Interfacial IMC Layer Research Articles

Hybrid solder joints: the effect of nanosized ZrO2 particles on morphology of as-reflowed and thermally aged Sn–3.5Ag solder joints

The main number of current researches has been focused on the microstructure and mechanical properties of the Sn-based Sn–Ag–Cu-based solders, while various kinds of nanosized particles have been added. The synthesis and handling of ceramic nanosized powder are much easier than of metal nanoparticles. In addition, metal nanoparticles solved in solder joints during the soldering process or by thermal aging could behave as an alloying element similar to bulk metal additions, while ceramic nanoparticles retain their chemically inactive behavior in various thermal, thermo-mechanical, and electrical constraints. In some cases, the solved metal nanosized inclusions could increase the growth kinetics of the present intermetallic phases or even create new phases, which leads to more complexity in the predictions and simulations of chemical processes in the solder joints. Based on the assertions mentioned above, ceramic nanosized particles are industrially more favorable as reinforcing inclusions. On the other hand, there is no direct comparison in the literature between Sn-based Sn–Ag–Cu and Sn–Ag solder joints with similar ceramic nanoinclusions based on microstructural features and mechanical properties. In the present research, the Cu/flux + NPs/SAC/flux + NPs/Cu solder joints were produced with a nominal amount of 0.2 wt%, 0.5 wt%, and 1.0 wt% nanosized ZrO2 powder. The solder joints prepared via the above-described method are called in the literature as hybrid solder joints. The microstructure of the as-reflowed and thermally aged samples has been studied, especially at the interface solder/substrate. It has been shown that the minor additions of ZrO2 NPs lead to a decrease in the thickness of the Cu6Sn5 interfacial layer in the as-reflowed solder joints and a reduction in the growth kinetics of this layer, while the Cu3Sn interfacial IMC layer remains practically unaffected. Similar investigations were performed in our previous study but for both the hybrid and nanocomposite Sn–3.0Ag–0.5Cu solder joints. A comparative analysis of the impact of the ZrO2 nanoinclusions on the hybrid solder joints using Sn–3.5Ag and Sn–3.0Ag–0.5Cu has been performed.

Study on the interfacial evolution and mechanical properties of Cu/SAC105/Cu solder joint modified by Si3N4 nanowires

In the present study, Si3N4 nanowires (NWs) with a mass fraction of 0.6 wt% were incorporated into Sn1.0Ag0.5Cu (SAC105) solder. Cu/SAC105/Cu and Cu/SAC105-Si3N4/Cu 3D structure joints were constructed by transient liquid-phase (TLP) bonding. To investigate the influence mechanism of Si3N4 NWs, the interfacial IMC evolution and mechanical properties of two solder joints were analyzed. It was found that the interfacial Cu3Sn IMC layer thickness of the joints containing Si3N4 NWs was thinner than that of the undoped ones at any given time. The growth of interfacial Cu6Sn5 IMC was inhibited, resulting in the retarded formation of interfacial IMC bridging and full-IMC joints. In addition, combined with the fracture morphology analysis, the brittle fracture behavior of the solder joints was suppressed, and the shear strength of the joints showed a significant improvement following the introduction of Si3N4 NWs. Moreover, Si3N4 NWs were found exposed on the fracture surface of Cu3Sn IMC layer, demonstrating that the doping of Si3N4 NWs shaped a joint similar to that of reinforced concrete structure, which facilitates the limitation of dislocations and crack expansion.

Microstructure and properties of Sn58Bi/Ni solder joint modified by Mg particles

In this paper, various mass fractions (0, 0.1, 0.2, 0.4, 0.6, 0.8 wt%) of Mg particles were added to the Sn58Bi solder to achieve the purpose of modification, through which the Ni substrate was connected successfully through bonding at 250 °C for 2 min. Meanwhile, the microstructure, interfacial IMC, and properties of the joint were focused on. The experimental results indicated that the appropriate addition of Mg particles is able to decrease the melting point of the solder quite slightly and significantly improve the wettability of the solder on the Ni substrate. When the content of Mg is less than 0.4 wt%, the presence of MgO on the surface does not affect the normal use of solder. Likewise, Mg particles are capable of refining the microstructure of the solder and reducing the thickness of the interfacial Ni3Sn4 IMC layer and the grain size. Accordingly, the mechanical property of the joint is markedly enhanced by 53.48%, whose fracture is mainly characterized by brittle fracture, and the Mg2Sn phase plays the central role of strengthening. In a word, it is the Sn58Bi-0.4Mg/Ni solder joint that processes the best combination performance.

Microstructure and shear property of Ni-coated carbon nanotubes reinforced InSn-50Ag composite solder joints prepared by transient liquid phase bonding

InSn-50Ag composite solder joints doping Ni-coated carbon nanotubes (Ni-CNTs) were fabricated by transient liquid phase (TLP) bonding, and then the effect of Ni-CNTs contents (x = 0, 0.01, 0.03, 0.05, 0.07, 0.1 wt%) on the microstructure and shear property of the composite solder joints was examined. The results showed that the microstructure of Cu/InSn-50Ag-x(Ni-CNTs)/Cu composite solder joints consisted of the interfacial region bamboo-type Cu3(In, Sn), solder center region Ag3In, Ni3Sn4, Ag particles and Ni-CNTs. The appropriate amounts of Ni-CNTs (about 0.07 wt%) are conducive to form a compact composite solder joint and restrain the growth of interfacial Cu3(In, Sn) IMC layer, and the morphologies of IMC layer are determined by Ni-CNTs suppressing atom diffusion and IMC grain orientation. The shear strength of composite solder joints relates to Ni-CNTs content, and the maximum shear strength reaches 42 MPa of Cu/InSn-50Ag-0.07(Ni-CNTs)/Cu. It is regulated by uniform distribution of Ni-CNTs, refined Ag3In grains and interface IMC layer with near-continuous voids. The fracture of the composite solder joints doped Ni-CNTs occurred near to the boundary between the interfacial IMC layer and solder center region, the fracture mechanism is ductile fracture.

Improvement on the mechanical properties of eutectic Sn58Bi alloy with porous Cu addition during isothermal aging

A comparison of the microstructure, interfacial IMC layer, shear behavior and hardness of the Sn58Bi, SnBi@110P-Cu/Cu and SnBi@500P-Cu/Cu solder joints was carried out during isothermal aging in this study. Experimental results reveal that the regions enclosed by Cu frames in SnBi@110P-Cu/Cu exhibit more refined microstructure and higher hardness before and after aging. In contrast, the hardness of the aged SnBi@500P-Cu solder bulk is closer to the SnBi eutectic solder due to their similar microstructure. With the aging time prolonging, β-Sn and Bi-rich phases do not only coarsen in solder joints, but also lead to the rise of the thickness of the interfacial IMC layers. The addition of porous Cu can suppress the formation of voids at the soldering interface and improves the shear strength of the SnBi/Cu solder joints during the aging process. The shear strength of the SnBi@500P-Cu/Cu solder joint is higher than the other two before and after aging.

Nanocomposite SAC solders: the effect of heat treatment on the morphology of Sn–3.0Ag–0.5Cu/Cu solder joints reinforced with Ni and Ni–Sn nanoparticles

The concept of nanocomposite lead-free solders has provided a promising approach to improve mechanical reliability of solder joints. Minor additions of metal nanoparticles into the commonly used Sn–3.0Ag–0.5Cu solder impact on the morphology of both the solder alloy and solder/Cu joint. For instance, it was stated that the average thickness of the interfacial intermetallic compound layer between Sn–3.0Ag–0.5Cu solder and Cu substrate decreases by addition up to 2.0wt.% of nano-sized Ni and Ni–Sn particles. The present research is the follow-up of our previous study and is focused on the investigations of the thermal aged Sn–3.0Ag–0.5Cu solder joints with up to 2.0wt.% of nano-sized Ni, Ni3Sn and Ni3Sn2 inclusions. The produced Cu/nanocomposite solder/Cu joints were kept at the temperature of 423 K for 10 and 20 days. Based on the average thickness of the interfacial intermetallic compound layer Cu6Sn5/Cu3Sn estimated for solder joints with different aging time, the growth kinetics of the interfacial IMC layer was examined.

The Effect of Thermal Annealing on the Microstructure and Mechanical Properties of Sn-0.7Cu-xZn Solder Joint

The microstructural properties of a Pb-free solder joint significantly affect its mechanical behaviours. This paper details a systematic study of the effect of the annealing process on the microstructure and shear strength of a Zn-added Sn-0.7Cu solder joint. The results indicated that the IMC layer’s thickness at the solder/Cu interface increases with annealing time. The interfacial IMC layer of the Sn-0.7Cu solder joint gradually thickened with increasing annealing time and annealing temperature, while the interfacial IMC layer’s morphology changed from scallop-type to layer-type after the annealing process. However, the addition of 1.0 wt.% and 1.5 wt.% Zn in the Sn-0.7Cu effectively altered the interfacial IMC phase to Cu-Zn and suppressed the growth of Cu3Sn during the annealing process. The single-lap shear tests results confirmed that the addition of Zn decreased the shear strength of Sn-0.7Cu. The interfacial IMC of the Cu6Sn5 phase in Sn-0.7Cu changed to Cu-Zn due to the addition of Zn. The shear fractures in the annealed solder joint were ductile within the bulk solder instead of the interfacial IMC layer. Increased annealing time resulted in the increased presence of the Cu-Zn phase, which decreased the hardness and shear strength of the Sn-0.7Cu solder joint.

Structural, Thermal and Wetting Characteristics of Novel Low Bi-Low Ag Containing Sn–x.Ag–0.7Cu–1.0Bi (x = 0.5 to 1.5) Alloys for Electronic Application

Low drop failure tendency and reduced cost of low silver SAC alloys has provoked researchers to exploit their potential for portable electronics. Low Bi addition in near-eutectic SAC alloys has proven confinement of IMC layer. In order to exploit potential of Ag variation (0.5–1.5 wt%) on structural, thermal, wettability and IMCs layers growth during thermal aging in low Bi-low Ag, Sn–xAg–0.7Cu–1.0Bi alloys were investigated. The resulting microstructure were characterized by scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. Furthermore, structure–property correlations were established. This study signifies that 1.5 wt% Ag is responsible for lower melting point (220.18 °C) and lower contact angle (33°) in low Bi-low Ag, Sn–xAg–0.7Cu–1.0Bi alloys. Moreover, increment in Ag content restricted the interfacial IMC layer growth during thermal aging making them reliable for use in portable electronics. The microstructural investigations inferred that 0.5 wt% Ag solder joint with Cu substrate had the highest growth rate (0.34 μm/h1/2) of IMC layer thickness while 1.5 wt% Ag solder joint had lowest growth rate (0.17 μm/h1/2).

Influence of an AlSi5 filler wire on microstructures and mechanical properties of EBW-brazed CP-Al to 304 SS joint

Electron beam welding-brazing experiments of commercial pure aluminum (CP-Al) to 304 stainless steel with and without an Al-Si5 filler wire were carried out. Si element existing in the filler wire could help to improve the wettability of aluminum on steel and formed a continuous interfacial IMC layer of Al8Fe2Si. The interface of the joint without filler metal was characterized by a discontinuous Fe2Al5 compound layer with cracks in it. Meanwhile, the nanohardness and Young’s modulus of Al8Fe2Si compound were both lower than those of Fe2Al5 compound. The tensile strength of the joint welded with filler wire was higher than that of the joint welded without filler metal. It implied that a continuous IMC layer with lower hardness is beneficial to improve the strength of the electron beam welding-brazed 304 SS to CP-Al joint.

Nanocomposite SAC solders: the effect of adding CoPd nanoparticles on the morphology and the shear strength of the Sn–3.0Ag–0.5Cu/Cu solder joints

The effect of bimetallic monodisperse CoPd nanoparticles on the microstructure and the shear strength of the Cu/ SAC305/Cu solder joint was investigated. The nanocomposite Sn–3.0Ag–0.5Cu (SAC305) solders with 0.1, 0.3, 0.5, and 1.0 wt% nanoCoPd were prepared through a paste mixing method. The employed bimetallic nanoparticles were produced via a modified oleylamine method. The microstructural analysis of as-solidified Cu/solder/Cu joints was performed by scanning electron microscopy. The results showed that initial additions of CoPd nanoparticles into the SAC305 solder promoted the growth of the interfacial planar-type Cu3Sn IMC layer; while the average thickness of the interfacial scallop-type Cu6Sn5 IMC layer slightly decreased. Further additions of the nanosized CoPd admixtures to the SAC305 solder paste lead to a significant increase of the average thickness of the Cu6Sn5 intermetallic compound layer up to 40%. The shear strength measurements were performed to investigate a relationship between the microstructure and mechanical properties of the investigated solder joints. The results indicated a decrease in the shear strength of the SAC305 solder joint by addition of 0.1 wt% CoPd NPs, while a difference in absolute values between solder joints with 0.3, 0.5, and 1.0 wt% nanoCoPd was practically insignificant.

Effect of Thermal Cycling on Interfacial Microstructure and Mechanical Properties of Sn-0.3Ag-0.7Cu-(α-Al2O3) Nanoparticles/Cu Low-Ag Solder Joints

The evolution of interfacial microstructures and mechanical properties of joints soldered with Sn-0.3Ag-0.7Cu (SAC0307) and SAC0307-0.12Al2O3 nanoparticles (NPs) subjected to thermal cycling were investigated. The joint soldered with SAC0307-0.12Al2O3 displayed an enhanced thermal cycling shear force with a ductile fracture mode when compared with the original alloy whose fracture mode showed a mixed feature of ductile and brittle. The enhanced thermal cycling shear force was attributed to a pinning effect by Al2O3 NPs on interfacial IMC grain growth. Even after 1200 thermal cycles, SAC0307-0.12Al2O3 solder was still structurally characterized by a much more refined microstructure than the non-reinforced solder alloy. Theoretical analysis on the growth of interfacial IMC layer showed that with the addition of Al2O3 NPs, the average growth coefficients of total interfacial IMCs (DT) and Cu3Sn IMCs (\( D_{{{\rm{Cu}}_{3} }} \)) were decreased from 9.2 × 10−11 cm2/h to 5.6 × 10−11 cm2/h, and from 6.9 × 10−11 cm2/h to 4.1 × 10−11 cm2/h, respectively. Hence, a much thinner IMC layer was produced at the SAC0307-0.12Al2O3/Cu interface, thus contributing to an enhanced shear resistance.

Coupling effects of rare-earth Pr and Al2O3 nanoparticles on the microstructure and properties of Sn-0.3Ag-0.7Cu low-Ag solder

Low-cost Al2O3 nanoparticles (NPs) are regularly incorporated into lead-free solder to improve its mechanical performance, but the poor wettability of Al2O3 NPs leads to weak Al2O3/solder interface bonding. In this work, a novel idea different from the common solution of pre-coating a metal layer on the surface of NPs was proposed to address the issue of weak interface bonding by simultaneously doping rare-earth (RE) Pr (0.06 wt%) and Al2O3 NPs (0–0.5 wt%) into Sn-0.3Ag-0.7Cu low-Ag solder. The coupling effects of RE Pr and Al2O3 NPs on the wettability, microstructures, shear force and creep resistance of prepared nanocomposite solder were systematically investigated. It was found that the coupling effects were concentration dependent. When the content of Al2O3 NPs was less than 0.06 wt%, a synergistic relationship between free Pr atoms and Al2O3 NPs was established. This is because the surface-active Pr atoms, acting as bridges between Al2O3 NPs and solder matrix, can be spontaneously adsorbed at the surfaces of Al2O3 NPs to in situ form Pr-coated Al2O3 NPs, which have much better wettability than the single Al2O3 NPs. However, increasing the doping content of Al2O3 NPs over 0.06 wt% led to a competitive relationship among free Pr atoms, Al2O3 NPs and Pr-coated Al2O3 NPs, which was probably caused by the emerged Al2O3 agglomerates occupying partial space of grain surfaces. The optimum doping content of Al2O3 NPs in SAC0307–0.06Pr solder was approximately 0.06 wt% and SAC0307–0.06Pr-0.06Al2O3 possessed the best wettability, a well-controlled growth of interfacial IMC layer, the highest shear force (∼61 N) and a superior room-temperature creep resistance with creep stress exponent (n) reaching approximately 11.7.

Effect of alternating current (AC) stressing on the microstructure and mechanical properties of low-silver content solder interconnect

The low-cost low-silver-content solder is receiving high attention while its reliability is not well studied. In this work, a copper/tin-silver-copper/nickel (Cu/Sn0.3Ag0.7Cu/Ni) line-type solder interconnect was designed for investigation of the effect of alternating current (AC) on the microstructure and mechanical properties. Three dimensional (3D) X-ray microcomputed tomography tests showed voids and protruding IMCs formed in the solder/Ni and solder/Cu interface regions of the interconnect after prolonged AC stressing, respectively. The thickness growth of interfacial IMC layers followed linear laws with faster rates than those in thermal aged sample. Atomic migration of Cu atoms in Sn0.3Ag0.7Cu solder was estimated using mathematical models with a thermal gradient of 206.77 °C cm−1. Significant degradation in mechanical properties happened. Nano-indentation results showed the hardness and elastic modulus in different solder areas were in the following order: central solder > solder near solder/Ni interface > solder near solder/Cu interface. Four mechanisms can be used to explain these results of AC damages: 1) elevated temperature due to Joule heating, which enhanced grain coarsening and dislocations redistribution; 2) thermal fatigue due to cyclic loading, which damaged the interconnect strength by accumulated stress induced by mismatch in the coefficients of thermal expansion; 3) thermo-migration due to thermal gradient from Ni side to Cu side, which released the thermal stress at the solder/Ni interface and prevented further deterioration, while increasing the thermal stress at the solder/Cu interface and deteriorated the mechanical strength; 4) atomic migration due to chemical gradient, which induced the mass redistribution and contribute to the growth of IMCs layers.

Comparative studies on microelectronic reliability issue of Sn whisker growth in Sn-0.3Ag-0.7Cu-1Pr solder under different environments

In this study, comparative studies on Sn whisker growth in Sn-0.3Ag-0.7Cu-1Pr solder under different environments were conducted to investigate factors like ambient temperature, oxygen level, and 3.5wt% NaCl solution on whisker growth. The experimental results revealed that ambient temperature and oxygen level are two important factors that could determine the oxidation rate of PrSn3 phase, thus indirectly affecting the growth rate of Sn whiskers. In addition, mechanisms of whisker growth under these three environments were established from the perspective of atom diffusion based on the “compressive stress-induced” theory. Although whiskers under different environments were all squeezed out from Pr oxides (hydroxides), the forms of their driving forces were different. For whiskers squeezed out in air whether at room temperature or 150°C, the driving force is the compressive stress produced by lattice expansion due to the oxidation of PrSn3 phase. The representative example was whiskers' growth at 150°C, which could be simplified as three stages: (1) squeezing out, (2) cracking and (3) bursting out. For whisker growth in 3.5wt% NaCl solution, the driving force for much fewer whiskers' growth was proposed to come from lateral stress provided by interfacial IMC layer growth. Moreover, Sn nanoparticles and their agglomerations were also found to form under the driving force of the potential difference between Sn atoms and Sn crystals. Their morphologies could also be affected by factors of ambient temperature, oxygen level and Cl− ions in corrosive liquid.

Influence of Zn additions on the interfacial reaction and microstructure of Sn37Pb/Cu solder joints

The effects of Zn (5 and 10 wt%) additions into Sn37Pb solder and isothermal solid state aging on the interfacial reactions between Sn37Pb–xZn solders and Cu substrates were investigated in this study. It was found that the addition of Zn changed the types and morphologies of interfacial IMC layers during reflowing and thereafter under aging condition. During reflowing, the planar-type Cu5Zn8 compound was the interfacial IMC for Sn37Pb–xZn (5 and 10 wt%) solder, while the scallop-type Cu6Sn5 was the interfacial IMC for Sn37Pb solder. After aging, the final interfacial structure for Sn37Pb–5Zn solder was solder/Cu5Zn8/Cu6(Sn,Zn)5/Cu, while solder/Cu6Sn5/Cu3Sn/Cu for Sn37Pb solder and solder/Cu5Zn8/Cu for Sn37Pb–10Zn solder, respectively. The Kirkendall voids disappeared with Zn addition into Sn37Pb solder. For the Sn37Pb–5Zn/Cu solder joint, the thickness of Cu6(Sn,Zn)5 layer increased, while the thickness of Cu5Zn8 layer decreased with aging time extended to 360 h due to the decomposition of the Cu5Zn8 IMC layer by diffusing Cu and Zn atoms into nether IMC layer, combining Sn atoms diffused from solder matrix to form Cu6(Sn,Zn)5 IMCs. Furthermore, the growth of Cu6Sn5 and Cu3Sn layers for Sn37Pb/Cu solder joint and the total IMC layer at the interface of Sn37Pb–xZn (x = 0, 5, and 10 wt%) solder with Cu substrate followed the diffusion control mechanism. Compared to the Sn37Pb–5Zn/Cu solder joint, higher Zn concentration depressed the growth of Cu5Zn8 layer for Sn37Pb–10Zn solder. In the end, refining effect on IMC grains was found by the addition of Zn into Sn37Pb solder and the 10 wt% Zn-doping significantly refined the interfacial IMC grains.

Combined effects of MIG and TIG arcs on weld appearance and interface properties in Al/steel double-sided butt welding-brazing

AA5052 aluminum alloy and Q235 low-carbon steel of 3.0mm thickness were butt joined without groove. The combined effects of the MIG-TIG double-sided arc on the weld appearance and interface properties were investigated in three aspects: weld appearance, interfacial properties and joint tensile strength. Form efficiency was defined to evaluate the quality of the weld appearance. The strength of the whole joint and the brazed interface were examined in the tensile tests of joints with and without excess weld metal, respectively. The weld appearance can be well characterized by the form efficiency. The form efficiency is found to be most sensitive to TIG current. Sensibility of the form efficiency to MIG voltage and welding speed is relatively low, indicating that the TIG arc at the back of the weld is the key factor for the double-sided weld appearance. Double-sided weld appearance can be achieved at a maximum welding speed of 2.2cm/s because a part of welding energy is input by TIG arc directly to the back weld. The interfacial properties of the joints are closely tied to the heat input ratio of MIG and TIG. Proper input ratio (1.35–1.64) leads to uniform deformation of the base metals and suppresses the initiation of the cracks in interfacial IMC layer. The differences between the interfacial strength and the whole joint strength indicate that the optimum weld appearance with desirable excess weld metal provides extra 20MPa strength for the whole joint. Benefitting from combined effects of the best weld appearance and toughest interfacial IMC layer, the highest tensile strength of the joint exceeds 70% the tensile strength of 5052 alloy.

Effects of CuZnAl Particles on Properties and Microstructure of Sn-58Bi Solder.

With the purpose of improving the properties of the Sn-58Bi lead-free solder, micro-CuZnAl particles ranging from 0 to 0.4 wt % were added into the low temperature eutectic Sn-58Bi lead-free solder. After the experimental testing of micro-CuZnAl particles on the properties and microstructure of the Sn-58Bi solders, it was found that the wettability of the Sn-58Bi solders was obviously improved with addition of CuZnAl particles. When the addition of CuZnAl particles was 0.2 wt %, the wettability of the Sn-58Bi solder performed best. At the same time, excessive addition of CuZnAl particles led to poor wettability. However, the results showed that CuZnAl particles changed the melting point of the Sn-58Bi solder slightly. The microstructure of the Sn-58Bi solder was refined by adding CuZnAl particles. When the content of CuZnAl addition was between 0.1 and 0.2 wt %, the refinement was great. In addition, the interfacial IMC layer between new composite solder and Cu substrate was thinner than that between the Sn-58Bi solder and Cu substrate.

Effect of Pr addition on properties and Sn whisker growth of Sn–0.3Ag–0.7Cu low-Ag solder for electronic packaging

In this study, trace amount of rare earth Pr was added into Sn–0.3Ag–0.7Cu low-Ag solder to enhance properties of solders. Experimental results indicated that optimal amount of Pr addition (~0.06 wt%) can improve properties of wettability, shear force and ductility of Sn–0.3Ag–0.7Cu low-Ag solder. This is because that solder with optimal Pr addition not only had a refined microstructure but also owned a regular and thin interfacial IMC layer after soldering on Cu substrate. Meanwhile, we have explained the change of morphology and thickness of interfacial IMC layer after Pr addition based on Kim and Tu’s kinetic model of interfacial IMC growth. However, it was found excessive Pr addition led to the formation of PrSn3 phase, which was easy to be oxidized and became a great deterioration on the properties referred above. Besides, the fracture mode of solder joint also gradually changed from ductile fracture to cleavage fracture. Moreover, this oxidized PrSn3 became the birthplace of Sn whisker since it provided abundant supply of Sn sources and compressive stress for Sn whisker growth. By the in-situ observation of Sn whisker growth in 0.5 wt% Pr-doped solder joint, we obtained that the incubation period of Sn whisker is very short and its growth rate may have a great decrease with time extending. Besides, it was found after 1 day at room temperature, spindly and rod-like Sn whiskers also grew in 0.5 wt% Pr-doped solder joint besides dot-shaped Sn whisker that grew in 0.5 wt% Pr-doped solder matrix. We suggest this may be related with the extra compressive stress provided by interfacial IMC layer growth in 0.5 wt% Pr-doped solder joint.

Microstructural characterization and mechanical properties of Al/Ti joint welded by CMT method—Assisted hybrid magnetic field

A novel external axial magnetic field (EMF) hybrid CMT welding-brazing process was adopted to join pure titanium TA2 and aluminum alloy 6061-T6. The effects of the magnetic intensity and frequency on microstructure and mechanical properties of the resultant joints were revealed. The experimental results indicated that the magnetic field can affect the flowability and surface spreadability of the molten filler metal on TA2 plate, thus forming the reliable Al/Ti joint. Grain refinement, decreased Al-Ti intermetallic compounds, and increased microhardness in both the heat-affected zone and the weld were observed with the applied of the EMF. The microstructure of the interfacial IMC layer was identified as TiAl3, and its thickness was decreased from 10μm to 5μm. In addition, the tensile shear strength of the Al/Ti joint reached a maximum value of 4.105KN and fractured at the Ti base metal with the coil current and magnetic frequency set at 1.0A and 10Hz respectively, increasing by 93.4% compared to the normal welding process. However, the additional magnetic field could be an inverse effect on the process of Al/Ti dissimilar metal joining because of the unstable welding process.

Mechanical properties and fracture mechanisms of Sn-3.0Ag-0.5Cu solder alloys and joints at cryogenic temperatures

For deep-space applications under cryogenic temperatures, the mechanical properties of Sn-3.0Ag-0.5Cu (SAC305) solder alloys and SAC305/Cu solder joints have been investigated over the temperature range 77–298K. The fracture surfaces were analyzed in the plan view to identify the fracture locations in the solder joints, and the fracture mechanisms in solder alloys and solder joints were also explored. Results showed the tensile strength of the solder alloys firstly increased and reached its maximum value at 123K, and then decreased with descending temperature. Meanwhile, the solder alloys displayed a shift in their fracture mechanism from dimple-ductile fracture to brittle fracture in a mixed model which was combined by transgranular and intergranular. It could be attributed to the asymmetric tetragonal structure of β-Sn which is the main constituent of SAC305 solder. The tensile strength of SAC305/Cu joints showed increment at first and then decrement as the temperature decreased, while increasing the strain rate enhanced the tensile strength. In addition, the fracture location of the joints changed from the solder to the solder/IMC interface or the interfacial IMC layer with the decrement of temperature, resulting in the variation of the fracture mechanism from ductile fracture to brittle fracture.