K. N. Tu

Six cases of reliability study of Pb-free solder joints in electronic packaging technology

Abstract 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.

Recent advances on electromigration in very-large-scale-integration of interconnects

Today, the price of building a factory to produce submicron size electronic devices on 300 mm Si wafers is over billions of dollars. In processing a 300 mm Si wafer, over half of the production cost comes from fabricating the very-large-scale-integration of the interconnect metallization. The most serious and persistent reliability problem in interconnect metallization is electromigration. In the past 40 years, the microelectronic industry has used Al as the on-chip conductor. Due to miniaturization, however, a better conductor is needed in terms of resistance–capacitance delay, electromigration resistance, and cost of production. The industry has turned to Cu as the on-chip conductor, so the question of electromigration in Cu metallization must be examined. On the basis of what we have learned from the use of Al in devices, we review here what is current with respect to electromigration in Cu. In addition, the system of interconnects on an advanced device includes flip chip solder joints, which now tend ...

Stress evolution due to electromigration in confined metal lines

Electromigration is an important concern in very large scale integrated circuits. In narrow, confined metal interconnects used at the chip level, the electromigration flux is resisted by the evolution of mechanical stresses in the interconnects. Solutions for the differential equation governing the evolution of back stresses are presented for several representative cases, and the solutions are discussed in the light of experimental as well as theoretical developments from the literature.

Tin-lead (SnPb) solder reaction in flip chip technology

Solder reactions between SnPb and one of the four metals, Cu, Ni, Au, and Pd have been reviewed on the basis of the available data of morphology, thermodynamics, and kinetics. The reactions on both bulk and thin film forms of these metals have been considered and compared. Also the two kinds of reactions, above and below the melting point of the solder, have been considered and compared. The rate of intermetallic compound formation in wetting reactions between the molten solder and the metals is three to four orders of magnitude faster than those between the solid state solder and the metals. The rate is controlled by the morphology of intermetallic compound formation. In the wetting reaction between molten SnPb and Cu or Ni, the intermetallic compound formation has a scallop-type morphology, but in solid state aging, it has a layer-type morphology. There are channels between the scallops, which allow rapid diffusion and rapid rate of compound formation. In the layer-type morphology, the compound layer itself becomes a diffusion barrier to slow down the reaction. Similar morphological changes occur between SnPb and Au or Pd. The stability of scallop-type morphology in wetting reaction and layer-type morphology in solid state aging have been explained by minimization of surface and interfacial energies. The unusually high rate of scallop-type intermetallic compound formation has been explained by the gain of rate of free energy change rather than free energy change. Also included in the review is the use of a stack of thin films as under-bump-metallization, such as Cr/Cu/Au, Al/Ni(V)/Cu, and Cu/Ni alloyed thin films.

Growth kinetics of planar binary diffusion couples: ’’Thin‐film case’’ versus ’’bulk cases’’

It is proposed that interfacial reaction barriers in binary A/B diffusion couples lead to the absence of phases predicted by the equilibrium phase diagram, provided that the diffusion zones are sufficiently thin (thin‐film case). With increasing thickness of the diffusion zones the influence of interfacial reaction barriers decreases and the simultaneous existence of diffusion‐controlled growth of all equilibrium phases is expected (bulk case). Selective growth of the first and second phases and the effect of impurities are discussed with the influence of interfacial reaction barriers and with references to the known cases of silicide formation.

Interdiffusion and reaction in bimetallic Cu-Sn thin films

Interdiffusion and intermetallic compound formation in Cu-Sn thin film couples have been investigated by X-rays using a Seemann-Bohlin diffractometer. The films were prepared by consecutive evaporation at room temperature on fused quartz substrates and subsequently annealed between − 2 and 100°C. The η′-phase (Cu6Sn5), which is ordered, was found to grow at all temperatures. In contrast the ordered e-phase (Cu3Sn) was found only in those specimens that had been annealed above 60°C. The formation of the η′-phase is interpreted in terms of interstitial diffusion of Cu into Sn at low temperatures. In specimens maintained at room temperature the unreactecl Sn and Cu layers were inferred to be in compression and tension, respectively. Tin whiskers were observed to grow spontaneously at room temperature from the Sn surface of the Cu-Sn bimetallic films, but not from Sn films without the Cu underlaver. The driving force for whisker growth is attributed to interdiffusion and reaction that occur in the bimetallic films.

Kirkendall void formation in eutectic SnPb solder joints on bare Cu and its effect on joint reliability

The electronic packaging industry has been using electroless Ni(P)∕immersion Au as bonding pads for solder joints. Because of the persistence of the black pad defect, which is due to cracks in the pad surface, the industry is looking for a replacement of the Ni(P) plating. Several Cu-based candidates have been suggested, but most of them will lead to the direct contact of solder with Cu in soldering. The fast reaction of solder with Cu, especially during solid state aging, may be a concern for the solder joint reliability if the package will be used in a high temperature environment and is highly stressed. In this work, the reaction of eutectic SnPb solder with electrodeposited laminate Cu is studied. Emphasis is given to the evolution of the microstructure in the interfacial region during solid state aging and its effect on solder joint reliability. A large number of Kirkendall voids were observed at the interface between Cu3Sn and Cu. The void formation resulted in weak bonding between solder and Cu and...

Low Schottky barrier of rare‐earth silicide on n‐Si

Disilicide of rare‐earth metals (Dy, Er, Ho, and Gd) and Y have been formed by reacting the metallic film on both n‐ and p‐type silicons at around 350 °C for Schottky‐barrier height measurement using I‐V technique. A passivation coating of W, or Pt, or both was used to prevent the rare earth from oxidation. Schottky‐barrier heights of about 0.4 eV on n‐Si and 0.7 eV on p‐Si were determined.

Physics and materials challenges for lead-free solders

At present, the electronic industry is actively searching for Pb-free solders due to environmental concerns over Pb-containing solders. Solder joints are widely used to bond chips to their substrates for electrical connection and packaging. Lacking reliability data, many electronic companies will be reluctant to adopt Pb-free solders in the advanced products. Hence, it is timely to review our understanding of structure-property relationship and potential reliability issues of Pb-free solders. A brief history of solder joint processes in electronic manufacturing is presented to serve as a background for the review. It emphasizes the unique phenomenon of spalling of interfacial intermetallic compound in solder reactions. Challenges for Pb-free solders from the point of view of physics and materials are given since the reliability issues of solder joints will remain with us when advanced Cu/low k dielectric interconnect technology is introduced into microelectronic devices.

Current-crowding-induced electromigration failure in flip chip solder joints

In a flip chip solder joint, the cross-section of the solder bump is one to two orders of magnitude bigger than that of an interconnect wire. At the contact interface between the bump and the wire, a very large current crowding occurs and it causes a unique and fast electromigration failure in the bump. Simulation of the current crowding phenomenon in a flip chip solder bump is reported here. Experimental results of real flip chip solder bumps show that void formation begins near the current crowding region of the contact, and after it is nucleated, it spreads quickly across the contact area. By designing the solder bump to achieve a uniform current distribution, we can improve its electromigration resistance and increase its current carrying capacity.