In this paper, the authors propose a highly conductive die attach material based on Ag@Sn powder for power devices operating at high temperatures or in other harsh environments. The preform can be reflowed at 250 °C (18 °C above the T m of Sn, 232 °C), but the resulting bondline can sustain high temperatures up to 400 °C with a high shear strength due to the high re‐melting temperature of the formed Ag 3 Sn ( T m = 480 °C) after the complete consumption of the outer Sn layers. In addition, the formed bondline exhibits excellent electrical and thermal conductivities due to the embedded Ag particles in the interconnections. The interconnections also exhibit excellent reliability under thermal shock cycling from −55 to 200 °C because of the increased bondline thickness and inherent ductility of the Ag particles embedded in the Ag 3 Sn.

A study was performed on the precipitation behavior of Cu6Sn5 during the solidification of Sn-based solders/Cu joints and its effect on the growth of interfacial intermetallic compounds (IMCs). During cooling, Cu in molten solder precipitates in the form of Cu6Sn5 both within the solder matrix and at the interface. The precipitation at the interface can greatly affect the interfacial IMC morphology and thickness but cannot change the interfacial IMC orientation. In general, additional Cu6Sn5 will precipitate at the solder/Cu interface when the Cu content is increased in the molten solder during soldering and when the cooling rate decreases during solder solidification. If the Cu content in the molten solder is sufficiently high, during cooling, the precipitated Cu6Sn5 will develop a prismatic morphology with the exposed surface indexed as (10-10) along the [0 0 0 1] direction of the existing Cu6Sn5 grains at the interface. (c) 2013 Elsevier B.V. All rights reserved.

Intermetallic compounds (IMCs) that form at eutectic Sn3.5Ag/Cu and pure Sn/Cu interfaces during solid-state aging are comparatively studied in terms of their respective morphological formations, orientation evolution behaviors, and growth kinetics. During solid-state aging, all the interfacial Cu6Sn5 grains evolve into a layer-type morphology, except for select grains that experience abnormal growth. This abnormal growth is caused by the preferential growth of the Cu6Sn5 at the grain boundary in solder matrix. Meanwhile, textured growth occurs in the Cu6Sn5 layer formed at the eutectic Sn3.5Ag/Cu interface. The morphology of each texture is determined by the initial joint preparation conditions and affects the growth of interfacial IMCs. The results reveal that Sn diffusion occurs faster along the [0001] direction of the Cu6Sn5 crystal than along angles from 25° to 50° relative to the [0001] direction. Additionally, the effects of solder composition on the interfacial IMC growth are evaluated. The results indicate that Ag addition retards IMC growth upon aging by inhibiting diffusion of Cu.

The thermomechanical response of Sn-based solder interconnects with differently oriented grains was investigated by electron backscattered diffraction technique under thermal cycling and thermal shock testing in this study. The results showed that deformation and cracking of solder interconnects have a close relationship with the unique characteristics of grain orientation and boundaries in each solder interconnect, and deformation was frequently confined within the high-angle grain boundaries. The micro Vickers hardness testing results showed that the hardness varied significantly depending on the grain orientation and structure, and deformation twins can be induced around the indents by the indentation testing.

Epitaxial growth of intermetallic compounds formed at eutectic Sn37Pb/ polycrystalline Cu interface during solid-state aging. The results show that the interfacial Cu 6 Sn 5 grains exhibit textured growth under solid-state condition and their preferred orientations are affected by the as-soldered joints. Cu 6 Sn 5 grains with [0001] direction normal to the interface are stable in solid and molten Sn37Pb solder at 200°C, but will be rapidly consumed at 280°C, which leads to the formation of different textures in Cu 6 Sn 5 layer during the solid-state aging treatment to the joints formed at 200°C and 280°C. Also, the effects of the texture evolution on the growth of interfacial IMCs are evaluated. The results show that Cu diffusion along [0001] direction of Cu 6 Sn 5 is faster and therefore more interfacial IMCs are generated in the joints formed at 200°C than those formed at 280°C under the same solid-state reaction conditions.

The reactive wetting properties of Sn0.7Cu-xZn (x=0,0.2,0.5,1.0) lead-free solder on Cu substrate were investigated by using the wetting balance test under the condition of air and N2 environment respectively. Even tiny addition of Zn could lead the surface tension of solders to increase greatly comparing to the original Sn0.7Cu solder due to the and oxidation of Zn in air, but in the N2 atmosphere, the effect of Zn on the surface tension of solder is small. And since the first IMC could affect the wetting property according to the reactive wetting theory, the effect of IMC formation on wetting was discussed based on the recent reactive-wetting theory. Using the CALPHAD (Thermo calc software) method, the first IMC that forms during wetting reaction were analyzed. And with the Zn content increasing, the IMC formed during wetting test changed from Cu6Sn5 into y-CuZn both from EDX analysis and CALPHAD method

The formation and growth of intermetallic compounds (IMCs) in lead-free solder joints, during soldering or subsequent aging, have a significant effect on the thermal and mechanical behavior of solder joints. In this study, the effects of a 0.2wt.%Zn addition into Sn-3.0Ag-0.5Cu (SAC) lead-free solder alloys on the growth of IMCs with Cu substrates during soldering and subsequent isothermal aging were investigated. During soldering, it was found that a 0.2wt.%Zn addition did not contribute to forming the IMC, which was verified as the same phase structure as the IMC for Sn-3.0Ag-0.5Cu/Cu. However, during solid-state isothermal aging, the IMC growth was remarkably depressed by the 0.2 wt.% Zn addition in the SAC solder matrix, and this effect tended to be more prominent at higher aging temperature. The activation energy for the overall IMC growth was determined as 61.460 and 106.903 kJ/mol for Sn-Ag-Cu/Cu and Sn-Ag-Cu-0.2Zn/Cu, respectively. The reduced diffusion coefficient was confirmed for the 0.2Zn-containing solder/Cu system. Also, thermodynamic analysis showed the reduced driving force for the Cu6Sn5 IMC with the addition of Zn. These may provide the evidence to demonstrate the depressing effect of IMC growth due to the 0.2wt.%Zn addition in the Sn-Ag-Cu solder matrix.

Using a wetting balance technique, it was found that for Sn-rich solders, the wetting time was smaller on Cu than that on Cu6Sn5/Cu3Sn/Cu, and the wetting forces were not markedly different for the two kinds of substrates. However, for high-Pb solders on Cu6Sn5/Cu3Sn/Cu, the wetting time became smaller while the wetting force became larger. The phase transformation from Cu6Sn5 to Cu3Sn was detected for 5SnPb solder wetting on Cu6Sn5/Cu3Sn/Cu substrate. Compared with the surface roughness, the energy released from interfacial reaction played a dominant role in the wetting occurrence and dynamic velocity. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

In order to explore the mechanism of reactive wetting occurred between solder and Cu substrate, the wetting behavior of high-Pb, say, Pb-xSn solder (x=0, 0.7, 1.4, 2.1, 2.8, and 3.5wt.%, respectively) was investigated using the wetting balance technique. It was found that small amount of addition Sn in Pb could improve the wettability of Pb-xSn solder on Cu severely. Since the current reactive wetting theory suggested that the first intermetallic compound formed during the wetting reaction could influence the wetting properties, the driving forces of the intermetallic compounds formation under metastable equilibrium were calculated using Thermo Calc software. And the first intermetallic compound was confirmed to be Cu3Sn by both theoretical computation and EDX analysis. The reactive wetting theories was employed, and combined with the analysis of wetting balance test results, to discuss the wetting phenomena herein.

Finite element (FE) simulations of visco-plastic indentation in Sn-37Pb eutectic solder alloy are performed to investigate the influence of loading rate on its creep characteristic. The resulting indentation load-displacement curves are rate-dependent and have varying creep penetration depths during the same hold time. Creep indentation hardness H, defined from the concept of “work of indentation”, varies with volume strain occurring during the creep hold time, which is a measure of creep strain rate \(\dot \varepsilon\)cr. Thus, creep stress sensitivity can be determined from the H versus \(\dot \varepsilon\)cr curve. This analysis can be verified by the good agreement between the derived value and the predefined value, and then be used to analyze the Berkovich indentation load-displacement curves of Sn-3.5Ag-0.75Cu lead-free solder. Such indentation tests and physical analysis provide a cheaper and more convenient method to determine the mechanical properties of the upcoming lead-free solder alloys.

With small amount of Zn addition, a refined microstructure of bulk Sn–0.7Cu solder alloy has been obtained. Meanwhile, retardation effect has been observed on the growth of IMC layer at the solder/Cu joint interface. The composition variation of the interface IMC layer with the amount of Zn addition is reported and thermodynamically analyzed.

Finite element method (FEM), using a viscoplastic constitutive model proposed by Yoshida [Int. J. Plasticity 16 (2000) 359], has been applied to simulate the propagation of Lüders band for an annealed low-carbon steel strip under uniaxial tension test. By reducing the thickness of corresponding element, additional stress concentration is introduced into the FEM model. Calculated results show that the formation and propagation of Lüders band are affected by the level of stress concentration existing at the ends of the specimen.

A method for aluminum brazed joint topology prediction based upon the observation of residual clad formation has been proposed in this work. It was found that the residual clad layer had a uniform topology regardless of brazing profiles. Therefore, the consumed clad mass, which flowed into the brazed joint during brazing process and is essential for brazed joint topology prediction, could be indirectly calculated upon the measured residual clad thickness and the mass conservation principle. Based on such obtained consumed clad mass and the principle of minimum potential energy, the aluminum brazed joint topology had been predicted and the results are in good agreements with the experimental data.

For annealed mild steel that exhibits yield-point phenomena under uniaxial tension, physical equations expressing the strain-rate dependencies of the strain and the Lüders-band velocity have been proposed in this work. In these expressions, both the Lüders strain and the Lüders-band velocity increase with gauge-length strain rate in the form of exponent functions. The proposed equations have been verified by our experimental data of uniaxial tension on mild steels, as well as by those reported by some other researchers. The proposed equations may provide a simple method to determine the stress-rate sensitivity exponent, in the equation of stress-dependent dislocation velocity, which is usually difficult to be measured by experiments.

The development of Cu–Sn intermetallic compound (IMC) at the solder/Cu joint interface had been studied using two Pb-free solders, Sn–3.8Ag–0.7Cu and Sn–2Ag–0.8Cu–0.6Sb alloys. Meanwhile, 100Sn/Cu joint was applied for comparison. Both Pb-free solder joints were found with thinner Cu–Sn IMC layers at as-soldered state due to the slower dissolution rate of intermetallic compound into the liquid Pb-free solders during reflow and, consequently, slower growth rates of Cu–Sn IMC during the solid-state thermal aging at 125 °C, where Sn–2Ag–0.8Cu–0.6Sb solder joint gave the minimum value. Thermodynamic analysis showed that such phenomena could be attributed to the reduction of the driving force for Cu–Sn IMC formation due to the existence of Ag and Sb atoms.

The shear stress distribution in an Al-Al2O3 soldering assembly after the cooling stage has been analysed via finite element numerical simulation. The effects of the coefficient of thermal expansion of the solder alloy and the fillet geometry have been investigated. The calculation results showed that, to achieve a minimal shear stress response, the coefficient of thermal expansion of the solder alloy should match that of the Al base metal and the geometry of the soldering fillet should be a concave shape having an extruding length somewhat less than the fillet height.

The influence of initial solidification temperature on the microstructure at the brazed joint zone was investigated. The volume fraction of α-(Al) phase at brazed joint zone at different initial solidification temperatures was obtained by the image-processing software Image-Pro Plus. The results show that this volume fraction increases with increasing initial solidification temperature. The characteristics of so-called silicon diffusion layer were analyzed through EPMA and hardness test. In addition, the micro-indentation method was utilized to determine the properties of similar microstructure produced at different initial solidification temperatures. The differential mechanical properties are found, which will impact the properties of the whole brazed joint.

Berkovich depth-sensing indentation tests with different loading rates have been performed on a creep material, Sn–3.5Ag–0.75Cu solder alloy. The resulting indentation load-depth curves are rate dependent and have varying creep penetration depths during the same hold time. Creep indentation hardness H, defined from the concept of “work of indentation,” varies with the volume strain occurring during the creep hold time, which is a measure of creep strain rate ε̇cr. Thus, rate sensitivity m of the indented material can be determined from the ln H vs ln ε̇cr curve. The derived value of m is consistent with the results from conventional uniaxial tensile and compression experiments of bulk solder alloy.