Mechanism of Ag3Sn grain growth in Ag/Sn transient liquid phase soldering

Transient liquid phase (TLP) bonding is a potential high-temperature (HT) electron packaging technology that is used in the interconnection of wide band-gap semiconductors. This study focused on the mechanism of intermetallic compounds (IMCs) evolution in Ag/Sn TLP soldering at different temperatures. Experimental results indicated that morphologies of Ag₃Sn grains mainly were scallop-type, and some other shapes such as prism, needle, hollow column, sheet and wire of Ag₃Sn grains were also observed, which was resulted from their anisotropic growths. However, the scallop-type Ag₃Sn layer turned into more planar with prolonging soldering time, due to grain coarsening and anisotropic mass flow of Ag atoms from substrate. Furthermore, a great amount of nano-Ag₃Sn particles were found on the surfaces of Ag₃Sn grains, which were formed in Ag-rich areas of the molten Sn and adsorbed by the Ag₃Sn grains during solidification process. Growth kinetics of the Ag₃Sn IMCs in TLP soldering followed a parabolic relationship with soldering time, and the growth rate constants of 250, 280 and 320 °C were calculated as 5.83×10^?15 m²/s, 7.83×10^?15 m²/s and 2.83×10^?14 m²/s, respectively. Accordingly, the activation energy of the reaction was estimated about 58.89 kJ/mol.     

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 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.     

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.     

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...     

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.     

Evolution of Ag3Sn at Sn-3.0Ag-0.3Cu-0.05Cr/Cu joint interfaces during thermal aging

The intermetallic compounds (IMC) in the solder and at the interface of Sn-3.0Ag-0.5Cu (SAC)/Cu and Sn-3.0Ag-0.3Cu-0.05Cr (SACC)/Cu joints were investigated after isothermal aging at 150 °C for 0, 168 and 500 h. Different shaped Ag₃Sn phases were found near the IMC layer of the latter joint. Interestingly, fine rod-shaped and branch-like Ag₃Sn were detected near the interface after soldering and long Ag₃Sn changed into shorter rods and small particles during aging. It is investigated that the Cr addition and thermal aging have effect on the evolution of Ag₃Sn morphologies and it is controlled by interfacial diffusion. Energy minimization theory and the redistribution of elements are used to explain the morphological evolution of Ag₃Sn. Small Ag₃Sn particles were also found on the IMC layer after aging, unlike the large Ag₃Sn at that of SAC/Cu joints. In conclusion, a favorable morphology of the joint interface leads to better bonding properties for SACC/Cu joints against thermal aging than that for SAC/Cu.     

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).     

Effect of soldering condition on formation of intermetallic phases developed between Sn-0.3Ag-0.7Cu low-silver lead-free solder and Cu substrate

In this paper, effect of soldering time and temperature on formation of intermetallic compounds developed between Sn-0.3Ag-0.7Cu lead-free solder and copper substrate was investigated. Dip soldering was performed at 250, 270, and 290 °C with soldering time of 5, 10, 15, and 20 s. Either ?-Cu₃Sn or η-Cu₆Sn₅ intermetallic phase was found at the interface between the solder and the substrate depending on the soldering condition, i.e., soldering time and soldering temperature. ?-Cu₃Sn was found only when the substrate was soldered at 250 °C for 5 and 10 s. At other soldering conditions, only η-Cu₆Sn₅ was found at the interfacial zone. Crystal structure of ?-Cu₃Sn intermetallic phase was orthorhombic, and it was hexagonal structure for η-Cu₆Sn₅. Transformation of the intermetallic phases was also discussed.     

Intermetallic Compounds Formed in Sn-20In-2.8Ag Solder BGA Packages with Ag/Cu Pads

The interfacial reactions in a Sn-20In-2.8Ag solder ball grid array (BGA) package with immersion Ag surface finish are investigated. After reflow, the Ag thin film dissolves quickly into the solder matrix, and scallop-shaped intermetallic layers, with compositions of (Cu_0.98Ag_0.02)₆(In_0.59Sn_0.41)₅, appear at the interfaces between Sn-20In-2.8Ag solder ball and Cu pad. No evident growth of the (Cu_0.98Ag_0.02)₆(Sn_0.59In_0.41)₅ intermetallic compounds was observed after prolonged aging at 100 °C. However, the growth accelerated at 150 °C, with more intermetallic scallops floating into the solder matrix. The intermetallic thickness versus the square root of reaction time (t ^1/2) shows a linear relation, indicating that the growth of intermetallic compounds is diffusion-controlled. Ball shear tests show that the strength of Sn-20In-2.8Ag solder joints after reflow is 4.4 N, which increases to 5.18 N and 5.14 N after aging at 100 and 150 °C, respectively.     

Evolution of CuSn intermetallics between molten SnAgCu solder and Cu substrate

The evolution of SnCu intermetallic compounds (IMCs) between a molten SnAgCu alloy and the Cu under-bump metallization (UBM) throughout reflow is presented based on interruption of soldering reactions in experiments by removing the liquid solder from the substrate. This allows to capture and visualize interfacial reactants at arbitrary moments of the soldering process, and to gain an insight into their formation characteristics. The results show that the interfacial Cu₆Sn₅/Cu₃Sn structure is formed at an early stage of reflow and is maintained throughout the process. Based on the experiments, formation mechanisms of interfacial CuSn IMCs are discussed.     

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.     

Interfacial reaction between Sn–Ag–Cu solder and Co–P films with various microstructures

We electroplated Co–P films with nanocrystalline, amorphous and nanocrystalline/amorphous mixed structures and used them as under-bump metallization (UBM) joined with Sn–Ag–Cu lead-free solder. We systematically investigated the interfacial reaction between the Sn–Ag–Cu solder and the Co–P UBM and analyzed the growth mechanisms of the intermetallic compounds formed at the interfaces of the Sn–Ag–Cu/Co–P joints through multiple reflows. Among the three kinds of Co–P films, the film with a mixed structure shows the best diffusion-barrier properties and is a good candidate for the UBM joined with the Sn–Ag–Cu solder. For the nanocrystalline Co–P UBM, Co diffuses quickly toward the solder and Sn does not diffuse into the UBM, whereas for the amorphous Co–P film not only does Co diffuse into the solder, but also Sn diffuses into the Co–P film with a large diffusion rate. In addition, the first-principles calculation shows that the exchange coupling between Co(3d⁷4s²) and Sn(5s²5p²) electrons and between Sn(5s²5p²) and P(3s²3p³) electrons results in the formation of CoSn and SnP₃, which originate from the diffusion and reaction of Co and Sn atoms, respectively; this is consistent with the experimental data of transmission electron microscopy characterization.     

Investigation of small Sn–3.5Ag–0.5Cu additions on the microstructure and properties of Sn–8Zn–3Bi solder on Au/Ni/Cu pads

The formation of intermetallic compounds and the shear strength of Sn–Zn–Bi solder alloys with various (0, 1, 3, 5 and 7 wt%) weight percentages of Sn–Ag–Cu were investigated on Au/Ni metallized Cu pads depending on the number of reflow cycles. In Sn–Zn–Bi solder joints, scallop-shaped AuZn₃ intermetallic compound (IMC) particles were found at the interfaces and in the solder ball regions, fine Bi- and needle-shaped Zn-rich phase were observed in the Sn matrix. After Sn–Ag–Cu additions, an additional Ag–Zn intermetallic compound layer was adhered to the top surface of the AuZn₃ layer at the interface and fine spherical-shaped AgZn₃ intermetallic compound particles were detected in the solder ball regions together with Bi- and Zn-rich phase volumes. After the addition of Sn–Ag–Cu, the shear strength of Sn–Zn–Bi solder joints increased due to the formation of the fine AgZn₃ intermetallic compound particles. The shear strengths of Sn–Zn–Bi and Sn–Zn–Bi/7 wt% Sn–Ag–Cu solder joints after one reflow cycle were about 44.5 and 53.1 MPa, respectively and their shear strengths after eight reflow cycles were about 43.4 and 51.6 MPa, respectively.     

The Nucleation of Sn in Undercooled Melts: The Effect of Metal Impurities

The dependence of the solidification temperature on the concentration x of impurity atoms, M, of Sn-M _x alloys after cooling from the melt was measured separately for M = Co, Ni, Ag, and Cu. For a comparison, similar measurements were performed on SAC305-Ni _x alloys. Large variations in undercooling were observed. It was found that the Ag atoms dissolved in the Sn-Ag melt significantly lowered undercooling, although the presence of Ag₃Sn intermetallic compounds did not. While Cu₆Sn₅ intermetallic compounds in Sn-Cu melts did not significantly lower undercooling, the undercooling of a Sn-Cu melt in contact with a Cu interface was significantly reduced. The addition of Ni to Pb-free solder SAC305 caused a factor of two reduction in the undercooling, similar to that observed after the addition of Ni to high-purity Sn.     

RE对Sn2.5Ag0.7Cu/Cu焊点性能的影响

利用X射线衍射分析仪(XRD)和JSM-5610LV扫描电镜(SEM)研究RE含量对Sn2.5Ag0.7Cu/Cu焊点界面区显微组织、剪切强度和蠕变断裂寿命的影响。结果表明:Sn2.5Ag0.7CuxRE焊点界面区金属间化合物由靠近钎料侧Cu6Sn5和靠近Cu基板侧Cu3Sn构成;添加微量RE可细化Sn2.5Ag0.7Cu焊点内钎料合金的显微组织和改善钎焊接头界面区金属间化合物的几何尺寸及形态;当RE添加量为0.1%时,焊点的剪切强度最高,蠕变断裂寿命最长。     

Dissolution and interfacial reactions of thin-film Ti/Ni/Ag metallizations in solder joints

The dissolution and interfacial reactions involving thin-film Ti/Ni/Ag metallizations on two semiconductor devices, diode and metal-oxide-semiconductor field-effect transistor (MOSFET), a Sn–3.0Ag–0.7Cu solder, and a Au-layer on the substrates are studied. To simulate the dissolution kinetics of the Ag-layer in liquid solder during the reflow process, the computational thermodynamics (Thermo-Calc) and kinetics (DICTRA: DIffusion Controlled TRAnsformations) tools are employed in conjunction with the assessed thermochemical and mobility data. The simulated results are found to be consistent with the observed as-reflowed microstructures and the measured Ag contents in the solder. In the as-reflowed joints two different intermetallic compounds (IMC) are found near the diode/solder interface. Both are in the form of particles of different morphologies, not a continuous layer, and are referred to as IMC-I and IMC-II. The former corresponds to Ni₃Sn₄ with Cu atoms residing in the Ni sublattice. It is uncertain whether IMC-II is Cu₆Sn₅ phase with Ni atoms residing in the Cu sublattice or a Cu–Ni–Sn ternary phase. Near the as-reflowed MOSFET/solder interface, both particles and a skeleton-like layer of Ni₃Sn₄ are observed. The primary microstructural dynamics during solid state aging are the coarsening of IMC particles and the reactions involving the unconsumed (after reflow) Ni- and the Ti-layer with Sn and Au. While the reaction with the Ni-layer yields only Ni₃Sn₄ intermetallic, the reaction involving the Ti-layer suggests the formation of Ti–Sn and Au–Sn–Ti intermetallics. The latter is due to the diffusion of Au from the substrate side to the die side. It is postulated that the kinetics of Au–Sn–Ti layer is primarily governed by the diffusion of Au through the Ni₃Sn₄ layer by a grain boundary mechanism.     

Growth mechanism of Ni3Sn4 in a Sn/Ni liquid/solid interfacial reaction

The chemical interfacial reaction of Ni plates with eutectic Sn–3.5Ag lead-free solder was studied by microstructural observations and mathematical calculations. Compared with the Sn–3.5Ag–0.75Ni/Ni interfacial reaction, based on a simple model of the growth of the liquid/solid chemical compound layer, the growth mechanism of Ni₃Sn₄ in the Sn–3.5Ag/Ni interfacial reaction is discussed and presented. The growth process of Ni₃Sn₄ in the Sn/Ni liquid/solid reaction interface involves the net effect of several interrelated phenomena, such as volume diffusion, grain boundary diffusion, grain boundary grooving, grain coarsening, and dissolution into the molten solder. The growth time exponent n and morphology of Ni₃Sn₄ were found to be dependent on these factors.     

Reaction diffusions of Cu<Subscript>6</Subscript>Sn<Subscript>5</Subscript> and Cu<Subscript>3</Subscript>Sn intermetallic compound in the couple of Sn-3.5Ag eutectic solder and copper substrate

The growth kinetics of intermetallic compound layers formed between Sn-3.5Ag solder and Cu substrate were investigated as a consequence of solid-state isothermal aging. Isothermal aging was carried out in a temperature range between 70°C and 200°C for 0 to 60 days. A quantitative analysis of the intermetallic compound layer thickness as a function of time and temperature was performed. The diffusion couples showed a composite intermetallic layer comprised of Cu₆Sn₅ and Cu₃Sn. The growth of intermetallic compounds followed diffusion-controlled kinetics and the layer thickness reached only 9 μm after 60 day of aging at 150°C. The apparent activation energies were calculated for the growth of the total intermetallic compound (Cu₆Sn₅+Cu₃Sn); Cu₆Sn₅ and Cu₃Sn intermetallic are 65.4, 55.4 and 75.7 kJ/mol, respectively.     

Sn3.0Ag0.5Cu/Cu回流焊点界面化合物尺寸分布特征及生长机制

在电子封装过程中,钎料与基体之间形成金属间化合物层,其主要成分为Cu₆Sn₅,Cu₆Sn₅晶粒的尺寸和形貌特征能够显著影响焊点的服役性能. 采用回流焊的方法制备了一系列Sn3.0Ag0.5Cu/Cu焊点,使用Image-Pro Plus软件对焊接界面化合物Cu₆Sn₅晶粒的尺寸分布和化合物层的厚度进行了统计分析. 结果表明,Cu₆Sn₅的平均粒径正比于t0.38(t为回流时间), 界面化合物层的平均厚度正比于t0.32. 随着回流时间的增加,界面化合物生长速度变慢,Cu₆Sn₅晶粒的尺寸分布更加均匀. 回流时间较长的样品中Cu₆Sn₅的粒径尺寸分布与FRD模型的理论曲线基本相符,而对于回流时间短的样品,晶粒尺寸分布与FRD理论偏离较大. 统计结果显示,出现频次最高的晶粒尺寸小于平均值. 最后讨论了界面Cu₆Sn₅晶粒的生长机制,分析了回流时间对界面Cu₆Sn₅晶粒生长方式的影响.