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Elementary theory of mass-transfer in two-phase alloy under electromigration with account of two competing mechanisms of the fluxes equilibration is presented. These two competing mechanisms are Kirkendall effect and backstress. Various versions of Monte Carlo models for simultaneous simulation of structure evolution kinetics and of mass-transfer kinetics under high-density current are presented. Possibility of self-organization with minimization of Joule heating is demonstrated. [ABSTRACT FROM AUTHOR]

• Electromigration in Cu RDLs capped with polyimide was investigated. • Failure of fine-pitched (2-µm) is very different from that of 10-µm RDLs. • Failure of the 2-µm RDLs is mainly caused by severe oxidation during EM. • Oxidation of nt-Cu and regular Cu RDLs during EM was characterized. • Oxidation equation was proposed to predict the resistance increase of RDLs. Polyimide (PI) has been adopted to protect the Cu from oxidation in the packaging industry. Electromigration (EM) of Cu redistribution layers (RDLs) capped with PI was investigated at 160 °C under 1.0 × 106 A/cm2. The results indicated that failure of fine-pitched (2-µm) is different from that of 10-µm RDLs, which is void formation. The failure of the 2-µm RDLs is mainly caused by severe oxidation during EM. To analyze the EM failure of the Cu RDLs with different pitches, the oxidation area of nanotwinned copper (nt-Cu) and regular Cu RDLs during EM was compared. We propose an oxidation equation to estimate the resistance increase of RDLs with various widths. [ABSTRACT FROM AUTHOR]

Solder is widely used to connect chips to their packaging substrates in flip chip technology as well as in surface mount technology. At present, the electronic packaging industry is actively searching for Pb-free solders due to environmental concern of Pb-based solders. Concerning the reliability of Pb-free solders, some electronic companies are reluctant to adopt them into their high-end products. Hence, a review of the reliability behavior of Pb-free solders is timely. We use the format of “case study” to review six reliability problems of Pb-free solders in electronic packaging technology. We conducted analysis of these cases on the basis of thermodynamic driving force, time-dependent kinetic processes, and morphology and microstructure changes. We made a direct comparison to the similar problem in SnPb solder whenever it is available. Specifically, we reviewed: (1) interfacial reactions between Pb-free solder and thick metalliztion of bond-pad on the substrate-side, (2) interfacial reactions between Pb-free solder and thin-film under-bump metallization on the chip-side, (3) the growth of a layered intermetallic compound (IMC) by ripening in solid state aging of solder joints, (4) a long range interaction between chip-side and substrate-side metallizations across a solder joint, (5) electromigration in flip chip solder joints, and finally (6) Sn whisker growth on Pb-free finish on Cu leadframe. Perhaps, these cases may serve as helpful references to the understanding of other reliability behaviors of Pb-free solders. [Copyright &y& Elsevier]

• Electromigration in Cu redistribution lines capped by organic layers is studied. • The measured EM lifetime of nt-Cu RDLs is 4 times longer than that of regular Cu. • Two main failure mechanisms in RDLs (EM-induced voiding and oxidation) are analyzed. Electromigration (EM) mechanisms are well studied in damascene copper lines capped with inorganic dielectrics. However, EM in Cu redistribution lines (RDLs) capped by organic layers is not well-understood. In this study, nanotwinned copper (nt-Cu) is adopted, which has high mechanical strength as well as excellent electrical resistivity. We designed EM test vehicles with four-point probes to measure the lifetime of RDLs and examine the failure mechanisms of the Cu RDLs. The length of the RDLs was 800 µm, the width was 10 µm, and the height was 5 µm. The samples were tested at 200 °C, with current densities of 1.0 × 106 A/cm2. The results indicated that there were two main failure mechanisms, including EM-induced voiding and oxidation of Cu RDLs. The measured EM lifetime of nt-Cu RDLs is 4 times longer than that of regular Cu RDLs. [ABSTRACT FROM AUTHOR]

• Based on Onsager's entropy production equation, the mean-time-to-failure equation for electromigration is derived. • Only need one set of data to predict the electromigration lifetime with this equation. • The cost and time can be saved for electromigration tests. Black's mean-time-to-failure (MTTF) equation has been applied to predict electromigration lifetime in electronics for decades. It is an empirical equation, and at least three sets of data tested under two temperatures and two current densities are needed to determine the parameter n, activation energy E , and pre-factor A in the equation. Based on Onsager's entropy production theory, we derived a modified MTTF equation, in which n = 2 becomes definite. The activation energy E is intrinsic for materials; for SnAg solder joints, we can take it as 1 eV. Therefore, we only need one set of data (one temperature and one current density) to determine the perfector A , for predicting the electromigration lifetime of the test samples. Our modified MTTF equation provides a fast and cost-saving method for accurate prediction of the electromigration lifetime for electronic products. [ABSTRACT FROM AUTHOR]

Abstract: In 3D IC technology, the vertical interconnection consists of through-Si-vias (TSV) and micro solder bumps. The size of the micro-bump is approaching 10μm, which is the diameter of TSV. Since joule heating is expected to be the most serious issue in 3D IC, heat flux must be conducted away by temperature gradient. If there is a temperature difference of 1°C across a micro-bump, the temperature gradient will be 1000°C/cm, which can cause thermomigration at the device operation temperature around 100°C. Thus thermomigration will become a very serious reliability problem in 3D IC technology. We review here the fundamentals of thermomigration of atoms in microbump materials; both molten state and solid state thermomigration in solder alloys will be considered. The thermomigration in Pb-containing solder joints is discussed first. The Pb atoms move to the cold end while Sn atoms move to the hot end. Then thermomigration in Pb-free SnAg solder joints is reviewed. The Sn atoms move to the hot end, but the Ag atoms migrate to the cold end. Thermomigration of other metallization elements, such as Cu, Ti and Ni is also presented in this paper. In solid state, copper atoms diffuse rapidly via interstitially to the cold end, forming voids in the hot end. In molten state, Cu thermomigration affects the formation of intermetallic compounds. [Copyright &y& Elsevier]

Vacancy flux in eutectic flip-chip SnPb solder joints driven by electromigration was studied using synchrotron radiation X-ray microtomography technique. The change in void volume and shape in three-dimensional images was measured quantitatively and the product of diffusivity and effective charge number of electromigration in eutectic SnPb alloy was calculated to be 3.3×10−9 and 9.5×10−9 cm2 s−1 at 100 and 120°C, respectively. The activation energy of the effective self-diffusivity in SnPb alloy was measured to be 0.66eV. [ABSTRACT FROM AUTHOR]

Abstract: Ultra-fine grained copper with a large amount of nano-scale twin boundaries has high mechanical strength and maintains normal electrical conductivity. The combination of these properties may lead to promising applications in future Si microelectronic technology, especially as interconnect material for air-gap and free-standing copper technologies. Based on first principles calculations of total energy and in-situ stress measurements, high stress followed by stress relaxation during the Cu film deposition seems to have contributed to nanotwin formation. Nanoindentation studies have shown a larger hardness for copper with a higher nanotwin density. The effect of Cu nanotwin boundaries on grain growth was investigated by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The presence of a high density of nanotwin boundaries may improve the reliability of Cu interconnects. [Copyright &y& Elsevier]

Abstract: The effect of a high electric current density on the interfacial reactions of micro ball grid array solder joints was studied at room temperature and at 150°C. Four types of phenomena were reported. Along with electromigration-induced interfacial intermetallic compound (IMC) formation, dissolution at the Cu under bump metallization (UBM)/bond pad was also noticed. With a detailed investigation, it was found that the narrow and thin metallization at the component side produced “Joule heating” due to its higher resistance, which in turn was responsible for the rapid dissolution of the Cu UBM/bond pad near to the Cu trace. During an “electromigration test” of a solder joint, the heat generation due to Joule heating and the heat dissipation from the package should be considered carefully. When the heat dissipation fails to compete with the Joule heating, the solder joint melts and molten solder accelerates the interfacial reactions in the solder joint. The presence of a liquid phase was demonstrated from microstructural evidence of solder joints after different current stressing (ranging from 0.3 to 2A) as well as an in situ observation. Electromigration-induced liquid state diffusion of Cu was found to be responsible for the higher growth rate of the IMC on the anode side. [Copyright &y& Elsevier]

Abstract: The electromigration of PbSn eutectic flip-chip solder joints was studied at 30°C with a nominal current density of 4×104 A/cm2. The void formation-and-propagation failure mechanism was observed in situ. In the joint with electrons flowing from the chip side to the substrate side, a void nucleated near the current crowding region after an incubation period that lasted between 30 and 50min. This void then propagated across the under bump metallurgy very quickly, and eventually resulted in the failure of the solder joint in about another 40min. The failure was concurrent with the melting of the joint. The joint with electrons flowing from the substrate to the chip side survived the current stressing. This failure mechanism directly reflects that current crowding is a dominant factor responsible for failure in flip-chip solder joints. This study shows that the incubation time of void nucleation is a good indicator of the real lifetime of a joint under current-stressing. [Copyright &y& Elsevier]

Electromigration (EM) has been the most persistent interconnect reliability issue over the decades. In general, EM damages tend to occur at atomic flux divergence sites. The EM failure rate can be further accelerated by current crowding, which occurs when current flows between inter-level wires. In this work, we used two different via etch schemes to study the effect of contact resistance on current crowding and EM. We found that the etch stop structures show longer EM lifetimes than the over etch. The contact resistance of the etch stop is higher than that of the over etch. Two-dimensional simulation results show that the higher contact resistance in the etch stop can suppress current crowding and improve EM lifetimes. Differences in void morphology between the over etch and etch stop as a result of current crowding are discussed. [Copyright &y& Elsevier]