An old procedure used to carry out a graphical derivation of curves, which is based on the optical properties of plane mirrors, has been adapted for the measurement of the contact angle (θ) formed between a liquid drop and a flat solid substrate in wettability experiments carried out by the so-called “sessile drop” method. The method was tested for mercury on soda-lime glass at room temperature in air as well as for Cusil (Ag-28wt.%Cu) and Incusil-ABA (Ag-27wt.%Cu-12wt.%In-2wt.%Ti) brazing alloys on pressureless-sintered silicon carbide (PLS-SiC) at 850 °C, under a vacuum of 10-4/10-5 Torr. The proposed method is fast, simple and accurate enough from high (∼140°) to relatively low (∼10°) contact angles. Although the proposed method has been tested for metal-ceramic systems, it is of general application, so that it would be useful for any liquid-solid system. The method is applicable for any temperature, pressure and atmospheric experimental conditions employed, as well as for any chemical composition of liquid and solid. It is also useful for both low and high contact angles, as well as for reactive and non-reactive systems, as long as a photograph of a liquid drop resting on a flat solid surface is available for the studied system.

Ni/Sn–xZn/Ni (x = 1, 5, 9 wt%) joints were used to investigate the effect of Zn content on interfacial reactions during reflow under a temperature gradient. Asymmetrical growth and transformation of intermetallic compounds (IMCs) occurred between the cold and hot end interfaces. Faster IMC growth at the cold end and a more prompt IMC transformation at the hot end in a lower Zn content solder joint were identified due to the more thermomigration-induced Zn and Ni atomic fluxes toward the cold end. The main diffusion species into IMC layers changed from Zn atoms at the early stage to Sn and Ni atoms at the later stage. As a result, the IMC evolution followed (Ni,Zn)3Sn4 → Ni3Sn4 in the Ni/Sn–1Zn/Ni joint, Ni5Zn21 → τ phase → Ni3Sn4 in the Ni/Sn–5Zn/Ni joint, and Ni5Zn21 → τ phase in the Ni/Sn–9Zn/Ni joint along with the reflow time. A higher Zn content could effectively inhibit the dissolution of the hot-end Ni substrate and restrain the growth rate of the cold-end interfacial IMCs.

For Sn–Pb eutectic solder alloy, uniaxial tensile tests were conducted to dog-bone type specimens annealed at different temperatures (60–180 °C) and durations (2–48 h). Low strain rates ranging from 10−4 s−1 to 10−3 s−1 were applied to study the competition between creep and plasticity and also the rate dependent effect of annealing condition. It is found that the influence of annealing temperature on material properties is more than that of annealing duration. Higher temperature up to 180 °C generally leads to higher yield and ultimate stresses and ultimate strain of annealed specimens. The optimal annealing condition is suggested to be 180 °C for 6 h for stable and efficient improvements in both strength and ductility. By proposing a concise unified creep and plasticity constitutive model, the sensitivity to strain rate and annealing condition is quantified with consideration of both creep and hardening properties. Parameter calibration theoretically confirms the observed optimal annealing condition in experiments.

Using in situ transmission electron microscopy, we report the observation of the melting behavior of one-dimensional nanostructures of Sn with different length/width aspect ratios. The melting of small aspect-ratio nanowires (nanorods) results in the expansion of liquid Sn along both axial and radial directions with the tendency to form an isometric or spherical particle, thereby minimizing the total surface area. For nanowires with the length/width aspect ratio of ∼10.5, perturbation along the liquid stream causes an unstable necking phenomenon and the whole wire tends to shrink into a spherical particle. In contrast, Rayleigh instability sets in for the melting of the nanowires with the length/width aspect ratio as large as ∼21, which gives rise to necking and fragmentation of the wire into particles. The amorphous native surface oxide (SnO x ) layer serves as a confinement tube and plays an important role in the melting induced morphological evolution of Sn nanowires. A thin SnO x layer is flexible with the ability to shrink or expand upon the flow of molten Sn. The increased rigidity for a thick SnO x surface layer kinetically suppresses bulging and necking formation in molten Sn nanowires.

Thermomigration (TM) and electromigration (EM) are two persistent reliability issues and they generally appear concurrently in solder joints. Many previous studies have attempted to understand the fundamental principles behind these phenomena with the majority of which focusing their interest into the faster migration elements in solders like Bi, Ni, or Cu. However, Sn as the slower migration element has not received that much attention. In the present study, a special linearly symmetrical structure was used. An unusual TM phenomenon of Sn atoms in the Sn58Bi solder joint was observed. The unusual TM of Sn atoms along the vertical edges was attributed to the coupled effect of the EM in the horizontal direction and the TM in vertical direction. The relationships between the microstructural characteristics and the temperature distribution were established. The results also indicated that elevated temperature and sufficient thermal gradient were the two major factors that caused TM.

The possibilities of Ni as contact material in electronic applications has motivated the interest on the intermetallic phases of the Ni-Sb system, in relation to their use in lead free micro-soldering processes. In this work, a detailed theoretical study of the cohesive and thermodynamic properties of the compound Ni3Sb in the (cF16) Fm-3m structure is reported. To this aim, the Full Potential Linearized Augmented Plane Waves method, within the framework of the Density Functional Theory and both Generalized Gradient and Local Density approximations, has been applied. The structural parameters, cohesive and elastic properties of this compound and its constituent elements have been determined. In particular, the equilibrium structural properties are determined through the minimization of the energy, including the full relaxation of the internal degrees of freedom of the cell. It is shown that the calculated properties agree well with the available experimental data. Moreover, various contributions to the electronic density of sates are studied. On this basis, a discussion is presented of the bonding characteristics of this compound, in the framework of the current ideas about cohesion in p-d bonded intermetallics.

With the electronics packaging industry shifting increasingly to three-dimensional packaging, microbumps have been adopted as the vertical interconnects between chips. Consequently, solder volumes have decreased dramatically, and the solder thickness has reduced to a range between a few and 10 microns. The solder volume of a microbump is approximately two orders of magnitude smaller than a traditional flip-chip joint. In contrast, the thickness of the under-bump metallization (UBM) remains almost the same as that in flip-chip solder joints. Therefore, many issues concerning materials and reliability of microbumps arise. This article reviews the challenges related to microbump materials for vertical interconnects, including transformation of solder joints into intermetallic (IMC) joints, necking or voiding induced by side wetting/diffusion on the circumference of the UBM, formation of porous Cu3Sn IMCs, early electromigration failures caused by specific orientations of Sn grains, and precipitation of plate-like Ag3Sn IMCs. An alternative way of fabricating vertical interconnects using direct Cu-to-Cu bonding is also discussed.

We report the implementation of an automated reel-to-reel fluidic self-assembly system based on surface-tension driven self-assembly for macroelectronics application. The reported system incorporates precisely controlled and automated agitation, web moving, component recycling, and dispensing system. The system enables continuous parallel assembly of semiconductor chips at a high rate (15,000 chips per hour using 2.5 cm wide web) and assembly yield (>99%) under optimal condition. In principle, scaling to any throughput should be possible considering the parallel nature of self-assembly. The process overcomes the limitations on area and throughput of prior methods. It provides a new platform for macroelectronics to enable the integration of microscopic high performance inorganic semiconductors on flexible substrates with any desired location, pitch, and integration density. As an example we demonstrate the fabrication of a solid state area lighting module.

To reinforce the reliability issue brought by excessive interfacial reaction with the dimensional scale-down of electronic device, an electroless Ni–P–ZrO2 (17.5 at.% of P) composite coating was developed as the under bump metallization (UBM) for lead-free solder interconnect. ZrO2 nanoparticles were proved to be homogeneously distributed and helped improve wetting ability of the layer. Both Sn–3.5Ag/Ni–P–ZrO2 and Sn–3.5Ag/Ni–P solder joints were prepared and aged at various conditions to study the interfacial reaction. Growth of intermetallic compounds (IMCs) without serious spalling in solder/Ni–P–ZrO2 joint was slowed down because of the barrier property of incorporation of ZrO2 nanoparticles, which blocked the diffusion of Ni and Cu atoms. Based on the IMC growth, the activation energy of solder/Ni–P–ZrO2 was estimated to be higher than that of plain solder joint. The top-view of IMCs demonstrated a much finer grain size compared with that of solder/Ni–P joint. A reactive diffusion-induced compound formation mechanism was proposed to address the microstructural evolution in detail. Moreover, solder/Ni–P–ZrO2 joint demonstrated higher shear strength than did solder/Ni–P joint for different aging durations. The fracture surface of solder/Ni–P joint after shear test showed ductile transition failure, with big dimples and plastic deformation.

A rapid nano-indentation measurement technique is employed to produce surface maps of hardness. Each indentation cycle requires less than three seconds, including surface approach, contact detection, force application, withdrawal, and movement to the next indentation site. Traditional nano-indentation analyses are applied to the force-displacement measurements from each indentation, but information storage and presentation owe much to scanning-probe technology. Consequently, this nano-indentation technique produces three-dimensional images of mechanical properties which are stored and manipulated just like scanned images.

In the present work, we use this technique to map the hardness of a SAC 305 solder joint with gold plating. After extended isothermal aging, the solder joint comprises three constituents: a tin-rich matrix, a bulk intermetallic AuSn4, and an interfacial intermetallic (Cu, Ni, Au)6Sn5. The softest material is the tin-rich matrix, which has a hardness of 0.51±0.07 GPa. The hardness of the bulk intermetallic is 2.12±0.18 GPa. The interfacial intermetallic has extraordinary hardness— greater than 8 GPa. Under uniform plastic strain, the mismatch in hardness between the interfacial intermetallic and surrounding material may increase the local stress intensity factor which drives interfacial fracture.

Cette étude expérimentale présente de nouveaux matériaux composites développés récemmentpour le brasage de pièces en cuivre. Il s’agit de compositions à base d’agarose quipeuvent être mises en forme par moulage par injection de poudres. Cette facilité de miseen œuvre ouvre comme possibilité d’application la fabrication de préformes de brasage deformes complexes. Deux exemples d’utilisations sont présentés, l’un porte sur le brasagetendre et l’autre concerne le brasage fort. Les brasures finales obtenues permettentd’obtenir une résistance mécanique et une qualité métallurgique similaires à cellesobtenues à partir des matériaux conventionnels (bases SnPb ou CuP) utilisés pour lebrasage manuel de tubes en cuivre.

Cu6Sn5 is a critical intermetallic compound in soldering operations. Conventional equilibrium phase diagrams show that this compound is of either a hexagonal or monoclinic structure at temperatures above and below 186 °C, respectively. Under nonequilibrium conditions, the crystal structure is dependent on composition, temperature, and processing history. The effect of Zn, Au, and In on the hexagonal to monoclinic polymorphic transformation in Cu6Sn5 intermetallics is investigated using variable temperature synchrotron powder x-ray diffraction and differential scanning calorimetry. It is revealed that, as in the case of trace Ni additions, the alloying elements Zn and Au completely stabilize the hexagonal Cu6Sn5 and prevent the phase transformation. In contrast, In additions only partially stabilize the hexagonal Cu6Sn5.

To enhance the reliability of Pb-free solders in high temperature and high humidity conditions, the minor alloying elements of Be and Co are investigated in terms of the growth of Sn whiskers and various properties of Sn-based Pb-free solders. Sn whisker growth is suppressed by adding up to 0.02 wt% Be to Sn-based solders. Adding Be and Co can effectively reduce the undercooling of Sn–1.0Ag–0.5Cu (wt%) solders. And the microstructures of Sn–1.0Ag–0.5Cu–0.02Be solders are similar to those of Sn–1.0Ag–0.5Cu. Furthermore, adding Co to solders increases the microhardness number as a result of the solid solution hardening. Adding Be causes no changes in the morphology or thickness of Cu6Sn5 at the Cu/OSP (organic solderability preservative) under bump metallurgy interface. However, the scallop-like Cu6Sn5 microstructure changes to a flat (Cu,Co)6Sn5 microstructure when 0.05 wt% of Co is added to Sn–1.0Ag–0.5Cu–0.02Be solders.

Reflow behavior of a Sn–8.5Zn–0.5Ag–0.01Al–0.1Ga (five-element) solder on the Ni/Cu substrate was investigated under different heating rates. Reflowed samples show decreased Zn and increased AgZn3 in the solder with a reduction in the heating rate. The Zn at the solder/substrate interface was found to be much lower than that in the Sn–Zn solder systems. Cu was observed to be diffused through the electroplated Ni layer and noticed only with the Ag–Zn compound in the solder. Ga was spotted at the interface in the Ag–Zn matrix, whereas Al was detected with the Zn at the interface. Small intermetallic compound (IMC) layer was formed at the interface; however, its amount enhanced with the reduction in the heating rate. Present study relates the reflow behavior of the five-element solder with the reactivity of different elements in the system and its influence on the formation of IMCs in the solder and at the solder/substrate interface.

Ni-based under-bump metallization (UBM) has attracted wide attention due to its low reaction rate with Sn, compared with Cu and Cu alloy. In this study, the interfacial reactions between eutectic Sn–3.5Ag solder and Ni-based UBM, including electroplated Ni (EP-Ni) and electroless Ni (EL-Ni) are investigated. Morphology and growth kinetics of Ni3Sn4 intermetallic compounds are studied at different reflow temperatures and durations. The growth rate and the growth activation energy of Ni3Sn4 were measured for the two sets of samples. The activation energies are measured to be 25 kJ/mol and 38 kJ/mol for the Ni3Sn4 growth on EP-Ni and EL-Ni, respectively. The Ni3Sn4 on EP-Ni UBMs shows a slower growth rate and the Ni3Sn4/solder interface is void free even after 20-min reflow at 240 °C. On the other hand, the interface of Ni3Sn4/EL-Ni has a lot of microvoids after reflowing at 240 °C for 20 min.

In this article, we investigated the effect of Sn grain structure on the electromigration (EM) reliability of Sn–2.5Ag (wt%) solder joints used in flip-chip packages. The electron backscattering diffraction technique was applied to characterize the Sn grain size and orientation of the solder joints. Failure analyses on Sn–2.5Ag solder joints after EM tests showed that the Sn grain structure was important in controlling the kinetics of the intermetallic compound growth and void formation under EM. Further microstructural analysis revealed that the grain sizes and orientations of the solder joints after multiple solder reflows were statistically different from those with a single solder reflow and resulted in an improved EM reliability. Thermal annealing effect was also investigated to separate the thermal effect from the EM-induced effect. Results obtained in this study demonstrated that EM reliability of Pb-free solder joints could be improved by optimization of the Sn grain structure.

Due to environmental concerns traditional eutectic tin-lead solder is gradually being replaced in electronic assemblies by “lead-free” solders. During this transition, nanoparticle technology is also being investigated to see whether improvements in joint reliability for high temperature applications can be made. Nanoparticles can be used to harden the solder via Zener pinning of the grain boundaries and reduce fatigue failure. This paper explores the effects of adding Silica nanoparticles to SnAgCu solder, and how the mechanical properties induced in the solder vary with temperature. It is found that above 100 °C the mechanical response and microstructure of the normal and nanoparticle enhanced solders converge.

Ternary electroless nickel, NiXP, films were produced by adding salts of Mo, Re, Tl, Cu, W, Co, Fe, Zn, and Mn to conventional electroless Ni baths and subsequently reacted with Sn-3.5Ag solder. From the full width at the half maximum (FWHM) data, as-plated NiXP films can be categorized into two groups: one is close to the FWHM value of nanocrystalline Ni5P film and the other is close to amorphous Ni9P film. Alloying elements in the electrolessly plated under-bump metallurgy that effectively suppressed intermetallic compound (IMC) spalling were Mn, Zn, Re, Fe, and W, whereas Tl exacerbated spalling. The roles of Cu, Mo, and Co were less clear due to a lack of data. Based on scanning electron microscopy observations, a spalling map was presented, which showed elemental demarcation lines of IMC spalling in the X-P coordinates.

Sn/Ni–8.0 at.%V (Ni–7.0 wt%V) couples are prepared and the interfacial reactions at 210 and 250 °C are examined. In the early stage of reaction at 250 °C, a T phase is formed as a result of fast diffusion of Sn into the Ni–8.0 at.%V substrate. With a longer reaction, the outer region of the T phase transforms to a Ni-depletion layer, which has not been observed previously. Both the T phase and the Ni-depletion layer are analyzed using transmission electronic microscopy. This newly found Ni-depletion layer is composed of Sn and nanosize “VSn2(V2Sn3)” particulates. The solid/solid reaction paths in the Sn/Ni–8.0 at.%V couples evolve from Sn/T/Ni–V, Sn/Ni3Sn4/T/Ni–V to Sn/Ni3Sn4/VSn2(V2Sn3). During the liquid/solid reactions, the paths are liquid/T/Ni–V, liquid/liquid + Ni3Sn4/T/Ni–V, liquid/liquid + Ni3Sn4/liquid + VSn2(V2Sn3)/T/Ni–V, and liquid/liquid + Ni3Sn4/liquid + VSn2(V2Sn3).

Cu6Sn5 is a common intermetallic compound formed during electrical packaging. It has an allotropic transformation from the low-temperature monoclinic η’-Cu6Sn5 to high-temperature hexagonal η-Cu6Sn5 at equilibrium temperature 186 °C. In this research, the effects of this allotropic transformation and Ni addition on the thermal expansion of η’- and/or η-Cu6Sn5 were characterized using synchrotron x-ray diffraction and dilatometry. A volume expansion during the monoclinic to hexagonal transformation was found. The addition of Ni was found to decrease the undesirable thermal expansion by stabilizing the hexagonal Cu6Sn5 at temperatures below 186 °C and reducing the overall thermal expansion of Cu6Sn5.