Sn Solder Research Articles

The Effect of Thermal Ageing on Solder/Substrate Interfacial Microstructures During Reflow of Sn–37Pb and Sn–3Ag–0.5Cu

In the current study, the influence of thermal ageing on evolution of microstructures in the interfacial region between solders (Sn–37Pb, Sn–3.5Ag–0.5Cu) and copper substrates was investigated. Pb-containing and Pb-free solders were reflowed on Cu substrates at 230 °C for 15 min and were isothermally aged at 100 °C for 24 h. As-reflowed Sn–Pb solder/Cu substrate interfacial region exhibited continuous and layered type of IMC at the interface, and this IMC morphology changed to scallop type with isothermal ageing. SAC solder/Cu as-reflowed samples showed continuous and needle-shaped Cu6Sn5 and Ag3Sn IMCs at the interface. However, in an isothermally aged condition, plate-shaped Cu6Sn5 and flower-shaped Ag3Sn IMCs were found inside the solder matrix. Scanning electron microscopic (SEM) study showed that the thickness of Cu6Sn5 IMC was higher in reflowed Sn–Pb/Cu region than in SAC/Cu region.

Relationship between free solder thickness to the solderability of Sn–0.7Cu–0.05Ni solder coating during soldering

The relationships between solderability, free solder thickness and intermetallic thickness of Sn–0.7Cu–0.05Ni solder coatings on Cu substrate has been investigated. The annealing method was proposed to control free solder thickness by controlling the ratio of free solder and interfacial intermetallics (IMCs). The solderability of solder coating has investigated using a purpose-designed microwetting balance. The interfacial intermetallic microstructure was analysed and the thickness measured in the as-coated condition with aging process. Results indicate that the solderability is related to free solder thickness, where the increase of aging time and temperature thickness of the free solder is reduced by the interfacial intermetallic compound growth. Onwards, the different composition of germanium (Ge) was added into Sn–0.7Cu–0.05Ni as an antioxidant to control drossing of the molten solder. The effect on solderability of a Ge addition to the coating alloy was measured. These results may be used as a basis to obtain an optimum wettability of solder coating during soldering.

Interface Growth and Void Formation in Sn/Cu and Sn0.7Cu/Cu Systems

In this work, the effects of electroplated Cu (EP Cu) and Cu addition (0.7%) in Sn solder on the intermetallic compounds (IMCs) growth and void formation were clarified by comparison with solder joints comprising of high purity Cu (HP Cu) substrate and pure Sn solder. After aging processes, a new IMC, Cu3Sn, was formed at the interface, in addition to Cu6Sn5 formed in the as-soldered joints. The EP Cu and Cu addition (0.7%) both had limited effects on the total IMCs thickness. However, the effects varied on the growth behaviors of different IMCs. Comparing to the void-free interface between Sn and HP Cu, a large number of voids were observed at the Cu3Sn/Cu interface in Sn/EP Cu joints. The formation of these voids may be induced by the impurities and fine grain, which were introduced during the electroplating process. The addition of Cu suppressed the inter-diffusion of Cu and Sn at the interface. Consequently, the growth of the Cu3Sn layer and formation of voids were suppressed.

Research on Interfacial Reaction and Growth Behavior of Intermetallic Compound of Dip-Soldered Sn/Ni System

Interfacial reaction and the growth behavior of intermetallic compound (IMC) between Sn solder and Ni substrate by dip soldering under different aging conditions (120 °C, 170 °C and 200 °C) by using scanning electron microscope were investigated. Experimental results showed that the only scallop-shaped Ni3Sn4 IMC layer formed at the interface after aging process and the thickness of IMC layer increased with prolonged aging time. Moreover, the cracks formed in the interface between IMC and solder could be resulted from the residual stress caused by the difference in thermal expansion coefficients. In addition, the spalling behavior of IMC from substrate was caused by cyclic shear stresses and strains induced and the morphological changes in IMC formation. During the aging process, it was found that the thickness of IMC always had a linear relationship with the square root of aging time under different aging temperatures (120 °C, 170 °C and 200 °C). The growth rate constants of IMC layers aged at 120 °C, 170 °C and 200 °C were obtained by using fitting method from the experimental data, which were 4.65 × 10−18 m2 s−1, 9.94 × 10−18 m2 s−1 and 1.76 × 10−16 m2 s−1, respectively. Based on these data, the activation energy of IMC layer was determined to be 63.23 kJ/mol. Likewise, the mean diameter of IMC grains increased with the increase in aging time. It is suggested that the mean diameter of IMC grains was always plotted against the aging time on a double logarithmic scale under different aging temperatures. The time rate constants of IMC grains aged at 120 °C, 170 °C and 200 °C were obtained by using same method, which were 0.10, 0.27 and 0.26, respectively.

Evolution of microstructure of Lead free cu/Sn solders and copper oxide phase precipitation in Cu3Sn intermetallic during thermal cycling

Intermetallic layers were formed after Solid Liquid Interdiffusion (SLID). Cu-Sn interdiffusion leads to the formation of two intermetallic layers: Cu6Sn5 and Cu3Sn. In this work, Sn solder was subjected to thermal cycling from −65 to +250 °C at 10 °C/min. After few cycles we noticed the appearance of a new –pillar– shaped phase made of copper oxide. The Intermetallic layers were mechanically tested using die-shear testing. The die-shear resistance appears to be increasing when the number of thermal cycles increases. The microstructure was observed using Backscatter Electron detector (BSD) and Scanning Electron Microscopy (SEM). The composition of the phase was measured using Energy Dispersive x-ray spectroscopy (EDX). It appears that a new copper oxide phase forms after few cycles. As the number of cycles increase, the size and the number of the copper oxide phases increase.

Geometrical effects on growth kinetics of interfacial intermetallic compounds in Sn/Cu joints reflowed with Cu nanoparticles doped flux

In this study, Cu nanoparticles prepared by chemical reduction method, were doped into flux (0–2 wt%) and disseminated to the pure Sn solder ball at 250. The enhanced spreading rate due to use of nanoparticles, increased the base diameter (W) and decreased the height (H) of the solder at constant volume. The finite element analysis for Cu concentration, temperature and velocity; in relation to the magnitudes of W and H show that larger W is responsible for enhancement of supersaturation and radial thermal gradient, whereas smaller H is responsible for reduction in flow velocity. The growth kinetics of interfacial Cu 6Sn 5 film during isothermal reflow is proportional to W 2/3 during isothermal reflow whereas linearly dependent on H during air cooling. As the ripening at isothermal stage and precipitation at cooling stage contribute to the gross growth behavior of Cu 6Sn 5 intermetallic compounds layer growth, the combined geometrical effect of base diameter and height of the solder specimen renders solder corresponding to flux with 2.0 wt% nanoparticles to have the overall thickest intermetallics.

Enhancement of melt-spun process Bi–Ag lead-free solder for high temperature applications

The enhancement of lead free solders has emerged as one of the most important issues in the electronics assembling industries. Bi–Ag hypo and eutectic alloys have been considerd as one of the lead free solders that can replace the Pb–Sn solder for high temperature applications. This study investigates the effect of various compositions of silver on the mechanical, thermal and electrical properties of the melt-spun process Bi–Ag hypo and eutectic alloys was directionally solidified using copper wheel technology.The microhardness (Hv), creep resistance were measured from solidified ribbons.The results showed that presence of α-Bi phase, Ag99.5Bi0.5 (with a cubic structure Fm-3m) and Ag0.97Bi0.03 (with a cubic structure Fm-3m) embedded in Bi matrix. Furthermore, the eutectic composition Bi—2.5 wt% Ag shows the more regular microstructure than hypo eutectic composition. It is also concluded that the solderability of solder is significantly imported due to the surface tension of molten Bi–Ag lead-free solder decreases with the Ag content. The obtained results show a surprising desirable decrease in melting temperature with increasing Ag from 269 to 261 °C. Also, it was found that the eutectic composition Bi—2.5 Ag exhibit good mechanical properties superior to hypo-eutectic composition.

Effect of zinc particle mixing on properties of copper–nanoparticle/bismuth–tin solder hybrid joints

The effects of Zn particle mixing into a Cu–nanoparticle/Bi–Sn solder hybrid bonding were examined. The bonding strength of an SiC/direct-bonded-copper joint exhibited limited dependence on the Zn mixing ratio, whereas that of an SiC/Cu joint increased with the Zn mixing ratio, up to 20 wt%. In the samples, Zn was present throughout the entire bonding layer, except for the Bi phase region. Bi segregation at the interface region, which occupies more volume in the SiC/Cu joints and impairs the bonding strength, reduced with the mixing ratio of Zn. Simultaneously, the volume fraction of the intermetallic compound involving Sn in the bonding layer increased. These phenomena could increase the interfacial strength, which reflects the bonding strength. However, at a Zn mixing ratio exceeding 20 wt%, the bonding strength decreased with increased Zn mixing ratio, independent of the joint structure, because of the increasing number of micro-voids generated in the bonding layer. The findings of this study indicate that mixing Zn powder into a Cu–nanoparticle/Bi–Sn solder hybrid bonding enables high bonding strength, even in joints composed of substances having strongly different thermal expansion coefficients, such as SiC and Cu.

Study of Fusion Thickness of Tin Solder Heating by Self-Propagating Exothermic Reaction

Heat released from aluminum-nickel (Al/Ni) self-propagating exothermic reaction has been developed as a rapid heating source and is called a localized heating source due to its short duration and narrow heat affected zone (HAZ). It is suitable for the interconnection of thermal mismatch material couples, temperature-sensitive devices, microbial systems and microanalysis systems. However, the fusion thickness of the solder layer has not been deeply studied to minimize the HAZ. In this study, Sn was electroplated on the AlNi nanofoil. Experimental and numerical methods were used to evaluate the heating capability of AlNi nanofoil through analyzing the size of fusion zone and HAZ within the Sn solder–AlNi nanofoil–Sn solder sandwich structure. Finite element analysis (FEA) was used for simulating the temperature field and temperature history of the structure. Different thicknesses of tin layer were considered to analyze the effect of the change of structures. The temperature of the surface of the tin layer and fusion zone was measured to validate the FEA model. Furthermore, the analysis showed that the maximum thickness for melting the Sn layer within the substrate–Sn solder–Al/Ni nanofoil–Sn solder–substrate sandwich structure changes with the thickness of substrates and preheating temperature. FEA predictions indicated that a preheating temperature of 125°C is effective to bond Cu substrates at the thickness of 0.3 mm with tin solder.

Study of Interfacial Reactions Between Lead-Free Solders and Cu-xZn Alloys

The solid/liquid reaction couple technique was employed to investigate the interfacial reactions between Cu-xZn (x = 0 wt.%, 5 wt.%, 15 wt.%, 30 wt.%, 40 wt.%) alloys and lead-free solders Sn, Sn-3.0Ag-0.5Cu (SAC, in wt.%), and Sn-9Zn (SZ, in wt.%) alloys at 240°C, 270°C, and 300°C for 0.5 h to 100 h. (Cu,Zn)6Sn5 and (Cu,Zn)3Sn phases were formed in the Sn/Cu-xZn (x = 5 wt.%, 15 wt.%, 30 wt.%) reaction couples, but with increasing reaction temperature and time, (Cu,Sn)Zn phase was formed, replacing (Cu,Zn)3Sn phase. Metastable T phase and (Cu,Sn)Zn phase were formed in the Sn/Cu-40Zn reaction couple at 300°C. (Cu,Zn)6Sn5 and (Cu,Zn)3Sn phases formed in the SAC/Cu-xZn (x = 5 wt.%, 15 wt.%) reaction couples. Furthermore, (Cu,Zn)6Sn5 and (Cu,Zn)Sn phases were observed when the SAC solders reacted with Cu-30Zn and Cu-40Zn alloys. T phase and (Cu,Sn)Zn phase were formed in the SAC/Cu-40Zn reaction couple reacted at 300°C for 100 h. (Cu,Sn)Zn5 and (Cu,Sn)5Zn8 phases were formed in the SZ/Cu-Zn reaction couples at 240°C. However, with increasing reaction time and temperature, only (Cu,Sn)5Zn8 phase was detected. Therefore, it can be concluded that the intermetallic compound (IMC) formation was sensitive to both the reaction temperature and the Zn content in the Cu-Zn alloy.

Kinetic study of solid-state interfacial reactions of p-type (Bi,Sb)2Te3 thermoelectric materials with Sn and Sn–Ag–Cu solders

P-type (Bi,Sb)2Te3 material system is extensively used in commercial thermoelectric (TE) applications. Soldering is commonly applied to fabricate bulk TE elements into TE modules. This study carefully examined the solid-state interfacial reactions of the commercial (Bi,Sb)2Te3 substrate with Sn and Sn–3.0Ag–0.5Cu solders. The joining plane was along the c-axis of the highly oriented [1 1 0] TE substrate. For the reactions with pure Sn solder, both the intermetallic compounds (IMCs) of SnTe and SnSb were rapidly formed at the interface. Most importantly, it was clearly demonstrated that both of them exhibited a linear growth with the aging time. The growth rates of SnTe and SnSb were 15.1 μm/h and 4.3 μm/h, respectively, at 180 °C. The IMC growth kinetics was further investigated systematically. The activation energies for the SnTe and SnSb growths were 151.6 kJ/mol and 165.3 kJ/mol, respectively. For the Sn–3.0Ag–0.5Cu/(Bi,Sb)2Te3 reaction, the SnTe and SnSb were also formed simultaneously. Similarly, they also had a linear growth, but their growth rates were significantly suppressed. A thin stripe-like Ag-rich phase was formed at the SnTe/(Bi,Sb)2Te3 interface, which could be the cause of the suppression of IMC growth. Additionally, minor addition of 0.1 wt.% Ga in Sn was also found to effectively suppress the interfacial IMC growth.

Effects of Dy substitution for Sn on the solderability and mechanical property of the standard near eutectic Sn–Ag–Cu alloy

In this study, the solderability of Sn–3.5Ag–0.5Cu–xDy solders were investigated and the shear strength properties of joints with Cu substrate were investigated. The results indicated that a small amount Dy addition can improve the solderability, and the optimal amount of Dy was 0.025 wt%. The maximum shear strength can be found with 0.025 wt% Dy addition, improved by 74%. With the observation of the fracture morphology, it was found that a small amount Dy can improve the ductility of the solder joints; but excessive amount of Dy would deteriorate the shear strength and form large dimples on the fracture surface.

Bonding of Si chips to low carbon steel boards using electroplated Sn solder

PurposeThe purpose of this research was to develop a new process to bond silicon (Si) chips to low carbon steel substrates using pure tin (Sn) without any flux.Design/methodology/approachIron (Fe) substrates were first electroplated with a Sn layer, followed by a thin silver (Ag) layer that inhibits Sn oxidation thereafter. It is this Ag capping layer that makes the fluxless feature possible. Fluxless processes are more environmentally friendly and more likely to produce joints without voids. The Si chips were deposited with Cr/Au dual layer structure. The bonding process was performed at 240°C in vacuum. The Sn joint thickness was controlled by spacers during the bonding. Scanning electron microscopy images on cross sections exhibited quality joints without visible voids. Energy dispersive X-ray spectroscopy analysis was used to detect joint compositions.FindingsIt was revealed that the Sn layer was bonded to a Si chip at the Cr–Sn interface and to the Fe substrate by forming an FeSn2 intermetallic compound (IMC). The IMC is only 1.1 to 1.5 µm in thickness. Thin IMC is highly preferred because IMC deforms a little in accommodating the coefficient of thermal expansion (CTE) mismatch between Si and Fe. Shear test results showed that the fracture forces of the samples passed the military criteria by a wide margin.Originality/valueThis new fluxless bonding process on Fe should make Fe or low carbon steel a more likely choice of materials in optical modules and electronic packages.

Gold-Tin Solder Wetting Behavior for Package Lid Seals

This study examined the cause of nonwetted regions of the gold (Au) finish on iron-nickel (Fe–Ni) alloy lids that seal ceramic packages using the 80Au-20Sn solder (wt %, abbreviated Au–Sn) and their impact on the final lid-to-ceramic frame solder joint. The Auger electron spectroscopy (AES) surface and depth profile techniques identified surface and through-thickness contaminants in the Au metallization layer. In one case, the AES analysis identified background levels of carbon (C) contamination on the surface; however, the depth profile detected Fe and Ni contaminants that originated from the plating process. The Fe and Ni could impede the completion of wetting and spreading to the edge of the Au metallization. The Au layer of lids not exposed to a Au–Sn solder reflow step had significant surface and through-thickness C contamination. Inorganic contaminants were absent. Subsequent simulated reflow processes removed the C contamination from the Au layer without driving Ni diffusion from the underlying solderable layer. An Au metallization having negligible C contamination developed elevated C levels after exposure to a simulated reflow process due to C contamination diffusing into it from the underlying Ni layer. However, the second reflow step removed that contamination from the Au layer, thereby allowing the metallization to support the formation of lid-to-ceramic frame Au–Sn joints without risk to their mechanical strength or hermeticity.

Interfacial behavior and joint strength of Sn–Bi solder with solid solution compositions

To obtain the interfacial behaviors and the joint strength of Sn–Bi solid solution solder, Sn–2.5Bi and Sn–5Bi solders were selected to be investigated in this work with pure Sn/Cu solder joints as a reference. The results showed that the growth behaviors of intermetallic compound (IMC) at Sn–Bi solder joints were different under reflow soldering and isothermal aging, which was related with the presence of Bi atoms and the diffusion rate of Cu atoms. Furthermore, the Cu6Sn5 particles in solder matrix and the Bi precipitation could change the diffusion path of Cu atoms, which caused the change on the distribution and size of Cu6Sn5 in pure Sn, Sn–2.5Bi and Sn–5Bi solder matrix after isothermal aging. The size and distribution of Cu6Sn5 in solder matrix finally affected the shear strength of solder joints. The shear strength of Sn–2.5Bi/Cu solder joints increased compared with pure Sn/Cu solder joints, and there was no obvious change with the aging times for Sn–2.5Bi/Cu solder joints due to the effect of the reinforcement of Bi atoms. However, the shear strength of Sn–5Bi/Cu solder joints increased firstly and then decreased with the prolongation of aging times. This phenomenon was attributed to the segregation of Bi and the large amounts of Cu6Sn5 particles distributed in solder matrix during the aging condition. Besides, the IMC thickness in Sn–5Bi/Cu solder joint played the important role on the shear strength due to the brittle nature of IMC. In pure Sn solder matrix, the size of Cu6Sn5 particles were larger than that in Sn–Bi solder matrix, which deteriorated the shear strength on pure Sn solder. From the observation on the fracture morphologies, Sn/Cu and Sn–2.5Bi/Cu solder joints showed the ductile fracture occurred in the solder matrix, while Sn–5Bi/Cu solder joints exhibited a mixed fracture.

Microstructure and reliability of Mo nanoparticle reinforced Sn–58Bi-based lead-free solder joints

ABSTRACTThe effect of Mo nanoparticles on the microstructure, wettability and mechanical properties of Sn–58Bi–xMo (x = 0–2.00) solder joints was investigated. The results showed that the optimal wettability of Sn–58Bi–0.25Mo solder was obtained and the microstructure was refined by adding small amounts of Mo. The intermetallic compounds (IMCs) thickness was increased from Sn–58Bi 2.17 µm to Sn–58Bi–2Mo 2.73 µm. The tensile strength of Sn–58Bi solder joint was improved by adding 0.25 wt-% Mo. The fracture mode of Sn–58Bi solder joint and Sn–58Bi–0.25Mo solder joint was brittle fracture and the mixed mode was ductile failure and brittle fracture, respectively. The microstructure of Sn–58Bi and Sn–58Bi–0.25Mo solder joints was coarsened, IMCs thickness was increased and the mechanical properties were deteriorated with increasing aging time.

Effects of Microstructure of Nickel Electrodeposits on the Growth of Tin-Nickel Intermetallic Compound and Joint Reliability

The mechanical breakage and electrical open failure of the flipchip bump interconnection are main issues that retarding the scale shrinkage of the 3D interconnections. In this study, the effects of the microstructure of the nickel electrodeposits on the growth of Sn-Ni intermetallic compound (IMC) and on the electro-migration (EM) reliability were investigatged. The crystal structure of the Ni diffusion barrier can be controlled by addition of additives in electroplating bath. When both coumarin and cis-2-butene-1,4-diol were contained in the bath, extremely fine grain Ni electrodeposits were deposited by the synergistic effects of the additives. The growth rate of Ni3Sn4 IMC formed on the interface between the extremely fine grain structured Ni layer and Sn solder is higher than that of the IMC formed on the interface between the large grain structured Ni layer and Sn solder. The EM failure stage corresponds to large void propagation at the Sn-Ni3Sn4 interface due to volume consumption of Sn for IMC formation. The results of STEM analysis in an early stage of the heat treatment of the solder demonstrated that substantial quantities of Ni atoms were dominantly dissolved into the liquid Sn layer. Ni3Sn4 IMC was mainly formed on the finely grained Ni layer due to the high dissolution of Ni atoms into the solute Sn during the reflow process. IMC growth was suppressed at the interface between the large and fine grain structured area. This study suggests a robust concept of the vertical structure of Ni diffusion barrier having a multi layered Ni microstructure, which can reinforce an adhesion property as well as diffusion barrier properties Figure 1

Spatial Chemical Analysis of Electrodeposited Metal Films By Femtosecond Laser Ablation Ionization Mass Spectrometry

In recent years, the development of three-dimensional large-scale integration (3D-LSI) has been accelerated to overcome the limitations of the classical 2D integration approach, which has begun to deviate from the ideal scaling trends1. Ever increasing complexity of these novel structures that assemble a great variety of materials with distinct physical properties pose new challenges for fabrication and subsequent composition analysis of microchips. In this context, metal electrodeposition plays a crucial role in the manufacturing of the interconnect networks inside the devices. Since these metallic interconnects are manufactured by an additive-assisted metal electrodeposition process, incorporation of the employed additives in the metallic matrices in trace amounts might heavily influence their performance and durability2. Herein we present case studies that analyze two key components of state-of-the-art integrated circuits (ICs), namely through-silicon-vias (TSV)2 and lead-free solder interconnects3 (Sn-Cu). In the former case, accurate targeting and chemical composition analysis of TSVs was addressed by means of three complementary techniques: femtosecond laser ablation ionization mass spectrometry (LIMS technique; τ ~ 190 fs, λ = 775 nm, laser crater diameter Ø ~ 15 µm)4, Auger electron spectroscopy and focused ion beam. Top-down laser ablation analysis of the TSVs allowed qualitative depth profiling that showed a decreasing trend of the C content with depth. This indicates preferential incorporation of organic impurities from the employed plating additives on the top level of the TSV feature. Quantitative C content analysis inside the copper lines was enabled by an alternative experimental approach, in which the chemical composition of the copper structures was analyzed over the feature cross-sections. In agreement with top-down profiling this approach revealed a 1.5-fold increased embedment of C in the upper part of the TSV with respect to the bottom section. Regarding the second case study, we focused on enabling quantitative depth profiling investigations of Sn-Cu bilayers as representative model system of the target Sn solder bumps deposited on Cu pillars. We report a dedicated study on the quantification of side-walls contributions in the course of fs-laser ablation mass spectrometry depth profiling experiments using the LIMS technique. We identified the prerequisites for quantitative depth profiling of multi-layer systems similar to these Sn/Cu solder bumps and compared different general approaches. Preliminary LIMS results on Sn/Cu solder bumps will be discussed. Overall, these investigations shed light on the spatial chemical distribution of contaminants in metal films that might enable further improvements of additive-assisted electrodeposition processes. 1 Motoyoshi, M. Through-Silicon Via (TSV). Proceedings of the IEEE 97, 43-48, (2009). 2 Gambino, J. P., Adderly, S. A. & Knickerbocker, J. U. Microelectronic Engineering 135, 73-106, (2015). 3 Bigas, M. & Cabruja, E. Microelectronics Journal 37, 308-316, (2006). 4 Grimaudo, V. et al. Anal. Chem. 87, 2037-2041, (2015).

Stability characterization of vacuum encapsulated MEMS resonators with Au–Sn solder bonding

This paper presents a vacuum encapsulation technique and stability characterization for MEMS resonator. Sn-rich Au–Sn solder bonding is used to achieve reliable hermetic packaging with high shear strength. Simple planar feedthrough structure is utilized to achieve electrical interconnection and low cost of packaging. The stabilities of the encapsulated resonator are systematically studied, including frequency stability, temperature stability, long-term hermeticity, and mechanical reliability. The short-term and medium-term frequency stability are ± 0.4 and ± 3 ppm, respectively. The temperature cycle test is introduced between − 20 and 80 °C, and the resonant-frequency drift of the packaged resonator is within ± 4 ppm between 40 temperature cycles. Furthermore, the packaged resonator is temperature compensated by micro-oven, which obtained a frequency stability range of ± 13 ppm between 20 and 100 °C. The packaged resonator shows favorable long-term stability of the Q-factor over 200 days and average shear strength of 43.93 MPa among 12 samples.

Interfacial reaction between liquid-state Sn-xBi solder and Co substrate

The interfacial reaction between liquid-state Sn-xBi (x = 0, 1, 5 and 10 wt%) solders and Co substrate at 260 °C for different soldering duration time was explored to investigate the effect of Bi addition and soldering time on the growth of intermetallic compound (IMC) and grains evolution by scanning electronic microscopy in this study. The results of research indicated that the Bi addition into Sn solder had not changed the formation of interfacial IMC and the single CoSn3 IMC layer formed at Sn-xBi/Co (x = 0, 1, 5 and 10 wt%) joints. The addition of 1 wt% Bi into Sn solder suppressed the growth of interfacial CoSn3 IMC, compared to Sn/Co system. Bi segregation occurred in Sn-10Bi/Co couple due to the supersaturated Bi element and would weaken the inhibiting effect on growth of IMC resulted from more Sn and Co atoms activated. The thickness of IMC at the interface of Sn-5Bi/Co at the minimum number was resulted from Bi solid solution strengthening, Bi segregation attributed to Bi supersaturation and gravity subsequently appeared and gradually influenced the IMC growth when the soldering time increased from 20 to 60 min. Accordingly, the law of IMC growth did not meet the linear relationship between the thickness of IMC and soldering time. Additionally, the thickness of interfacial IMC increased with extending soldering time, regardless of Bi content in Sn solder. Prism-like CoSn3 grain formed at Sn-xBi/Co (x = 0, 1, 5 and 10 wt%) joints and the law of grain mean size was in well agreement with the change of thickness of IMC in cross-sectional images.