Interfacial Voiding Research Articles

Electrodeposition Complexity and the Root Cause of Interfacial Voiding in Solder Joints with Plated Nickel

Electrodeposition Complexity and the Root Cause of Interfacial Voiding in Solder Joints with Plated Nickel

Effects of initial Cu grain size and impurity residues on the liquid phase reactions between molten solder and electroplated Cu

Copper (Cu) electroplating is a cost-effective and scalable technology used to fabricate interconnects/metallizations in microelectronic products. Liquid-phase soldering reactions between electroplated Cu and Sn-based alloys are essential for constructing heterogeneous joints for mechanical support and electrical conduction applications. In this study, three Cu electroplated layers with different grain sizes and impurity contents are fabricated by formulating additives in the plating solution. The Cu electroplated layers are reacted with molten Sn-3.5 wt% Ag alloy at 260 °C for 20–60 min to investigate the liquid phase reactions. Microstructural characterizations of the electroplated Cu layers and the Cu/solder/Cu joints are carried out using electron microscopy and X-ray diffraction processes. A time-of-flight secondary ion mass spectrometer is used to analyze the impurity contents in the Cu electroplated layers. The Cu/solder/Cu joints after liquid phase reactions are mechanically analyzed to measure the shear strength. The results show that the additive formulas affect the microstructural features and the impurity contents in the Cu electroplated layers. Electroplating of Cu with an ultrafine grain texture accompanies a high impurity content, and the solder joint constructed by the ultrafine-grained Cu exhibits a rapid liquid phase reaction and interfacial voiding propensity, leading to a low shear strength.

An Experimental Study on the Thermal Stability of Mg2Si/Ni Interface under Thermal Cycling

Mg2Si is a promising eco-friendly thermoelectric material, and Ni is suited for electrical contact on it. In this study, Bi-doped Mg2Si ingots with Ni contacts were fabricated by co-sintering, and thermal stability was investigated by long-time (500 h, 500 cycles) temperature cycling from 25 °C to a peak temperature (Th = 400 and 450 °C) in N2. The as-sintered Ni/Mg2Si interfacial region is a multilayer consisting of Mg3Bi2, a series of MgxSiyNiz ternary compounds (ω, ν, ζ, and η-phases), and MgNi2. In the complex microstructure, the MgNi2 / η-phase interface was vulnerable to stress-induced voiding at Th = 450 °C, which arises from the mismatch of the thermal expansion coefficients. Interfacial voiding was avoided by adding 10 mol% Ag in Ni, which is probably due to the suppression of vacancy migration by the Ag-containing 2nd phase formation at the MgNi2/η-phase interface.

Void formation and its impact on Cu[sbnd]Sn intermetallic compound formation

Void formation in the CuSn system has been identified as a major reliability issue with small volume electronic interconnects. Voids form during the interdiffusion of electrochemically deposited Cu and Sn, with varying magnitude and density. Electroplating parameters include the electrolytic chemistry composition and the electroplating current density, all of which appear to effect the voiding characteristics of the CuSn system. In addition, interfacial voiding affects the growth kinetics of the Cu3Sn and Cu6Sn5 intermetallic compounds of the CuSn system. The aim here is to present voiding data as a function of electroplating chemistry and current density over a duration (up to 72 h) of isothermal annealing at 423 K (150 °C). Voiding data includes the average interfacial void size and average void density. Voids sizes grew proportionally as a function of thermal annealing time, whereas the void density grew initially very quickly but tended to saturate at a fixed density. A morphological evolution analysis called the physicochemical approach is utilised to understand the processes that occur when a voided Cu/Cu3Sn interface causes changes to the IMC phase growth. The method is used to simulate the intermetallic thickness growths' response to interfacial voiding. The Cu/Cu3Sn interface acts as a Cu diffusion barrier disrupting the diffusion of Cu. This resulted in a reduction in the Cu3Sn thickness and an accelerated growth rate of Cu6Sn5.

Effect of interfacial solute segregation on ductile fracture of Al–Cu–Sc alloys

Three-dimensional atom probe analysis is employed to characterize the Sc segregation at θ′/α-Al interfaces in Al–2.5wt.% Cu–0.3wt.% Sc alloys aged at 473, 523 and 573K, respectively. The interfacial Sc concentration is quantitatively evaluated and the change in interfacial energy caused by Sc segregation is assessed, which is in turn correlated to yield strength and ductility of the alloys. The strongest interfacial Sc segregation is generated in the 523K-aged alloy, resulting in an interfacial Sc concentration about 10 times greater than that in the matrix and a reduction of ∼25% in interfacial energy. Experimental results show that the interfacial Sc segregation promotes θ′ precipitation and enhances the strengthening response. A scaling relationship between the interfacial energy and precipitation strengthening increment is proposed to account for the most notable strengthening effect observed in the 523K-aged alloy, which is ∼2.5 times that in its Sc-free counterpart and ∼1.5 times that in the 473 and 573K-aged Al–Cu–Sc alloys. The interfacial Sc segregation, however, causes a sharp drop in the ductility when the precipitate radius is larger than ∼200nm in the 523K-aged alloy, indicative of a transition in fracture mechanisms. The underlying fracture mechanism for the low ductility regime, revealed by in situ transmission electron microscopy tensile testing, is that interfacial decohesion occurs at the θ′ precipitates ahead of crack tip and favorably aids the crack propagation. A micromechanical model is developed to rationalize the precipitate size-dependent transition in fracture mechanisms by taking into account the competition between interfacial voiding and matrix Al rupture that is tailored by interfacial Sc segregation.

Atomistic Modeling of Voiding Mechanisms at Oxide/Alloy Interfaces

For the first time, voiding mechanisms resulting from the condensation of atomic vacancies injected at oxide/alloy interfaces by the growth of oxide layers have been studied by means of periodic density functional theory (DFT) calculations. Several interfaces were built by superimposing ultrathin films of alumina on the γ-TiAl(111) surface, and their relative stabilities were compared by calculating the interface energy variation. The formation energy of single Ti or Al vacancies and clustered and dispersed ensembles of 2Ti + 1Al or 2Al + 1Ti vacancies injected into the alloy were calculated. The results show that it is easier to inject the vacancies into the oxide/alloy interface than into the bare alloy surface and into the bulk alloy. The injected vacancies, trapped at the oxide/alloy interface, condense in the topmost plane of the alloy to form 2D clusters. The minimization of the coordination number of the vacancies with metal atoms of the alloy and O atoms of the overlaying oxide favors vacancy condensation and interfacial voiding. The data are relevant for a detailed understanding of the adherence and breakdown of protective oxide films and the lifetime of metallic materials.

Consideration of temperature and current stress testing on flip chip solder interconnects

This work discusses the experimental set-up and data interpretation for high temperature and current stress tests of flip chip solder joints using the four-point Kelvin measurement technique. The single solder joint resistance responses are measured at four different four-point Kelvin structure locations in a flip chip package. Various temperatures (i.e., 125–165 °C) and electric current (i.e., 0.6–1.0 A) test conditions are applied to investigate the solder joint resistance degradation behavior and its failure processes. Failure criterion of 20% and 50% joint resistance increases, corresponding to solder and interfacial voiding, are employed to evaluate the solder joint electromigration reliability. The absolute resistance value is substantially affected by the geometrical layout of the metal lines in the four-point Kelvin structure, and this is confirmed by finite element simulation. Different current flow directions and strengths yielded different joint resistance responses. The anode joint, where electrons flow from the die to the substrate, usually measured an earlier resistance increase than the cathode joint, where electrons flow in the opposite direction. The change in measured joint resistances can be related to solder and interfacial voiding in the solder joint except for ±1 A current load, where resistance drop mainly attributed to the broken substrate Cu metallization as a result of “hot-spot” phenomenon. The solder joint temperature increases above the oven ambient temperature by ∼25 °C, ∼40 °C and ∼65 °C for 0.6 A, 0.8 A and 1.0 A stress current, respectively. It is found that two-parameter log-normal distribution gives a better lifetime data fitting than the two-parameter Weibull distribution. Regardless of failure criterion used, the anode joint test cells usually calculated a shorter solder joint mean life with a lower standard variation of 0.3–0.6, as compared to the cathode joint test cells with a higher standard variation of 0.8–1.2. For a typical flip chip solder joint construction, electromigration reliability is mainly determined by the under bump metallization consumption and dissolution, with intermetallic compound formation near the die side of an anode joint.

A theory for stress-driven interfacial damage upon cationic-selective oxidation of alloys

Imagine a void at an interface, separating an outwardly growing oxide and a substitutional solid solution of two metallic elements A and B. Assume the metal interface oxidizes, but the void-free surface does not. Interdiffusion inside the metal, and misfit dislocation activities at the oxidizing interface, both generate a stress-free strain rate field. The compositional and material constraints in the presence of a non-oxidizing void give rise to a multi-axial tensile stress field, while a viscoplastic strain field arises to relax stress. The tensile stress at the interface enforces a concave curvature near the void tip through the continuity condition of the chemical potential. Atoms interflow along the void surface under the combined action of curvature, stress and composition gradients. They enter the metal/oxide interface and flow under the action of local stress, curvature and composition fields. The void grows. The stress at the interface relaxes, and the interface recedes partially and non-uniformly. Interfacial voiding upon cationic-selective oxidation is a long-standing topic in the world of thermal barrier coating and interconnect systems. This paper develops governing equations, within the alloy, for stress generation upon composition evolution and induced plastic strain. Governing equations at the interface and the void surface are next formulated to describe a moving boundary problem that accounts for the simultaneous void extension and interface recession. These governing equations are boundary conditions for the bulk formulation.

Influence of plating parameters and solution chemistry on the voiding propensity at electroplated copper–solder interface

Interfacial voiding in solder joints formed with Sn–Ag–Cu solder alloys and electroplated Cu was examined as a function of the plating solution chemistry and parameters. Galvanostatic Cu plating of ~10 μm thick Cu films was performed in a commercially available plating solution, and in model generic plating solutions. Analysis of the current voltage behavior along with Secondary Ion Mass Spectrometry studies of organic impurity content of two plated and a wrought copper samples, yielded a conclusion that for certain chemistry solutions (e.g., H2SO4 + CuSO4 + Cl− + PEG) and current density ranges above 2.5 mA cm−2, organic impurities were incorporated into the growing Cu. Solder joints were produced with a variety of electroplated Cu samples. These joints were, then, annealed at a temperature of 175 °C for 1 week, cross sectioned and examined. In general, it was observed that interfacial voiding in laboratory electroplated Cu layers was qualitatively similar to the unexplained voiding observed in some industrially plated Cu products. More specifically, it was found that the propensity for voiding could be correlated with specific electroplating parameters that in turn were associated with significant incorporation of organic impurities in the Cu deposit.

Study on poly(ethylene terephthalate)/polypropylene microfibrillar composites. II. Solid‐state drawing behavior

AbstractA (20/80) blend of poly(ethylene terephthalate)/polypropylene (PET/PP) was solid‐state drawn to enhance the molecular orientation of the PET microfibers. Effects of drawing temperature (23–140°C) and drawing speed (max. 1000 mm/min) on the morphology and draw ratio of the blend were studied and discussed based on the drawing behaviors of the pure polymers. In cold drawing, there seemed to be a critical drawing speed below which the natural draw ratios of the polymers remained constant, but above which the draw ratios first decreased slightly because of suppression of molecular relaxation and then increased because of breakage of highly strained molecules and disintegration of lamellar crystals into finer mosaic blocks. Macroscopically, the pure PP and the PET/PP composite extrudates gave similar draw ratios at the same speeds. SEM showed that the PET microfibers suffered a smaller elongation than the PP matrix and severe voiding occurred at the PET/PP interface. Furthermore, substantial fiber breakage occurred during cold drawing at speeds above 200 mm/min. In comparison, drawing at 100°C caused no obvious interfacial voiding and fiber breakage. Furthermore, the natural draw ratio of the blend was lower than that of the pure PP extrudate, indicating that the PET microfibers had constrained the deformation of the PP matrix. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1989–2000, 2004

The influence of interfacial interactions on the morphology and thermal properties of binary polymer blends

In order to understand the influence of interfacial modification in polyamide/ionomer/ polypropylene ternary blends, the binary blends were investigated. In this study two multiphase blends have been chosen, PP/ionomer and PA/ionomer, where the ionomer was a copolymer of polyethylene and of a mixture of methacrylic acid, zinc methacrylate and isobutylacrylate. The PP/ionomer blend is a classic example of an incompatible system which displays a high degree of interfacial voiding in SEM fracture surfaces, a strong negative deviation from additivity in the viscosity/composition relationship and an important coalescence effect on increasing dispersed phase concentration. This blend demonstrated increased nucleation and crystallization rate, compared to pure polypropylene. The second blend, PA/ionomer, demonstrates a very fine multiphase structure evident only after microtoming/etching of the sample, strong additive behaviour in the viscosity/composition curve. Formation of an amide bond was identified by Fourier transform-infrared spectroscopy. This evidence supports the conclusion that much stronger interactions exist between the ionomer and the polyamide than between the ionomer and the polypropylene.

Thermal Studies of Au-Ti Thin Films Deposited on Oxide Substrates

ABSTRACTCo-evaporated Au-Ti thin films deposited onto sapphire and silicon dioxide substrates were thermally treated over a temperature range of 300–900°C in both vacuum and reactive amb ients. Vacuum annealing produced negligible reaction between the oxide substrate and film, the Au-Ti, however, reacted to form intermetallic compounds with little net elemental redistribution in the film. Heat treating in oxygen resulted in segregation of titanium to the surface where it formed both rutile and anatase structures. In addition, the high interfacial energy between the gold and the oxide phases led to the formation of large (5 micron diameter) gold particles on the film surface or voids at the titania-gold interface. Annealing in Ammomia produced an oxynitride surface in addition to the Ti-Au separation, Au particulate formation, and/or interfacial voiding already observed. The extent of surface degradation observed during these reactive ambient anneals varied with Ti concentration and temperature. The film did not react with SiO2 under any circumstances, however, at temperatures above 650°C in reactive ambients the titanium accumulated at the Al2,O3 interface. These results were obtained by RBS, X-ray diffraction, TEM and SEM.

High-temperature corrosion of LPPS-coated inconel alloy MA 6000

A low-pressure plasma spray (LPPS) coating of a commercial NiCoCrAlY(Ta) alloy on Inconel alloy MA 6000 was examined metallographically after 500 h of cyclic oxidation and burner rig testing for coating performance and interfacial reactions. Uncoated Inconel alloy MA 6000 was also examined for oxidation and hot corrosion resistance. Bare Inconel alloy MA 6000 has excellent oxidation resistance to 2010 °F (1100 °C) but is subject to internal sulfidation in burner rig testing at 1700 °F (925 °C) and 2000 °F (1095 °C). An LPPS coating of a NiCoCrAlY(Ta) alloy protected the Inconel alloy MA 6000 substrate from oxidation and sulfidation for 312 h at both temperatures but developed appreciable interfacial voiding after 500 h at 2000 °F (1095 °C). Spalling of the coating was observed only on burner rig test specimens run at 2000 °F (1095 °C).