Growth Behavior Of Intermetallic Compounds Research Articles

Growth behavior of intermetallic compounds during reactive diffusion between aluminum alloy 1060 and magnesium at 573–673 K

A potential new research reactor fuel design proposes to use U–Mo fuel in a Mg matrix clad with Al. Interdiffusion between the Mg containing fuel core and Al cladding can result in the formation of intermetallic compounds that can be detrimental to fuel element performance. The kinetics of the reactive diffusion in the binary Al–Mg system was experimentally studied. Layers of the intermetallic compounds, β (Al3Mg2) and γ (Al12Mg17) phases, were formed between the Al alloy 1060 and Mg during annealing. The β layer was observed to grow faster than the γ phase. The thickness of each layer can be expressed by a power function of the annealing time with the exponent n close to 0.5 for the β phase and less than 0.5 for the γ phase. The results suggest that the growth of β phase is controlled by lattice diffusion and that of the γ phase by grain boundary and lattice diffusion. Metallographic examination showed the grain boundary diffusion in the form of columnar growth of γ phase during annealing. Based on the reactive diffusion equation developed in this work, in the absence of irradiation effects, it will take more than 110h to consume a half thickness of 400μm of the cladding.

Microstructure, thermal analysis and damping properties of Ag and Ni nano-particles doped Sn–8Zn–3Bi solder on OSP–Cu substrate

This paper investigates the effects of reaction time and temperature on the morphology and growth mechanism of intermetallic compound (IMC) layers of plain Sn–8Zn–3Bi solder and solders containing Ag, Ni nano-particles on Organic Solderability Preservative (OSP)–Cu substrate. At the interface of the plain Sn–8Zn–3Bi solder/OSP–Cu system, a scallop-shaped Cu5Zn8 IMC layer was clearly observed. However, after adding Ag nano-particles into the Sn–8Zn–3Bi solder, an additional island-type AgZn3 IMC layer was found at the top surface of the scallop-shaped Cu5Zn8 IMC layer. Moreover, in the Sn–8Zn–3Bi–0.5Ni solder/OSP–Cu system, a ternary (Cu, Ni)–Zn IMC layer was well adhered at their interfaces. Furthermore, in solder ball regions a needle-shaped α-Zn and spherically-shaped Bi phases were clearly observed in the β-Sn matrix of plain Sn–8Zn–3Bi solder. However, in the solder alloys containing Ag and Ni nano-particles an additional very fine AgZn3 IMC particles for Ag doped solder and Zn–Ni IMC particles for Ni doped solder was uniformly distributed as well as the needle-shaped α-Zn and spherically-shaped Bi phases in the β-Sn matrix. These IMC layer thicknesses were increased with the reaction time and temperature. However, after the addition of the Ag and Ni nano-particles, the IMC growth behavior was slower than that of the plain solder system.Damping capacity of plain Sn–8Zn–3Bi solder alloy and solder alloys containing Ag, Ni nano-particles were measured by dynamic mechanical analyzer (DMA). The damping capacity of composite solder alloys exhibited higher values than that of plain Sn–8Zn–3Bi solder alloy which is essential to suppress the mechanical vibration and wave propagation. Furthermore, the composite solder alloys doped with Ag, Ni nano-particles improved the oxidation resistance than that of plain Sn–8Zn–3Bi solder alloy by the formation of fine Zn containing IMC particles which block the penetration of vapor and oxygen into the solder matrix.

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 (e-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 Cu–Sn intermetallic compound reactions on the Kirkendall void growth characteristics in Cu/Sn/Cu microbumps

Growth behaviors of intermetallic compounds (IMCs) and Kirkendall voids in Cu/Sn/Cu microbump were systematically investigated by an in-situ scanning electron microscope observation. Cu–Sn IMC total thickness increased linearly with the square root of the annealing time for 600 h at 150 °C, which could be separated as first and second IMC growth steps. Our results showed that the growth behavior of the first void matched the growth behavior of second Cu6Sn5, and that the growth behavior of the second void matched that of the second Cu3Sn. It could be confirmed that double-layer Kirkendall voids growth kinetics were closely related to the Cu–Sn IMC growth mechanism in the Cu/Sn/Cu microbump, which could seriously deteriorate the mechanical and electrical reliabilities of the fine-pitch microbump systems.

Electromigration in thin-film solder joints

Intermetallic compound (IMC) formation and growth are critical reliability issues with interconnected microelectronics, especially for small-scale solder joints in three-dimensional integrated circuits (3D IC) packaging. In this study, the growth behavior of IMCs in ultrathin 3D-IC solder joints under electromigration was investigated using a field-emission scanning electron microscope (FE-SEM) equipped with an electron backscatter diffraction (EBSD) analysis system. The joints consisted of a Cu/Sn/Cu configuration with a Sn thickness of ~30μm. We determined a strong dependence of the IMC growth and Cu pad depletion on the β-Sn orientation upon electron current stressing. When the c-axis of the Sn grain was oriented in the direction of the electron flow, aggressive CuSn IMC growth and Cu pad depletion occurred at the anode and cathode, respectively. A mathematical analysis based on crystallography and electromigration theory was used to rationalize the strong dependences that were observed.

In situ study of the real-time growth behavior of Cu6Sn5 at the Sn/Cu interface during the soldering reaction

The growth behavior of intermetallic compound (IMC) at the Sn/Cu interface during the soldering reaction was studied using synchrotron radiation real-time imaging technology. The annexation behavior of adjacent IMC grains, which slowed down with the soldering process, was directly observed for the first time. The IMC grains undergoing annexation tended to maintain a hemispherical morphology which was influenced by the radius of curvature of the nearest IMC grain. The annexation behavior was driven by the Gibbs–Thomson effect.

Effect of Trace Diamond Nanoparticle Addition on the Interfacial, Mechanical, and Damping Properties of Sn-3.0Ag-0.5Cu Solder Alloy

In the present study, small amounts of nanodiamond particles were added to Sn-3.0Ag-0.5Cu eutectic solder to investigate the growth kinetics of the intermetallic compound (IMC) layer at ball grid array pad/solder interfaces. The IMC growth behavior was found to be comparatively slower than that of plain solder joints due to a change in the diffusivity of the constituent atoms and the thermodynamic parameters of elemental affinity. The solder joints containing nanodiamond particles consistently showed higher hardness and strength than plain Sn-3.0Ag-0.5Cu solder joints, after a number of reflow cycles. The hardness of the doped solder was enhanced due to the homogeneous dispersion of nanoparticles, refining IMC phases which act as reinforcements in the solder matrix and as barriers to movement of dislocations. The fracture surface after shear testing of plain solder exhibited a relatively smooth fracture surface, while doped solder joints showed ductile failures with very rough dimpled surfaces. The damping capacity of the doped solder was better than the plain solder at a wide range of temperatures, and the solder with nanoparticle diamond exhibited lower internal friction.

Interfacial reactions between Sn–3.0Ag–0.5Cu solder and Cu–xZn (x=0–35wt%) or Cu–xZn–yNi (x=20 and 25wt%, y=15 and 10wt%) substrates

Abstract The interfacial reactions of Sn–3.0Ag–0.5Cu (SAC 305) solder on Cu– x Zn ( x = 0 wt%, 10 wt%, 15 wt%, 30 wt%, and 35 wt%) substrates and Cu– x Zn– y Ni ( x = 20 wt% and 25 wt%, y = 10 wt% and 15 wt%) were investigated after soldering and isothermal aging. SAC 305 solder was reacted with Cu– x Zn and Cu– x Zn– y Ni substrates by dipping Cu– x Zn or Cu– x Zn– y Ni wires into molten solder at 250 °C for 90 s. After soldering, Cu 6 Sn 5 -based intermetallic compound (IMC) formed at the solder interfaces. After aging at 150 °C, a bi-layer structure of Cu 6 Sn 5 /Cu 3 Sn formed, and microvoids were observed in the Cu 3 Sn layer at the SAC/Cu solder interfaces. For SAC/Cu–Zn samples, Cu 6 (Sn, Zn) 5 was the predominant reaction product, and formation of Cu 3 Sn was not observed. Interfacial IMC growth was delayed as the Zn concentration increased. In addition, the Cu(Zn, Sn) phase formed at the Cu 6 (Sn, Zn) 5 /Cu–Zn interfaces in the Cu– x Zn substrates with a high Zn concentration (30 wt% and 35 wt%). In the case of the Cu– x Zn– y Ni samples, the major interfacial IMC was (Cu, Ni) 6 (Sn, Zn) 5 phase. The Ni atoms from the Cu– x Zn–Ni substrates participated in the interfacial reaction at the SAC/Cu– x Zn– y Ni interfaces and resulting in the formation of a thick and layered IMC phase, comparing to those at the SAC/Cu–Zn interfaces. Although the effect of Ni addition to the Cu–Zn substrates exhibited delayed IMC growth, the Zn concentration in the Cu– x Zn– y Ni substrates influenced the IMC growth behavior more than the Ni concentration.

Growth behaviors of intermetallic compounds at Sn–3Ag–0.5Cu/Cu interface during isothermal and non-isothermal aging

The morphology and growth mechanism of intermetallic compound (IMC) layer at Sn–3Ag–0.5Cu/Cu interface during isothermal aging, thermal cycling and thermal shock were investigated in this study by microstructural observations and phase analysis. The results showed that the IMC layer flattened with aging duration because the grooves in scallop-like IMC provide a more convenient access for Cu atoms to dissolve and react with solders and previous IMCs. When isothermal aging was subjected, the growth rate of Cu6Sn5 was lower than that of Cu3Sn. While for thermal cycling, it was Cu6Sn5 that contributed much to the growth of IMC layer. This had much to do with abundant supply of Sn and Cu atoms at the interface of solder/Cu6Sn5 caused by recrystallization of solder and compression stress during thermal cycling. Growth mechanisms of total IMC layer both during isothermal and non-isothermal aging are stated as empirical power-law relationship by using an equivalent aging time parameter. And the growth time exponent depended on the evolution of grain structures of solder matrix, thermal stress and propagation of cracks during non-isothermal conditions.

Intermetallic growth study on SnAgCu/Cu solder joint interface during thermal aging

The intermetallic compound (IMC) growth behavior at SnAgCu/Cu solder joint interface under different thermal aging conditions was investigated, in order to develop a framework for correlating IMC layer growth behavior between isothermal and thermomechanical cycling (TMC) effects. Based upon an analysis of displacements for actual flip-chip solder joint during temperature cycling, a special bimetallic loading frame with single joint-shear sample as well as TMC tests were designed and used to research the interfacial IMC growth behavior in SnAgCu/Cu solder joint, with a focus on the influence of stress–strain cycling on the growth kinetics. An equivalent model for IMC growth was derived to describe the interfacial Cu-Sn IMC growth behavior subjected to TMC aging as well as isothermal aging based on the proposed “equivalent aging time” and “effective aging time”. Isothermal aging, thermal cycling (TC) and TMC tests were conducted for parameter determination of the IMC growth model as well as the growth kinetic analysis. The SnAgCu/Cu solder joints were isothermally aged at 125, 150 and 175 °C, while the TC and TMC tests were performed within the temperature range from −40 to 125 °C. The statistical results of IMC layer thickness showed that the IMC growth for TMC was accelerated compared to that of isothermal aging based on the same “effective aging time”. The IMC growth model proposed here is fit for predicting the IMC layer thickness for SnAgCu/Cu solder joint after any isothermal aging time or thermomechanical cycles. In addition, the results of microstructure evolution observation of SnAgCu/Cu solder joint subjected to TMC revealed that the interfacial zone was the weak link of the solder joint, and the interfacial IMC growth had important influence on the thermomechanical fatigue fracture of the solder joint.

Interfacial reaction and growth behavior of IMCs layer between Sn–58Bi solders and a Cu substrate

This paper reports on the interfacial reaction and growth behavior of intermetallic compounds (IMCs) layer (η-Cu6Sn5 + e-Cu3Sn) between molten Sn–58Bi solder and Cu substrate for various liquid–solid soldering temperatures and times. In addition, the Bi segregation at the Cu3Sn/Cu interface was also discussed, too. It was found that the Cu6Sn5 IMC could be observed as long as the molten solder contacted with the Cu substrate, while the Cu3Sn IMC was formed at the interface between Cu6Sn5 and Cu substrate as the higher soldering temperature and/or longer soldering time were applied. Both thickness of total IMCs layer and Cu6Sn5 grains size increased with increased soldering temperature or time. The growth of the Cu-Sn IMCs layer during soldering exhibited a linear function of the soldering temperature and 0.27 power of soldering time. With soldering temperature increasing (above 280 °C in this present study), Bi was accumulated at the Cu3Sn/Cu interface and resulted in some isolated Bi particles were formed.

Correlation of intermetallic compound growth behavior and melt state of Sn–3.5Ag–3.5Bi/Cu joint during soldering and isothermal aging

The intermetallic compound (IMC) growth behavior of Sn–3.5Ag–3.5Bi/Cu joint was investigated with a change in the solder melt structure during soldering and 180 °C isothermal aging. The results show that when the solders undergo liqiud–liquid structure transition (LLST), the IMC of the joint is thinner and more evenly distributed during soldering. The interface IMC is also thinner, and the quantity of Ag3Sn as well as Cu6Sn5 in the solder is relatively lower. However, the IMCs are more bulky after long-time aging at 180 °C. When the solders do not undergo LLST, microcracks form in the solder. Kirkendall voids are more abundant and interconnected after long, high-temperature aging. This finding indicates better joint reliability after than before LLST. The growth rate constants of the interface IMC for the two kinds of joints are calculated to be 1.94 × 10−12 and 9.71 × 10−13. The correlation of IMC growth behavior and melt state is analyzed from the viewpoints of LLST and atom diffusion.

Effect of Constant and Cyclic Current Stressing on the Evolution of Intermetallic Compound Layers

In this paper the effects of constant and cyclic power loads on the evolution of interfacial reaction layers in lead-free solder interconnections are presented. Firstly, the differences in the growth behavior of intermetallic compound (IMC) layers at the cathode and anode sides of the interconnections are rationalized. This is done by considering the changes in the intrinsic fluxes of elements owing to electromigration as well as taking into account the fact that the growth of Cu3Sn and Cu6Sn5 are coupled via interfacial reactions. In this way, better understanding of the effect of electron flux on the growth of each individual layer in the Cu-Sn system can be achieved. Secondly, it is shown that there is a distinct difference between steady-state current stressing (constant current, constant temperature) and power cycling with alternating on- and off-cycle periods (accompanied by a change of temperature). The reasons behind the observed differences are subsequently discussed. Finally, special care is taken to ensure that the current densities are chosen in such a way that there is no risk for even partial melting of the solder interconnections.

Bismuth segregation enhances intermetallic compound growth in SnBi/Cu microelectronic interconnect

There was a sudden increase of intermetallic compound (IMC) Cu 6Sn 5 growth rate in the eutectic Sn58wt. %Bi/Cu joint during aging process. With aging time increasing, Bi accumulated at the Cu 3Sn/Cu interface and gradually induced the fracture mode of the joint to change from ductile to brittle one along this interface. Bi segregation enhanced IMC Cu 6Sn 5 growth by means of promoting the interfacial reaction at Cu 3Sn/Cu interface, which was concluded from IMCs (Cu 6Sn 5 and Cu 3Sn) growth behavior for pure Sn/Cu and Sn10wt. %Bi/Cu interconnects at the same temperature.

Growth Behavior of Intermetallic Compounds at SnAgCu/Ni and Cu Interfaces

The growth behavior of reaction-formed intermetallic compounds (IMCs) at Sn3.5Ag0.5Cu/Ni and Cu interfaces under thermal-shear cycling conditions was investigated. The results show that the morphology of (CuxNi1–x)6Sn5 and Cu6Sn5 IMCs formed both at Sn3.5Ag0.5Cu/Ni and Cu interfaces gradually changed from scallop-like to chunk-like, and different IMC thicknesses developed with increasing thermal-shear cycling time. Furthermore, Cu6Sn5 IMC growth rate at the Sn3.5Ag0.5Cu/Cu interface was higher than that of (CuxNi1–x)6Sn5 IMC under thermal-shear cycling. Compared to isothermal aging, thermal-shear cycling led to only one Cu6Sn5 layer at the interface between SnAgCu solder and Cu substrate after 720 cycles. Moreover, Ag3Sn IMC was dispersed uniformly in the solder after reflow. The planar Ag3Sn formed near the interface changed remarkably and merged together to large platelets with increasing cycles. The mechanism of formation of Cu6Sn5, (CuxNi1–x)6Sn5 and Ag3Sn IMCs during thermal-shear cycling process was investigated.

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.

Interfacial Reactions in Sn-xZn-Cu/Cu Couples during Soldering

The microstructure evolution and the growth behavior of intermetallic compounds (IMCs) at Sn-xZn-2Cu/Cu (x=6.5, 8.8, 10 and 12, wt%) interfaces during soldering were investigated. The results indicate that planar Cu5Zn8 layer is formed at each interface of Sn-8.8Zn-2Cu/Cu, Sn-10Zn- 2Cu/Cu and Sn-12Zn-2Cu/Cu couples for all soldering time. However, for Sn-6.5Zn-2Cu/Cu couple, it is Cu5Zn8 phase that formed at the interface within shorter soldering time (1 h and 4 h), but the interfacial reaction products become a double layer structure of Cu6Sn5 phase (near Cu substrate) and CuZn phase (near solder) for longer soldering time (25 h and 49 h). The thickness of IMC layers in all couples increases exponentially with the soldering time. It is also found that for the same soldering time, the thickness of IMC layers increases with increasing Zn content in the solder.

Growth behavior of Cu/Al intermetallic compounds and cracks in copper ball bonds during isothermal aging

Copper wires are increasingly used in place of gold wires for making bonded interconnections in microelectronics. There are many potential benefits for use of copper in these applications, including better electrical and mechanical properties, and lower cost. Usually, wires are bonded to aluminum contact pads. However, the growth of Cu/Al intermetallic compounds (IMC) at the wire/pad interfaces is poorly understood, and if excessive would increase the contact resistance and degrade the bond reliability. To study the Cu/Al IMC growth in Cu ball bonds, high temperature aging at 250 °C for up to 196 h has been used to accelerate the aging process of the bonds. The Cu/Al IMCs growth behavior was then recorded and the IMC formation rate of 6.2 ± 1.7 × 10 −14 cm 2/s was obtained. In addition to the conventional yz-plane cross-section perpendicular to the bonding interface, a xy-plane cross-section parallel through the interfacial layers is reported. Three IMC layers were distinguished at the Cu/Al interfaces by their different colors under optical microscopy on the xy-plane cross-sections of ball bonds. The results of micro-XRD analysis confirmed that Cu 9Al 4, and CuAl 2 were the main IMC products, while a third phase is found which possibly is CuAl. During the aging process, IMC film growth starts from the periphery of the bond and propagates inward towards the centre area. Subsequently, with increased aging time, cavities are observed to develop between the IMC layer and the Cu ball surface, also starting at the bond periphery. The cavitation eventually links up and progresses toward the centre area leading to a nearly complete fracture between the ball and the intermetallic layer, as observed after 81 h.

Elemental redistribution and interfacial reaction mechanism for the flip chip Sn-3.0Ag-(0.5 or 1.5)Cu solder bump with Al/Ni(V)/Cu under-Bump metallization during aging

In flip chip technology, Al/Ni(V)/Cu under-bump metallization (UBM) is currently applicable for Pb-free solder, and Sn−Ag−Cu solder is a promising candidate to replace the conventional Sn−Pb solder. In this study, Sn-3.0Ag-(0.5 or 1.5)Cu solder bumps with Al/Ni(V)/Cu UBM after assembly and aging at 150°C were employed to investigate the elemental redistribution, and reaction mechanism between solders and UBMs. During assembly, the Cu layer in the Sn-3.0Ag-0.5Cu joint was completely dissolved into solders, while Ni(V) layer was dissolved and reacted with solders to form (Cu1−y,Niy)6Sn5 intermetallic compound (IMC). The (Cu1−y,Niy)6Sn5 IMC gradually grew with the rate constant of 4.63 × 10−8 cm/sec0.5 before 500 h aging had passed. After 500 h aging, the (Cu1−y,Niy)6Sn5 IMC dissolved with aging time. In contrast, for the Sn-3.0Ag-1.5Cu joint, only fractions of Cu layer were dissolved during assembly, and the remaining Cu layer reacted with solders to form Cu6Sn5 IMC. It was revealed that Ni in the Ni(V) layer was incorporated into the Cu6Sn5 IMC through slow solid-state diffusion, with most of the Ni(V) layer preserved. During the period of 2,000 h aging, the growth rate constant of (Cu1−y,Niy)6Sn5 IMC was down to 1.74 × 10−8 cm/sec0.5 in, the Sn-3.0Ag-1.5Cu joints. On the basis of metallurgical interaction, IMC morphology evolution, growth behavior of IMC, and Sn−Ag−Cu ternary isotherm, the interfacial reaction mechanism between Sn-3.0Ag-(0.5 or 1.5)Cu solder bump and Al/Ni(V)/Cu UBM was discussed and proposed.

Phase field simulations of morphological evolution and growth kinetics of solder reaction products

Two-dimensional simulations of the morphological and growth evolutions of the intermetallic compound (IMC) formed between a Sn-based solder and a Cu pad are performed using a multiphase field model, in order to examine the difference between the IMC growth behavior for the soldering reaction and that for the solid-state aging after soldering. The simulations show that the scallop-type IMC morphology formed in the soldering reaction is governed not only by the surface energy difference between the solder/IMC interface and the IMC grain boundary (GB), but also by the difference in diffusivities in the liquid solder and the IMC GB. It is also shown that the orders of magnitude difference in the IMC growth rate between the soldering reaction and the subsequent aging process is mainly attributed to the increased IMC GB distance resulting from the morphological change of the IMC from a scallop type to a layer type. The change in the solder diffusivity itself, however, has only a minor influence on the IMC growth kinetics of the layer-type IMC in the subsequent aging process.