Morphology and growth kinetics of Ag3Sn during soldering reaction between liquid Sn and an Ag substrate

For the soldering of recycled Ag sputtering targets, the interfacial reaction between liquid Sn and an Ag substrate at temperatures ranging from 250 –425°C has been investigated. Experimental results show that a scallop-shaped layer of Ag₃Sn intermetallic compounds formed during the soldering reaction. Kinetics analysis indicated that the growth of such interfacial Ag₃Sn intermetallic compounds is diffusion-controlled with activation energy of 70.3kJ/mol. During the reaction, the Ag substrate dissolves into the molten Sn solder and causes the appearance of needle-shaped Ag₃Sn precipitates in the Sn matrix.     

Microstructural and mechanical analysis on Cu–Sn intermetallic micro-joints under isothermal condition

This study focuses on the mechanism of phase transformation from Cu₆Sn₅ into Cu₃Sn and the homogenization process in full intermetallics (IMCs) micro-joints, which were prepared by soldering the initial Cu/Sn/Cu structure through high temperature storage in vacuum environment as the Transient Liquid Phase (TLP) process. From the microstructural observation by electron backscatter diffraction (EBSD), a mixture of IMCs phases (Cu₆Sn₅ and Cu₃Sn) has been found to constitute the sandwich-structured Cu/IMCs/Cu joints. With the dwell time increasing at 533 K, there were two layers of Cu₃Sn emerging from both sides of copper substrates with the depletion of Cu₆Sn₅ layer, toward merging each other in the IMCs interlayer. Then the Cu₃Sn grains with various sizes became more homogenous columnar crystallites. Meanwhile, some equiaxial ultra-fine grains accompanied with the Kirkendall voids, were found only in adjacent to the electroplated copper. In addition, a specific type of micropillar with the size ∼5 μm × 5 μm × 12 μm fabricated by focus ion beam (FIB) was used to carry out the mechanical testing by Nano-indentation, which confirmed that this type of joint is mechanically robust, regardless of its porous Cu₃Sn IMC interconnection.     

62Sn36Pb2Ag钎料中Ag元素对AgCu/SnPbAg/CuBe焊缝性能的影响

分别采用62Sn36Pb2Ag钎料和63Sn37Pb共晶钎料焊接AgCu合金块和CuBe合金片进行试验,对比分析两种钎料形成的焊缝性能和显微组织结构,阐述了62Sn36Pb2Ag钎料中Ag元素的存在对AgCu/SnPbAg/CuBe焊缝性能的影响机制.结果表明,62Sn36Pb2Ag钎料中的Ag元素对于润湿铺展状态的改...     

Scaling effect on Ag3Sn growth behaviours in micro-joints for flip chip assemblies

The scaling effect on Ag₃Sn growth behaviours in Sn–3.0Ag–0.5Cu (SAC305) micro-joints of flip chip assemblies was investigated using thermal shock (TS) tests. After assembly reflow, the large plate-like Ag₃Sn compounds only emerged from the Cu interface of small joints, which was strongly related with a higher local Ag concentration in the remaining solder. During TS cycling, the growth of Ag₃Sn grains exhibited comparatively more pronounced growth in the solder matrix of small joints, due to the stronger strain-enhanced coarsening induced by more cycle stress and strain. Coarsening kinetic models based on TS experiments were employed to predict Ag₃Sn growth, the kinetic constants N were determined to clarify the correlation of the joints scaling and Ag₃Sn coarsening in depth.     

Effect of adding 1 wt% Bi into the Sn–2.8Ag–0.5Cu solder alloy on the intermetallic formations with Cu-substrate during soldering and isothermal aging

Intermetallic compound (IMC) formations of Sn–2.8Ag–0.5Cu solder with additional 1 wt% Bi were studied for Cu-substrate during soldering at 255 °C and isothermal aging at 150 °C. It was found that addition of 1 wt% Bi into the Sn–2.8Ag–0.5Cu solder inhibits the excessive formation of intermetallic compounds during the soldering reaction and thereafter in aging condition. Though the intermetallic compound layer was Cu₆Sn₅, after 14 days of aging a thin Cu₃Sn layer was also observed for both solders. A significant increase of intermetallic layer thickness was observed for both solders where the increasing tendency was lower for Bi-containing solder. After various days of aging, Sn–2.8Ag–0.5Cu–1.0Bi solder gives comparatively planar intermetallic layer at the solder–substrate interface than that of the Sn–2.8Ag–0.5Cu solder. The formation of intermetallic compounds during aging for both solders follows the diffusion control mechanism and the diffusion of Cu is more pronounced for Sn–2.8Ag–0.5Cu solder. Intermetallic growth rate constants for Sn–2.8Ag–0.5Cu and Sn–2.8Ag–0.5Cu–1.0Bi solders were calculated as 2.21 × 10⁻¹⁷ and 1.91 × 10⁻¹⁷ m²/s, respectively, which had significant effect on the growth behavior of intermetallic compounds during aging.     

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.     

Effect of Purging Gas on Wetting Behavior of Sn-3.5Ag Lead-Free Solder on Nickel-Coated Aluminum Substrate

The wetting characteristics of Sn-3.5Ag lead-free solder alloy on nickel-coated aluminum substrates in air (ambient), nitrogen, and argon atmospheres were investigated. The contact angles for the solder alloy obtained under air and argon atmospheres were in the range of 36°-38°. With nitrogen atmosphere the contact angle was found to be significantly lower at about 26°. Solder solidifying in air exhibited needle-shaped tin-rich dendrites surrounded by a eutectic matrix. The amount of tin dendrites decreased in argon atmosphere. However, the morphology of tin dendrites transformed from needle-shaped to nearly non-dendritic shape as the soldering atmosphere was changed from air to nitrogen. The interfacial microstructures revealed the presence of Ni₃Sn and Ni₃Sn₄ IMCs at the interface. The enhanced wettability observed under nitrogen atmosphere is attributed to the higher thermal conductivity of nitrogen gas and the formation of higher amount of Ni₃Sn IMCs at the interface compared to air and argon atmospheres.     

Morphologies,orientation relationships and evolution of Cu6Sn5 grains formed between molten Sn and Cu single crystals

The morphologies and orientation relationships of Cu₆Sn₅ grains formed between Sn and (0 0 1), (0 1 1), (1 1 1) and (1 23) Cu single crystals under liquid- and solid-state aging conditions were systematically investigated. The regular prism-type Cu₆Sn₅ grains formed on (0 0 1) and (1 1 1) Cu single crystals are elongated either along two perpendicular directions or along three preferential directions with an angle of 60° between each pair of directions. The orientation relationships between Cu and Cu₆Sn₅ lattice structures were determined by electron backscatter diffraction and were explained in terms of their minimum misfit. However, on (0 1 1) and (1 2 3) Cu single crystal surfaces, the Cu₆Sn₅ grains were mainly scallop-type, with only a few regular prism-type grains. Furthermore, the regular prism-type Cu₆Sn₅ grains will change into scallop-type after long reflow or aging times. Meanwhile it is considered that the growth of the scallop-type grains is supplied by two fluxes: the flux of the interfacial reaction and the flux of ripening. However, the growth of the prism-type grains is only supplied by the flux of the interfacial reaction. The kinetics of IMCs growth between Sn and Cu single crystals was also investigated.     

Kinetics of Ag3Sn growth in Ag–Sn–Ag system during transient liquid phase soldering process

The kinetics of the interfacial reaction of a thin layer of Sn sandwiched between two pieces of Ag foil has been investigated at temperatures of 260 °C, 300 °C and 340 °C. A time dependence of the form t^1/n with n = 3 was obtained for the kinetics of both the consumption of the Sn remaining and the thickening growth of the Ag₃Sn scallops formed between Sn and Ag. Such a result can be explained well using the model of grain boundary/molten channel-controlled growth of intermetallic compounds. In this case, the diffusion of Ag atoms through the molten channels existing between the previously formed Ag₃Sn scallops is the controlling mechanism for the kinetics. We also report here the derived kinetic constants including reaction constants and the associated activation energy for guiding the practical transient liquid phase soldering of the Ag–Sn–Ag system.     

Interfacial reactions between high-Pb solders and Ag

Interfacial reactions between high-Pb solders (Pb-10Sn, Pb-5Sn, and Pb-3Sn, in wt.%) and immersion Ag layer at 350 °C are investigated. Upon decreasing the Sn concentration from 10 wt.% to 5 wt.%, the reaction product formed at the solder/Ag interface changes from the Ag₃Sn phase to the Ag₄Sn phase. When the Sn concentration reduces to only 3 wt.%, the reaction product is the Ag₄Sn phase at the initial stage of reaction but transforms to the (Ag) phase dissolved with Sn at the later stage of reaction. Pb penetrates across the (Ag) phase via grain boundary and forms a continuous Pb-rich layer between the (Ag) phase and the bottom Cu layer. The correlation between the phase transformation and the solder composition is discussed based on the calculated Sn-Pb-Ag isothermal section.     

瞬态电流键合对Ag修饰石墨烯Sn-Ag-Cu复合焊点界面反应的影响

为了研究Ag修饰石墨烯增强的Sn-Ag-Cu(SAC/Ag-GNSs)焊点在传统回流焊过程中Ag-GNSs在熔池中上浮聚集问题,利用电流密度(1.0×104 A/cm2)在几百毫秒内实现Cu-SAC/Ag-GNSs-Cu接头快速键合.结果表明,Ag-GNSs均匀分散在焊点中,为Cu6Sn5晶粒成核提供更多形核位点,从而...     

Growth Behavior of Intermetallic Compounds at SnAgCu/Ni and Cu Interfaces

The growth behavior of reaction-formed intermetallic compounds (IMCs) at Sn3.5Ag0.5Cu/Ni and Cu interfaces under thermal-shear cycling conditions was investigated. The results show that the morphology of (Cu _x Ni_1–x )₆Sn₅ and Cu₆Sn₅ IMCs formed both at Sn3.5Ag0.5Cu/Ni and Cu interfaces gradually changed from scallop-like to chunk-like, and different IMC thicknesses developed with increasing thermal-shear cycling time. Furthermore, Cu₆Sn₅ IMC growth rate at the Sn3.5Ag0.5Cu/Cu interface was higher than that of (Cu _x Ni_1–x )₆Sn₅ IMC under thermal-shear cycling. Compared to isothermal aging, thermal-shear cycling led to only one Cu₆Sn₅ layer at the interface between SnAgCu solder and Cu substrate after 720 cycles. Moreover, Ag₃Sn IMC was dispersed uniformly in the solder after reflow. The planar Ag₃Sn formed near the interface changed remarkably and merged together to large platelets with increasing cycles. The mechanism of formation of Cu₆Sn₅, (Cu _x Ni_1–x )₆Sn₅ and Ag₃Sn IMCs during thermal-shear cycling process was investigated.     

Interfacial reactions of liquid Sn and Sn-3.5Ag solders with Ag thick films

The interfacial reactions of liquid Sn and Sn-3.5Ag solders with Ag thick films are investigated in the temperature range from 250–325 °C, and the morphology of intermetallic compounds formed after such soldering reactions is observed. In kinetics analysis of the growths of intermetallic compounds, it was found that both Sn/Ag and Sn-3.5Ag/Ag reactions were interfacial-controlled, and the growth rates for both cases were similar. The rate of Ag dissolution into liquid solder attendant on the formation of interfacial intermetallic compounds after Sn/Ag reaction was about four times higher than that after Sn-3.5Ag/Ag reaction, as evidenced by experimental results.     

Effect of crystallographic anisotropy of β-tin grains on thermal fatigue properties of Sn–1Ag–0˙5Cu and Sn–3Ag–0˙5Cu lead free solder interconnects

Abstract

An effect of the crystallographic anisotropy of β-tin grains on thermal fatigue properties of Sn–1Ag– 0˙5Cu and Sn–3Ag–0˙5Cu lead free solder interconnects were discussed. From an orientation imaging microscopic observation, three types of microstructures (single crystal-like, fine grain type and large grain type) were observed in both solders. The single crystal-like microstructure disappeared and the large grain type occurred by further fatigue due to recrystallisation. Because single crystal-like microstructure had the {100} plane approximately parallel to strain concentrated areas, recrystallisation could be retarded if the slip systems of {100}<011> or {100}<010> operate and an amount of thermal strain decreases because these slip systems have the larger critical resolved shear stress due to an anisotropic nature of β-tin. One of the reasons Sn–3Ag–0˙5Cu had longer thermal fatigue life than Sn–1Ag–0˙5Cu can be the number of the single crystal-like or the fine grain type microstructures in Sn–3Ag–0˙5Cu were larger.     

Reaction properties and interfacial intermetallics for Sn−xAg−0.5Cu solders as a function of Ag content

Wetting and interfacial reactions were investigated for Sn−xAg−0.5Cu alloys, in which the Ag content had a variation from x=1.0 to x=4.0. Differential scanning calorimetry (DSC) was used to investigate the range of the melting temperature and the solidification temperature by measuring the endothermic and the exothermic heat flow, respectively. Low Ag contents increased the melting temperature ranges and deteriorated the wetting properties such as zero cross time and wetting force measured at two seconds. The extent of undercooling increased and the thickness of intermetallic compounds (IMC) decreased as the Ag content decreased. As the Ag content decreased, the initial IMC thickness decreased due to the large undercooling and, during the solid aging at 170°C, the IMC growth slightly decelerated because of the small diffusion coefficient. For the application of good drop shock reliability, Sn−Ag−Cu solder of low Ag content should be beneficial due to the restraint of the IMC growth (Cu₆Sn₅ and Cu₃Sn) and of the coarse plate-like IMC (Ag₃Sn).     

Effects of in situ nickel particle addition on the microstructure and microhardness of Sn–Ag solder

Abstract

In this study, various amounts of Ni particles were added in situ to Sn–3·5 wt-%Ag lead free solder to form new composite solders. Copper substrates were then dipped into these solders and aged at 150°C for 0, 25, 225, or 1000 h. The microstructure and microhardness of the as solidified solder and the aged solder/copper couples were investigated. Experimental results revealed that the addition of Ni particles increased the microhardness of the composite solder. Ni additions of less than 3 wt-% yielded a microstructure of β-Sn grains surrounded by a eutectic mixture of Ag₃Sn and a Sn rich matrix. An intermetallic compound of Ni₃Sn₄ particles was dispersed throughout the eutectic. For 5 wt-%Ni addition, the Ni₃Sn₄ phase and the remaining Ni particles were agglomerated. In the case of copper substrate dipped with a thick layer of composite solder, water quenched and then aged at 150°C, the induced (Ni, Cu)₃Sn₄ particles coarsened and agglomerated. Additionally, the intermetallic (Cu, Ni)₆Sn₅ compound layer formed at the solder/Cu interface thickened with increasing Ni content.     

Effects of sulfide-forming element additions on the Kirkendall void formation and drop impact reliability of Cu/Sn–3.5Ag solder joints

Ternary Pb-free solders, Sn–3.5Ag–X, containing 0.5 wt.% of Zn, Mn and Cr, were reacted with Cu UBM, which was electroplated using SPS additive. Characteristics of Cu–Sn IMCs and Kirkendall void formation at the Cu/Sn–3.5Ag solder joints were significantly affected by the third element, and the potency to suppress Kirkendall voids at the solder joint increased in the order of Cr, Mn, Zn, which was indeed the order of the drop reliability improvement. From the AES analyses, it was suggested that the sulfide-forming elements in the solder diffused into the Cu UBM and reduced the segregation of S atoms to the Cu/Cu₃Sn interface by scavenging S, which led to the suppression of Kirkendall void nucleation at the Cu/Cu₃Sn interface and the drop reliability improvement. In the case of the Zn-containing solder joint, Cu₃Sn phase, known to be a host of Kirkendall voids, did not form at all even after extended aging treatments. The magnitude of the tensile stress at the Cu₃Sn/Cu interface which drove the Kirkendall void growth was estimated to be 10–100 MPa.     

Phase equilibria of the Au–Sn–Zn ternary system and interfacial reactions in Sn–Zn/Au couples

Phase equilibria of the Au–Sn–Zn ternary system and interfacial reactions between Sn–Zn alloys and Au were experimentally investigated at 160 °C. Experimental results reveal that no equilibrium-stated ternary phases were found and the ternary element solubility in the binary phase is insignificant. When the Zn content was less than 3 wt% in the Sn–Zn alloy, only the Au–Sn binary intermetallic compounds (IMCs), such as AuSn, AuSn₂ and AuSn₄ phases, were formed at the Sn–Zn/Au interface. When the Zn content in Sn–Zn alloys was greater than 7 wt%, the AuZn, AuZn₂ and Au₃Zn₇ phases were formed in the Sn–Zn/Au couples at 160 °C. However, both Sn–Zn and Au–Zn IMCs, and the Au–Zn–Sn ternary IMC (T phase) were observed between Au and the Sn–Zn alloys with 3–5 wt% added Zn. This T phase might be the metastable phase. The evolution of IMCs in the Sn–Zn/Au couples is very sensitive to the Zn content in Sn–Zn alloys.     

Investigation of diffusion and electromigration parameters for Cu–Sn intermetallic compounds in Pb-free solders using simulated annealing

An efficient numerical method was developed to extract the diffusion and electromigration parameters for multi-phase intermetallic compounds (IMC) formed as a result of material reactions between under bump metallization (UBM) and solder joints. This method was based on the simulated annealing (SA) method and applied to the growth of Cu–Sn IMC during thermal aging and under current stressing in Pb-free solder joints with Cu-UBM. At 150 °C, the diffusion coefficients of Cu were found to be 3.67 × 10¹⁷ m² s⁻¹ for Cu₃Sn and 7.04 × 10¹⁶ m² s⁻¹ for Cu₆Sn₅, while the diffusion coefficients of Sn were found to be 2.35 × 10¹⁶ m² s⁻¹ for Cu₃Sn and 6.49 × 10¹⁶ m² s⁻¹ for Cu₆Sn₅. The effective charges of Cu were found to be 26.5 for Cu₃Sn and 26.0 for Cu₆Sn₅, and for Sn, the effective charges were found to be 23.6 for Cu₃Sn and 36.0 for Cu₆Sn₅. The SA approach provided substantially superior efficiency and accuracy over the conventional grid heuristics and is particularly suitable for analyzing many-parameter, multi-phase intermetallic formation for solder systems where quantitative deduction for such parameters has seldom been reported.     

微量锌对Sn-3.5Ag无铅钎料/铜界面金属间化合物生长的抑制

在钎焊和时效条件下,研究了Sn-3.5Ag无铅钎料中添加0.2%的Zn元素后对钎料/铜界面组织形貌的影响.结果表明,钎焊条件下,将0.2%的Zn元素加入到Sn-3.5Ag钎料中对Cu-Sn间的金属间化合物及其扇贝形形状不产生影响.时效处理后,0.2%Zn元素的加入对界面IMC层的厚度、组成及形态都有影响.在Sn-3.5...