Bi Solder Research Articles

Low Melting Temperature Sn-Bi Solder: Effect of Alloying and Nanoparticle Addition on the Microstructural, Thermal, Interfacial Bonding, and Mechanical Characteristics

Sn-based lead-free solders such as Sn-Ag-Cu, Sn-Cu, and Sn-Bi have been used extensively for a long time in the electronic packaging field. Recently, low-temperature Sn-Bi solder alloys attract much attention from industries for flexible printed circuit board (FPCB) applications. Low melting temperatures of Sn-Bi solders avoid warpage wherein printed circuit board and electronic parts deform or deviate from the initial state due to their thermal mismatch during soldering. However, the addition of alloying elements and nanoparticles Sn-Bi solders improves the melting temperature, wettability, microstructure, and mechanical properties. Improving the brittleness of the eutectic Sn-58wt%Bi solder alloy by grain refinement of the Bi-phase becomes a hot topic. In this paper, literature studies about melting temperature, microstructure, inter-metallic thickness, and mechanical properties of Sn-Bi solder alloys upon alloying and nanoparticle addition are reviewed.

Influence of Sb2O3 Nanoparticles Addition on the Thermal, Microstructural and Creep Properties of Hypoeutectic Sn–Bi Solder Alloy

Here we investigate the effects (thermal, microstructural, and creep properties) of adding Sb2O3 nanoparticles to a hypoeutectic Sn-5 wt% Bi solder alloy. The Sb2O3-containing solder alloy was prepared by mechanically incorporating 0.5 wt% Sb2O3 nanoparticles into the Sn-5 wt% Bi solder alloy. The addition of nano-sized Sb2O3 particles to the Sn–Bi solder alloy increases the melting temperature, but only slightly. The main phases of the investigated solder alloys include the β-Sn and Bi-rich phases in addition to the crystalline phase of Sb2O3 nanoparticles. No other intermetallic compounds were observed in the β-Sn matrix. The tensile creep experiments have been carried out in the 303–363 K temperature interval under constant stresses ranging from 5.1 to 7.64 MPa. The creep parameters of both solders increased gradually with increasing creep temperature up to 333 K, after which they increased rapidly with relatively higher values. The creep parameters of the Sb2O3-containing solder alloys are smaller than that of Sb2O3-free solder alloys. The present solder alloys exhibit class-M creep behavior. The calculated stress exponent values and activation energy data for both solders could be related to dislocation climb through core diffusion as the dominant operating mechanism.

Effect of Bi addition on the shear strength and failure mechanism of low-Ag lead-free solder joints

To optimize the mechanical properties of low-Ag lead-free solder, different amount of Bi element was added into the Sn–1.0Ag–0.5Cu (SAC105) to form SAC105-xBi (0 ≤ x ≤ 4) solder alloy. The microstructural evolution and shear fracture of SAC105-xBi solder joints were systematically investigated to clarify the failure mechanism after reflow and after 175 °C aging compared to SAC305. It was found that Bi element could provide solution strengthening and second-phase strengthening in the low-Ag SAC solder joints depending on the amount of added Bi element. The shear strength after reflow increased from 34.79 to 70.16 MPa with the increase of Bi addition from x = 0.0 to x = 4.0, while that after aging at 175 °C for 168 h increased from 28.97 MPa (x = 0.0) to 55.02 MPa (x = 4.0). The thickness of IMC decreased with the addition of Bi, and excessive Bi would precipitate in the matrix when the Bi content reached 2 wt% or even form brittle Bi barrier layer after 4 wt% addition. During shear tests, three kinds of fracture mode including ductile fracture, quasi-cleavage fracture, and cleavage brittle fracture were observed, and three kinds of failure position including solder matrix, Cu6Sn5/solder interface, and Cu3Sn/Cu interface were revealed. The failure mechanism of different solder joints was closely related to the coupling effects among the Bi amount in the matrix, the thicknesses of IMC, and the status of Kirkendall voids, which significantly affected the reliability of SAC solder joints. Finally, SAC105-1.0Bi solder alloy was considered having the most suitable Bi addition with anticipated comprehensive properties compared with SAC305.

Pengaruh Agen Kimia Berbeza terhadap Penguraian Terma dan Pembentukan Fasa Bahan La0.6SR0.4CoO3- δ yang Disediakan melalui Kaedah Sol-Gel

Sifat penguraian terma dan pembentukan fasa bahan lantanum strontium kobalt oksida, La0.6Sr0.4CoO3-δ (LSC) yang disediakan melalui kaedah sol-gel berbantu agen kimia berbeza, iaitu agen serakan, agen pempolimeran dan agen permukaan aktif atau surfaktan telah dicirikan secara sistematik masing-masing melalui analisis termogravimetrik (TG) dan pembelauan sinar-X (XRD). Penguraian terma bahan organik dan bahan bukan organik yang tidak diperlukan dalam serbuk pelopor bahan LSC telah lengkap pada suhu kurang daripada 1000 °C bagi serbuk pelopor yang disediakan dengan menggunakan agen serakan dan agen pempolimeran, dan suhu melebihi 1000 °C bagi serbuk pelopor yang disediakan dengan menggunakan surfaktan. Sifat penguraian terma ini dipengaruhi oleh suhu pengeringan serbuk pelopor tersebut dan berat molekul agen kimia. Pembentukan fasa tunggal perovskit LSC telah disahkan dalam serbuk pelopor yang disediakan dengan menggunakan agen serakan, iaitu karbon teraktif dan agen pempolimeran, iaitu etilena glikol selepas serbuk pelopor tersebut dikalsin pada suhu 900 °C. Sebaliknya, fasa tunggal perovskit LSC tidak terbentuk secara lengkap dalam serbuk pelopor yang disediakan dengan menggunakan surfaktan (polietilena glikol, Triton-X-100, Brij-97 dan Tween-80) walaupun selepas serbuk pelopor tersebut telah dikalsin pada suhu yang lebih tinggi iaitu 1100 °C. Kepekatan surfaktan, nisbah molar surfaktan kepada logam kation dan nilai pH larutan bahan pelopor yang tidak sesuai telah menyumbang kepada keputusan tersebut.

Effect of Bismuth Additions on Wettability, Intermetallic Compound, and Microhardness Properties of Sn-0.7Cu on Different Surface Finish Substrates

The influence of bismuth (Bi) addition on wettability, thickness of interfacial intermetallic compound (IMC), and microhardness properties of Sn-0.7Cu + xBi solder alloy using different types of substrate were examined. The 0.5, 1.0, 1.5, and 2.0 wt. % Bi was added into Sn-0.7Cu and fabricated using the casting process. The result shows that the influence of 1.5 wt. % Bi in the Sn-0.7Cu solder soldered on copper organic solderability preservative (Cu-OSP) and immersion tin (Im-Sn) surface finish has improved the wettability and microhardness. Subsequently, the IMC thickness of Sn-0.7Cu+1.5Bi solder alloy on Im-Sn surface finish gives a better result than reflowed on Cu-OSP. Generally, with the addition of 1.5 wt. % Bi in Sn-0.7Cu solder alloy reflowed on the Im-Sn surface finish had enhanced the performance in terms of wettability, thickness of IMC and microhardness properties compared to on Cu-OSP surface finish.

Effect of Sb additions on the creep behaviour of low temperature lead-free Sn–8Zn–3Bi solder alloy

PurposeThis paper aims to investigate the creep behaviour of the recently developed Sn–8Zn–3Bi–xSb (x = 0, 0.5, 1.0 and 1.5) low temperature lead-free solder alloys.Design/methodology/approachAn in-house compressive test rig was developed to perform creep tests under stresses of 20–40 MPa and temperature range 25°C–75 °C. Dorn power law and Garofalo hyperbolic sine law were used to model the secondary creep rate.FindingsHigh coefficient of determination R2 of 0.99 is achieved for both the models. It was found that the activation energy of Sn–8Zn–3Bi solder alloy can be significantly increased with addition of Sb, by 60% to 90 kJ/mol approximately, whereas the secondary creep exponent falls in the range 3–7. Improved creep resistance is attributed to solid solution strengthening introduced by micro-alloying. Creep mechanisms that govern the deformation of these newly developed lead-free solder alloys have also been proposed.Originality/valueThe findings are expected to fill the gap of knowledge on creep behaviour of these newly developed solder alloys, which are possible alternatives as lead-free interconnecting material in low temperature electronic assembly.

Fatigue Properties and Microstructure of SnAgCu Bi-Based Solder Joint

Abstract The reliability of solder joints plays a critical role in electronic assemblies. SnAgCu solder alloys with doped elements such as Bi and Sb is one of the candidates for high reliability applications. However, the mechanical and fatigue properties of the actual solder joint structure have not been studied for these new alloys. In this paper, a cyclic fatigue test was conducted on individual real solder joints of different alloys, including SnAgCu, SnCu–Bi, SnAgCu–Bi, and SnAgCu–BiSb. The fatigue property of those solder joints was analyzed based on the characteristic fatigue life and stress–strain, hysteresis, loops. The results show that solder joints with both Ag and Bi content have a better fatigue resistance than the solder joints with Ag or Bi content only. The results of SnAgCu and SnCu–Bi solder alloys show similar fatigue performance. Also, the fatigue performance of SnAgCu–Bi is close to SnAgCu–BiSb in the accelerated test. But the SnAgCu–Bi alloy is estimated to have a longer characteristic life under low-stress amplitude cycling. The microstructure analysis shows a bismuth-rich phase formed around the Ag3Sn precipitates. Adding bismuth in the solder alloy can significantly improve the fatigue properties through solid solution hardenings. On another hand, the plastic strain range and work dissipation were measured from the hysteresis loops for all tests. The Morrow Energy and the Coffin–Manson models were developed from the fitted data to predict the fatigue life as a function of work dissipation and plastic strain range.

Effects of Temperature–Humidity Treatment on Bending Reliability of Epoxy Sn–58Bi Solder with Electroless Nickel Immersion Gold (ENIG) and Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG) Surface Finishes

An epoxy Sn–58wt.%Bi solder joint was evaluated by a three-point bending test with electroless nickel immersion gold (ENIG) and electroless nickel electroless palladium immersion gold (ENEPIG) surface finishes aged at 85 °C and 85% relative humidity. Scanning electron microscopy and electron probe microanalysis were carried out to study intermetallic compound variation. The morphology, total thickness, and chemical composition of intermetallic compound in epoxy Sn58Bi solder joints were the same as those of Sn–58wt.%Bi solder joints with each surface finish. The average number of bending-to-failure cycles for the epoxy Sn–58wt.%Bi solder/ENIG joints and epoxy Sn–58wt.%Bi solder/ENEPIG was more than 4000 and 5000, respectively. The average number of bending-to-failure cycles of the epoxy Sn–58wt.%Bi solder joint decreased with increasing age. Three-point bending reliability of epoxy Sn–58wt.%Bi solder joints was higher than that of Sn–58wt.%Bi solder with both surface finishes. Cracking of all solder joints subjected to as-reflowed was propagated through the solder matrix. However, after aging for 1000 h, cracking occurred primarily between intermetallic compound layers.

Microstructures and Mechanical Properties of Sn-58 wt.% Bi Solder with Ag-Decorated Multiwalled Carbon Nanotubes Under 85°C/85% Relative Humidity Environmental Conditions

The mechanical properties of Sn-58 wt.% Bi solder with different amounts (0 wt.%, 0.05 wt.%, 0.1 wt.%, and 0.2 wt.%) of Ag-decorated multiwalled carbon nanotube (MWCNT) nanoparticles under 85°C/85% relative humidity environmental conditions for 0 h to 1000 h was investigated. Sn-58 wt.% Bi solder is a lead-free option for use in solder joints due to its low melting temperature and good creep resistance; however, it is brittle and has reliability issues. Ag-decorated MWCNT nanoparticles were used to improve these weaknesses of Sn-58 wt.% Bi solder. A ball shear test was performed using a bond tester to investigate the solder's mechanical properties. The microstructures of the solder joints and fracture mode were analyzed using a field-emission scanning electron microscope. The results demonstrated that the addition of Ag-decorated MWCNT nanoparticles to Sn-58 wt.% Bi increased the shear strength and fracture energy by approximately 15% and 14%, respectively, compared with Sn-58 wt.% Bi alone. After a high-temperature, high-humidity test for 1000 h, the shear strength and fracture energy of Sn-58 wt.% Bi with 0.1 wt.% Ag-decorated MWCNT nanoparticles were 13% and 21% greater than for Sn-58 wt.% Bi alone.

Mechanical, electrical, and thermal reliability of Sn-58wt.%Bi solder joints with Ag-decorated MWCNT for LED package component during aging treatment

The effect of aging on reliability of Sn-58 wt%Bi solder joint with Ag-decorated multi-walled carbon nanotubes (Ag-MWCNTs) was investigated for the LED package components. The mechanical, electrical and thermal characteristics of the solder joints with various Ag-MWCNT contents were examined under aging treatment. Ag-MWCNTs were synthesized by using a colloidal method and the nanocomposite solder paste was fabricated by mechanically mixing the Ag-MWCNTs with Sn-58 wt%Bi solder paste. The flip-chip light emission diode (FC-LED) package component was bonded on the substrate with Cu pad finished organic solderability preservative (OSP). And then these were isothermally aged at 85 °C up to 1000 h in an oven. The mechanical, electrical and thermal property of solder joints was evaluated by using shear test, measuring the electrical resistance and thermal transient test, respectively. The bonding strength of all of the solder joints decreased and the electrical resistance of the solder joints increased with increasing aging time. The bonding strength of the solder joints with 0.05Ag-MWCNTs was 12% higher than those of other solder joints. And the electrical and thermal resistance of the solder joints exhibited the lowest resistance with 0.1Ag-MWCNTs and 0.05Ag-MWCNTs after aging for 1000 h, respectively.

Mechanical properties of Sn-58 wt%Bi solder containing Ag-decorated MWCNT with thermal aging tests

The mechanical properties and microstructure of Sn-58 wt%Bi solder including various levels of Ag-decorated MWCNTs (0, 0.05, 0.1, and 0.2 wt%) were investigated for use in solder joints. The electronic industry relies on Pb-free solders because lead can have harmful effects on the environment and human health. The Sn-58 wt%Bi solder is a representative low temperature Pb-free solder with high tensile strength and good creep resistance. However, the Sn-58 wt%Bi solder has poor ductility for relieving shock impact. To address this drawback, Ag-decorated MWCNTs were used to reinforce the mechanical properties of Sn-58 wt%Bi solder. The Sn-58 wt%Bi-nanocomposite solder paste was fabricated by mechanically mixing Ag-decorated MWCNTs with solder paste. The Sn-58 wt%Bi solder pastes including Ag-decorated MWCNTs were printed on a printed circuit board (PCB), and the joints were aged in an isothermal environment. Inclusion of 0.1 wt% Ag-decorated MWCNTs increased the shear strength of the solder by about 15%. The thickness of the IMC of Sn-58 wt%Bi solder increased with age.

Effect mechanism of Ag and Bi elements on the corrosion process in Sn–9Zn–1Ag and Sn–9Zn–3Bi solders

Sn–9Zn lead–free solder is widely used in low melting temperature welding field due to its low melting point (198.5 °C), good connection reliability, and low cost. However, its poor wettability and corrosion resistance weaken its performance. According to previous studies, addition of 1% Ag and 3% Bi to Sn–9Zn alloy, respectively, can optimize the wettability and mechanical properties of solder. Nonetheless, the effect of added elements on corrosion process has not been determined. In this study, the effect mechanism of Ag and Bi elements on the corrosion process in Sn–9Zn–1Ag and Sn–9Zn–3Bi solders was analyzed. The changes of the surface state of each solder in the corrosion process were detected by electrochemical polarization experiment. The microstructural compositions of solders in different corrosion stages were compared through the immersion test. The results indicated that compared to Sn–9Zn solder, Sn–9Zn–1Ag solder exhibited the existence of a finer Zn-rich phase, which improved the pitting corrosion resistance of solder by blocking the entry of corrosive solution. In passivation film forming process, Ag–Zn compounds were formed on the surface of Sn–9Zn–1Ag solder, which reduced the content of Zn-rich phase, leading to the decrease of the overall corrosion rate. After the formation of passivation film, Ag compounds were precipitated on the surface of passivation film of solder, which prevented further destruction of the passivation film. However, Sn–9Zn–3Bi solder produced a large number of loose structural corrosion products on the surface of the corroded sample, thus the substrate could not be effectively protected. Furthermore, Sn–9Zn–3Bi solder showed a larger Zn-rich phase, thus it was easier for the corrosive solution to enter the sample, resulting in severe corrosion in the interior of sample. In this study, the effect mechanism of Ag and Bi addition on the corrosion process of Sn–9Zn–1Ag and Sn–9Zn–3Bi solders was revealed, which provides a theoretical basis for the study on improving the corrosion resistance of Sn–9Zn solder.

Reliability Challenges in Fabrication of Flexible Hybrid Electronics for Human Performance Monitors: A System-Level Study

Flexible hybrid electronics (FHE) interface rigid electronic components with flexible sensors, circuits, and substrates. This paper reports the reliability improvement of an FHE human performance monitor (HPM), designed to monitor electrocardiography (ECG) signals. The $50.8 {\,\,\mathrm {mm}} \times 50.8 {\,\,\mathrm {mm}}$ HPM is fabricated on Kapton HN polyimide (PI) substrate with flexible gold (Au) ECG electrodes on one side of the substrate and rigid electronic components for signal processing and communication mounted on the other side of the substrate. Our previously reported HPMs demonstrated reliability issues due to: 1) cracking of the copper (Cu) circuitry and 2) thinning and lack of adhesion at the printed Au and plated Cu interface that connected printed sensors to the Cu circuitry. Both failure mechanisms resulted in electrical opens in the circuit, which caused device failure. We explored effects of different design parameters, such as PI substrate thickness (50 versus $125~\mu \text{m}$ ), Cu circuit thickness (2 versus 6 $\mu \text{m}$ ), solder reflow temperature [205 °C for Tin-Lead (Sn-Pb) versus 175 °C for Tin-Bismuth (Sn–Bi) solder], solder pad design, and optimized inkjet printing (printing on bare Cu versus Au-plated Cu) on improving FHE reliability. Test vehicles (TVs) with different combinations of these factors were fabricated and bend-tested to determine the most robust configuration. TVs with 50- $\mu \text{m}$ -thick PI substrate, 6- $\mu \text{m}$ -thick Cu circuit, Sn–Bi solder, redesigned solder pads with rounded corners, and printed Au traces on Au plated Cu pads demonstrated the best reliability results.

Experimental Investigation on the Effect of Ag Addition on Ternary Lead Free Solder Alloy –Sn–0.5Cu–3Bi

Lead cannot be used in the solder material anymore due to environmental legislations which is related to the inherent toxicity of lead. This paper investigates the effect of silver (Ag) addition on the melting behavior, microstructure, and microhardness of Ternary lead free solder alloy Sn–0.5Cu–3Bi. The contact angle between Sn–0.5Cu–3Bi and Cu-substrate were also analyzed. Samples with different Ag percentages (0, 0.25, 0.5, 0.75 and 1.0 wt%) in Sn–0.5Cu–3Bi were prepared using an induction furnace, annealing furnace with argon gas. Tests were conducted for melting temperature using TG–DTA analysis, chemical composition using ICP–OES, Hardness using Vickers’s hardness tester and the microstructure using Field emission scanning electron microscopy. The obtained results were thoroughly analyzed. The results show that the Ag addition has striking positive effects on enhancing the properties of the base solder alloy. Melting point is found to be decreasing with the increase in Ag content. Hardness and contact angle were improved with the addition of Ag. The microstructure observations revealed that the Ag was uniformly distributed on the surface of the solder matrix. The recommended content of the Ag to be added into the Sn–0.5Cu–3Bi solder alloy is 1.0 wt%. With the observed better properties it can be considered as the potential alternative to lead–tin alloy.

Fine Microstructured In-Sn-Bi Solder for Adhesion on a Flexible PET Substrate: Its Effect on Superplasticity and Toughness.

A novel In-Sn-Bi solder with a low electrical resistivity of 14.3 × 10-6 Ω cm and a melting temperature of 99.3 °C was produced for use in adhesive joining on a flexible poly(ethylene terephthalate) substrate. We determined that the fine microstructure of the In-based solder (which had an average phase size of 62.2 nm) strongly influenced its superplasticity and toughness at diffusive temperatures of 55-85 °C because the late-forming BiIn intermetallic compound (IMC) suppressed the growth of two other IMCs, In3Sn and In0.2Sn0.8, which formed earlier in the soldering process. Thus, an elongation of 858.3% and toughness of 36.0 MPa were obtained at a temperature of 85 °C and a strain rate of 0.0020 s-1. However, due to phase boundary fracturing, the phase-refined solder exhibited a slightly more brittle nature (with an elongation of 74.3%) at room temperature compared with a standard In-Sn solder consisting only of the In3Sn and In0.2Sn0.8 IMCs, which had a slightly larger phase size of 84.9 nm and higher ductility (with an elongation of 80.7%). In terms of superplastic deformation, the conventional fracture system based on the Hall-Petch effect transformed into phase boundary sliding at the solder operating temperature, significantly enhancing ductility.

Abnormal migration behavior and segregation mechanism of Bi atoms undergoing liquid–solid electromigration

The migration behavior of Bi atoms in Cu/Sn–58Bi/Cu solder interconnects undergoing liquid–solid electromigration (L–S EM) was in situ observed using synchrotron radiation real-time imaging technology. Bi atoms tend to migrate from the cathode toward the anode resulting in the linear growth of Bi-rich phase layer at the anode in both the heating and cooling stages but not in the dwelling stage. Since the temperature of Sn–Bi solder in the dwelling stage is higher than those in both the heating and cooling stages, the electrically driven migration effect diminishes with increasing temperature. As a result, a three-layer equilibrium distribution of Bi-rich, Sn–Bi and Sn-rich phases across the solder is observed in the dwelling stage of 140 °C, and a uniform Bi distribution across the solder is observed in the dwelling stage of a higher temperature of 170 °C, i.e., Bi atoms fully diffuse backward at such a high temperature. The abnormal migration behavior of Bi atoms is determined by the combined effect of chemical potential gradient-induced flux (Jchem) and EM-induced flux (Jem). The EM-induced flux (Jem) is determined by the effective charge number (Z*) of Bi atoms that is calculated as − 3.57 at 140 °C based on the growth kinetics of the Bi-rich layer. Furthermore, the Z* value of Bi atoms is calculated as − 3.04 by using a modified model for calculating Z* of liquid metals, which agrees well with the experimental value. The present work provides a reference for purifying refractory metals from alloys.

Recent progress on the development of Sn–Bi based low-temperature Pb-free solders

With the implementation of legislations on inhibiting the usage of Sn–Pb solder in consumer electronic products, Sn–Ag–Cu series solder has been gotten the most application. However, there are some stimulations from electronic manufacturers to adopt low temperature soldering such as the economic driver from the reduction in manufacturing assembly cost and the reliability driver to avoid the dynamic warpage of area array components caused from Sn–Ag–Cu solder. Sn–Bi series solder is one of the promising candidates, which met the requirements for low melting point, low cost and environment friendly. However, the disadvantage of brittleness characteristic prevented its wide practical application. In order to promote the better application of Sn–Bi based solders, many efforts have been made to improve the wettability, mechanical properties and reliability of Sn–Bi based solders. This paper will summarize the related results about Sn–Bi solder alloys from wettability, interfacial reaction, mechanical properties of Sn–Bi solder and reliabilities of Sn–Bi solder joints. Moreover, in order to improve the properties of Sn–Bi solders, researchers have done lots of works on effect of addition of element dopants. The corresponding works of effect of alloying elements on the properties of Sn–Bi solder were also focused. According to the existing research results, it provides an important basis of understanding the current development of Sn–Bi solders.

Effect of nickel (Ni) on the growth rate of Cu6Sn5 intermetallic compounds between Sn–Cu–Bi solder and Cu substrate

In this work, the lead-free composite solder was fabricated by mixing Ni element with Sn–0.7Cu–10Bi solder. The effect of nickel (Ni) addition on the growth behavior of intermetallic compounds (IMCs) between Sn–0.7Cu–10Bi–xNi (x = 0, 0.05, 0.10, 0.15 and 0.20, in wt%) solder and Cu substrate during the soldering process was studied. The microstructure and the IMCs growth of the solder joints under thermal aging were systematically investigated. The results shown that the addition of Ni element has a slightly influence on melting point of the solder. Moreover, the addition of Ni element can change Cu6Sn5 shape from scalloped-like structure into flat-like one. Moreover, the results reveal that Ni can considerably inhibit the growth and reduce the thickness of Cu6Sn5. The thickness of Cu6Sn5 ranges from 3.07 to 8.42 µm after aging process The diffusion coefficient (D) is 1.80 × 10−3 µm2 h−1 and growth rate (dH/dt) is 4.08 × 10−7 µm s−1 of the Cu6Sn5 when the Ni content come to 0.15 wt%. The (Cu,Ni)6Sn5 phase formed in the IMCs layer when Ni was added to the solder, and it can effectively hinder the diffusion of Cu atoms and depressed the growth rate of Cu6Sn5.

Electromigration behaviors in Sn–58Bi solder joints under different current densities and temperatures

Sn–58Bi eutectic solder is attracted much attention to replace Sn–Ag–Cu Pb-free solder due to its lower melting temperature in recent years. However, its low melting temperature also raised serious electromigration (EM) reliability due to the accelerated atomic migration from the high local temperature induced by Joule heating. This paper mainly studied the EM behaviors occurred in Sn–58Bi solder joints with consideration the effect of different current densities and temperature. Variation on the atomic migration and phase coarsening were observed in Sn–58Bi solder joint, and the threshold current densities to trigger EM damage were estimated. The results showed that Bi-rich layer was produced at anode side during stressing with low current density while caused Cu cathode to dissolve into Sn–Bi solder and formed Cu6Sn5 intermetallic compounds near anode side during stressing with high current density. Accordingly, the corresponding dominant diffusing species in Sn–58Bi solder were Bi atoms at low temperature and Cu and Sn atoms at high current density, respectively. High temperature applied with EM accelerated the atomic migration. The threshold current densities at different temperature were then calculated from the thickness of Bi-rich layer at anode side. There existed a lower threshold current density at higher temperature due to the active atomic mobility. Finally, Bi phase coarsening was observed in Sn–Bi solder under different current densities and temperatures. Higher current density or higher temperature induced faster grain growth, but the applied current had a greater effect on Bi phases coarsening in Sn–Bi solder matrix than the applied temperature. The controlling kinetics on the growth of Bi phases were also suggested for the effect of current densities and temperatures.

Electromigration behaviors of Sn58%Bi solder containing Ag-coated MWCNTs with OSP surface finished PCB

We investigated the effect of Ag-coated multi-walled carbon nanotubes (Ag-MWCNT) on the microstructures and electromigration behaviors of a Sn58%Bi solder and organic solderability preservative (OSP) surface finished on the FR-4 printed circuit board (PCB) joint under a current stress of 3000 A/cm2 at 100 °C. Electromigration of Ag-MWCNT Sn58%Bi composite solder was investigated by daisy-chain test-kit. Reaction layers formed at the anode side and cathode side of the Sn58%Bi solder joints consisted of three microstructures; Bi-rich layer, Sn-rich layer, and intermetallic compounds (IMCs, Cu6Sn5 and Cu3Sn). The Bi-rich layer was mainly formed at the anode side in the couple of the Sn58%Bi solder joint with various times of applying current stress. The Bi-rich layer of the Ag-MWCNT Sn58%Bi composite solder joint was approximately 2 times thinner than that of the Sn58%Bi solder joint because the Ag-MWCNT acts as a diffusion barrier. Also, the Cu6Sn5 and Cu3Sn IMCs that formed at the interface between the Ag-MWCNT Sn58%Bi composite solder joints were thinner than those of the Sn58%Bi solder joints. The time to failure (TTF) was longest at the 0.05% Ag-MWCNT Sn58%Bi composite solder joint. Therefore, Ag MWCNT is expected to improve the reliability of electromigration in the Sn58%Bi composite solder joint.