Intermetallic Growth Kinetics Research Articles

Electromigration enhanced growth kinetics of intermetallics at the Cu/Al interface

The growth kinetic behaviors of the Cu/Al system under current were investigated. The current was shown to significantly promote the growth of the intermetallic (IMC) layer. The growth rate constant of the Al4Cu9 layer under current was approximately 3.26 times higher than that processed without current. Moreover, the current direction leads to a significant difference in the growth rate of the IMC layer, which was attributed to the electromigration effect. Based on the traditional thermal-activated theory and electromigration theory, an electromigration and thermal dual-activated growth kinetic model incorporating the contributions of Joule heat and the nonthermal effect was constructed. The growth kinetic behaviors of the Al4Cu9 layer were investigated by applying this model. The calculated growth activation energies under different current densities were almost constant and close to 130 kJ/mol. The experimental data were applied to verify the model, and the results showed that the model was in good agreement with the experimental results.

Bond shear strength of Al2O3 nanoparticles reinforced 2220-capacitor/SAC305 solder interconnects reflowed on bare and Ni-coated copper substrate

The influence of Al2O3 nanoparticles addition in trace amounts and electroless Ni–P substrate coating on the microstructure development and bond shear strength of Sn-3.0Ag–0.5Cu (SAC305) solder joint were investigated. The performance and reliability of the 2220-capacitor joints with Al2O3 nanoparticle reinforced nanocomposites reflowed on Cu and Ni–P coated substrate were analyzed under varying high-temperature environments. The addition of nanoparticles enhanced the wettability and microhardness of the solder and considerably refined the joint microstructure. The dispersion and adsorption of Al2O3 nanoparticles resulted in the suppression of intermetallic (IMC) growth at the interface and refinement of the β-Sn grains as well as IMC precipitates into the matrix. The Ni–P coating on the substrate significantly retarded the IMC growth kinetics resulting in the formation of a thin and uniform IMC layer at the joint interface. The thermal stability and performance of the joint under high-temperature environments were enhanced due to the Ni–P coating on the substrate. Compared to the unreinforced SAC305 solder joint with bare Cu substrate, joints with SAC305 + 0.05Al2O3 composite showed about 17% higher shear strength with bare Cu substrate and about 27% higher strength with Ni–P coated substrate. The Weibull distribution analysis indicates a significant improvement in joint reliability of the 2220-capacitor/SAC305 solder assembly using SAC305 + 0.05Al2O3 nanocomposite and Ni-coated substrate. The ANOVA study suggests that the solder joint performance majorly depends on the operating environment, solder composition, and the substrate finish.

Intermetallic Compounds at the Interfaces of Ag–Pd Alloy Stud Bumps With Al Pads

Stud bumping using a traditional wire bonding process followed by the severing of the wire material from the ball bond has been widely applied in flip chip packages. Traditionally, Au stud bumps were employed because of the physical properties of gold, but the mediocre reliability of Au with Al pads and the cost of gold make this material unfavorable for industrial needs. Copper stud bumps were also considered, but the intrinsic features of Cu, such as its high hardness and ease of oxidation, present no advantages over gold stud bumps. However, Ag-alloy stud bumps exhibit satisfactory bondability and have been proposed as the optimal substitute for Au stud bumps to ensure better reliability in soldering and better intermetallic growth kinetics with Al bond pads. In this article, the intermetallic growth of Ag stud bumps alloyed with 2%, 10%, and 12% Pd after high-temperature storage tests (HTSTs) is investigated. The results indicated that the Pd atoms remained immobile in the bump-side matrix without diffusing into the aluminum pad side. Alloying with higher Pd content enhanced the formation of the favorable Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Al intermetallic compound (IMC), whereas lower Pd content resulted in severe cracking due to the formation of Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Al IMC. Compositional analysis showed that continuous layers of Ag-Al IMCs doped with large amounts of Pd formed at the interfaces close to the Ag stud bumps, but no Pd was detected on the Al pad side. The bonding strength increased after aging at 150 °C and 200 °C for 200 h, but it decayed drastically after aging at 250 °C for 200 h. The relationship between intermetallic growth and failure mode was established.

Intermetallic growth kinetics in gold ball bonds on Al-1%Si-0.5%Cu bond pads at 175∘C

Purpose The purpose of this study is to demonstrate that isothermal intermetallic growth data for gold ball bonds can be non-parabolic with explanations of why deviation from parabolic kinetics may occur. Design/methodology/approach Intermetallic thickness measurements were made at the centre of cross-sectioned ball bonds that were isothermally annealed at 175°C. Intermetallic growth kinetics were modelled with a power law expression(x(t) − x0)2 = α1tα2. The parameters of the power law model were obtained by transformation of the response and explanatory variables followed by data fitting using simple linear regression (SLR). Findings Ball bonds made with 4 N (99.99%Au) and 3 N (99.9%Au) gold wires exhibited two consecutive time regimes of intermetallic growth denoted Regime I and Regime II. Regime I was characterised by reactive diffusion between the gold wire and the aluminium alloy bond pad, during which Al was completely consumed in the formation of Au–Al intermetallics with non-parabolic kinetics. In Regime II, the absence of a free supply of Al to sustain intermetallic growth led to the conclusion that thickening of intermetallics was caused by phase transformation of Au8Al3 to Au4Al. Ball bonds made with 2 N (99%Au) wire also exhibited non-parabolic kinetics in Regime I and negligible intermetallic thickening in Regime II. Research limitations/implications The analysis of intermetallic growth is limited to total intermetallic growth at a single temperature (175°C). Originality/value The value of this study lies in showing that the assumption that only parabolic intermetallic growth is observed in isothermally aged gold ball bonds is incorrect. Furthermore there is no need to assume parabolic growth kinetics because with an appropriate data transformation, followed by fitting the data to a power law model using SLR and with the use of statistical diagnostics, both the suitability of the kinetic model and the nature of the growth kinetics (parabolic or non-parabolic) can be determined.

Intermetallic growth kinetics and microstructure evolution in Al-Cu-Al metal-matrix composite processed by high pressure torsion

Constrained high pressure torsion and subsequent annealing was used to fabricate Al-Cu-Al metal-matrix multilayered composite. The composite has a heterogeneous structure consisting of a plastic aluminum matrix and hard intermetallic inclusions with a gradient decrease of layer thickness from the center to periphery of the sample. Intermetallic growth kinetics was retrieved from precise X-ray phase analysis using a narrow parallel X-ray beam of increased intensity. It was found that the growth of Al2Cu phase at temperatures from 150 °C to 210 °C can be fitted by t1/2 time relation, indicating to the bulk diffusion-controlled growth kinetics. The growth activation energy of Al2Cu was calculated to be 0.48 eV.

Contents

Abstract The Kirkendall-effect induced migration of inert markers during a diffusion-controlled growth of intermetallic compounds in the Cu/Sn and Au/Sn couples at 215 and 180°C were studied. It was shown that the behaviour of markers in the multiphase reaction zones can be rationalized in terms of the Kirkendall velocity construction. Observations on the microstructural features of the product intermetallic layers and the role of the Kirkendall effect in the morphogenesis of the interdiffusion systems are discussed. It was demonstrated that the velocity of markers in a product layer, the appearance of the Kirkendall plane(s), their location(s) and the morphological evolution of the reaction products can also be explained from a purely chemical point of view considering the diffusion-controlled interactions at the interphase interfaces. A representation of the reaction scheme and kinetics of intermetallic growth in each of the two systems is given.

In Situ Electromigration in Cu-Sn and Ni-Sn Critical Solder Length for Three-Dimensional Integrated Circuits

An in situ electromigration study has been conducted on U-groove Cu/Sn-3.5Ag/Cu and Ni/Sn-3.5Ag/Ni sandwich structures; the results were used to simulate microsolder joints passing current density of 1 × 104 A/cm2 at 150°C. The solder gap was only 15 μm, shorter than the critical length of Sn-3.5Ag solder. Backstress was proved to exist at critical solder lengths and to influence the electromigration mechanism. Theoretical calculations of the diffusivity of Cu and Ni in Sn solder indicated that the degree to which the dominant diffusion species (Cu or Ni atoms) diffused through the solder line is retarded by the backstress effect. The morphologies of intermetallic compounds (IMCs) were observed, and the grain boundaries in Sn solder were measured using electron backscatter diffraction to determine the kinetics of intermetallic growth. The results reveal that the unique electromigration characteristics of microbump joints, including the diffusivity, morphology, and backstress, can be determined. The retardation of atomic migration improves the reliability against electromigration.

Microstructural evolution at Cu/Sn–Ag–Cu/Cu and Cu/Sn–Ag–Cu/Ni–Au ball grid array interfaces during thermal ageing

The effect of metallization chemistry on the solid state growth kinetics of intermetallics formed in an industrial metallization/solder/metallization arrangement during isothermal ageing has been studied. During isothermal ageing of the Cu/Sn–Ag–Cu/Cu arrangement at temperatures lower than 125°C, Cu was the main mobile element and diffused either through or along interfacial Cu6Sn5 grain boundaries to react with Sn on the solder side of the Cu6Sn5 layer; while for temperatures greater than 125°C, both Cu and Sn were relatively mobile. Isothermal ageing of the Cu/Sn–Ag–Cu/Ni–Au asymmetric arrangement showed a concentration gradient of Ni and Cu across the entire ball in the solder joints leading to an inhomogeneous precipitation of (Cu, Ni)6Sn5 in the vicinity of the Ni metallization. Ni alloying of the interface reaction products also occurred in the vicinity of the Cu metallization, reducing the growth rate of Cu6Sn5 and Cu3Sn by 20% and 70% respectively when compared with symmetric Cu/Sn–Ag–Cu/Cu arrangement. Growth rate studies and microscopy suggested the presence of the Cu6Sn5 η'→η transformation during extended ageing in the region of 150–175°C, which is generally under-reported in these important commercial systems.

Effects of silver addition on Cu–Sn microjoints for chip-stacking applications

The solid-state reactions under confined space between Cu substrate and Sn and Sn–Ag solders are systematically investigated, with the objective of providing fundamental data that are critical to the design of robust microjoints for chip-stacking applications. Sandwich structures of Cu/Sn/Cu and Cu/Sn3.5Ag/Cu are prepared by thermal compression bonding. The thickness of Sn and Sn3.5Ag layer is controlled at 10μm. High temperature storage tests are conducted at 120°C, 150°C, 180°C, and 200°C for different time periods. Unlike the well-known void formation in the Ni/Sn/Ni system, there is no such void at all in the Cu/Sn/Cu system. The addition of Ag consistently slows down the growth kinetics of intermetallics. The time-to-impingement of intermetallics is established for the first time for both systems under solid-state reactions.

Effects of Bonding Wires and Epoxy Molding Compound on Gold and Copper Ball Bonds Intermetallic Growth Kinetics in Electronic Packaging

This paper discusses the influence of bonding wires and epoxy mold compounds (EMC) on intermetallic compound (IMC) diffusion kinetics and apparent activation energies (E aa) of CuAl and AuAl IMCs in a fineline ball grid array package. The objective of this study is to study the CuAl and AuAl IMC growth rates with different epoxy mold compounds and to determine the apparent activation energies of different combination of package bills of materials. IMC thickness measurement has been carried out to estimate the coefficient of diffusion (D o) and E aa various aging conditions of different EMCs and bonding wires. Apparent activation energies (E aa) of both wire types were investigated after high temperature storage life tests (HTSL) for both molding compounds. Au bonds were identified to have faster IMC formation, compared to slower IMC growth of Cu. The E aa obtained for CuAl IMC diffusion kinetics are 1.08 and 1.04 eV with EMC A and EMC B, respectively. For AuAl IMC diffusion kinetics, the E aa obtained are 1.04 and 0.98 eV, respectively, on EMC A and EMC B. These values are close to previous HTSL studies conducted on Au and Cu ball bonds and are in agreement to the theory of HTSL performance of Au and Cu bonding wires.Overall, EMC B shows slightly lower apparent activation energy (E aa) valueas in CuAl and AuAl IMCs. This proves that the different types of epoxy mold compounds have some influence on IMC growth rates.

Intermetallic Growth: Structure-Sensitive Effects

The kinetics of intermetallic growth in the bulk, along the surface and grain boundaries (GBs) was studied in the Cd-Ni, Cd-Cu and Cu-Sn systems. Bulk dynamic diffusion coefficients exceed by a few orders of magnitudes the tracer self-diffusion coefficients in homogeneous phases. The reasons for this difference are discussed in terms of departure of growing intermetallics from stoichiometry due to the simultaneous existence of two processes: interdiffusion through the growing phase layer and chemical reactions at interfaces. Accelerating contribution of GBs and free surfaces, as fast diffusion paths, into diffusion penetrability of growing intermetallics has been investigated. It was found that the rates of lateral phase spreading along free surfaces and GBs exceed several times the rate of phase growth in the bulk. Accelerating GB contribution depends on the grain size. Nano-dispersed thin films demonstrate maximal phase propagation rates, which several times exceed even the rates of lateral phase spreading along free surfaces in coarse-grained polycrystals.

Intermetallic growth in gold ball bonds aged at 175°C: comparison between two 4N wires of different chemistry

Gold wires of different chemistry (denoted as Types A and B) are ballbonded on identical metallization and isothermally aged at 175°C. In both wire types the same intermetallic compounds Au4Al and Au8Al3 grow with parabolic time dependence as long as there is a supply of Al from the bondpad. After Al is completely reacted, composition of intermetallics in Type A wire remains unchanged but in Type B wire, the Au8Al3 compound undergoes a phase transformation into a second layer of Au Al. The phase transformation does not affect mechanical reliability of the ballbonds. Intermetallic growth kinetics, growth sequence and the possible origins of the phase transformation are discussed.

Whisker formation in Sn and Pb–Sn coatings: Role of intermetallic growth, stress evolution, and plastic deformation processes

We have simultaneously measured the evolution of intermetallic volume, stress, and whisker density in Sn and Pb–Sn alloy layers on Cu to study the fundamental mechanisms controlling whisker formation. For pure Sn, the stress becomes increasingly compressive and then saturates, corresponding to a plastically deformed region spreading away from the growing intermetallic particles. Whisker nucleation begins after the stress saturates. Pb–Sn layers have similar intermetallic growth kinetics but the resulting stress and whisker density are much less. Measurements after sputtering demonstrate the important role of the surface oxide in inhibiting stress relaxation.

Lateral Diffusion Spreading of Two Competitive Intermetallic Phases over Free Surface

The kinetics of growth and lateral spreading of intermetallic layers during surface interdiffusion in Cu – Sn system has been studied in a temperature range 160 – 200oC by the methods of optical microscopy, SEM provided with X-ray microprobe, and AFM. Lateral phase spreading over the surface is characterized by competition between two phases: Cu6Sn5 and Cu3Sn. A steady state solution for concentration distribution on the surface of growing intermetallic phases, as well as kinetic equations of lateral spreading of growing phase layers have been obtained. By comparison of experimental data on intermetallic growth kinetics with the proposed theory, the dynamic surface diffusion coefficients have been calculated.

Stress Relaxation Mechanisms during Cd&lt;sub&gt;21&lt;/sub&gt;Ni&lt;sub&gt;5&lt;/sub&gt; Intermetallic Growth under High Hydrostatic Pressure

Stress relaxation processes accompanying intermetallic growth during reactive diffusion between Cd and Ni have been studied by the methods of optical and scanning electron microscopy, provided with X-ray microanalysis. The experiments were carried out with the two-layer Cd-Ni samples at 250 and 280oC under hydrostatic pressures 350-900 MPa. The observed processes have been compared with those occurred at low pressures to demonstrate that the mechanisms of stress relaxation and thus the kinetics of intermetallic growth essentially depend on applied hydrostatic pressure. New mechanisms of stress relaxation were found, such as Cd extrusion and Cd whisker growth, which accompanied formation of Cd21Ni5 compound. It is shown, that the whisker growth is more probable at lower temperatures when the grain size is smaller, and the stress gradient, which support a driving force for whisker formation, is higher. An atomic mechanism for whisker growth, based on diffusion climbing of dislocation loops produced by Bardeen-Herring source for building new atomic layers, has been discussed.

Intermetallic growth kinetics for Sn-Ag, Sn-Cu, and Sn-Ag-Cu lead-free solders on Cu, Ni, and Fe-42Ni substrates

Soldering with the lead-free tin-base alloys requires substantially higher temperatures (∼235–250°C) than those (213–223°C) required for the current tin-lead solders, and the rates for intermetallic compound (IMC) growth and substrate dissolution are known to be significantly greater for these alloys. In this study, the IMC growth kinetics for Sn-3.7Ag, Sn-0.7Cu, and Sn-3.8Ag-0.7Cu solders on Cu substrates and for Sn-3.8Ag-0.7Cu solder with three different substrates (Cu, Ni, and Fe-42Ni) are investigated. For all three solders on Cu, a thick scalloped layer of η phase (Cu6Sn5) and a thin layer of e phase (Cu3Sn) were observed to form, with the growth of the layers being fastest for the Sn-3.8Ag-0.7Cu alloy and slowest for the Sn-3.7Ag alloy. For the Sn-3.8Ag-0.7Cu solder on Ni, only a relatively uniform thick layer of η phase (Cu,Ni)6Sn5 growing faster than that on the Cu substrate was found to form. IMC growth in both cases appears to be controlled by grain-boundary diffusion through the IMC layer. For the Fe-42Ni substrate with the Sn-3.8Ag-0.7Cu, only a very thin layer of (Fe,Ni)Sn2 was observed to develop.

Formation and growth kinetics of interfacial intermetallics in Pb-free solder joint

The formation and growth kinetics of the intermetallic compound (IMC) between lead-free solder and Cu substrate in the surface mount process were studied. Optical and scanning electron microscope (OM and SEM) were used to measure the thickness of intermetallic layers and observe the microstructure evolution of solder joint. The IMC phases were identified by energy dispersive X-ray (EDX) and X-ray diffraction (XRD). The grain size of the intermetallic compound /spl eta/-phase Cu/sub 6/Sn/sub 5/ was observed to increase with the increase in peak temperature hold time. The results show that a /spl eta/-phase Cu/sub 6/Sn/sub 5/ IMC layer is formed at solder-Cu substrate interface at a very short time. The growth of intermetallics formation was found not to follow the Fick's law that predicts the mean total thickness increases linearly with the square root of the time. It deviates the Fick's law at the early stage of the growth process and then approaches the parabolic law. To explore the growth kinetics, the IMC growth mechanism is suggested and a lagging diffusion model is presented for predicting the intermetallic compound layer growth. Comparison between the model and experimental results demonstrates that the proposed phase-lag model captures the growth history of IMC layers quite well.

The growth of intermetallic compounds at Sn-Ag-Cu solder/Cu and Sn-Ag-Cu solder/Ni interfaces and the associated evolution of the solder microstructure

The evolution of intermetallics at and near SnAgCu/Cu and SnAgCu/Ni interfaces was examined, and compared to the behavior, near PbSn/metal and Sn/metal interfaces. Two different solder compositions were considered, Sn93.6Ag4.7Cu1.7 and Sn95.5Ag3.5Cu1.0 (Sn91.8Ag5.1 Cu3.1 and Sn94.35Ag3.8Cu1.85 in atomic percent). In both cases, phase formation and growth at interfaces with Cu were very similar to those commonly observed for eutectic SnPb solder. However, the evolution of intermetallics at SnAgCu/Ni interfaces proved much more complex. The presence of the Cu in the solder dramatically altered the phase selectivity at the solder/Ni interface and affected the growth kinetics of intermetallics. As long as sufficient Cu was available, it would combine with Ni and Sn to form (Cu,Ni)6)Sn5 which grew instead of the Ni3Sn4 usually observed in PbSn/Ni and Sn/Ni diffusion couples. This growing phase would, however, eventually consume essentially all of the available Cu in the solder. Because the mechanical properties of Sn-Ag-Cu alloys, depend upon the Cu content, this consumption can be expected to alter the mechanical properties of these Pb-free solderjoints. After depletion of the Cu from the solder, further annealing then gradually transformed the (Cu,Ni)6Sn5 phase into a (Ni,Cu)3Sn4 phase.

Kinetics of Intermetallic Growth under High Pressure

Kinetics of Intermetallic Growth under High Pressure

Kinetics and Mechanisms of Intermetallic Growth by Surface Interdiffusion

ABSTRACTKinetics of growth and lateral spreading of an intermetallic layer along a free surface during surface interdiffusion in binary systems have been studied both theoretically and experimentally. By comparison of experimental data on intermetallic growth kinetics with the proposed theory, the surface diffusion constants have been calculated for intermetallics Cd21Ni5 and Cu5Zn8, and mechanisms of surface diffusion have been discussed.