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With miniaturization and heterogeneous integration in packaging, there has been a drive toward developing lower temperature solders. Sn-58Bi eutectic solder provides an attractive alternative with its meting point at 138°C. Due to the lower melting temperature of Sn-Bi solder, Bi coarsening may occur even at room temperature. In this paper, the microstructural evolution in Sn-58Bi joints was observed during room temperature storage. Room temperature aging induced the dissolution and coarsening of Bi phases in the solder matrix, especially in the primary Sn phases and Sn-Bi dendrites. The mechanical properties of individual Sn-rich and Bi-rich phases were measured by nanoindentation. The results showed that the Sn-rich phases had higher Young’s modulus and hardness than Bi-rich phases in aged solder joints due to solution strengthening. Bi phases were more compliant and had lower hardness than Sn.

To optimize the mechanical properties of low-Ag lead-free solder, different amount of Bi element was added into the Sn–1.0Ag–0.5Cu (SAC105) to form SAC105-xBi (0 ≤ x ≤ 4) solder alloy. The microstructural evolution and shear fracture of SAC105-xBi solder joints were systematically investigated to clarify the failure mechanism after reflow and after 175 °C aging compared to SAC305. It was found that Bi element could provide solution strengthening and second-phase strengthening in the low-Ag SAC solder joints depending on the amount of added Bi element. The shear strength after reflow increased from 34.79 to 70.16 MPa with the increase of Bi addition from x = 0.0 to x = 4.0, while that after aging at 175 °C for 168 h increased from 28.97 MPa (x = 0.0) to 55.02 MPa (x = 4.0). The thickness of IMC decreased with the addition of Bi, and excessive Bi would precipitate in the matrix when the Bi content reached 2 wt% or even form brittle Bi barrier layer after 4 wt% addition. During shear tests, three kinds of fracture mode including ductile fracture, quasi-cleavage fracture, and cleavage brittle fracture were observed, and three kinds of failure position including solder matrix, Cu6Sn5/solder interface, and Cu3Sn/Cu interface were revealed. The failure mechanism of different solder joints was closely related to the coupling effects among the Bi amount in the matrix, the thicknesses of IMC, and the status of Kirkendall voids, which significantly affected the reliability of SAC solder joints. Finally, SAC105-1.0Bi solder alloy was considered having the most suitable Bi addition with anticipated comprehensive properties compared with SAC305.

The effect of Ag solutes on the solid-state Cu dissolution in the Sn3.5Ag solder was studied. The solid-state Cu dissolution depends on the dissolution of the interfacial Cu6Sn5 compound layer, which is constant with time. In a low annealing temperature range (150, 165, and 180 °C), the Ag solutes forming Ag3Sn phase attaching (near) the interfacial Cu-Sn compound layer, which retards dissolution of the interfacial Cu6Sn5 compound layer. The activation energy of Cu dissolution into Sn3.5Ag solder is 0.37 eV, which is less than Cu in pure Sn solder (1.48 eV). Interestingly, the consumption mechanism of the Cu wire changes at a high annealing temperature of 200 °C, where the consumption of the Cu wire in the Sn3.5Ag/Cu-wire/Sn3.5Ag system is faster than in the Sn/Cu-wire/Sn system. Therefore, the higher Ag solubility in the Sn3.5Ag solder matrix causes fewer Ag3Sn precipitates in the Sn lattice than in pure Sn solder, which is the main factor promoting Cu6Sn5 dissolution at the high annealing temperature of 200 °C. Additionally, the extensive dissolution of the interfacial Cu6Sn5 compound layer in the Sn3.5Ag/Cu-wire/Sn3.5Ag system resulted in an irregular decomposition with prolonged annealing (greater than 10 h) at the high annealing temperature of 200 °C. After 35 h of annealing, the interfacial Cu6Sn5 compound layer broke off and dissolved into the Sn3.5Ag solder matrix.

In fusion applications, cables are exposed to high currents and magnetic fields. Cable-in-conduit conductors (CICC) made of several high temperature superconductor (HTS) strands, like the HTS CrossConductor (HTS CroCo), have the ability to meet these requirements. HTS CroCos are made of a cross-shaped stack consisting of two different widths of REBCO and copper tapes embedded in a round solder matrix. The round outer shape allows an easy encapsulation with copper tubes for electrical and mechanical reasons. The triplet-CICC of HTS CroCos is considered as the next step towards a full size fusion conductor. First, a straight triplet sample and later a high field winding demonstrator is planned to be tested at high operational currents (>10 kA) and high magnetic fields (>10 T). Operating the triplet under such conditions will inevitably lead to high Lorentz forces acting on the strands of the triplet-CICC. In order to investigate the electromechanical performance of HTS CroCo triplets, short triplet samples were loaded with transversal mechanical pressure at 77 Kelvin, self-field, to simulate the Lorentz forces of the intended high-current, high-field application. The maximum allowable transversal load was investigated for different triplet configurations in order to determine design limits for the triplet winding demonstrator.

Over the past years, high temperature superconductors (HTS) Direct Current (DC) power distribution has received increased attention. One particular scenario is the application of HTS in energy-intensive industries to substitute conventional copper or aluminum bus bar systems. The use of HTS in these scenarios creates significant benefits regarding space, weight, and system losses. In industrial electrolysis, currents of up to several hundred kiloamperes are commonly encountered. One way toward a HTS cable with such high currents is to assemble multiple high current HTS strands into a cable. The HTS CroCo, developed at KIT, can serve as a high current strand for such cables. This paper reports on the progress in HTS CroCo manufacturing as well as the results of the CroCo characterization. With a new manufacturing machine, 8-m-long HTS CroCos within a round solder matrix were produced and cut into pieces, each 3.6 m in length. Twelve HTS CroCo pieces were characterized at 77 K, self-field. The tested HTS CroCos will be assembled to a HTS dc cable demonstrator that is designed to have a critical current of 35 kA at 77 K in self-field. To allow an individual measurement of the different HTS CroCos within the cable assembly, the strands will be connected in a first step individually to the feeding bus bar. The impact of the lead resistance and critical current distribution of the individual HTS CroCos on the current distribution within the assembled 35 kA cable demonstrator is discussed.

The mechanical properties of solder joints highly depend on the interfacial reaction between the solders and the metallization on substrates. In this work, we electroplated Co–P films with various compositions on the Cu pads of printed circuit boards and fabricated Sn-3.8 wt% Ag-0.7 wt% Cu/Co–P ball grid array (BGA) solder joints. The BGA solder joints were annealed at 150 °C for 100, 200, 500, and 1000 h and the shear strength of these joints was measured. When the P content of the Co–P metallization was increased from 2.3 to 18.8 at.%, the shear strength after 1000 h annealing initially rose to 107.9 MPa at a P content of 8.5 at.%, then decreased to 84.3 MPa at a P content of 12.5 at.%, and again increased to 96.0 MPa at a P content of 18.8 at.%. The enhancement of the shear strength of the joints with Co-8.5 at.% P, Co-12.5 at.% P, and Co-18.8 at.% P films was 109.5%, 63.7%, and 86.4% in comparison to the joints without Co–P metallization, respectively. The interfacial reaction between the Sn–Ag–Cu (SAC) solder and Co–P films during annealing and the fractured surfaces of the solder joints after the shear test were studied. For the joints with Co-8.5 at.% P and Co-18.8 at.% P films, a thick layer of CoSn3 was formed at the interfaces during annealing, which enhanced the shear strength. For the joints with Co-12.5 at.% P metallization, a thin layer of Co–Sn–P was formed at the interfaces and was peeled off layer by layer with prolongation of the annealing time. The spalled Co–Sn–P was mixed with the solder matrix, increasing the shear strength of the solder joints. The shear strength of the SAC/Co-12.5 at.% P joints was less than that of the joints with Co-8.5 at.% P and Co-18.8 at.% P films because no CoSn3 formed. Therefore, the composition of Co–P metallization played an important role in the interfacial reaction of the SAC/Co–P solder joints, which in turn affected the shear strength of the solder joints. Our experimental results show that the electroplated Co–P film is a promising candidate as the metallization for BGA solder joints.

The Sn-3.0Ag-0.5Cu solder alloy is a prominent candidate for the Pb-free solder, and SAC305 solder is generally employed in today’s electronic enterprise. In this study, the formation of intermetallic compounds (Cu6Sn5 and Ag3Sn) at the interface, average neighbour’s particle distance, and the morphological mosaic are examined by the addition of SiC and nickel-coated silicon carbide reinforcements within Sn-3.0Ag-0.5Cu solder. Results revealed that the addition of SiC and SiC(Ni) particles are associated with a small change to the average neighbor’s particle distance and a decrease of clustering rate to a certain limit of the Sn-3.0Ag-0.5Cu solder composites. Moreover, the development of the Cu6Sn5 and the structure of the Ag3Sn are improved with the addition of SiC and Ni coated SiC.

In this study, Sn-Zn solder samples containing 2 to 12 wt.% Zn were fabricated and reflowed into a Cu substrate. The microstructure of solder samples were observed after reflow and aging for up to 1000 h at 150 °C. Thermodynamically stable intermetallics (IMCs) Cu-Zn and Cu-Sn formed at the interface depending on the solder composition. Formation of different interfacial IMCs during soldering and after prolonged aging is explained by the spalling mechanism that resulted from the depletion of Zn from the solder matrix.

Superconducting joints are essential for persistent-mode operation in a superconducting magnet system to produce an ultra-stable magnetic field. Herein, we report rationally designed niobium-titanium (Nb-Ti) superconducting joints and their evaluation results in detail. For practical applications, superconducting joints were fabricated by using a solder matrix replacement method with two types of lead-bismuth (Pb-Bi) solder, including Pb42Bi58 as a new composition. All the joints attained a critical current of >200 A below 1.43 T at 4.2 K. Our optimized superconducting joining method was tested in a closed-loop coil, obtaining a total circuit resistance of 3.25 × 10−14 Ω at 4.2 K in self-field. Finally, persistent-mode operation was demonstrated in an Nb-Ti solenoid coil with a persistent-current switch. This work will pave the way to developing high-performance Nb-Ti superconducting joints for practical applications.

In this study, the shear performance and fracture behavior of micro-scale BGA structure Cu/Sn–3.0Ag–0.5Cu/Cu joints under electro-mechanically coupled loads with different loading rates (from 0.1 N/min to 1 N/min) and current densities (6.0×103 A/cm2, 8.0×103 A/cm2 and 1.0×104 A/cm2) have been investigated. The results show that shear strength of joints increases with the increase of loading rate at various current densities, and the relationship between the current density and the increasing rate of shear strength can be fitted by the Boltzmann function. In addition, for joints under high-density current stressing with low loading rate, fracture is prone to occur at the solder/IMC interface by a brittle fracture mode. While for joints under low-density current stressing with high loading rate, fracture tends to happen in the solder matrix by a ductile fracture mode.

The properties and microstructure of Sn–0.3Ag–0.7Cu solder bearing trace amount of rare earth Nd were investigated in this paper. It was found that adding appropriate Nd can improve the properties of solder, the wettability of Sn–0.3Ag–0.7Cu–xNd solder was greatly improved and the mechanical properties were significantly enhanced when the addition of Nd reached 0.1 wt%. It was also found that the microstructure of solder was refined with the increasing content of Nd up to 0.1 wt%. In addition, when the Nd content exceeded 0.25 % a Sn–Nd phase appeared in the solder matrix. Comparing all the results, the comprehensive properties of Sn–0.3Ag–0.7Cu–0.1Nd solder were close to that of the traditional Sn–3.8Ag–0.7Cu solder, even though the content of Ag has been significantly reduced from 3.8 to 0.3 wt%, which had shown a great application prospect to satisfy the increasing requirements of low cost and high performance in the electronics industry.

A composite solder was prepared by adding different amount Ni modified multi-walled carbon nanotubes (MWCNTs) into the Sn-3.0Ag-0.5Cu (SAC305) solder. Experiments were conducted to investigate the thermal behavior, mechanical properties of composite solder alloys, shear fracture behavior of solder joints were evaluated as well. The modification process was carried out on the MWCNTs using electroless modification method to synthesis the Ni modified carbon nanotubes (Ni-CNTs), which was manually mixed into SAC305 solder powder with different amount. The melting point of the composite solder alloys was slightly increased with the increment of the amount of Ni-CNTs. The nanoindentation test results confirmed that the hardness and modulus of composite solder alloys were enhanced after the addition of Ni-CNTs. The aging experiment was implemented for the joints and the results indicated that the Ni-CNTs could effectively inhibit the growth of IMC layer as an additive in SAC305 solder. The shear strength of SAC305-xNi-CNTs/Cu (x = 0, 0.1 and 0.2 wt.%) joints was reinforced with the increment of Ni-CNTs amount. Furthermore, the SAC305-0Ni-CNTs/Cu and SAC305-0.1Ni-CNTs/Cu solder joints were broken partially between the solder and IMC, mostly between the solder matrix, and SAC305-0.2Ni-CNTs/Cu solder joint was broken completely inside solder matrix. It was suggested that the fracture mode of the solder joints changed from mix fracture mode to ductile fracture mode as the amount of Ni-CNTs increased.

Due to the toxicity of Pb in the soldering alloys, new alloys (lead free) are suggested by scholars especially for high-temperature applications. One of the alloys recommended so far is Bi–Ag. This alloy has certain similar features to Pb–Sn alloys. Thus, the study focused on several candidate alloys (Bi–1.5–2.5–3.5 wt%Ag) as alternative solders. In this paper, the mechanical properties were studied using shear test. It can be found from the shear test results that increasing the amount of Ag content in the solder matrix positively impact the maximum shear stress and shear strain. The highest maximum shear stress and strain were observed at Bi–3.5 wt%Ag with the value of 13.355 MPa and 44.136 MPa, respectively. Furthermore, the lowest maximum shear stress and strain were observed at Bi–1.5 wt%Ag with the value of 11.032 MPa and 24.955 MPa, respectively. Hence, the fracture behavior shifted from brittle to ductile.

It is important to develop Pb-free solder alloys suitable for automotive use instead of traditional Sn–Pb solder due to environmental regulations (e.g., Restriction of Hazardous Substances (RoHS)). Al addition has been spotlighted to enhance solder properties. In this study, we investigated the microstructural change of Sn–0.5Cu wt.% based Pb-free solder alloys with Al addition (0.01–0.05 wt.%). The small amount of Al addition caused a remarkable microstructural change. The Al was favored to form Cu–Al intermetallic compounds inside the solder matrix. We identified the Cu–Al intermetallic compound as Cu 33 Al 17 , which has a rhombohedral structure, using EPMA and TEM analyses. This resulted in refined Cu 6 Sn 5 networks in the Sn–0.5Cu based solder alloy. In addition, we conducted thermal analysis to confirm its stability at a high temperature of approximately 230 °C, which is the necessary temperature range for automotive applications. The solidification results were substantiated thermodynamically using the Scheil solidification model. We can provide criteria for the minimum aluminum content to modify the microstructure of Pb-free solder alloys.

Purpose – The purpose of this paper is to study the intermetallic compound (IMC) thickness, composition and morphology in 100-μm pitch and 200-μm pitch Sn–Ag–Cu (SAC305) flip-chip assemblies after bump reflow and assembly reflow. In particular, emphasis is placed on the effect of solder joint size on the interfacial IMCs between metal pads and solder matrix. Design/methodology/approach – This work uses 100-μm pitch and 200-μm pitch silicon flip chips with nickel (Ni) pads and stand-off height of approximately 45 and 90 μm, respectively, assembled on substrates with copper (Cu) pads. The IMCs evolution in solder joints was investigated during reflow by using 100- and 200-μm pitch flip-chip assemblies. Findings – After bump reflow, the joints size controls the IMC composition and dominant IMC type as well as IMC thickness and also influences the dominant IMC morphology. After assembly reflow, the cross-reaction of the pad metallurgies promotes the dominant IMC transformation and shape coarsened on the Ni pad interface for smaller joints and promotes a great number of new dominate IMC growth on the Ni pad interface in larger joints. On the Cu pad interface, many small voids formed in the IMC in larger joints, but were not observed in smaller joints, combined with the drawing of the IMC growth process. Originality/value – With continued advances in microelectronics, it is anticipated that next-generation microelectronic assemblies will require a reduction of the flip-chip solder bump pitch to 100 μm or less from the current industrial practice of 130 to150 μm. This work shows that as the packaging size reduced with the solder joint interconnection, the solder size becomes an important factor in the intermetallic composition as well as morphology and thickness after reflow.

A study was performed on the precipitation behavior of Cu 6 Sn 5 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 Cu 6 Sn 5 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 Cu 6 Sn 5 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 Cu 6 Sn 5 will develop a prismatic morphology with the exposed surface indexed as (1 0 −1 0) along the [0 0 0 1] direction of the existing Cu 6 Sn 5 grains at the interface.

The electromigration behaviors of line-type Cu/Sn–Ag–Cu/Cu interconnects with and without Ni-Coated multi-walled Carbon Nanotubes addition were investigated in this work. After soldering, the (Cu,Ni) 6 Sn 5 intermetallic compounds formed at the solder/Cu interface. The electromigration analysis shows that the presence of Carbon Nanotubes can suppress the atomic diffusion in the solder induced by electromigration effectively. And finite element simulation indicates that the Carbon Nanotube networks can reduce the current density in the solder matrix, which results in the improvement of electromigration resistance of composite solders.

The morphology and growth mechanism of intermetallic compound (IMC) layer at Sn–3Ag–0.5Cu/Cu interface during isothermal aging, thermal cycling and thermal shock were investigated in this study by microstructural observations and phase analysis. The results showed that the IMC layer flattened with aging duration because the grooves in scallop-like IMC provide a more convenient access for Cu atoms to dissolve and react with solders and previous IMCs. When isothermal aging was subjected, the growth rate of Cu6Sn5 was lower than that of Cu3Sn. While for thermal cycling, it was Cu6Sn5 that contributed much to the growth of IMC layer. This had much to do with abundant supply of Sn and Cu atoms at the interface of solder/Cu6Sn5 caused by recrystallization of solder and compression stress during thermal cycling. Growth mechanisms of total IMC layer both during isothermal and non-isothermal aging are stated as empirical power-law relationship by using an equivalent aging time parameter. And the growth time exponent depended on the evolution of grain structures of solder matrix, thermal stress and propagation of cracks during non-isothermal conditions.

(Cu,Ni)6Sn5 is an important intermetallic compound (IMC) in lead-free Sn-Ag-Cu solder joints on Ni substrate. The formation, growth, and microstructural evolution of (Cu,Ni)6Sn5 are closely correlated with the concentrations of Cu and Ni in the solder. This study reports the interfacial behaviors of (Cu,Ni)6Sn5 IMC (Sn-31 at.%Cu-24 at.%Ni) with various Sn-Cu, Sn-Ni, and Sn-Cu-Ni solders at 250°C. The (Cu,Ni)6Sn5 substrate remained intact for Sn-0.7 wt.%Cu solder. When the Cu concentration was decreased to 0.3 wt.%, (Cu,Ni)6Sn5 significantly dissolved into the molten solder. Moreover, (Cu,Ni)6Sn5 dissolution and (Ni,Cu)3Sn4 formation occurred simultaneously for the Sn-0.1 wt.%Ni solder. In Sn-0.5 wt.%Cu-0.2 wt.%Ni solder, many tiny (Cu,Ni)6Sn5 particulates were formed and dispersed in the solder matrix, while in Sn-0.3 wt.%Cu-0.2 wt.%Ni a lot of (Ni,Cu)3Sn4 grains were produced. Based on the local equilibrium hypothesis, these results are further discussed based on the liquid–(Cu, Ni)6Sn5–(Ni,Cu)3Sn4 tie-triangle, and the liquid apex is suggested to be very close to Sn-0.4 wt.%Cu-0.2 wt.%Ni.

Sn–Ag–Cu lead-free solder have been shown to have inferior mechanical shock resistance to that of Pb–Sn alloy. Sn-rich solders containing rare earth elements have been shown to have superior mechanical properties when compared to conventional Sn–Ag–Cu solder, in terms of strain-to-failure. In this paper, we report on the mechanical shock behavior of Sn–3.9Ag–0.7Cu and Ce-containing Sn–3.9Ag–0.7Cu alloys over the strain rate range of 10 −3 –12 s −1 . Failure mechanisms of solders in different strain regimes are investigated based on the fractography analysis. It will be shown that the shock performance of Sn–3.9Ag–0.7Cu solders can be improved with addition of trace amount of Ce in the solder matrix controlled regime. The role of CeSn 3 intermetallics on the enhanced dynamic performance is discussed.

The migration of interphase interfaces in Sn-based solder joints was investigated by using the upper surface of unreacted Cu substrate (the original reaction interface) as a reference. Then a model was built to study the interdiffusion of Cu and Sn in Cu 6 Sn 5 and Cu 3 Sn layers. After being aged at 150 °C, both solder/Cu 6 Sn 5 and Cu 6 Sn 5 /Cu 3 Sn interfaces moved toward the solder matrix, and the Cu 3 Sn/Cu interface migrated toward the Cu substrate. Compared with Sn, Cu was found to be the faster diffuser in Cu 6 Sn 5 and Cu 3 Sn layers. The diffusion fluxes (DFs) of Cu and Sn in both intermetallic compound (IMC) layers decreased with the extension of aging period. The DF of Cu in Cu 3 Sn layer got close to that in Cu 6 Sn 5 layer. So did the DF of Sn. The diffusion of Cu and Sn can be suppressed by the addition of alloying elements (Ag 3.5 wt.% or Cu 0.7 wt.%), which can be indicated by various descending rates of the DFs of Cu and Sn in different joints.

This study investigated the interfacial reactions between Sn-4.0Ag-0.5Cu (wt.%) (SAC405) and Sn-4.0Ag-0.5Cu-0.05Ni-0.01Ge (wt.%) (SACNG) lead free solders with an Au/Ni(P)/Cu substrate reflowed using the CO2 laser and hot-air methods. A mixture of two intermetallic compounds, (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4 phases, was formed at the interface when the solder/substrate couples were reflowed using the CO2 laser. When the specimen was aged at 150°C, the thickness of the (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4 phases increased as the aging time increased. The (Cu, Ni)6Sn5, (Ni, Cu)3Sn4, Ni–Sn–P and Ni3P phases were formed at the interface when the specimens were reflowed using the hot-air method at 240°C and aged at 150°C for a long reaction period. Due to a rapid cooling-rate in the CO2 laser reflowing system, solidification phases with finer microstructure were found in the solder matrix.

It is known that rare-earth elements exhibit high chemical activity and adding trace amount of rare-earth Y in the solder can significantly improve the properties of solder alloy. However, adding excessive rare-earth Y in the solder will result in rapid growth of tin whiskers. The research results show that: if the YSn3 precipitates formed in the interior of Sn3.8Ag0.7Cu1.0Y were exposed to the air, YSn3 precipitates will oxidize and expand in volume, and the tremendous compressive stress produced by the surrounding solder matrix constraining the volume expansion will accelerate the growth of tin whiskers.

The effect of Ti on the solid state reactions between Sn and Ni has been investigated in this work. Based on the experimental results the following statements can be made: Firstly, the presence of Ti does not have measurable effects on the thickness evolution of Ni3Sn4 during solid state annealing. Secondly, the results from long term heat treatments show that there is no marked solubility of Ti to Ni3Sn4. Rather Ti reacts with Sn to form large Ti2Sn3 platelets inside the solder matrix. The Sn-rich part of the Ni–Sn–Ti phase diagram was assessed in order to rationalize the experimental results. By utilizing this information, the absence of any marked effects of Ti on the growth of Ni–Sn intermetallic compounds (IMC) was analysed. As there is no solubility of Ti to SnAg solder or to Ni–Sn IMC’s, Ti cannot change activities of components in the solder nor influence the stability of the IMC layers. Hence, these results throw significant doubts over the concept of trying to influence the Ni–Sn IMC layer thickness or quality by Ti alloying.

In this paper, varying weight percentages of Ni-coated carbon nanotubes (Ni-CNTs) was incorporated into Sn-Ag-Cu (SAC) solder matrix, to form composite solder. Up to 0.05% of Ni-CNTs were successfully incorporated. After that, the electrochemical behavior of Sn-Ag-Cu with varying weight percentages of Ni-coated carbon nanotubes (Ni-CNTs) composite solder were investigated in 0.3% Na2SO4 solution by using potentiodynamic polarization method. Scanning microscopy was used to characterize the samples after the electrochemical tests. It was observed that the corrosion resistance decreased when the content of Ni-CNTs reached a certain level.

The effect of Ti on the solid state reactions between Sn and Cu has been investigated in this work. Based on the experimental results the following statements about the effect of Ti can be made: Firstly, the presence of Ti does not have measurable effect on the thickness of either Cu6Sn5 or Cu3Sn during solid state annealing. However, the unevenness of both Cu6Sn5 and Cu3Sn layers is increased by the addition of Ti. Secondly, there is no marked solubility of Ti to either Cu6Sn5 or Cu3Sn. Rather Ti reacts with Sn to form large Ti2Sn3 platelets inside the solder matrix. These findings were subsequently rationalized with the help of the assessed Cu–Sn–Ti phase diagram. By utilizing this phase diagram information, the absence of any marked effects of Ti on the growth of Cu–Sn intermetallic compound (IMC) formation was rationalized. As there is a very low solubility of Ti to SnAg solder and to Cu–Sn IMC’s, Ti cannot change activities of components in the solder nor influence the stability of the IMC layers. Hence, these results throw significant doubts over the concept of trying to influence the Cu–Sn IMC layer thickness or quality by Ti alloying.

Due to its high Young’s modulus and strong bond with Sn, Co has been considered as one of the most promising alloying elements to improve the strength and reliability performance of Pb-free solders. In this study, the microstructual characteristics and mechanical properties of Sn-3.0Ag-0.5Cu + XCo (X = 0 wt.%, 0.1 wt.%, 0.2 wt.%, 0.45 wt.%, and 1.0 wt.%) solder joints on Cu substrates were investigated. Moreover, an exploration on different joint fabrication methods and undercooling of solder alloys with both solder preform and paste was carried out. Experimental results indicated that the undercooling of the solder alloy can be altered by Co additions. During the metallurgical process, the added Co can dissolve into the solder matrix before rolling the preform. In addition, Co can form reinforcement particles in the solder paste. The tendency for the formation and growth of an intermetallic compound (IMC) layer can be enhanced after incorporation of Co elements into the preform. However, a proper amount of Co addition to the paste can suppress IMC growth. The shear strength of solder joints reinforced with 0.1 wt.% or 0.2 wt.% Co addition can be increased. The solder joint made by paste with 0.2 wt.% Co addition exhibited the highest shear strength among all the solder joints.

Microstructures and fatigue life of SnAgCu and SnAgCu bearing nano-Al particles in QFP (Quad flat package) devices were investigated, respectively. Results show that the addition of nano-Al particles into SnAgCu solder can refine the microstructures of matrix microstructure. Moreover, the nano-Al particles present in the solder matrix, act as obstacles which can create a back stress, resisting the motion of dislocations. In QFP device, it is found that the addition of nano-Al particles can increase the fatigue life by 32% compared with the SnAgCu solder joints during thermal cycling loading.

The growth mechanism of an interfacial (Cu,Ni)6Sn5 compound at the Sn(Cu) solder/Ni(P) interface under thermal aging has been studied in this work. The activation energy for the formation of the (Cu,Ni)6Sn5 compound for cases of Sn-3Cu/Ni(P), Sn-1.8Cu/Ni(P), and Sn-0.7Cu/Ni(P) was calculated to be 28.02 kJ/mol, 28.64 kJ/mol, and 29.97 kJ/mol, respectively. The obtained activation energy for the growth of the (Cu,Ni)6Sn5 compound layer was found to be close to the activation energy for Cu diffusion in Sn (33.02 kJ/mol). Therefore, the controlling step for formation of the ternary (Cu,Ni)6Sn5 layer could be Cu diffusion in the Sn(Cu) solder matrix.

Trace elements are increasingly being incorporated into lead-free solder compositions. This paper analyses the distribution of trace elements in solder joints when commercial purity Sn-based alloys are soldered onto Cu substrates. Analysis techniques include μ-XRF (X-ray fluorescence) mapping performed at the SPring-8 synchrotron radiation facility. The mapping results indicate that Ni is present in the Cu6Sn5 intermetallic reaction layer, and is distributed in a relatively homogeneous fashion as (Cu,Ni)6Sn5. In alloys containing trace levels of Ge (60 ppm), this element is comparatively concentrated within the oxide at the solder surface, and a lower concentration is distributed homogeneously in the solder matrix and the intermetallic reaction layer. In Sn–Pb alloys, the Pb was found to segregate to the boundaries between adjacent Cu6Sn5 grains.

It has previously been established that adding 0.2 wt.% Zn into a Sn–3Ag–0.5Cu–0.5Ce alloy improves the mechanical properties and eliminates the problem of rapid whisker growth. However, no detailed studies have been conducted on electromigration behavior of Sn–3Ag–0.5Cu–0.5Ce–0.2Zn alloy. The electromigration damage in solder joints of Sn–3Ag–0.5Cu and Sn–3Ag–0.5Cu–0.5Ce–0.2Zn with Ag/Cu pads and Au/Ni(P)/Cu pads was studied after current stressing at room temperature with an average current density of 3.1 × 10 4 A/cm 2 . With additions of 0.5 wt.% Ce and 0.2 wt.% Zn, the electromigration processes of Sn–Ag–Cu solder joints were accelerated due to refinement of the solder matrix when joint temperature was around 80 °C. Since Ni is more resistant than Cu to diffusion driven by electron flow, solder joints of both alloys (Sn–3Ag–0.5Cu and Sn–3Ag–0.5Cu–0.5Ce–0.2Zn) with Au/Ni(P)/Cu pads possess longer current-stressing lifetimes than those with Ag/Cu pads.

During the reflow process of In-3Ag solder ball grid array (BGA) packages with electroless nickel immersion gold (ENIG) and immersion silver (ImAg) surface finishes, continuous (Au0.9Ni0.1)In2 and scallop-shaped (Ag0.9Cu0.1)In2 intermetallic layers form at the interfaces of In-3Ag solder with Au/Ni/Cu and Ag/Cu pads, respectively. The (Au0.9Ni0.1)In2 layer breaks into clusters with increases in the aging time and temperature. Aging at 115 °C results in the formation of an additional continuous Ni10In27 layer on the Ni/Cu pads and the migration of (Au0.9Ni0.1)In2 intermetallic clusters into the solder matrix. In contrast, the (Ag0.9Cu0.1)In2 scallops grow into a continuous layer after aging treatment. Accompanying the interfacial reactions, AgIn2 precipitates in the interior of In-3Ag solder balls and coarsens during aging, causing the ball shear strengths of reflown ENIG (1.18 N) and ImAg (1.11 N)-surface-finished solder joints to decrease gradually. However, the migration of (Au0.9Ni0.1)In2 clusters into the solder matrix of ENIG-surface-finished In-3Ag packages leads to an increase in their ball shear strengths after aging at 115 °C over 300 h. Both the ENIG- and ImAg-surface-finished In-3Ag solder joints, after ball shear tests, have fractured across the solder balls with ductile characteristics.

The effect of electromigration (EM) on Sn(Cu)/Ni/Cu solder joint interfaces under current stressing of 104 A/cm2 at 160°C was studied. In the pure Sn/Ni/Cu case, the interfacial compound layer was mainly the Cu6Sn5 compound phase, which suffered serious EM-induced dissolution, eventually resulting in serious Cu-pad consumption. In the Sn-0.7Cu case, a (Cu,Ni)6Sn5 interfacial compound layer formed at the joint interface, which showed a strong resistance to EM-induced dissolution. Thus, there was no serious consumption of the Cu pad under current stressing. In the Sn-3.0Cu case, we believe that the␣massive Cu6Sn5 phase in the solder matrix eased possible EM-induced dissolution at the interfacial compound layer due to current stressing.

Intermetallic compounds formed in Sn‐51In solder ball grid array (BGA) packages with Ag/Cu pads are investigated. After reflow, the Ag thin film dissolves quickly into the solder matrix, and scallop‐shaped intermetallic layers with compositions of (Cu0.89Ag0.11)(In0.79Sn0.21)2/η‐Cu6(Sn0.54In0.46)5 appear at the interfaces between the Sn‐51In solder balls and Cu pads. Aging at 75°C and 100°C caused the η‐Cu6 (Sn0.54In0.46)5 intermetallics to replace the (Cu0.89Ag0.11)(In0.79Sn0.21)2 at the solder/pad interfaces. The thickness of η‐ intermetallic versus the square root of reaction time (t 1/2) shows a linear relation, indicating that the growth of intermetallics is diffusion‐controlled. Ball shear tests show that the strength of these Sn‐51In solder joints is initially 1.9 N, decreasing to 1.7 N and 1.6 N after aging at 75°C and 100°C, respectively.

A significant reduction of the undercooling of Sn-based solder alloys was previously reported when they were reacted with various under bump metallurgies (UBMs). In the present study, new experiments have been designed and carried out to understand the undercooling behavior of various Cu- and Ni-doped solders on Ni UBM. Two competing mechanisms were further investigated that include the formation of intermetallic compounds (IMCs) at solder/UBM interface and the change of solder composition because of the dissolution of Ni UBM into solder. Two types of IMCs, including both Ni3Sn4 and Cu6Sn5 that were formed at the interface, were correlated with the undercooling of Sn–0.2Cu and Sn–3.8Ag–0.2Cu solders. In addition, the compositional changes of various Sn-based solders after reactions with Ni UBM were analyzed. On the basis of the experimental results, it was found that the significant reduction in undercooling is primarily caused by dissolved Ni atoms from Ni UBM and the concurrent formation of Ni3Sn4 IMC in the solder matrix. Finally, the beneficial effect of Ni dissolution is thermodynamically favorable as confirmed by the thermodynamic calculations and differential scanning calorimetry measurements with various Ni-doped solder alloys.

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 (Cu0.98Ag0.02)6(In0.59Sn0.41)5, appear at the interfaces between Sn-20In-2.8Ag solder ball and Cu pad. No evident growth of the (Cu0.98Ag0.02)6(Sn0.59In0.41)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.

The electromigration behavior of the Cu/Au/SnAgCu/Cu combination was investigated under 103 A/cm2 of current stressing at ambient temperature. The Au layer, when it acts as a cathode, was consumed continuously, and no significant compound was found at the interface. Meanwhile, Cu6Sn5 was formed at the anodic Cu layer, and the thickness of the compound increased with increasing time. The Au atoms were found to be trapped in Cu6Sn5 within the solder matrix. The AuSn4 compound precipitated while attaching to Cu6Sn5 at the Cu6Sn5/solder interface. The thermomigration effect was found to be insignificant in this work as no obvious reaction occurred at the cathode/anode sides or in the solder matrix without current stressing.

Solder pots used in wave soldering are usually made using different kinds of steel. Dissolution and interfacial reactions of the Fe substrate in molten Sn-Pb and Sn-Cu solders are investigated in this study. FeSn2 phase is formed in the Sn-0.7wt%Cu/Fe couples reacted at 250, 400, and 500 °C, as well as in the Sn-37wt%Pb/Fe couples reacted at 250 and 400 °C. The activation energies of formation are 123 and 121 kJ/mol in the Sn-Cu/Fe and Sn-Pb/Fe couples, respectively. FeSn phase is the reaction product in the Sn-37wt%Pb/Fe couples reacted 500 °C. The dissolution rates of Fe in the Sn-0.7wt%Cu melt are much higher than those in-the Sn-37wt%Pb melt. The FeSn2 phase layer in the Sn-Cu/Fe couple is not as dense as that in the Sn-Pb/Fe couple and accounts for the very different dissolution rates. Detachment of the reaction FeSn2 phase into the solder matrix is observed in the Sn-Cu/Fe couples, and is a potential contaminant source in wave soldering.

The Sn-3.5Ag and Sn-3.5Ag-0.2Co-0.1Ni lead-free solders were investigated on common electronics substrates, namely, organic solderability preservative (OSP) and electroless Ni/immersion Au (ENIG) surface finishes. The formation of Kirkendall voids at the interfacial region during isothermal solid aging was explored. For the Sn-3.5Ag-0.2Co-0.1Ni/OSP solder joint, the Kirkendall voids were present after isothermal solid-state aging at higher temperature (e.g., 150°C); however, the size of voids did not change remarkably with prolonged aging time due to the depressed Cu 3 Sn layer growth. For ENIG surface finishes, the 0.2Co-0.1Ni additions seemed to enhance the longitudinal groove-shaped voids at the Ni 3 P layer; however, void formation at the solder/Ni 3 Sn 4 interface was effectively reduced. This might be attributed to the reduced Sn activity in the solder matrix and the suppressed Ni-P-Sn layer formation.

The reflow of Sn-58Bi solder joints in a BGA package with Au/Ni/Cu pads has been performed by employing various temperature profiles, which results in the formation of (Au0.66Ni0.34)(Sn0.82Bi0.18)4 intermetallic flakes in the solder matrix. The reflow operation performed at a peak temperature of 180 °C for a melting time of 80 s gives a ball shear strength of 9.1 N, which decreases drastically to lower values between 6.4 and 4.6 N after further aging at temperatures from 75 to 125 °C. Double layers of intermetallic compounds with the compositions of (Au0.30Ni0.70)(Sn0.90Bi0.10)4/Ni3Sn4 can be found at the solder/pad interfaces of the aged Sn-58Bi solder joints. Ball shear testing of the reflowed specimens shows ductile fracture through the solder matrix, which changes to brittle cleavage fracture mainly along the (Au0.30Ni0.70)(Sn0.90Bi0.10)4 intermetallic layer after aging at various temperatures. The measurement of ball shear strengths (S) reveals a linear relation with the thicknesses (X) of (Au0.30Ni0.70)(Sn0.90Bi0.10)4 intermetallic layers: S = 7.13 − 0.33X.

In spite of the many beneficial effects obtained from the addition of rare earth elements to solder alloys, rapid growth of tin-whiskers has been found in Sn–3Ag–0.5Cu–1.0Ce solder joints. The morphology of the whiskers changes from fiber-shaped to hillock-shaped when the storage temperature increases from 25 to 150 °C. The driving force for the whisker growth is the compressive stress resulting from the volume expansion of the oxidized CeSn3 phase, which is constrained by the surrounding solder matrix.

We chose Sn−2.8Ag−20In and Sn−10Bi−10In (numbers are in weight percentages unless specified otherwise) as Pb-free solder materials for intermediate-step soldering. We then investigated how the two solders reacted with the under bump metallurgy (UBM) of Au/Ni (Au: 1.5 μm and Ni: 3 μm) at 210°C, 220°C, 230°C, and 240°C for up to 4 min. All, of the Au UBM was dissolved into the solder matrix as soon as the interfacial reaction started. The reaction formed Au(In, Sn)2 in the case of SnAgIn, and it formed Au(Sn, In)4 and Au(In, Sn)2 in the case of SnBiIn. The formation mechanism of the intermetallic phases is explained thermodynamically. The exposed Ni layer reacted with the solder and formed Ni28Sn55In17 in case of SnAgIn, and formed Ni3(Sn, In)4 in case of SnBiIn, at the solder joint interface. Under the same soldering conditions, the Ni3(Sn,In)4 layer in the SnBiIn/UBM is thicker than the Ni28Sn55In17 layer in the SnAgIn/UBM. Because of the thicker intermetallic compound layer, the SnBiIn solder joint has weaker shear strength than the SnAgIn solder joint.

Composite approaches have been developed in lead-free solder research in an effort to improve the service temperature capabilities and thermal stability of the solder joints. This article overviews the background for composite lead-free solder development, the roles of reinforcements and their requirements, composite solder processing techniques, as well as current status of composite solder research. Examples of several representative lead-free composite solders produced with various methods and reinforcement types are presented. The effects of reinforcement addition on processing and mechanical properties are analyzed. Difficulties and problems that exist in composite solder research are proposed and tentative solutions are attempted with examples of newly emerging novel lead-free composite solders.

The interfacial morphologies for the Sn–3Ag–0.5Cu/Ni–P joint were investigated using a field emission electron probe microanalyzer (FE-EPMA). As the Ni–Sn–P formed, several layers of P-rich layers including Ni 3 P, Ni 12 P 5 , and Ni 2 P were observed. The relationship between Ni–Sn–P formation and the evolution of P-rich layers was quantitatively analysed by FE-EPMA and by using the phase diagram of binary Ni–P. It was also proposed that once the Ni 2 P phase was formed, Sn atoms diffused from the solder matrix into Ni 2 P. As a result, the original Ni 2 P transformed to Ni 2 SnP due to the in-diffusion of Sn.