well as fracture behavior of solder bumps was investigated, and the composition of the fracture surface was identified by means of EPMA. The results indicate that the fracture is inside the bulk solder in low speed shear test regardless of the aging effect, thus the maximum load reflects the solder strength rather than the interfacial strength. It is also found that under 1 m/s impact loading, the crack initiation position is changed from solder/Cu₆Sn₅ interface to Cu₃Sn/Cu interface after long time isothermal aging, and the fracture occurs inside the bulk solder accompanying with intermetallic compound in both of the as-soldered and aged joints. The thickened multiple IMC layers during isothermal aging account for the degraded impact resistance, and the change of the solder matrix is another factor for reduced impact resistance owing to Sn residue on the fracture surface.     

Microstructure of GaN1−x Bi x

In this paper we describe detailed transmission electron microscopy studies of GaN_1?x Bi _x with 0.05 < x < 0.18 grown by low-temperature molecular beam epitaxy under Ga-rich conditions. Microstructural transformation from columnar growth separated by thin amorphous areas in the films with lowest Bi content (5%) to pseudo-amorphous structure with crystalline grains embedded in the amorphous matrix in the samples with higher Bi content (13% to 18%) was observed. In addition, metallic Bi segregation occurred in the samples with the highest Bi concentration. An abrupt decrease in absorption edge energy is found in samples with higher Bi content.     

Microstructure of Sn–1Ag–0.5Cu solder alloy bearing Fe under salt spray test

Understanding the behavior of lead-free solder alloys within a high humidity environment is a serious topic in the deployment of products in various electronics applications. The work reported herein investigates this specific impact on Sn–1.0Ag–0.5Cu–0.5Fe solder alloy. Specimens were treated with 5% NaCl salt spray. All specimens showed strong resistance to corrosion. Microstructural deformations after the test were analyzed using Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDX). Concerns were at the localized corroded area, as this would cause significant degradation at the solder joints. The mechanisms leading to these disadvantageous results as well as the microstructural evolution and correlation with the intrinsic properties of the solder alloy are discussed.     

Effects of Cu on the interfacial reactions between Sn–8Zn–3Bi–xCu solders and Cu substrate

The microstructure, thermal property, and interfacial reaction with Cu substrate of Sn–8Zn–3Bi–xCu (x = 0, 0.5, 1) lead-free solders were investigated in this work. Cu–Zn intermetallics formed in the solder matrix and the melting temperature increases slightly with Cu addition. After soldering at 250 °C for 90 s, a flat Cu₅Zn₈ layer and a scallop CuZn₅ layer formed at the interfaces of all samples. The CuZn₅ intermetallic compound (IMC) transformed to Cu₅Zn₈ IMC with longer reaction time due to the diffusion of Cu atoms from Cu substrate. The interfacial IMC layer grew thicker with the reaction time following a parabolic law which suggested the interfacial reactions were diffusion controlled. The calculation results show that the activation energy of IMC growth for Cu-containing solders is larger than that of Sn–8Zn–3Bi solder, which demonstrated that a small amount of Cu addition to the solder can effectively suppressed the growth of the interfacial IMC.     

Electrodeposited and characterization of Ag–Sn–S semiconductor thin films

Thin film of silver tin sulfides (Ag–Sn–S) has been deposited on indium tin oxide coated glass (ITO) substrates using potentiostatic cathodic electrodeposition technique. New procedure for the growth of Ag–Sn–S film is presented. An electrolyte solution containing Silver Nitrate (AgNO₃), Tin(II) Chloride (SnCl₂) and Sodium Thiosulfate (Na₂S₂O₃)in acidic solution (pH ~2) and at temperature of the bath 55 °C were used for the growth of Ag–Sn–S thin film. Prior to the deposition, a cyclic voltammetry technique was performed in binary (Ag–S, Sn–S) and ternary (Ag–Sn–S) systems. This study was carried out to examine the behavior of electroactive species at the electrode surface. Based on these results, the cathodic applied potential was fixed at −1000 mV versus Ag/AgCl to obtain a uniform and good adhesion of ternary thin film. After that, structural, morphological and optical performances of films have been investigated. The X-ray diffraction patterns of the samples demonstrate the presence of the orthorhombic phase of Ag₈SnS₆ at applied potential of −1000 mV versus Ag/AgCl. Based on the scanning electron microscopy (SEM), it was found that the surface morphology and grain size were strongly influenced by the presence of Sn and/or Ag in the electrolyte bath. The band gaps of binaries and ternary compound are evaluated from optical absorption measurements. Band gap of Ag₈SnS₆ determined from transmittance spectra is in the range 1.56 eV. Flat-band potential and free carrier concentration have been determined from Mott–Schottky plot and are estimated to be around 0.18 V and 2.21×10¹⁴ cm⁻³ respectively. The photoelectrochemical test of Ag₈SnS₆ was studied and the experimental observations are discussed in detail.     

Effect of nano Al2O3 particles doping on electromigration and mechanical properties of Sn–58Bi solder joints

In the present study, the effect of Al₂O₃ nanoparticles on performances of Sn–58Bi solder were investigated in aspects of electro-migratio, shear strength and microhardness. The experimental results show that the Al₂O₃ nanoparticles significantly improved microstructure and mechanical performances of solder joints. With the addition of 0.5 wt% Al₂O₃, the intermetallic compounds (IMC) reduced from 2.5 μm to 1.27 μm after 288 aging hours at 85 °C. Furthermore, after electromigration test under a current density of 5 × 10³ A/cm² at 85 °C, Bi-rich layers formed at the anode side of both Al₂O₃ doped and plain solder. Moreover, the addition of Al₂O₃ nanoparticles reduced the mean thickness of Bi-rich layer. In addition, the growth rate of the IMC layer of Al₂O₃ doped solder decreased by 8% compared with the plain solder. Besides, the Al₂O₃ doped solder exhibited better performance than plain solder in microhardness after different aging times. While, the addition of Al₂O₃ significantly impeded the degradation of the shear strength of solder joint after aging for 48 and 288 h. Furthermore, it was worth noting that the fracture surface of doped solder showed a typical rough and ductile structure. However, plain solder exhibited a relatively smooth and fragile surface.     

Impact of Ni concentration on the intermetallic compound formation and brittle fracture strength of Sn–Cu–Ni (SCN) lead-free solder joints

Cu₆Sn₅ and Cu₃Sn are common intermetallic compounds (IMCs) found in Sn–Ag–Cu (SAC) lead-free solder joints with OSP pad finish. People typically attributed the brittle failure to excessive growth of IMCs at the interface between the solder joint and the copper pad. However, the respective role of Cu₆Sn₅ and Cu₃Sn played in the interfacial fracture still remains unclear. In the present study, various amounts of Ni were doped in the Sn–Cu based solder. The different effects of Ni concentration on the growth rate of (Cu, Ni)₆Sn₅/Cu₆Sn₅ and Cu₃Sn were characterized and compared. The results of characterization were used to evaluate different growth rates of (Cu, Ni)₆Sn₅ and Cu₃Sn under thermal aging. The thicknesses of (Cu, Ni)₆Sn₅/Cu₆Sn₅ and Cu₃Sn after different thermal aging periods were measured. High speed ball pull/shear tests were also performed. The correlation between interfacial fracture strength and IMC layer thicknesses was established.     

Interrelationship of thermal parameters,microstructure and microhardness of directionally solidified Bi–Zn solder alloys

Due to the health and environmental concerns associated with lead usage, the research on alternative lead-free alloys for replacing lead-based solders is a global demand. Despite numerous studies on Sn-based lead-free solders in recent years, there is not yet a standard solder alloy able to cover the spectrum of properties furnished by the classic Sn-Pb alloy. In this sense, particular lead-free alloys compositions have been suited for specific needs and the options have been broadening when elements other than tin are used as the base component, such as indium, gold and bismuth. The later element is well known in the hall of lead-free alloys as an alloying option rather than as a base component. This study aims to establish interrelations of solidification thermal parameters (growth and cooling rates), microstructure features (primary and secondary dendrite arm spacings – λ₁ and λ₂; eutectic spacing – λ_E and interphase spacing – λ_int) and hardness of Bi-Zn alloys (1.5 wt% Zn-hypoeutectic, 2.7 wt% Zn-eutectic, and 5 wt% Zn-hypereutectic alloys) samples, which were directionally solidified in unsteady-state conditions under cooling rates similar to those of found in industrial soldering practice. Examination of the resulting microstructures by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS) permitted the different phases morphologies to be characterized: Bi-rich trigonal dendrites and long Zn fibers, as primary phases of, hypoeutectic and hypereutectic alloys, respectively, immersed in a fiber-like eutectic mixture. The combined effects of macrosegregation, Zn alloying and representative scales of the phases forming the microstructure (λ_int, λ₂ and λ_E) on hardness of the Bi-Zn alloys are evaluated and Hall–Petch type equations relating λ_int, λ₂ and λ_E to hardness are proposed.     

Sol–Gel Synthesis of Mn–Sn–Ti-Substituted Strontium Hexaferrite Nanoparticles: Structural,Magnetic, and Reflection-Loss Properties

Substituted strontium hexaferrite nanoparticles with the chemical formula SrFe_12?x (MnSn_0.5Ti_0.5) _x/2O₁₉(x = 0–2.5, in a step of 0.5) were prepared by a sol–gel method. Phase identities and crystal structure of the synthesized nanoparticles were investigated by x-ray diffraction. The morphology of the nanopowders was determined by field emission scanning electron microscopy. Results obtained from Fourier-transform infrared spectroscopy revealed the presence of stretching and bending modes in the citrate complex. Mössbauer spectroscopy (M _S) revealed occupancy of the hexagonal lattice structure by non-magnetic Mn²⁺ –Sn⁴⁺ –Ti⁴⁺ cations. Magnetic properties were measured by use of a vibrating sample magnetometer. The results showed that saturation magnetization and coercivity decreased with increasing x content. Microwave absorption properties were investigated by use of a vector network analyzer. It was found that the maximum reflection loss of substituted Sr-ferrite 1.6 mm thick reached ?41.8 dB at a frequency of 4.3 GHz and a bandwidth of 7.5 GHz, with reflection loss being higher than ?25 dB. These results imply that the prepared composites are good candidates for absorbers in the gigahertz frequency range.     

Miniaturized Heat-Flux Sensor Based on a Glass-Insulated Bi–Sn Microwire

Semiconductors - Thermoelectric-energy conversion based on a single element made of an anisotropic material is considered. In such materials, the heat flux generates a transverse electric field. We...     

Effect of Isothermal Aging on the Long-Term Reliability of Fine-Pitch Sn–Ag–Cu and Sn–Ag Solder Interconnects With and Without Board-Side Ni Surface Finish

The combined effects on long-term reliability of isothermal aging and chemically balanced or unbalanced surface finish have been investigated for fine-pitch ball grid array packages with Sn–3.0Ag–0.5Cu (SAC305) (wt.%) and Sn–3.5Ag (SnAg) (wt.%) solder ball interconnects. Two different printed circuit board surface finishes were selected to compare the effects of chemically balanced and unbalanced structure interconnects with and without board-side Ni surface finish. NiAu/solder/Cu and NiAu/solder/NiAu interconnects were isothermally aged and thermally cycled to evaluate long-term thermal fatigue reliability. Weibull plots of the combined effects of each aging condition and each surface finish revealed lifetime for NiAu/SAC305/Cu was reduced by approximately 40% by aging at 150°C; less degradation was observed for NiAu/SAC305/NiAu. Further reduction of characteristic life-cycle number was observed for NiAu/SnAg/NiAu joints. Microstructure was studied, focusing on its evolution near the board and package-side interfaces. Different mechanisms of aging were apparent under the different joint configurations. Their effects on the fatigue life of solder joints are discussed.     

Monitoring extent of curing and thermal–mechanical property study of printed circuit board substrates

Precise control of curing conversion for epoxy-based printed circuit board (PCB) substrates and clarification of curing–property relationship are critical for the performance and reliability assessment, and for the design optimization of electronic systems. In this article, various epoxy composites for PCB substrates were analyzed by infrared spectroscopy (IR), differential scanning calorimetry (DSC), rheometry, dynamic mechanical analysis (DMA), and scanning electron microscope (SEM). Compared with mid-IR and DSC, near-IR (NIR) is found to be a reliable method for the characterization of curing conversion process by detecting the consumption of epoxy groups. And DMA is a powerful method for measuring the conversion of PCB materials by testing glass transition temperatures (T_g) and viscoelastic properties. The curing behaviors of a variety of epoxy composites show distinct differences in both curing rate and activation energy, and the growth tendency of T_g with curing conversion also changed depending on the material compositions. Correlation of curing conversion versus thermal properties shows that the activation energy of curing at different stage by DSC resembles the tendency of T_g transitions tested by DMA. Mechanical properties of the composites show close relationship with the curing conversions. Peel strength, the indicator of adhesion strength between copper foil and epoxy composites, was tested on all the specimens of different curing conversions, and the results showed a maximum value at curing conversion between ca. 90 and 95%.