Zhidong Xia

Study on the Microstructure and Wettability of an Al-Cu-Si Braze Containing Small Amounts of Rare Earth Erbium

The effect of adding small amounts of rare earth Er on the microstructure of an Al-Cu-Si braze alloy has been investigated. Several Al-20Cu-7Si braze alloys containing various contents of Er were prepared, and their melting temperature, microstructure, hardness, and wettability in contact with 3003 aluminum alloy substrates were determined. The results indicate that the constituents of the microstructure of Al-20Cu-7Si-Er braze alloys are similar to those in the Al-20Cu-7Si alloy, and comprise of solid solutions of aluminum, silicon, and the intermetallic compound CuAl2. When the Er content increases, the size of the Al phase decreases, and the needle-like Si phase is thickened, and transformed to a blocky shape. Moreover, small amounts of Er can improve the wettability and hardness of the Al-20Cu-7Si braze alloy; however, the melting temperature of the Al-20Cu-7Si alloy does not change.

Comparative study of microstructures and properties of three valuable SnAgCuRE lead-free solder alloys

Lead-free solders with excellent material properties and low cost are essential for the electronics industry. It has been proved that mechanical properties of SnAgCu alloys can be remarkably improved with a minute addition of rare earth (RE) elements. For comparison and optimization, three valuable solder candidates, Sn3.8Ag0.7Cu0.05RE, Sn3Ag0.5Cu0.05RE, and Sn2.9Ag1.2Cu0.05RE, were chosen due to the excellent properties of their own SnAgCu basic alloys. Wetting properties, melting temperature, bulk tensile properties, and joint tensile and shear properties were investigated. In addition, the microstructures of solder joints were observed and the effects of microstructure on mechanical properties were analyzed. Experimental results indicated that the tensile and shear strengths of solder joints were decreased from Sn3.8Ag0.7Cu0.05RE, Sn2.9Ag1.2Cu0.05RE, to Sn3Ag0.5Cu0.05RE, in order. Such difference in mechanical properties could be attributed to the influence of slightly coarse or strong Cu6Sn5 scallops in the reaction layer as well as superior eutectic network and large volume percentage of large primary intermetallic compounds (IMCs) inside the solder joints. It is also suggested that the size and volume percentage of large primary IMCs inside the solder be controlled. In addition, serration morphology was observed at the edge of large primary and eutectic IMCs in the three solder joints, which could be related to the content of Ag, Cu, and RE. The serration morphology was proved to be beneficial to mechanical properties theoretically. Furthermore, the three alloys investigated possessed similar wetting properties, melting temperatures, and bulk tensile properties.

Interfacial reaction between the electroless nickel immersion gold substrate and Sn-based solders

Abstract The electroless nickel immersion gold (ENIG) surface finish is widely used in electronic packaging. The ENIG induced Au embrittlement has been investigated in SnPb/ENIG/Cu solder joint since several years ago. However, in Sn-based lead-free solder joint, discrepancies still exist about the influence of Au finish on the reliability of the solder joint. This study investigated the effects of ENIG surface finish on the interfacial reaction and thus the mechanical property of Sn-based solder joints. Experimentally, two types of ENIG with different thickness of Au layer were fabricated. The results indicated that the Au layer dissolved into the solder matrix readily in the soldering stage, and then affected the shear strength of the solder joint significantly. The Au migration occurred in the solder joint during isothermal aging. The Au migration is more apparent when the Cu 6 Sn 5 type compound formed at the interface. The embrittlement caused by the weak interface between the (Au 1− x Ni x )Sn 4 and Ni 3 Sn 4 layers cannot be observed in this study.

Effect of Ramp Rate on Microstructure and Properties of Thermomechanically-Fatigued Sn-3.5Ag Based Composite Solder Joints

Solder joints experience repeated reverse thermal stress in service, caused by the mismatch in the coefficient of thermal expansion (CTE) between solder and Cu substrate. Lead-free composite solder was fabricated by mechanically mixing Sn-3.5Ag solder paste with Cu reinforcement particles to improve service capability of the base solder alloy. Thermomechanical fatigue (TMF) tests were carried out in environmental chambers. Two temperature profiles, with the same extreme temperature of -40 degC to 125 degC and a 10-minute dwell time at temperature extremes, were applied in the tests. One was a 5-minute ramp time and another was a 20-minute ramp time. Microstructural characteristics and residual mechanical strengths of the solder joints were investigated after 0, 100, 250 and 500 TMF cycles. Composite solder joints showed better TMF performance than the Sn-3.5Ag eutectic solder joints. The results proved that ramp rate was one of the key issues for the damages in solder joints

Effects of nano-sized Ag reinforcing particulates on the microstructure of Sn-0.7Cu solder joints

Abstract Composite solders were prepared by mechanically dispersing different volumes of nano-sized Ag particles into the Sn-0.7Cu eutectic solder. The effects of Ag particle addition on the microstructure of Sn-0.7Cu solder joints were investigated. Besides, the effects of isothermal aging on the microstructural evolution in the interfacial intermetallic compound (IMC) layer of the Sn-0.7Cu solder and the composite solder reinforced with 1vol% Ag particles were analyzed, respectively. Experimental results indicate that the growth rate of the interfacial IMC layer in the Ag particles reinforced composite solder joint is much lower than that in the Sn-0.7Cu solder joint during isothermal aging. The Ag particles reinforced composite solder joint exhibits much lower layer-growth coefficient for the growth of the IMC layer than the corresponding solder joint.

Constitutive modeling on creep deformation for a SnPb-based composite solder reinforced with microsized Cu particles

In the present work, the creep strain of solder joints is measured using a stepped load creep test on a single specimen. Based on the creep strain tests, the constitutive modeling on the steady-state creep rate is determined for the Cu particle-reinforced Sn37Pb-based composite solder joint and the Sn37Pb solder joint, respectively. It is indicated that the activation energy of the Cu particle-reinforced Sn37Pb-based composite solder joint is higher than that of Sn37Pb solder joint. In addition, the stress exponent of the Cu particle-reinforced Sn37Pb-based composite solder joint is higher than that of the Sn37Pb solder joint. It is expected that the creep resistance of the Cu particle-reinforced Sn37Pb-based composite solder joint is superior to that of the Sn37Pb solder. Finally, the creep deformation mechanisms of the solder joint are discussed.

Creep properties of Sn‐0.7Cu composite solder joints reinforced with nano‐sized Ag particles

Purpose – The purpose of this paper is to investigate the creep properties of Sn‐0.7Cu composite solder joints reinforced with optimal nano‐sized Ag particles in order to improve the creep performance of lead‐free solder joints by a composite approach.Design/methodology/approach – The composite approach has been considered as an effective method to improve the creep performance of solder joints. Nano‐sized Ag reinforcing particles were incorporated into Sn‐0.7Cu solder by mechanically mixing. A systematic creep study was carried out on nano‐composite solder joints reinforced with optimal nano‐sized Ag particles and compared with Sn‐0.7Cu solder joints at different temperatures and stress levels. A steady‐state creep constitutive equation for nano‐composite solder joints containing the best volume reinforcement was established in this study. Microstructural features of solder joints were analyzed to help determine their deformation mechanisms during creep.Findings – The creep activation energies and stress...

Environmentally friendly solders 3–4 beyond Pb-based systems

Based on environmental considerations, global economic pressures, enacted by legislations in several countries, have warranted the elimination of lead from solders used in electronic applications. Sn3.5Ag, SnAgCu, and Sn0.7Cu have emerged among various lead-free candidates as the most promising solder alloys to be utilized in microelectronic industries. However, with the vast development and miniaturization of modern electronic packaging, new requirements such as superior service capabilities have been posed on lead-free solders. In order to improve the comprehensive property of the solder alloys, two possible approaches were adopted in the current research and new materials developed were patented. One approach was involved with the addition of alloying elements to make new ternary or quaternary solder alloys. Proper addition of rare earth element such as La and Ce have rendered solder alloys with improved mechanical properties, especially creep rupture lives of their joints. Another approach, the composite approach, was developed mainly to improve the service temperature capability of the solder alloys. Composite solders fabricated by mechanically incorporating various reinforcement particles to the solder paste have again exhibited enhanced properties without altering the existing processing characteristics. The recent progress and research efforts carried out on lead-free solder materials in Beijing University of Technology were reported. The effects of rare earth addition on the microstructure, processing properties, and mechanical properties were presented. The behaviors of various Sn-3.5Ag based composite solders were also explicated in terms of the roles of reinforcement particles on intermetallic growth, steady-state creep rate, the onset of tertiary creep, as well as the overall creep deformation in the solder joints. Thermomechanical fatigue (TMF) behavior of the solder alloys and composite solders were investigated with different parameters such as ramp rate, dwell time, etc. The damage accumulation features and residual mechanical properties of the thermomechanically-fatigued composite solder joints were compared with non-composite solder joints. To match the lead-free alloys, various types of water soluble no-clean soldering flux have also been developed and their properties were presented.

Electric anisotropy of carbon fiber-filled conductive composite vulcanized in electric field

Effects of carbon fiber’s (CF’s) orientation on resistivity of CF-filled conductive composite vulcanized in electric field were investigated experimentally and theoretically. The result exhibited that the filler amount of CF and the applied electric field strength primarily determined the CF’s distribution and orientation, and corresponding resistivity features of the composite. The composite exhibited two kinds of resistivity anisotropy depending upon the combination action from driving and obstacle forces, and according distribution of CF. When a network-like structure of CF established in the composite with medium CF addition, ‘positive resistivity anisotropy’ appeared, where the resistivity in parallel direction of the electric field was lower than that in perpendicular direction. However, the resistivity in the direction parallel to the electric field was higher than that in its vertical direction when CF addition was less and all CF aligned strictly along the electric field. The same phenomenon occurred in composite with higher addition of CF where CF aligned perpendicular to the field. The latter two phenomena were all called as ‘pseudo resistivity anisotropy’. However, their resistivity mechanisms were totally different. Calculation based on effective medium theory and theory of coulomb blockade effect were applied to paraphrase this ‘pseudo resistivity anisotropy’ appeared in composite with less CF addition.

Corrosion Behavior of Different Steel Substrates Coupled With Conductive Polymer Under Different Serving Conditions

The corrosion behaviors of stainless steel and nickel-plated carbon steel coupled with conductive polymer were investigated in both hot humid environment and simulated marine environment. The corrosion currents of different steel substrates and conductive polymer in simulated marine environment at room temperature were measured. The corrosion surfaces of different steel couples were observed under a scanning electron microscope (SEM) and chemical compositions were examined by energy dispersive spectrum (EDS) analysis. The corrosion mechanism was discussed. The results showed that the stability of both stainless steel and nickel-plated carbon steel in hot humid environment was excellent and no corrosion happened in the blank test for 360 h while slight corrosion existed in the contact area of coupled steel substrates. In simulated marine environment, the corrosion current of the stainless steel was lower than 100 μA and some directional rod-like particles formed on the surface of the stainless steel, which are mainly caused by oxidative corrosion among different phases. The corrosion current of the nickel-plated carbon steel couples was much greater than that of stainless steel couples and nickel plate cracking resulted in the corrosion of the internal iron because the coated nickel layer was not dense enough.