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Consumer electronic applications are the primary target of the Pb-free initiative and package assembly and performance is affected by the move from eutectic Sn-Pb to Pb-free solder alloys. This paper outlines the key issues and mitigation possibilities for package assembly using Pb-free solders: High temperature reflow, Interfacial reactions, and Reliability. At the high temperatures required to reflow Pb-free alloys, moisture absorbed into the package can result in delamination and failure. The reaction of the Pb-free solder with Ni and Cu metallizations results in interfacial intermetallics that are not significantly thicker than with Sn-Pb but provide a path for fracture under mechanical loading due to the increased strength of the Pb-free alloys. The reliability issues discussed include thermomechanical fatigue, mechanical shock, electromigration and whiskering. The Pb-free alloys tend to improve thermomechanical fatigue and electromigration performance but are detrimental to mechanical shock and whiskering. Design trade-offs must be made to successfully implement Pb-free alloys into consumer applications.

Intermetallic compound (IMC) growth during solid-state aging at 125, 150, and 170 °C up to 1500 h for four solder alloys (eutectic SnPb, Sn–3.5Ag, Sn–3.8Ag–0.7Cu, and Sn–0.7Cu) on Cu under bump metallization was investigated. The samples were reflowed before aging. During the reflow, the solders were in the molten state and the formation of the IMC Cu6Sn5 in the cases of eutectic SnPb and Sn–3.5Ag had a round scallop-type morphology, but in Sn–0.7Cu and Sn–3.8Ag–0.7Cu the scallops of Cu6Sn5 were faceted. In solid-state aging, all these scallops changed to a layered-type morphology. In addition to the layered Cu6Sn5, the IMC Cu3Sn also grew as a layer and was as thick as the Cu6Sn5. The activation energy of intermetallic growth in solid-state aging is 0.94 eV for eutectic SnPb and about 1.05 eV for the Pb-free solders. The rate of intermetallic growth in solid-state aging is about 4 orders of magnitude slower than that during reflow. Ternary phase diagrams of Sn–Pb–Cu and Sn–Ag–Cu are used to discuss the reactions. These diagrams predict the first phase of IMC formation in the wetting reaction and the other phases formed in solid-state aging. Yet, the morphological change and the large difference in growth rates between the wetting reaction and solid-state aging cannot be predicted.

A variety of lead-free solder alloys were studied for use as flip-chip interconnects including Sn-3.5Ag, Sn-0.7Cu, Sn-3.8Ag-0.7Cu, and eutectic Sn-37Pb as a baseline. The reaction behavior and reliability of these solders were determined in a flip-chip configuration using a variety of under-bump metallurgies (TiW/Cu, electrolytic nickel, and electroless Ni-P/Au). The solder micro-structure and intermetallic reaction products and kinetics were determined. The Sn-0.7Cu solder has a large grain structure and the Sn-3.5Ag and Sn-3.8Ag-0.7Cu have a fine lamellar two-phase structure of tin and Ag3Sn. The intermetallic compounds were similar for all the lead-free alloys. On Ni, Ni3Sn4 formed and on copper, Cu6Sn5Cu3Sn formed. During reflow, the intermetallic growth rate was faster for the lead-free alloys, compared to eutectic tin-lead. In solidstate aging, however, the interfacial intermetallic compounds grew faster with the tinlead solder than for the lead-free alloys. The reliability tests performed included shear strength and thermomechanical fatigue. The lower strength Sn-0.7Cu alloy also had the best thermomechanical fatigue behavior. Failures occurred near the solder/intermetallic interface for all the alloys except Sn-0.7Cu, which deformed by grain sliding and failed in the center of the joint. Based on this study, the optimal solder alloy for flip-chip applications is identified as eutectic Sn-0.7Cu.

The reaction kinetics of eutectic SnPb solder on Cu were studied and compared in the liquid state at 200 to 240 °C and in the solid state aged at 125–170 °C. The ternary phase diagrams of SnPbCu, the morphology of intermetallic compound (IMC), and the kinetics of growth of the intermetallics were used in the comparison. The temperature difference between these two reactions is only 30 °C, but the kinetics of reaction, as well as the morphology of IMC formation, are very different. The kinetics in the wetting reaction is four orders of magnitude faster than that in solid state aging. The Cu6Sn5 intermetallic morphology in solid state aging is a layer type, but it has a scallop-type morphology in the wetting reaction. The morphology strongly affects the kinetics. While the kinetic difference can be attributed to the difference in atomic diffusivity between the liquid state and the solid state, it is the morphology that determines the kinetic path in these reactions. We conclude that a fast rate of reaction,...

The electromigration of eutectic SnPb solder interconnects between a Si chip and a FR4 substrate was studied at 120 °C for up to 324 h with current stressing of 104 amp/cm2. Hillocks were observed at the anode and voids at the cathode. The dominant diffusing species was found to be Pb, confirmed by its accumulation at the anode. Diffusion markers were used to measure the electromigration flux and calculate the effective charge of atomic diffusion in the solder. Extensive microstructural evolution was also observed in the two-phase solder alloy that occurred by a ripening process.

A variety of Pb-free solder alloys have been proposed for use as interconnects for electronic packaging including Sn−Ag, Sn−Cu, Sn−Ag−Cu, Sn−Ag−Bi, and Sn−Sb, among others. This paper presents a review of the behavior of promising Pb-free solder alloys as related to their microstructure. Recommendations of optimal alloy composition as a function of performance requirements are given. For surface mount applications, eutectic Sn−Ag−Cu is recommended as the optimal alloy. For flip chip interconnects, the eutectic Sn-Cu alloy has the best performance. The materials and process trends of Pb-free packaging are summarized with optimal conditions identified.

The wetting reaction between molten eutectic SnPb solder and a sputtered trilayer Cu/Ni(V)/Al thin film metallization was studied using cross-sectional transmission electron microscopy and scanning electron microscopy. Reaction temperatures were from 200 to 240 °C and reaction times ranged from 1 to 40 min. The initial reaction products were Cu6Sn5 and Cu3Sn. The latter transforms to the former after an annealing greater than 1 min at 220 °C. The Cu6Sn5 grains adhere well to the Ni(V) surface and no spalling of them was observed, even after 40 min at 220 °C. This surprising result indicates that the Cu/Ni(V)/Al or Cu6Sn5/Ni(V)/Al is a stable thin film metallization for low temperature eutectic SnPb solder direct chip attachment to organic substrates. Additionally, Kirkendall voids accompanied Cu3Sn formation, yet the voids disappear when the Cu3Sn transforms to Cu6Sn5.

In interfacial reactions, a short-circuit diffusion along grain boundaries or interfaces can be accompanied by intermetallic compound formation. The compound penetrates the grain boundaries or the interfaces. This is a generic reliability issue for layered thin film structures because it causes a decrease in adhesion strength of the thin films. We have modified Fisher’s grain boundary diffusion model to include this reactive kinetic process, and an analytical solution was obtained. A t1/4 dependence of penetration is found, the same as Fisher’s model. The important kinetic parameters in the solutions are a diffusion coefficient along the short-circuit path, an intrinsic interdiffusion coefficient in the compound, and a partition coefficient. A comparison between the calculated and measured data from the lateral penetration of eutectic SnPb solder along the interface between electroless Ni and oxysilicon nitride dielectric, accompanied by Ni3Sn4 compound formation, is given.

The compressive creep and stress-strain behavior of the near-eutectic 60Sn-40Pb solder alloy has been investigated over the temperature range of −55 °C to 125 °C. The total primary creep strain is a strong function of stress and temperature: at lower temperatures and high applied stresses (i.e., near the power-law breakdown regime), it is quite large, while it is much smaller at higher temperatures and lower applied stresses. The compressive minimum creep rate as a function of stress and temperature is fit well by the Garofalo sinh equation. A discussion of the effective stress exponent, n eff, in the context of the Garofalo sinh equation is presented to understand trends in the creep data. The values of n eff, for the applied stress levels studied, are found to range from 3.09 to 5.00 at 125 °C, while they have a range of 10.75 to 15.79 at −55 °C. These trends are consistent with the interpretation of climb-dominated creep at higher temperatures and plasticity-dominated power law breakdown behavior at the lower temperatures. The microstructural observations suggest that, at elevated temperatures, deformation occurs by relative displacement of eutectic colonies in the solder microstructure accompanied by extensive grain coarsening in the colony boundaries. At lower temperatures (

The most commonly used solder for electrical interconnects in electronic packages is the near eutectic 60Sn‐40Pb alloy. This alloy has a number of processing advantages(suitable melting point of 183°C and good wetting behaviour). However, under conditions of cyclic strain and temperature (thermomechanical fatigue) the microstructure of this alloy undergoes a heterogeneous coarsening and failure process that makes the prediction of solder joint lifetime complex. A finite element simulation methodology to predict solder joint mechanical behaviour, that includes microstructural evolution, has been developed. The mechanical constitutive behaviour was incorporated into the time‐dependent internal state variable viscoplastic model through experimental creep tests. The microstructural evolution is incorporated through a series of mathematical relations that describe mass flow in a temperature/strain environment. The model has been found to simulate observed thermomechanical fatigue behaviour in solder joints.

A variety of Pb-free solders and under bump metallurgies (Cu, Ni, NiP) were investigated for flip chip packaging applications. The result shows that Sn-0.7Cu exhibits the most desirable mechanical properties (shear, tensile, aging, etc.) during deformation under a variety of stress conditions and has the most favorable failure mechanism under both mechanical and thermomechanical stress testing regardless of UBM type. The eutectic Sn-0.7Cu failed through bulk solder while the eutectic Sn-37Pb, Sn-3.5Ag and Sn-3.8Ag-0.7Cu failed at the solder and UBM interface, involving their respective intermetallic compounds. Cu UBM is more favorable for better reliability than NiP UBM from both interface IMC morphology and electromigration points of view. The current carrying capability for all alloys had no failures when stressed up to 2,338 hours at 2.6/spl times/10/sup 4/ A/cm/sup 2/ and 170/spl deg/C. However, when stressed at 5.1/spl times/10/sup 4/ A/cm/sup 2/, there is a significant migration of Pb toward the anode, creating a multiple layered Pb-rich and Sn-rich microstructure. An observation of excessive Ni migration away from the NiP UBM towards the anode after only 30 hours of current stressing at 5.1/spl times/10/sup 4/ A/cm/sup 2/ and 150/spl deg/C raised the reliability concern for solders with NiP UBM, especially for high power applications. The Sn-0.7Cu/Cu UBM and Sn0.7Cu/Ni UBM exhibit greater than 5,300 cycles of thermal fatigue characteristic life under -55/spl deg/C 1+150/spl deg/C and -40/spl deg/C /+125/spl deg/C air-to-air thermal cycling conditions, respectively.

The thermomechanical fatigue failure of solder joints is increasingly becoming an important reliability issue for electronic packages. The purpose of this Laboratory Directed Research and Development (LDRD) project was to develop computational tools for simulating the behavior of solder joints under strain and temperature cycling, taking into account the microstructural heterogeneities that exist in as-solidified near eutectic Sn-Pb joints, as well as subsequent microstructural evolution. The authors present two computational constitutive models, a two-phase model and a single-phase model, that were developed to predict the behavior of near eutectic Sn-Pb solder joints under fatigue conditions. Unique metallurgical tests provide the fundamental input for the constitutive relations. The two-phase model mathematically predicts the heterogeneous coarsening behavior of near eutectic Sn-Pb solder. The finite element simulations with this model agree qualitatively with experimental thermomechanical fatigue tests. The simulations show that the presence of an initial heterogeneity in the solder microstructure could significantly degrade the fatigue lifetime. The single-phase model was developed to predict solder joint behavior using materials data for constitutive relation constants that could be determined through straightforward metallurgical experiments. Special thermomechanical fatigue tests were developed to give fundamental materials input to the models, and an in situ SEM thermomechanical fatiguemore » test system was developed to characterize microstructural evolution and the mechanical behavior of solder joints during the test. A shear/torsion test sample was developed to impose strain in two different orientations. Materials constants were derived from these tests. The simulation results from the two-phase model showed good fit to the experimental test results.« less

The electromigration behavior of Sn–2.5Ag and Sn–0.7Cu (in wt %) flip chip solder joints on electroless NiP/Cu metallization at a current density of 1.3×104 A/cm2 was studied. For Sn-2.5 Ag solder, electromigration at 115 °C for 250 h showed a selective dissolution of Ni from the electroless NiP layer forming crystallized Ni3P. At 140 °C, the damage to the NiP layer was accelerated and instability of the NiP/Cu interface was observed. For eutectic Sn–0.7Cu solder, the electromigration behavior at a higher temperature was evaluated. At 180 °C, the NiP/Cu under bump metallurgy (UBM) started to show damage after 50 h. At 200 °C, the entire NiP/Cu layer was damaged, and P in the NiP layer moved to the edge of the anode much faster than the other species forming CuP2 intermetallics. NiP/Cu UBM experiences selective dissolution of Ni at lower temperatures, and the damage of the entire UBM occurred abruptly at the higher temperature.

This paper discusses the microstructures of solder joints and the mechanism of thermal fatigue, which is an important source of failure in electronic devices. The solder joints studied were near‐eutectic Pb‐Sn solder contacts on copper. The microstructure of the joints is described. While the fatigue life of near‐eutectic solder joints is strongly dependent on the operating conditions and on the microstructure of the joint, the metallurgical mechanisms of failure are surprisingly constant. When the cyclic load is in shear at temperatures above room temperature the shear strain is inhomogeneous, and induces a rapid coarsening of the eutectic microstructure that concentrates the deformation in well‐defined bands parallel to the joint interface. Fatigue cracks propagate along the Sn‐Sn grain boundaries and join across the Pb‐rich regions to cause ultimate failure. The failure occurs through the bulk solder unless the joint is so thin that the intermetallic layer at the interface is a significant fraction of the joint thickness, in which case failure may be accelerated by cracking through the intermetallic layer. The coarsening and subsequent failure are influenced more strongly by the number of thermal cycles than by the time of exposure to high temperature, at least for hold times up to one hour. Thermal fatigue in tension does not cause well‐defined coarsened bands, but often leads to rapid failure through cracking of the brittle intermetallic layer. Implications are drawn for the design of accelerated fatigue tests and the development of new solders with exceptional fatigue resistance.