Investigation of the Growth of Intermetallic Compounds Between Cu Pillars and Solder Caps

In flip chip applications, Cu pillars with solder caps are regarded as next-generation electronic interconnection technology, because of high input/output density. However, because of diffusion and reaction of Sn and Cu during the high-temperature reflow process, intermetallic compounds (IMC) are formed, and grow, at the interface between the cap and the pillar. Understanding the growth behavior of interfacial IMC is critical in the design of solder interconnections, because excessive growth of IMC can reduce the reliability of connections. In this study, the growth of IMC during thermal cycling, an accelerated method of testing the service environment of electronic devices, was studied by use of focused ion beam–scanning electron microscopy. Under alternating high and low-temperature extremes, growth of Cu₆Sn₅ (η-phase) and Cu₃Sn (ε-phase) IMC was imaged and measured as a function of the number of cycles. The total IMC layer grew significantly thicker but became more uniform during thermal cycling. The Cu₃Sn layer was initially thinner than the Cu₆Sn₅ layer but outgrew the Cu₆Sn₅ layer after 1000 cycles. It was found that, with limited Cu and Sn diffusion, consumption of Cu₆Sn₅ for growth of the Cu₃Sn layer can result in a thinner Cu₆Sn₅ layer after thermal cycling.     

Effect of Cross-Interaction between Ni and Cu on Growth Kinetics of Intermetallic Compounds in Ni/Sn/Cu Diffusion Couples during Aging

The solid-state, cross-interaction between the Ni layer on the component side and the Cu pad on the printed circuit board (PCB) side in ball grid array (BGA) solder joints was investigated by employing Ni(15 μm)/Sn(65 μm)/Cu ternary diffusion couples. The ternary diffusion couples were prepared by sequentially electroplating Sn and Ni on a Cu foil and were aged isothermally at 150, 180, and 200°C. The growth of the intermetallic compound (IMC) layer on the Ni side was coupled with that on the Cu side by the mass flux across the Sn layer that was caused by the difference in the Ni content between the (Cu_1−x Ni _x )₆Sn₅ layer on the Ni side and the (Cu_1−y Ni _y )₆Sn₅ layer on the Cu side. As the consequence of the coupling, the growth rate of the (Cu_1−x Ni _x )₆ Sn₅ layer on the Ni side was rapidly accelerated by decreasing Sn layer thickness and increasing aging temperature. Owing to the cross-interaction with the top Ni layer, the growth rate of the (Cu_1−y Ni _y )₆Sn₅ layer on the Cu side was accelerated at 150°C and 180°C but was retarded at 200°C, while the growth rate of the Cu₃Sn layer was always retarded. The growth kinetic model proposed in an attempt to interpret the experimental results was able to reproduce qualitatively all of the important experimental observations pertaining to the growth of the IMC layers in the Ni/Sn/Cu diffusion couple.     

Influence of External Strain on the Growth of Interfacial Intermetallic Compounds Between Sn and Cu Substrates

This study used a four-point bending procedure to investigate the influence of compressive and tensile strain on the growth of an interfacial Cu-Sn intermetallic compound (IMC) layer. The test specimens were prepared by depositing 25?μm layers of matte or bright tin atop a copper substrate using electroplating. Samples were then placed in a furnace at 200°C, and external bending strain was applied through a strained substrate. Comparisons were made between samples undergoing tensile strain or compressive strain, and those without strain. We observed the influence of strain levels and aging time on the formation of the IMC. Both tensile and compressive strain influenced the formation of the Cu/Sn IMC. In matte tin samples, the IMC thickness increased under compressive strain and decreased under tensile strain. In contrast, in bright tin samples, the IMC thickness increased under both compressive and tensile strained substrate conditions. The growth rate of IMC was faster in strained bright tin samples than in strained matte tin samples. Moreover, the formation of IMC microscopic structures under external strain differed considerably according to the source of tin.     

Effect of Bi Segregation on the Asymmetrical Growth of Cu-Sn Intermetallic Compounds in Cu/Sn-58Bi/Cu Sandwich Solder Joints During Isothermal Aging

An asymmetrical interfacial microstructure was observed at both top and bottom interfaces of Cu/Sn-58Bi/Cu solder joints after isothermal aging at 120°C for different times. The asymmetrical interfacial microstructure resulted from asymmetrical Bi segregation, which was attributed to the density difference between Bi and Sn atoms. Bi atoms were driven to the bottom solder/Cu interface by gravity during the liquid soldering procedure since Bi atoms are more massive than Sn atoms. With increasing aging time, Bi accumulated at the bottom Cu₃Sn/Cu interface and the Bi segregation enhanced Cu₆Sn₅ intermetallic compound growth, blocked Sn transport to the Cu₃Sn intermetallic compound, and facilitated growth of the Cu₆Sn₅, based on the measured thicknesses of intermetallic compounds (including Cu₆Sn₅ and Cu₃Sn) at both bottom and top interfaces for Cu/Sn-58Bi/Cu sandwich joints under the same aging process.     

Thermal Aging Effects on Cu Ball Shear Strength and Cu/Al Intermetallic Growth

Intermetallic growth and ball shear behavior of annealed Cu wire bonds on Al have been studied. The shear strength of Cu ball bonds decreased with time, and ductile fracture was the dominant failure mode from 125°C to 150°C. Al pad peel-off occurred as the aging temperature was increased above 150°C. The overall Cu/Al intermetallic thickness exhibited parabolic behavior as a function of time. A linear correlation was established between ball shear strength, metal peel-off occurrence, and intermetallic growth. The Cu/Al intermetallic growth activation energy was 0.23 eV, and the intermetallics identified in the experiment were CuAl₂ and CuAl.     

Condensation and Growth of Kirkendall Voids in Intermetallic Compounds

A model for the simulation of Kirkendall voiding in metallic materials is presented based on vacancy diffusion, elastic-plastic and rate-dependent deformation of the material. Starting with a phenomenological explanation of the Kirkendall effect we briefly discuss the consequences on the reliability of microelectronic components. Then, a constitutive model for void nucleation and growth is introduced, which can be used to predict the temporal development of voids in solder joints during thermal cycling. We present numerical studies and discuss the potential of the results for the failure analysis of joining connections.     

Microstructure Changes and Physical Properties of the Intermetallic Compounds Formed at the Interface Between Sn-Cu Solders and a Cu Substrate Due to a Minor Addition of Ni

The influence of Cu content and added Ni on the morphology of the intermetallic compound (IMC) layer formed at the interface between liquid Sn-Cu-based solders and a Cu substrate and on the strength of simulated solder joints was investigated. The reaction of a Sn-0.7Cu alloy with the substrate led to the formation of a thin layer of Cu₆Sn₅ (η-phase) with typical scallop morphology that did not grow with longer reaction times. Higher Cu content such as in the Sn-1.4Cu alloy led to extensive growth with increased reaction time; at long reaction times, Cu₃Sn (ε-phase) was observed at the interface between the Cu substrate and the Cu₆Sn₅ layer. A small nickel addition to the Sn-0.7Cu alloy significantly changed the IMC morphology, accelerated its growth kinetics, prevented formation of the Cu₃Sn layer, and reduced the rate of substrate dissolution.     

Temperature Effect on Intermetallic Compound Growth Kinetics of Cu Pillar/Sn Bumps

The in situ intermetallic compound (IMC) growth in Cu pillar/Sn bumps was investigated by isothermal annealing at 120°C, 150°C, and 180°C using an in situ scanning electron microscope. Only the Cu₆Sn₅ phase formed at the interface between the Cu pillar and Sn during the reflow process. The Cu₃Sn phase formed and grew at the interfaces between the Cu pillar and Cu₆Sn₅ with increased annealing time. Total (Cu₆Sn₅ + Cu₃Sn) IMC thickness increased linearly with the square root of annealing time. The growth slopes of total IMC decreased after 240 h at 150°C and 60 h at 180°C, due to the fact that the Cu₆Sn₅ phase transforms to the Cu₃Sn phase when all of the remaining Sn phase in the Cu pillar bump is completely exhausted. The complete consumption time of the Sn phase at 180°C was shorter than that at 150°C. The apparent activation energy for total IMC growth was determined to be 0.57 eV.     

Early Interfacial Reaction and Formation of Intermetallic Compounds in the Sn-3.5Ag/Cu Soldering System

The early interfacial reaction in the Sn-3.5Ag/Cu soldering system and the system’s premelting behavior were found and characterized by differential scanning calorimetry incorporated into the reflow process. The results show that the early interfacial reaction occurs by way of melting and wetting of the solder layer adjacent to the Cu substrate at a temperature nearly 4°C below the actual melting point of Sn-3.5Ag solder due to solid-state diffusion of Cu atoms into the Sn-3.5Ag binary solder. Consequently, the early interfacial reaction brings about formation of Cu-Sn intermetallic compounds (IMCs) at a temperature below the melting point of Sn-3.5Ag, and a prolonged early interfacial reaction can lead to change of the Cu-Sn IMC morphology from planar-like to scallop-like and promote excessive growth of IMCs at the interface.     

Kinetics of Intermetallic Compound Formation at the Interface Between Sn-3.0Ag-0.5Cu Solder and Cu-Zn Alloy Substrates

The growth kinetics of an intermetallic compound (IMC) layer formed between Sn-3.5Ag-0.5Cu (SAC) solders and Cu-Zn alloy substrates was investigated for samples aged at different temperatures. Scallop-shaped Cu₆Sn₅ formed after soldering by dipping Cu or Cu-10 wt.%Zn wires into the molten solder at 260°C. Isothermal aging was performed at 120°C, 150°C, and 180°C for up to 2000 h. During the aging process, the morphology of Cu₆Sn₅ changed to a planar type in both specimens. Typical bilayer of Cu₆Sn₅ and Cu₃Sn and numerous microvoids were formed at the SAC/Cu interfaces after aging, while Cu₃Sn and microvoids were not observed at the SAC/Cu-Zn interfaces. IMC growth on the Cu substrate was controlled by volume diffusion in all conditions. In contrast, IMC growth on Cu-Zn specimens was controlled by interfacial reaction for a short aging time and volume diffusion kinetics for a long aging time. The growth rate of IMCs on Cu-Zn substrates was much slower due to the larger activation energy and the lower layer growth coefficient for the growth of Cu-Sn IMCs. This effect was more prominent at higher aging temperatures.     

The Effects of Solid-State Aging on the Intermetallic Compounds of Sn-Ag-Bi-In Solders on Cu Substrates

The growth behavior of the intermetallic compounds that formed at the interfaces between Sn-Ag-Bi-In solders and Cu substrates during solid-state aging is investigated. The compositions of the intermetallic compounds are Cu₃(Sn,In) near the Cu substrates and Cu₆(Sn,In)₅ near the solders; very little Bi or Ag was dissolved in the compounds. The aging temperatures were 120°C, 150°C, and 180°C for 5 days, 10 days, 20 days, and 40 days. The change in the morphology of Cu₆(Sn,In)₅ from scallop type to layer type was prominent at the aging temperature of 180°C. The thickness of the compound layers did not vary much at the lower aging temperatures but followed the diffusion- controlled mechanism at 180°C. Massive Kirkendall voids were observed in Cu₃(Sn,In) layers at the aging temperature of 180°C.     

Growth of Intermetallic Compounds in Thermosonic Copper Wire Bonding on Aluminum Metallization

Interface evolution caused by thermal aging under different temperatures and durations was investigated by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that approximately 30-nm-thick and discontinuous Cu-Al intermetallic compounds (IMCs) were present in the initial bonds before aging. Cu-Al IMCs grew under thermal aging with increasing aging time. The growth kinetics of the Cu-Al IMCs was correlated to the diffusion process during aging; their combined activation energy was estimated to be 1.01 eV. Initially, Al-rich Cu-Al IMCs formed in the as-bonded state and early stage of aging treatment. Cu₉Al₄ was identified by selected-area electron diffraction (SAD) as the only type of Cu-Al IMC present after thermal aging at 250°C for 100 h; this is attributed to the relatively short supply of aluminum to the interfacial reaction.     

Intermetallic Compound Growth and Reliability of Cu Pillar Bumps Under Current Stressing

Fine-pitch Cu pillar bumps have been adopted for flip-chip bonding technology. Intermetallic compound (IMC) growth in Cu pillar bumps was investigated as a function of annealing or current stressing by in situ observation. The effect of IMC growth on the mechanical reliability of the Cu pillar bumps was also investigated. It is noteworthy that Sn exhaustion was observed after 240 h of annealing when current stressing was not applied, and IMC growth rates were changed remarkably. As the applied current densities increased, the time required for complete Sn consumption became shorter. In addition, Kirkendall voids, which would be detrimental to the mechanical reliability of Cu pillar bumps, were observed in both Cu₃Sn/Cu pillars and Cu₃Sn/Cu under-bump metallization interfaces. Die shear force was measured for Cu pillar samples prepared with various annealing times, and degradation of mechanical strength was observed.     

Effect of Stress State on Growth of Interfacial Intermetallic Compounds Between Sn-Ag-Cu Solder and Cu Substrates Coated with Electroless Ni Immersion Au

Excessive intermetallic compound (IMC) growth in solder joints will significantly decrease the reliability of the joints. IMC growth is known to be influenced by numerous factors during the component fabrication process and in service. It is reported that, other than temperature and holding time, stress can also influence the IMC growth behavior. However, no existing method can be used to study the effect of stress state on IMC growth in a controlled manner. This paper presents a novel method to study the effect of stress on interfacial IMC growth between Sn-Ag-Cu solder and a Cu substrate coated with electroless Ni immersion Au (ENIG). A C-ring was used and in-plane bending induced tensile and compressive stresses were applied by tightening the C-ring. Results revealed that in-plane compressive stress led to faster IMC growth as compared with in-plane tensile stress.     

Morphology and Growth of Intermetallics at the Interface of Sn-based Solders and Cu with Different Surface Finishes

Several types of surface finishes have been applied on Cu substrates in an effort to facilitate bonding and improve the reliability of lead-free solder joints. In the current research, the effects of printed circuit board surface finishes on the reliability of the solder joints were investigated by examining the morphology and growth behavior of the intermetallic compounds (IMCs) between Sn-based solders and different surface finishes on Cu. Three types of Cu substrates with different surface finishes were fabricated in this study: organic solderability preservative (OSP)/Cu, Ni/Cu, and electroless nickel immersion gold (ENIG)/Cu. Sn-3.5Ag and Sn-3.0Ag-0.5Cu were used as the solders. In the experiment, the solder joint specimens were aged isothermally at 150°C for up to 1000 h. Experimental results revealed that the OSP surface finish promoted the interdiffusion between Cu and Sn during soldering. The composition and morphology of the IMC layer at the solder/Ni/Cu interface were sensitive to the Cu concentration in the solder. Meanwhile, the solder joints with different morphological features of the IMCs exhibited significant differences in shear strengths. The Au-containing ENIG surface finish affected the shear strength of the solder joint significantly at the initial stage of isothermal aging.     

Fabrication of Mg-Based Intermetallic Compounds by Liquid Electromigration

In this study, we successfully produced the compound phase Mg₂Ni by using the liquid electromigration (EM) method. The application of a high-density current flow through the Ni/molten Mg/Ni sandwich structure caused the Ni atoms in the molten Mg to electromigrate toward the anode interface to form a thick layer of compound Mg₂Ni at the anode’s molten Mg/Ni interface. The formation of the interfacial Mg₂Ni compound was much larger than that which occurs in the no-current case. This implies that the growth of the interfacial Mg₂Ni compound can be enhanced by current stressing. Remarkably, as can be seen from the X-ray diffraction (XRD) analysis, the EM-grown Mg₂Ni phase is shown to be indicative of texture in nature, that is, only one single peak [(110) plane] appears in the XRD diffraction pattern. This observation suggests that the formation of EM-grown Mg₂Ni compound at the anode interface closely corresponds to the direction of electron flow.     

Morphological Evolution of the Reaction Product at the Sn-9wt.%Zn/Thin-Film Cu Interface

The morphological evolution of the reaction product formed at the Sn-9wt.%Zn/thin-film Cu interface under reflow and solid-state aging was investigated. The Cu thin film was rapidly consumed and converted to CuZn₅ and Cu₅Zn₈ at the interface after reflow for 1 min. Upon increasing reflow time, the Cu₅Zn₈ compound was transformed into CuZn₅, followed by grain ripening. CuZn₅ grains grew bigger and the number of grains decreased. During the reflow process, CuZn₅ grains showed a round and convex surface morphology. During solid-state aging, a different morphological evolution of CuZn₅ grains was observed. The surface morphology of the grains became more planar after solid-state aging for 24 h. In addition, the grain surface fractured severely, implying that a compressive stress was created at the interface. After a longer duration of solid-state aging, the grain surface changed into a more faceted morphology. Potential mechanisms of the morphological evolution and fracture of the CuZn₅ grains are also discussed.     

Effect of Cu Thickness on the Evolution of the Reaction Products at the Sn-9wt.%Zn Solder/Cu Interface During Reflow

Interfacial reactions between Sn-9wt.%Zn solder and Cu substrates at 230°C were investigated. The substrate thickness was found to have noticeable effects on the evolution of the reaction products formed at the solder/Cu interface. The CuZn₅ and Cu₅Zn₈ phases were formed at the early stage of reflow, regardless of the Cu thickness, while, with increasing reflow time, the two phases displayed different growth behaviors on the Cu substrates with various thicknesses. For the thicker Cu substrates with a thickness of 6 μm, 10 μm, and 0.5 mm, CuZn₅ disappeared but Cu₅Zn₈ kept on growing after a longer reflow time. In contrast, for the thinner Cu substrates with a thickness less than 3 μm, Cu₅Zn₈ shrank with increasing reflow time but CuZn₅ grew dominantly. A different evolution of the grain morphology of CuZn₅ was also observed between the thicker and thinner Cu substrates. When the reflow time was increased, the CuZn₅ grains retained a rounded shape on the thinner Cu substrates; however, the grain structure became faceted on the thicker Cu substrates.     

铜线键合Cu/Al界面金属间化合物微结构研究

由于铜线具有较高的热导率、卓越的电学性能以及较低的成本,被普遍认为将逐渐代替传统的金线而在IC封装的键合工艺中得到广泛的应用。铜线键合工艺中Cu/Al界面金属间化合物(IMC)与金线键合的Au/Al IMC生长情况有很大差别,本文针对球焊键合中键合点的Cu/Al界面,将金属间化合物生长理论与分析手段相结合,研究了Cu/Al界面IMC的生长行为及其微结构。文中采用SEM测试方法,观察了IMC的形貌特点,测量并得到了IMC厚度平方正比于热处理时间的关系,计算得到了生长速率和活化能数值,并采用TEM,EDS等测试手段,进一步研究了IMC界面的微结构、成分分布及其金相结构。