Search

Pulsed gas metal arc welding (GMAW) is widely applied in industrial manufacturing. The use of pulsed GMAW was found superior to the traditional direct-current (DC) welding method with respect to spatter, welding performance, and adaptability of all-position welding. These features are closely related to the special pulsed projected metal transfer process. In this paper, a monitoring system based on a high-speed camera and laser backlight is proposed. High-quality images with clear droplets and a translucent arc can be obtained at the same time. Furthermore, a novel image-processing algorithm is proposed in this paper, which was successfully applied to remove the interference of the arc. As a result, the edge and region of droplets were precisely extracted, which is not possible using only the threshold method. Based on the algorithm, centroid coordinates of undetached and detached droplets can be calculated, and more parameters of the kinematic characteristics of droplets can be derived, such as velocity, acceleration, external force, and momentum. The proposed monitoring system and image-processing algorithm give a simple and feasible way to investigate kinematic characteristics, which can provide a new method for possible applications in studying mathematic descriptions of droplet flight trajectory and developing a precise automatic welding system.

The effects of trace mount of rare earth elements Pr and Nd addition on Sn–0.3Ag–0.7Cu–0.5Ga lead free solder were investigated by observation of microstructure and the morphology of interface layer, as well as the test of shearing strength of the solder joints. The results show that the microstructure of the solder matrix can be optimized by appropriate addition of Pr and Nd, but the scalloped and “region-like” compounds (IMCs) appear in the solder matrix with excessive Pr and Nd addition respectively which may result in the stress concentration and become the birthplace of the cracks. The behavior of absorption of rare earth elements can retard the interface reaction between solder and Cu substrate, refine the morphology of the interface layer. The shearing strength of the solder joints can be enhanced better with appropriate addition of Pr than Nd by improvement of nucleation rate and control of growth rate of the grains near the interface layer.

Ag is used as a beneficial alloy element in no matter solders or brazing filler metals. Obviously, the addition of Ag has a positive function on melting temperature, wettability, mechanical property and conductivity of filler metals. Therefore, Ag is still widely used in many researches and production in spite of that Ag is very expensive. Respectively, three kind of typical solders (Sn–Ag–Cu, Sn–Zn and Sn–Bi) and brazing filler metals (Ag–Cu–Zn, Cu–P, and Zn–Al) have been chosen for illustration. This article summarizes research status on the studying of Ag-contained solders and brazing filler metals, also analyses influence rules of Ag addition on the change of filler metals’ physical property, microstructure as well as mechanical property. Moreover, the problems and difficulties in the process of study Ag-contained solders and brazing filler metals have been presented. Synchronously, some suggestions have been put forward which may solve the problems and difficulties mentioned above, which provides theory guide for the follow-up study of Ag-contained solders and brazing filler metals, and their prospects are also looked ahead.

Currently, accompanying the development of microelectronic joining and packaging, the integrated level of integrated circuit among the electronic devices has been substantially improved, which means the number of the soldered joints on a device is becoming more and more while the size of the micro-joint turns to be smaller and smaller. Given that soldered joints do play an important role in electronic packaging, serving as both mechanical bridges and electrical interconnections between the components and the bonding pads, to investigate the sensitivity of reliability of lead-free solders to various severe conditions deserves considerable concerns. This paper introduces the recent reliability research of lead-free solders under several specific conditions (e.g., isothermal aging, corrosive environment, drop impact, radiation) comprehensively, producing a fundamental summarization for the further reliability research.

The reaction mechanism of the intermetallic compounds of SnxNdy and GaxNdy in the joint soldered with Sn–9Zn–1Ga–0.5Nd solder was investigated in order to evaluate the reliability of the soldered joint of Sn–9Zn–1Ga–0.5Nd. The results showed that the IMC of SnxNdy, which the compound may be induced the spontaneous growth of Sn whisker, could be substituted by GaxNdy, so that the risk of the growth of Sn whisker was eliminated. Thermodynamic calculation results showed the compounds of GaxNdy were much easier to form than those of SnxNdy which was adequately verified by SEM, EDS and XRD tests and analyses.

Cerium and titanium were added to an Al-42Zn-6.5Si brazing alloy, and the subsequent microstructures of the brazing alloy and the 6061 Al alloy brazing seam were investigated. The microstructures of filler metals and brazed joints were characterized by scanning electron microscopy and X-ray energy dispersion spectrometry. A new Ce-Ti phase formed around the silicon phase in the modified filler metal and this saturation phenomenon was analyzed. Interestingly, following brazing of the 6061 alloy, there is no evidence of the Ce-Ti phase in the brazing seam. Because of the mutual solubility of the brazing alloy and base metal, the quantity of the solvent increases, and the solute Ce and Ti atoms assume an undersaturated state.

Recently, the Sn–Zn family of alloys, which possesses many attractive advantages such as relatively low melting point, cheap cost and the environmentally friendly component of Zn, has been widely used in electronic industry as one of the most potential replacements for the traditional Sn–Pb solders. However, there’re still some arguments on its shortcomings about the poor wettability and the weak oxidation resistance, which definitely limits its further application in lead-free electronic manufacturing. In order to overcome these disadvantages and further enhance the properties of Sn–Zn lead-free solders, alloying elements such as RE, Bi, Ag, Al, Ga, Cu, etc. were selected by lots of researchers as alloys addition into the solders. This paper summarizes the effects of alloying elements on the wettability, oxidation resistance, mechanical properties and microstructures of Sn–Zn lead-free solder alloys.

Brazing 6061 Al alloy to 304 stainless steel by flame brazing has been carried out with an improved CsF–RbF–AlF 3 flux which matched Zn– x Al filler metals. The results showed that, the spreading area on stainless steel of Zn– x Al filler metals has been improved with the addition of RbF to CsF–AlF 3 flux. It is found that a Zn-rich phase appeared between the brazing seam and the intermetallic compound (IMC) layer in the joints brazed with Zn–2Al and Zn–5Al filler metals, and the thickness of the IMC layer was approximately 1.76–6.45 μm which increased with the increase of Al added to the filler metals. Moreover, a Fe 4 Al 13 phase formed in the IMC layer, while a Fe 2 Al 5 phase appeared as the second layer in Zn–25Al brazed joint. Neither the Zn-rich phase nor Fe 2 Al 5 phase was found in the joint brazed with Zn–15Al filler metal, so that the joint was exhibited the maximum shear strength which was up to 131 MPa. All the lap joints were fractured at the interfacial layer of the brazing seam and stainless steel.

This work was focused on the influence of Ga on the thermal properties, microstructural evolution and interfacial morphology with aging treatment at 150 °C of low-silver Sn–0.5Ag–0.7Cu (SAC) lead-free solder. The melting temperature of the SAC–Ga solder was decreased owing to the low melting point element Ga and the formations of intermetallic compound (IMC) and growth at the interfaces of SAC/Cu and SAC–0.5Ga/Cu were studied for different aging time ranging from 0 to 720 h. The results indicated that for both solders, the thickness of the IMC increased with aging time prolonging. However, compared with the interface of SAC/Cu, the thickness of the interface of SAC–Ga/Cu was obviously suppressed and the growing speed was slowed down, which may be attributed to the decreased activity of copper atoms by Ga addition, so that the SAC–Ga solder showed relatively planar-like IMC instead of scallop-like at the interface.

The interfacial microstructures and mechanical properties of Sn–9Zn–0.5Ga– x Nd on Cu substrate with aging treatment were investigated. Unlike the previous results, no Sn–RE phase was formed near the interface of Sn–9Zn–0.5Ga– x Nd soldered joint after aging treatment at 150 °C for 1200 h. The combined action of Ga and Nd inhibited the formation of Sn–RE phase and resulted in an enhanced reliability of the soldered joint. The shear force of Sn–Zn–0.5Ga–0.08Nd soldered joint after aging treatment for 1200 h was twice the amount of Sn–9Zn joint and approaching the origin as-soldered joint of Sn–9Zn. Further, the fracture type of soldered joint still performed ductile after aging treatment for 1200 h. Synthesized the results of interfacial microstructures, mechanical test and fracture morphologies, Sn–9Zn–0.5Ga–0.08Nd solder has shown great potential to satisfy the increasing reliability requirements in electronics industry.

Growth kinetics of intermetallic compound layers (IMCs) formed between SnZnGa/SnZnGaNd solders for re-flow soldering and Cu substrate during aging was investigated at temperatures between 100 and 150 °C. The Cu5Zn8 IMCs could be found by reacting SnZnGa/SnZnGaNd solders on Cu substrate, and it was found that the addition of rare earth Nd can decrease the thickness of the IMCs of SnZn/Cu solder joints. The apparent activation energies of Cu5Zn8 which were calculated as 48.76 kJ/mol (SnZnGa) and 56.99 kJ/mol (SnZnGaNd). The shear force of Sn9Zn0.5Ga0.08Nd solder joint after aging treatment was remarkably higher than those of Sn–9Zn and Sn9Zn0.5Ga solder joints. The results would provide support to the superiority of Sn9ZnGaNd solder which can be used in the electronic packaging instead of traditional SnPb solder.

The mechanism of reaction between Nd and Ga in Sn-Zn-0.5Ga-xNd solder was investigated in order to enhance the reliability of soldered joints. It was found that, after aging treatment at ambient temperature and 125°C for over 3000 h, no Sn whisker growth was observed in Sn-9Zn-0.5Ga-0.08Nd soldered joints. X-ray diffraction (XRD) analysis and thermodynamic calculations indicated that Ga reacted with Nd instead of Sn-Nd intermetallic compound (IMC), eliminating Sn whisker growth. Shear force testing was carried out, and the results indicated that Sn-9Zn-0.5Ga-0.08Nd solder still had excellent mechanical properties after aging treatment. This new discovery can provide a novel approach to develop high-reliability solder without risk of Sn whiskers.

The effects of rare element Ga on solderability, microstructure, and mechanical properties of Sn–0.5Ag–0.7Cu lead-free solder were investigated. The experimental results show that Ga plays a positive role in improving the wettability and the microstructure of the solder. When the content of Ga is at 0.5 wt%, the grain size of the solder is smaller and the shear force is enhanced greatly. It is also found that the thickness of the IMCs at the solder/Cu interface is reduced with proper addition of Ga. The increase of mechanical properties may be related to the refining of IMCs of the solder due to Ga addition.

The wettability and interfacial whiskers of Sn–9Zn–0.5Ga–0.08Nd solder were investigated. The results indicated that Sn–9Zn–0.5Nd–0.08Nd solder has shown good wettability using water soluble flux and self made flux based on wetting balance testing, and solder with SnBi coating has shown superiority wettability among all three coatings. Moreover, no whisker was observed in Sn–9Zn–0.5Ga–0.08Nd solder with different coatings. But in Sn–9Zn–0.5Ga–1Nd solder, fewer whiskers were observed in solder with SnBi coating compared with Sn and Au/Ni coatings, which can be attributed to the refinement effect of Bi.

The effect of Nd and Ga additions on Sn whisker growth inhibition in Sn–9Zn solder is newly reported in this work. After ageing treatment at room temperature and 125 °C for over 4 months, no Sn whisker growth was observed in the joints soldered with Sn–9Zn–0.5 Ga–0.08Nd and Sn–9Zn–0.5 Ga–1Nd solders. XRD analysis shows that the formation of GaNd and GaNd3 compounds instead of SnNd compound is the key factor. Because the Gibbs formation free energies of GaNd and GaNd3 are more negative than that of SnNd, the GaNd and GaNd3 compounds are relatively more stable, no free Sn atoms released during exposure and oxidizing reaction to feed the Sn whisker growth. EDX analysis was also identified that the formation of Ga–Nd compounds substituted for the Sn–Nd IMC is the main factor that inhibited the spontaneous growth of Sn whisker.

With the development of microelectronic packaging and increasingly specific service environment of solder joints, much stricter requirements have been placed on the properties of lead-free solders. On account of small size effect and high surface energy, nanoparticles have been widely used to improve the microstructure and properties of lead-free solders. Therefore, the composite solders bearing nanoparticles have recently attracted wide attention. This article reviewed the recent research on SnAgCu, SnBi, and SnZn composite solder alloys and introduced the effect of nanoparticles on their microstructure, mechanical properties, wettability, and reliability. The mechanism of nanoparticles strengthening was analyzed and summarized. In addition, the shortcomings and future development trends of nanoparticle-reinforced lead-free solders were discussed, which is expected to provide some theoretical reference for the application of these composite solder in 3D IC package.

PurposeThe purpose of this paper is to investigate the growth behavior and mechanism of Sn whisker induced by RE addition in Sn‐Zn‐Ga‐Pr solder at ambient condition.Design/methodology/approachBy means of aging treatment, FIB and SEM microstructure analysis, the whisker growth behavior was investigated.FindingsIt was found that the morphologies of tin whisker are changed during air exposure. After 60 days aging, the average length of the longest whiskers could reach up to 70 μm, some whiskers even can grow to a length of 100 μm. It was discussed that the oxidation of Pr‐Sn intermetallics provides driving force for whisker growth.Originality/valueTin whisker growth is a complex reliability issue for lead‐free solder. The current research can be helpful in re‐understanding the issue of tin whisker growth as well as an enriched understanding on the effects of REs on lead‐free solders.

The addition of rare earth (RE) into Pb-free solders has been shown to induce spontaneous Sn whisker growth on RE-rich intermetallics (IMCs). In this research, whisker growth behaviors under ambient and thermal cycling conditions were investigated to address the RE-induced Sn whisker growth mechanism. Unlike the needle-like whiskers grown mainly in ambience, it was found that coarse Sn hillocks were the dominant growth morphology under thermal cycling condition. It was discussed that the CTE mismatch between RE-rich PrSn 3 IMCs and surrounding solder matrix induces compressive stress within the IMCs during thermal cycling, which contributes to the driving force for hillock growth.

Al-6.5Si-42Zn and Al-6.5Si-42Zn-0.09Sr filler metals were used for brazing 6061 aluminum alloy. Air cooling and water cooling were applied after brazing. Si phase morphologies in the brazing alloy and the brazed joints were investigated. It was found that zinc in the Al-Si filler metals could reduce the formation of eutectic Al-Si phase and lower the brazing temperature at about 520°C. Adding 0.09wt% Sr element into the Al-6.5Si-42Zn alloy caused α-Al phase refinement and transformed acicular Si phase into the finely fiber-like. After water cooling, Zn element dissolved into the Al-Si eutectic area, and η-Zn phase disappeared in the brazed joints. Tensile strength testing results showed that the Sr-modified filler metal could enhance the strength of the brazed joints by 13% than Al-12Si, while water-cooling further improved the strength at 144 MPa.

To develop new aluminum filler metals used for brazing 6061 alloy, the microstructure and properties of Al–6.5Si–42Zn– x Sr alloys with different Sr content were studied. The results showed that the melting point of Al–6.5Si–42Zn alloy is around 520 °C, and Sr addition did not change the melting point of the filler metals, yet trace amounts of them greatly improved the microstructure of the alloys. As Sr content increased, the number of primary Si particles decreased and the eutectic Si changed from acicular to fine fibrous morphology in the Al–6.5Si–42Zn– x Sr alloys. Meanwhile, Sr also refined the β-Fe intermetallic compound in the alloy. The wettability of the Al–6.5Si–42Zn– x Sr brazing alloys improved with increasing Sr content, in addition, it was found that Sr addition also modified the microstructure of the brazing joints. From the tensile test results, the strength of the lap joints and the butt joints could be increased by 14% and 11%.

PurposeThe purpose of this paper is to improve the properties of Sn−9Zn solder, so as to meet the requirements of industrial applications.Design/methodology/approachThe effects of Praseodymium on property and Sn whisker growth under aging treatment in Sn−9Zn lead‐free solder were investigated.FindingsThe results indicate that with the addition of rare earth Pr, the wettability and mechanical properties of Sn−9Zn solder were improved. The best wettability and comprehensive property of soldered joint is obtained when the content of Pr is 0.08 wt.%. After aging treatment at 150°C for 360 h, the mechanical properties of Sn−9Zn−0.08Pr decreased but are still obviously higher than that of Sn−9Zn. Moreover, when the content of Pr reached 0.1 wt.%, plenty of Sn−Pr compounds were found in the Sn−9Zn solder. The inevitable oxidation of Sn−Pr compounds would cause a high stress accumulated within PrSn3 phases, which would be served as driving force to induce the Sn whisker sprout and growth after aging treatment at 150°C for 120 h to 360 h. Compared with the results in Sn−9Zn−0.5Ga−0.7Pr solder that Sn whisker observed until the addition of Pr reached 0.7 wt.%, it could be inferred that the addition of Ga may react against the sprout of Sn whisker.Originality/valueIt is found that the addition of Pr can improve the properties of solder and avoid Sn whisker growth in the right range and proper conditions. The cost of the solder with added Pr is limited to RMB 2 yuan/kg so it can be widely used in industry.

In spite of previous reports documented that many beneficial effects can be obtained by adding rare earth (RE) elements to Pb-free solders, this paper presents the risk of Sn whisker growth in the Sn–9Zn–0.5Ga Pb-free solders due to the addition of RE Pr. Results showed that solder microstructures are refined with the addition of trace amount of Pr. However, excessive Pr addition led to the formation of Pr–Sn intermetallic compounds (IMCs) and spontaneous growth of Sn whiskers on the IMC surfaces. It was found that the IMC size has a dramatic impact on whisker growth. Sn whiskers grew in slow-cooled solder with larger IMC particles are much longer and more prolific than that in fast-cooled solder with smaller IMC size. It was proposed that the driving force for whisker growth is originated from the oxidation of the RE-rich Pr–Sn IMCs. Our results indicated that the effects of RE on Pb-free solders should be reevaluated.

In this article, the microstructures and the properties of Sn–9Zn/Cu and Sn–9Zn–0.06Nd/Cu soldered joints with aging at a constant temperature 150 °C for different hours were studied. It was found that the microstructure of Sn–9Zn–0.06Nd solder aged for 240 h remarkably changed and formed a combination net of Sn grains due to diffusion and aggregation of rare earth element Nd. Moreover, the interfacial reaction of solder/Cu was systematically investigated. Cu 5 Zn 8 intermetallic compound (IMC) can be found at the soldered joints interface, and its thickness increased with the increased aging time. Moreover, the thickness of intermetallic compound layer (IML) can be repressed with the addition of 0.06% Nd compared to that of Sn–9Zn. For the tensile property, the pull strength of soldered joints gradually decreased as the aging time prolonged and even decreased by 55.1% and 44.9% respectively after aging for 720 h. Furthermore, the fracture of Sn–9Zn–0.06Nd soldered joints indicated ductile fracture before aging treatment, while after aging for 720 h it presented both ductile and brittle fracture.

The mechanical properties and microstructural distribution of the Cu/Al brazing joints formed by torch-brazing with different Zn–Al filler metals were investigated. The microstructure of the Zn–Al alloys was studied by optical microscopy and scanning electron microscopy, and the phase constitution of the Cu/Al joints was analyzed by energy dispersion spectrometry. The results show that the spreading area of the Zn–Al filler metals on the Cu and Al substrates increases as the Al content increases. The mechanical results indicate that the shear strength reaches a peak value of 88 MPa when Al and Cu are brazed with Zn–15Al filler metal. Microhardness levels from HV122 to HV515 were produced in the three brazing seam regions corresponding to various microstructure features. The Zn- and Al-rich phases exist in the middle brazing seam regions. However, two interface layers, CuZn 3 and Al 2 Cu are formed on the Cu side when the Al content in the filler metals is 2% and more than 15%, respectively. The relationship between intermetallic compounds on Cu side and Zn– x Al filler metals was investigated.

Ternary Al­Si­Zn filler metals were designed in order to join the 6061 aluminum alloy. The microstructure, phase constitution and fracture morphology of the brazed joint were investigated. Results of the microstructure observation showed that eutectoid i(Al) + ©(Zn), i(Al) solid solution, as well as Si particles formed in the filler metal. The melting points of these filler metals are much lower than Al­12Si alloy. The ©(Zn), i(Al) solid solution, and the primary Si particles were found in the 6061 Al brazing seam when using Al­0.2Si­78Zn and Al­2Si­62Zn filler metal, while the Al­Si eutectic were found when using Al­6.5Si­42Zn alloy. Results also indicate that the tensile strength of the 6061 Al brazed joints using Al­0.2Si­78Zn, Al­2Si­62Zn and Al­6.5Si­42Zn is 101, 109, 129MPa, respectively. The fracture morphology of the brazed joints showed intergranular fracture mode while some transgranular fracture could be found in the joint of Al­6.5Si­42Zn filler metal. [doi:10.2320/matertrans.M2012110]

Using Zn—Al filler metal with Al content of 2%–22% (mass fraction) and improved CsF–AlF 3 flux, wetting properties of Zn–Al filler metal on 3003 Al substrate were investigated. The mechanical property as well as the microstructure of the brazed joints was also studied. The results indicate that excellent joints can be produced by means of torch brazing when the Al content is less than 8%. The metallographic structure in the brazing seam is mainly composed of Al based solid solutions and Zn based solid solutions. The high hardness value of brazing seam of the 3003 aluminum alloy is higher than that of the base metal due to the effect of solid solution strengthening. The results also show that three microstructure zones could be found at the brazing interface; i.e., base metal, diffusing zone and interface zone. The distribution of the solid solution in the brazing seam is the main factor of the tensile strength rather than the diffusion zone width near the interface.

The effects of Nd on wettability, microstructure and mechanical properties of Sn–9Zn–Ga–xNd lead-free solder were investigated. The results indicate that adding moderate amount of rare earth Nd, the wettability as well as mechanical properties of Sn–9Zn–0.5Ga solder were evidently improved, and when the content of Nd is at 0.08 wt%, the best wettability and comprehensive properties of soldered joint were obtained. It was also found that the addition of rare earth Nd could refine the microstructure of the solder, but some dark NdSn3 phase appeared when the addition of Nd exceeded 0.15 wt%. Moreover, the IMCs thickness at the solder/Cu interface was reduced with the addition of Nd which gave a favorable influence on the mechanical property of the soldered joints.

The creep-rupture lives of Sn3.8Ag0.7Cu and Sn3.8Ag0.7Cu0.03Ce lead-free solder joints for electronic packaging were investigated, respectively. And the relationship between creep behavior and intermetallic compound (IMC: Ag3Sn, Cu6Sn5, CeSn3) particles in SnAgCu/SnAgCuCe solder joints has been obtained. Meanwhile, rare earth Ce concentration gradient and retardation effect of Ce on the IMC layer have been observed at the solder/Cu interface. Moreover, aging reaction of Sn and Cu, and the effect mechanism of rare earth Ce on two IMCs (Cu6Sn5 and Cu3Sn) are reported.

PurposeThe purpose of this paper is to investigate the laser soldering of fine pitch quad flat package (QFP) devices using lead‐free solders and solder joint reliability during thermal cycling.Design/methodology/approachQFP devices were selected as the test vehicles and were soldered with four alloy types, Sn37Pb, Sn3.5Ag, Sn3.8Ag0.7Cu and Sn3.8Ag0.7Cu0.03Ce. The experimental samples were QFP‐256 devices with lead‐free solder paste on the printed circuit boards. The packages were dried for 24 h at 125°C prior to reflow soldering. Soldering experiments on the QFP devices were carried out with an infrared (IR) reflow soldering oven and a diode laser (DL) soldering system. Reflow soldering was performed at peak temperatures of 210°C (SnPb), 240°C (SnAgCu and SnAgCuCe) and 250°C (SnAg), as determined on the boards. Pull testing was adopted to evaluate the tensile strength of the four solders using an STR–1000 micro‐joint strength tester.FindingsThe tensile force of the QFP micro‐joints increased as laser intensity increased when it was less than an “optimal” value. The maximum tensile force of the QFP micro‐joints was gained when the laser intensity had increased to 2,165, 2,127, 2,165 and 2,064 W/cm2, depending on the alloy used. The thermal fatigue performance of three lead‐free solder joints, SnAgCuCe, SnAgCu and SnAg, was determined to be superior to that of the eutectic SnPb alloy. After soldering without thermal cycling tests, the fracture morphology of soldered joints exhibited characteristic toughness fracture with both of the soldering methods. After 700 thermal cycles, the fracture mechanism was also toughness fracture, nevertheless, the dimples became large. The fracture morphology of the soldered joints subjected to 1,500 thermal cycles indicated brittle intergranular fracture on the fracture surface and no intense plastic deformation appeared before fracture with IR soldering. For DL soldering, the pull fracture model of the SnAgCuCe was completely ductile in the soldered joint with 1,500 thermal cycles.Originality/valueThe paper usefully investigates the influence of laser intensity on the tensile strength of different soldered joints and the solder joint reliability during thermal cycling.

Interfacial microstructure and properties of Sn–0.7Cu–0.05Ni/Cu solder joints with trace amounts Nd additions have been investigated in this paper. The evolution of interfacial morphology of SCN solder joints with and without the presence of Nd under long-term room ambience was also studied. The greatest improvement to the solderability and tensile strength of SCN- x Nd are obtained at 0.05 wt.% Nd. Meanwhile, the morphology and growth of interfacial intermetallic (IMC) layer are greatly improved by adding Nd, and the propensity for IMC spalling from the interface of solder joint is definitely reduced with the presence of rare earth Nd, since the sponge-like structure was completely inhibited in the solder joint containing Nd. In addition, a significant amount of Sn whiskers were present on the surface of NdSn 3 phases in the SCN0.15Nd/Cu solder joints, and the reason for this phenomenon has been briefly discussed.

Nowadays, a major concern of Sn–Cu based solder alloy today is focused on continuously improving the comprehensive properties of the solder joints formed between the solders and substrates. The key issues and improvements about Sn–Cu–X (X = Ni, rare earths, Zn, Co, Ga, In, Bi, secondary particles etc.) solder are outlined and evaluated in this paper which compared to Sn–Cu solder. It can be summarized that by adding appropriate amounts of certain alloying elements X to Sn–Cu solder, and it is possible to tailor the properties of the solder, such as the melting and solidification behaviors, wettability, microstructure, interfacial reactions and mechanical properties of the solder. The reliability issues related to the implementation of Sn–Cu–X solder in advanced electronics system are also introduced, which indicates that further development on the Sn–Cu–X solders are to be underway.

PurposeThe purpose of this paper is to explore the formation and growth mechanism of bulk Cu6Sn5 intermetallic compounds, selecting Sn‐Ag‐Cu‐Ce solders as specimens.Design/methodology/approachIn order to further enhance the properties of SnAgCu solder, trace amount of rare earth Ce was selected as alloying addition into the alloy; in previous investigations, the enhancements include better wettability, physical properties, creep strength and tensile strength. In this paper, the microstructure of Sn‐Ag‐Cu‐Ce soldered joints and its interfacial intermetallic compounds were investigated. Moreover, different morphologies of Cu6Sn5 IMCs were enumerated and described, and Ostwald ripening theory was employed to interpret the formation mechanism of bulk Cu6Sn5 IMCs.FindingsIn addition, based on finite element simulation, it is found that the von Mises stress concentrate around the bulk Cu6Sn5 IMCs inside the Sn‐Ag‐Cu‐Ce soldered joints after three thermal cycling loading (−55‐125°C). From the stress distribution, the failure site was predicted to fracture near the bulk Cu6Sn5 IMCs interface. This coincides with the experimental findings significantly.Originality/valueThe results presented in this paper may provide a theory guide for developing novel lead‐free solders as well as reliability investigation of lead‐free soldered joints.

The spontaneous growth of Sn whisker in Sn–Zn series solder is newly reported in this work. It is found that during the exposure of Sn–9Zn–0.5Ga–0.7Pr bulk solder to ambient conditions for a few hours, many different lengths of needle-like Sn whiskers originate spontaneously from the Sn–Pr intermetallic compounds of the solder and grow rapidly at a rate of about 3.5 A/s. It is proposed that the driving force for whisker formation is the compressive stress resulting from the oxidation of Sn–Pr compounds, and that the free Sn atoms released from oxidation reaction feed the whisker growth during exposure.

Effects of trace amount of rare earth element Pr on properties and microstructure of Sn–0.7Cu–0.05Ni solder were investigated in this paper. The solderability of Sn–Cu–Ni–xPr alloy and shear strengh of Sn–Cu–Ni–xPr soldered micro-joints were determined by means of the wetting balance method and shear test, respectively. Moreover, microstructure of solder alloys bearing Pr, as well as intermetallic compound (IMC) layer formed at solder/Cu interface after soldering were observed. It was concluded that the major benefits of rare earth element Pr on Sn–Cu–Ni lead-free solder are: improving solderability, refining microstructure, and depressing IMC (IMC) growth, which exhibited improved mechanical properties. It also revealed that (Cu,Ni)6Sn5 is the majority IMC phase at the interface of Sn–Cu–Ni–xPr/Cu solder joints. Ni added into the solder effectively suppressed the growth of Cu3Sn and consequently also the total IMC layer thickness. Above all, the thickness and morphology of the interfacial (Cu,Ni)6Sn5 IMC were optimized due to alloying Pr. It can be inferred that Pr and Ni would play an important role in improving the reliability of Sn–Cu–Ni lead-free solder joints.

The reliability of Sn–Ag–Cu–Ce lead-free soldered joints in quad flat packages under thermal cycling was investigated based on finite element simulation and experiments. The stress and strain response of fine pitch QFP device lead-free soldered joints were analyzed using finite element method based on Garofalo–Arrhenius model. The simulated results indicate that the creep distribution is not uniform, the heel of joints is the maximum creep strain concentrated sites. And comparisons were then made with experimental results of the cracks observed in the Sn–Ag–Cu–Ce soldered joints subjected to the temperature cycled experiment. In addition, the relative mechanical and metallurgical factors, which dominate the failure of soldered joints, were utilized to analyze the phenomena. The fracture surfaces indicate that crack initiate and propagate along the interface among bulk Cu 6 Sn 5 phases in Sn–Ag–Cu–Ce soldered joints.

Recent years, the SnAgCu family of alloys has been found a widely application as a replacement for the conventional SnPb solders in electronic industry. In order to further enhance the properties of SnAgCu solder alloys, alloying elements such as rare earth, Bi, Sb, Fe, Co, Mn, Ti, In, Ni, Ge and nano-particles were selected by lots of researchers as alloys addition into these alloys. Rare earth (RE) elements have been called the ''vitamin'' of metals, which means that a small amount of RE elements can greatly enhance the properties of metals, such as microstructure refinement, alloying and purification of materials and metamorphosis of inclusions. In addition, a small amount of Zn addition has the ability to reduce undercooling efficiently and suppress the formation of massive primary Ag"3Sn plates, and Bi/Ga has the ability to enhance the wettability of SnAgCu alloys as well as Ni. Moreover, adding Co/Fe/Ge can effectively refine microstructure, modify interfacial Cu-Sn compounds and increase the shear strength of joints with Cu. This paper summarizes the effects of alloying elements on the wettability, mechanical properties, creep behavior and microstructures of SnAgCu lead-free solder alloys.

The influences of different Ag contents on the properties of Sn-9Zn lead-free solder were investigated. The results indicate that Ag plays an important role not only in the solderability and the structure of the solder, but also in the mechanical property of the soldered joint. In particular, adding 0.3% (mass fraction) Ag can improve the oxidation resistance of the solder, so the solderability of the solder is significantly improved. Sn-9Zn-0.3Ag shows finer and more uniform microstructure than Sn-9Zn. Results also indicate that when the content of Ag is 0.3%, the best mechanical property of the soldered joint can be obtained, and the fracture micrographs show that plenty of small and uniform dimples are found on the soldered joints fractures. Some Cu-Zn and Ag-Zn intermetallic compounds appear at the bottom of dimples when the content of Ag is up to 1%, and the mechanical property of the soldered joint is reduced.

Cosmic radiation has always been the most obvious barrier to planetary travels, especially in long-duration deep space exploration missions. Therefore, the reliability of satellite materials and the requirements of satellite miniaturization have received considerable attention. In this paper, the effect of &gamma, ray irradiation on the reliability of Sn50Pb49Sb1/Cu solder joints was investigated. It was found that the influence of &gamma, ray irradiation on the thickness and morphology of the intermetallic compound layer in Sn50Pb49Sb1/Cu was not obvious. However, the formation and growth of micro-voids and micro-cracks was observed in Pb-based solid solutions. Due to the Compton effect, the &gamma, ray photon could knock the electron out of its orbit, which created the energetic electron. The accumulation of dislocated atoms and lattice vacancies generated by energetic electrons could be the main factor that caused the formation of micro-voids and micro-cracks. The pull force of Sn50Pb49Sb1/Cu solder joints was reduced by 22% after being irradiated at the dose rate of 0.25 Gy(Si)/s for 960 h. Fractographic analysis showed that after irradiation, the fracture type of solder joints was still ductile but the ductility of the solder joints decreased with slightly inconspicuous dimples.

PurposeThe purpose of this paper is to numerically evaluate the reliability of SnAgCuCe solder joints compared with that of SnAgCu. A trace amount of the rare earth (RE) element Ce was added into SnAgCu solder in order to improve the reliability of lead‐free solder joints, which was evaluated based on finite element simulation and experiments.Design/methodology/approachA finite element method and an Anand constitutive model were employed to analyze the reliability of SnAgCuCe and SnAgCu solder joints in fine pitch quad flat packages under thermal cycling. The mechanical properties and reliability of solder joints were characterized by using thermal fatigue and creep tests, while the microstructure of the solder alloy and SnAgCu/SnAgCuCe solder joints were also investigated in the experimental procedure.FindingsThe simulation results indicated that SnAgCuCe solder joints had better reliability than SnAgCu. In addition, the experimental results accorded well with those of simulation, the thermal fatigue property and creep resistance of solder joints was increased by adding cerium. SnAgCuCe alloy can get its microstructure refinement improved and the thickness of the intermetallic compound layer at the solder/Cu interface decreased significantly compared to that of SnAgCu.Originality/valueThe findings provide certain guidelines to the reliability evaluation of solder joints when applying novel RE containing solder alloys in practical electronics industry applications. In the meantime, the reason for the superior reliability of SnAgCuCe solder joints can be explained from the property and microstructural point‐of‐view.

In the present work, effects of trace amount of Pr on the microstructure, wettability and mechanical properties of Sn-9Zn solder were studied. The range of Pr content in Sn-9Zn solder alloys varied from 0.01 to 0.25 wt%. Test results indicate that adding appropriate amount of Pr can evidently improve the wettability and mechanical properties of the Sn-9Zn solder alloy. The Sn-9Zn-0.08Pr solder shows the best comprehensive properties compared to other solders with different Pr content. At the same time, notable changes in microstructure were found compared with the Pr-free alloy. The needle-like Zn-rich phase in the Sn-9Zn solder was refined and became more uniform. Moreover, the thickness of the IMC layer at Sn-9Zn/Cu substrate was remarkably depressed with Pr addition.

Finite element method and Taguchi method were combined and related to evaluate the reliability of CSP soldered joints with various sets of parameters under temperature cycling. Garofalo–Arrheninus model was implemented to simulate the creep behaviour of soldered joints. It is found that the maximum creep strain is located at the upper surface of the soldered joint which is under the outermost of CSP chip. Subsequently, the L9(34) orthogonal array was applied for Taguchi experiment to investigate the effects of solder alloy, height of the soldered ball, chip thickness and substrate thickness on the reliability of soldered joints, results indicate that substrate thickness and height of soldered ball possess an important effect on the reliability. Furthermore, the above parameters were selected optimally towards the improvement of reliability, and result shows that the optimal parameter settings are that the solder is Sn3.8Ag0.7Cu, the ball height is 0.18 mm, the chip thickness is 0.1 mm and the substrate thickness is 0.42 mm, which could reduce the creep strain energy density by 78.4% compared with the original parameters.

Rapid and reliable generation of the landing footprint of an entry vehicle is the key capability required for on-board determination of landing options. Under the conventional formulation of the maximum crossrange problem at specified downranges, the footprint problem is difficult to numerically solve. When inequality trajectory constraints are imposed, accurate determination of footprints becomes even more challenging. Using the quasi-equilibrium glide condition (QEGC) in entry flight, a near-optimal, closed-form bank angle control law is first developed for the class of problems including maximum-crossrange problem. Then it is shown that the footprint problem is equivalent to seeking the solutions of a series of much simpler problems of closest approach to a moving virtual target. The closest-approach problem is a univariate root-finding problem using the near-optimal control law. Common inequality entry trajectory constraints are enforced without any additional difficulty with appropriate velocity-dependent bounds of the bank angle by using the QEGC. As a result, an accurate and fully constrained landing footprint from any feasible initial condition can be obtained rapidly and very reliably. The validity of the method is verified with independent trajectory optimization software. Footprints for an entry vehicle in several different mission scenarios and constraint settings are generated to demonstrate the method.

Purpose The purpose of this paper is to investigate the effects of minor addition of the rare earth (RE) element cerium, Ce, on the microstructures and creep properties of Sn-Ag-Cu solder alloys. Design/methodology/approach The pure Sn, Sn-Cu alloy, Sn-Ag alloy and Cu-Ce alloy were used as raw materials. Sn-Ag-Cu alloys with different contents of RE Ce were chosen to compare with Sn-Ag-Cu. The raw materials of Sn, Sn-Cu alloy, Sn-Ag alloy, Cu-Ce alloy were melted in a ceramic crucible, and were melted at 550°C ± 1°C for 40 minutes. To homogenize the solder alloy, mechanical stirring was performed every ten minutes using a glass rod. During the melting, KC1 + LiCI (1.3:1), were used over the surface of liquid solder to prevent oxidation. The melted solder was chill cast into a rod. Findings It is found that the microstructure exhibits smaller grains and the Ag3Sn/Cu6Sn5 intermetallic compound (IMC) phases are modified in matrix with the addition of Ce. In particular, the addition of 0.03 wt.% Ce to the Sn-Ag-Cu solder can refine the microstructures and decrease the thickness of the IMC layers of Sn-Ag-Cu solder alloys. Meanwhile, thermodynamic analysis showed that these phenomena could be attributed to the reduction of the driving force for Cu-Sn IMC formation due to the addition of Ce. Results calculated using the thermodynamic method are close to the above experimental data. Thus, the optimum content of Ce in Sn-Ag-Cu solder alloys should be about 0.030 percent. Additionally, the effect of Ce on the creep rupture life of Sn-Ag-Cu soldered joints was studied. It was found that the creep rupture life may be increased up to 7.5 times more than that of the original Sn-Ag-Cu alloy, when Ce accounts for 0.030 percent. Originality/value This paper usefully investigates the effects of the RE cerium (Ce), on the microstructures and creep properties of Sn-Ag-Cu solder alloys, optimizing the quantity of Ce in the Sn-Ag-Cu solder alloy through a thermodynamic method and by creep-rupture life testing.

The wetting properties and interfacial microstructures of Sn–9Zn–xGa lead-free solders with Cu substrate were investigated. The wetting property is improved remarkably with the increase of Ga content in the Sn–9Zn lead-free solder. The lower surface tension, which results from the decrease of the oxidation of the Zn atoms owing to the formation of the Ga-rich protective film covered on the liquid solder, is the key reason for the better wettability. During soldering, the Cu5Zn8 compounds layer form at the interface of Sn–9Zn/Cu and the IMCs formed at the solder/Cu surface become much thicker when the Ga content is from 0.1 wt.% to 3 wt.%. However, neither Cu–Sn compounds nor Ga-rich phases are observed at the solder/Cu surface.

The objective of this review is to study the interfacial intermatallic compounds (IMCs) between Sn–Ag–Cu based solders and common substrates, which play a crucial role in solder joints typically present in Pb-free electronics manufacturing. The microstructural evolution of IMCs at the solder/substrate interfaces is analyzed, while the models and theories describing the formation/growth mechanism of interfacial IMCs are also introduced. We focus on the influence of a variety of factors that have been reported recently, including substrates, minor alloying, mechanical stress, electromigration and thermomigration etc., as full understanding of the mechanisms that determine the formation and growth of interfacial IMCs is important to reach for developing high reliability solder joints. In the end of this review, the characteristics of the IMCs are compared and illustrated, which have marked effect on the mechanical properties and fracture behavior as well as reliability of solder joints.

Extensive testing was carried out to study the effects of rare earth Ce doping on the properties of SnAgCu solder alloys. The addition of 0.03% (mass fraction) rare earth Ce into SnAgCu solder may improve its mechanical properties, but slightly lower its melting temperature. The tensile creep behavior of bulk SnAgCuCe solders was reported and compared with SnAgCu solders. It is found that SnAgCuCe solders show higher creep resistance than SnAgCu alloys. Moreover, Dorn model and Garofalo model are successfully used to describe the creep behavior of SnAgCu and SnAgCuCe alloys. The parameters of the two creep constitutive equations for SnAgCu and SnAgCuCe solders are determined from separated constitutive relations and experimental results. Nonlinear least-squares fitting is selected to determine the model constants. The experimental data of the stress-creep strain rate curves are in good agreement with the theoretical ones.

An orthogonal method was used to evaluate the effects of Ga, Al, Ag, and Ce multi-additions on the wetting characteristics of Sn-9Zn lead-free solders by wetting balance method. The results show that the optimal loading of Ga, Al, Ag, and Ce was 0.2 wt.%, 0.002 wt.%, 0.25 wt.%, and 0.15 wt.%, respectively. Intermetallic compounds (IMCs) formed at the interface between Sn-9Zn-0.2Ga-0.002Al-0.25Ag-0.15Ce solder and Cu substrate were investigated by scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) analysis. The SEM images illustrate that the IMCs can be divided into two portions from the substrate side to the solder side: a planar Cu5Zn8 layer and an additional continuous scallop-like AgZn3 layer. The EDS analysis also shows that Ga segregates in the solder abutting upon the interface. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) of the surface components of Sn-9Zn-0.2Ga-0.002Al-0.25Ag-0.15Ce solder indicate that Al aggregates at the surface in the form of Al2O3 protective film, which prevents the further oxidation of the solder surface. On the other hand, Ce aggregates at the subsurface, which may reduce the surface tension of the solder and improve the wettability in consequence.

The eutectic Sn-9Zn alloy was doped with Ag (0 wt.%-1 wt.%) to form Sn-9Zn-xAg lead-free solder alloys. The effect of the addition of Ag on the microstructure and solderability of this alloy was investigated and intermetallic compounds (IMCs) formed at the solder/Cu interface were also examined in this study. The results show that, due to the addition of Ag, the microstructure of the solder changes. When the quantity of Ag is lower than 0.3 wt.%, the needle-like Zn-rich phase decreases gradually. However, when the quantity of Ag is 0.5 wt.%-1 wt.%, Ag-Zn intermetallic compounds appear in the solder. In particular, adding 0.3 wt.% Ag improves the wetting behavior due to the better oxidation resistance of the Sn-9Zn solder. The addition of an excessive amount of Ag will deteriorate the wetting property because the glutinosity and fluidity of Sn-9Zn-(0.5, 1)Ag solder decrease. The results also indicate that the addition of Ag to the Sn-Zn solder leads to the precipitation of e-AgZn3 from the liquid solder on preformed interfacial intermetallics (Cu5Zn8). The peripheral AgZn3, nodular on the Cu5Zn8 IMCs layer, is likely to be generated by a peritectic reaction L + γ-Ag5Zn8 → ɛ-AgZn3 and the following crystallization of AgZn3.