Au-Al Intermetallic Compounds Research Articles

Insight on the Electronic, Elastic and Thermal Properties of Au-Al Intermetallic Compounds Based on First-Principles Calculations

Insight on the Electronic, Elastic and Thermal Properties of Au-Al Intermetallic Compounds Based on First-Principles Calculations

Calculation of Mechanical Properties, Electronic Properties, and Thermodynamic Properties of AuAl Crystal: First‐Principles Calculation

AbstractThe structural stability, elastic properties, electronic properties, and thermodynamic property (Debye temperature) of AuAl intermetallic compound (IMC) are systematically investigated using first‐principles calculations based on density functional theory. The elastic constants, elastic moduli (including bulk modulus, shear modulus, Young's modulus, and Poisson's ratio) and Vickers hardness, the density of states, and Debye temperature of AuAl under hydrostatic pressure are calculated. The calculation results show that AuAl is an elastic anisotropic material with low rigidity and high toughness under zero pressure. Under hydrostatic pressure, the anisotropy index decreases while the hardness and Debye temperature increase. The research in this paper provides a theoretical basis for the study of the properties of bonding gold wire intermetallics.

Ultrasonic Bonding Interface Degradation Characteristics of Gold-Coated Silver Wire for Semiconductor Packaging

Gold-coated silver wire was developed to alleviate the high cost of Au wire used in semiconductor packaging. Ball-bonding and stitch-bonding techniques were used to fabricate the dummy packaging material, comprising 97.3 % Ag, 97.3% Au-Coated Ag, and 99.99 %Au wires. The wire ball shear test (BST), the wire ball pull test (BPT), and the microstructural attributes of the ultrasonic bonding interfaces were compared with the initial properties, both before and after the highly accelerated stress test (HAST), conducted at 130 ℃ and 85% relative humidity (RH). The initial bonding strength for all the wire variants was ?23~24 gf. Following the HAST, the bonding strength of the Ag wire, the Au-coated Ag wire, and the Au wire decreased by approximately 75 %, 47 %, and 17 %, respectively. The microstructure analysis revealed that cracks developed and propagated at the ends of the interface and that the Au-rich Au-Al intermetallic compound (IMC) inhibited the propagation of the crack at the ACA/Au wire interface. Additionally, it was discovered that the presence of the Ag-Au-Al IMC at the interface of the ACA wire reduced Kirkendall voids, which act as a barrier to Au-Al interdiffusion.

Au-Al intermetallic compounds: A series of more efficient LSPR materials for hot carriers-based applications than noble metal Au

Abstract Hot carriers generated by the non-radiative decay of localized surface plasmon resonance (LSPR) of noble metal nanoparticles are widely investigated in applications ranging from solar energy conversion to photocatalysis, and photodetection. From the perspective of sustainable development of the LSPR based applications, it is important to find metallic materials that are cheap and plentiful and show excellent hot carrier properties. In this context, we investigate the possibility of alloying Au with Al to develop efficient LSPR materials for hot carriers-based applications. Taking thermodynamically stable AuAl2 and AuAl as examples, we study their electronic structures, generation rates, energetic distribution, relaxation times, and mean free paths of hot carriers originating from direct and phonon-assisted indirect electronic transitions, which are two of the main channels for generating high-energy hot carriers from the decay of LSPR, and compare the results with that of Au and Al metals. It is found that both AuAl2 and AuAl show larger hot carrier generation rates than Au and the resulting hot carriers are energetically higher than that in Au. Meanwhile, the transport properties of hot carriers in AuAl2 and AuAl outperform that of Au, which is manifested through comparable or even longer relaxation times and mean free paths. Consequently, it is believed that the Au-Al intermetallic compounds should be a series of efficient LSPR materials for hot carriers-based applications, which is comparable with or even better than the noble metal Au. These results are helpful for the sustainable development of related fields.

The reliability assessment of Au–Al bonds using parallel gap resistance microwelding

The Au–Al bonding joint was prepared by the parallel gap resistance microwelding. The microstructural evolution of Au–Al bonds was investigated under thermal aging, and the mechanical properties of Au–Al intermetallic compounds (IMCs) were computed by the first-principles calculations. We found that the Au–Al IMCs along with cracks grew during aging time and temperature increasing. The formation of the cracks is explained by the Kirkendall effect, volumetric shrinkages and thermal mismatch. Based on the experimental data, the growth of the IMCs is fitted to the diffusion equation to describe the performance degradation of Au–Al bonds. The Arrhenius relationship is introduced as the acceleration model to describe the effects of thermal stress on lifetime. Furthermore, the Weibull–Arrhenius model is built by the combination of the Weibull distribution and the Arrhenius relationship, which provide a method to estimate the lifetime distribution under work condition.

Au/Al Wire Bond Interface Resistance Degradation Rate Simulations

Gold wires are often used to connect aluminum terminals on semiconductor chips for signal transmission. Au-Al bonds degrade over time with temperature due to formation of intermetallic compounds at the gold-aluminum interface. A literature overview of the research on the Au-Al intermetallic compounds is outlined. Formation of these intermetallic compounds is governed by diffusion rules; however, the observed interface failure rates do not always follow the expected Fickian t1/2 time dependence. Gold-aluminum intermetallic compounds in turn cause Kirkendall voiding. The proliferation of Kirkendall voids over time separates the two interfaces. A literature overview of research on Kirkendall voiding is also outlined. In this paper, time-dependence generated by simulations of the Au/Al interface degradation due to Kirkendall voids is compared with the observed data time-dependence and the empirical failure rate time-dependences and their diverse rates of progression.

Ductile Au4Al intermetallic compound with crack resistance

The Au–Al intermetallic compounds (IMCs) were commonly regarded as brittle within wire bonding interfaces. Here the in-situ micro-pillar compression tests of Au4Al IMCs were carried out to verify their intrinsic mechanical property. The result clearly shows that, surprisingly, the Au4Al IMC is ductile, while the yield stress appeared to be about 1 GPa. Furthermore, upon the compression tests of Au4Al IMC micro-pillars with initial crack, no brittle fracture was observed; instead, the plastic deformation of Au4Al was shown via shear band. This work suggests that pristine Au4Al IMC is ductile and crack-resistant, which provides new insights for wire bonding process and technology.

Brittle-to-ductile transition of Au2Al and AuAl2 intermetallic compounds in wire bonding

Au–Al intermetallic compounds (IMCs) were conventionally regarded as damage sources within wire bonding interfaces. Among the Au–Al IMCs system, Au2Al and AuAl2 IMCs were commonly observed after long-term service. Here, the intrinsic mechanical properties of Au2Al and AuAl2 IMCs were investigated by in-situ bending tests. The results clearly revealed that the transition from brittle to ductile facture behavior was determined by the atomic ratio of Au and Al elements, while Al-rich Au–Al IMC exhibits higher bending strength than that of Au-rich Au–Al IMC. The Al-rich Au–Al IMC such as AuAl2 indeed showed brittle fracture behavior, in which cracks propagated through the grain boundaries. In contrast, the Au-rich Au–Al IMC such as Au2Al appeared to be quite ductile and crack resistant, and its plastic deformation was undertaken by slip bands. This work suggested that not all Au–Al IMCs cause brittleness and lead to damages, only Al-rich IMCs should be avoided at wire bonding interfaces.

Multiple Quality Characteristics Optimization of Ball Grid Array Wire Bonding Process

This paper presents an integrated approach combining principal component analysis (PCA) and Taguchi methods to develop a ball grid array (BGA), gold (Au) wire bonding process with multiple quality characteristics optimization. Eight main process factors of BGA wire bonding technology are selected as the control factors for parameter design. They are the factor A (seating ultrasonic generator (USG)), factor B (TIP height), factor C (C/V), factor D (USG current), factor E (USG bond time), factor F (bond force), factor G (FS threshold), and factor H (FAB size). The quality characteristics of the process in the study, including the wire pull strength, the ball shear strength, the ball thickness difference, the ball size difference, and the percentage of the Au–Al intermetallic compound (IMC) are measured. The optimal process parameters that meet the requirements for multiple quality characteristics are A1B3C1D3E3F1G1H2. They are then used to be tested for verification. Experimental results confirm that the optimal process design indeed enhances the quality characteristics investigated. The analysis of variance (ANOVA) results also show that the most important control factors affecting the quality characteristics are factor B (TIP height), factor C (C/V), and factor G (FS threshold), which accounts for 72.34% of total process variance. Thus, they must be strictly monitored during processing.

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.

Superior performance and reliability of copper wire ball bonding in laminate substrate based ball grid array

PurposeThe purpose of this paper is to provide a systematic method to perform long‐term reliability assessment of gold (Au) and copper (Cu) ball bonds in fineline ball grid array package. Also with the aim to study the apparent activation energies (Eaa) and its associated wearout mechanisms of both Au and Cu wire in semiconductor device packaging. This paper discusses the influence of wire type on the long‐term reliability and mechanical performance after several component reliability stress tests.Design/methodology/approachA fineline ball grid array (FBGA) package with Cu and Au wire bonds was assembled with green molding compound and substrate. Samples are subjected for long‐term high temperature storage bake test at elevated temperatures of 150°C, 175°C and 200°C. Long‐term reliability plots (lognormal plots) are established and Eaa of both ball bonds are determined from Arrhenius plots. Detailed failure analysis has been conducted on failed sample and HTSL failure mechanisms have been proposed.FindingsReliability results show Au ball bond in FBGA package is observed with higher hour‐to‐failure compared to Cu ball bonds. The Eaa value of high temperature storage life (HTSL) reliability for Au ball bond is lower than Cu ball bond. Typical HTSL failure mechanism of Au ball bond is induced by micro‐voiding and AuAl intermetallic compound (IMC) micro‐cracks while CuAl IMC micro‐cracking (induced by Cl− corrosion attack and micro‐cracking) caused wearout opens in Cu ball bond. These test results affirm the test‐to‐failure data collected is a useful method for lifetime prediction and Eaa calculation.Practical implicationsThe paper reveals higher reliability performance of Cu ball bond in FBGA flash memory package which can be deployed in flash memory FBGA packaging with optimised package bill of materials.Originality/valueThe test‐to‐failure methodology is a useful technique for wearout reliability prediction and Eaa calculation.

Interfacial Characteristics and Dynamic Process of Au- and Cu-Wire Bonding and Overhang Bonding in Microelectronics Packaging

Microstructure characteristics of Cu-wire bonding interface are investigated first by using the X-ray microdiffractometer, and the dynamic process of Cu- and Au-wire overhang bonding is recorded first with a high-speed camera system. It was found that the success rate of Cu-wire bonding is lower than that of Au-wire bonding, the formation of Cu-Al intermetallic compounds (IMCs) is more difficult than that of Au-Al IMCs, and the impact and deflection of Cu-wire overhang bonding process are much bigger than those of Au-wire overhang bonding process and affect bonding performance. In order to improve the bondability of Cu-wire bonding and overhang bonding, an approach with thick Al pad on the die which reduces its hardness is more compliant during Cu bonding process; another idea is to activate the formation of Cu-Al IMCs by increasing the energy. The effectiveness of the suggested approach is confirmed. In particular, Al2Cu and Al4Cu9 IMCs can form within 18 ms due to diffusion and reaction activated by high thermal (to 340 ° C) combined with ultrasonic energy at the bonding interface, and the success rate and strength of Cu-wire bonding increased significantly when Cu-Al IMCs form. For overhang bonding, the approach with thick Al layer improves dynamics features of hard Cu-wire overhang bonding process; thereby, it enhances the performance of Cu-wire overhang bonding.

Intermetallic phase transformations in Au–Al wire bonds

This paper describes the crystallochemical mechanisms that underpin the migration of nano-size alumina, intermetallic growth and phase transformations in Au–Al wire bonds during annealing from 175°C to 250°C by utilizing high-resolution transmission electron microscopy (HRTEM). Alumina, encapsulated within the Au–Al intermetallic compounds (IMCs), migrates towards the Au ball during annealing, with Au diffusion into the Al pad. The sequence of Au–Al IMC phase development during annealing was investigated. Initially, Au8Al3 appears as a third phase between the Au4Al and AuAl2 layers that form during bonding, and gradually becomes the dominant compound. Both AuAl2 and Au8Al3 are transformed into the Au-rich alloyAu4Al when Al is completely consumed, and this is the terminal product.

Behavior of aluminum oxide, intermetallics and voids in Cu–Al wire bonds

Nanoscale interfacial evolution in Cu–Al wire bonds during isothermal annealing from 175 °C to 250 °C was investigated by high resolution transmission electron microscopy (HRTEM). The native aluminum oxide film (∼5 nm thick) of the Al pad migrates towards the Cu ball during annealing. The formation of intermetallic compounds (IMC) is controlled by Cu diffusion, where the kinetics obey a parabolic growth law until complete consumption of the Al pad. The activation energies to initiate crystallization of CuAl 2 and Cu 9Al 4 are 60.66 kJ mol −1 and 75.61 kJ mol −1, respectively. During IMC development, Cu 9Al 4 emerges as a second layer and grows together with the initial CuAl 2. When Al is completely consumed, CuAl 2 transforms to Cu 9Al 4, which is the terminal product. Unlike the excessive void growth in Au–Al bonds, only a few voids nucleate in Cu–Al bonds after long-term annealing at high temperatures (e.g., 250 °C for 25 h), and their diameters are usually in the range of tens of nanometers. This is due to the lower oxidation rate and volumetric shrinkage of Cu–Al IMC compared with Au–Al IMC.

Effects of alloying elements on microstructure and thermal aging properties of Au bonding wire

We studied the effects of alloying elements Cu and Ni on the microstructure and the thermal aging properties of Au bonding wire. The thermal aging properties of samples bonded with an Al pad, and annealed at 200 °C for durations ranging from 0 to 300 h was investigated using mechanical tests. Both of the alloyed specimens showed higher thermal aging properties than the 4 N Au-bonded specimen, as investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), an electron probe micro analyzer (EPMA) and transmission electron microscopy (TEM). The Cu-alloyed Au bonding wire formed, at the Au–Al interface, a discontinuous Cu-rich layer that was considered to have delayed the growth of the Au–Al intermetallic compound (IMC). Meanwhile, at the Au–Al interface, the Ni-alloyed Au bonding wire formed secondary phase particles that were believed to have improved the bonding strength by interrupting micro-crack propagation.

Formation and Growth of Intermetallic Compounds in Cu–Au and Au–Al Systems for Copper on Gold Bonding

Copper (Cu) ball on gold (Au) bump (COG) bonding is developed as a practicable approach to solving the challenges posed by Cu wire bonding on large integrated circuit devices. In the COG bonding, there are two different interfaces, Cu-Au and Au-Al couples, which is totally different from either Cu or Au wire bonding. In this paper, the COG bonding was studied on a typical experimental chip of 69-μm bond pad opening and four-layer central via pattern design with current wire bonders. Tests of wire pull, ball shear, and crater were adopted in investigating the bonding strength at the interfaces and the potential damage of the underlying metals of bond pads after the COG bonding. The intermetallic compounds (IMCs) formation at the two interfaces, Cu-Au and Au-Al couples, were studied in detail on the cross-sectioned assembled samples. Two reliability tests, high-temperature baking (HTB) and temperature cycle (TC) were also used to investigate the IMC change and growth. The results showed that qualified bonding strength on all of bond pads existed and no damage such as crack and crater was observed on the underlying metals of the bond pads after removing Al metallization. At the Au-Al interface, a comparable Au-Al IMC was formed on as-bonded samples, and it gradually grew thick after the reliability tests and its corresponding content also changed along with temperature and time. At the Cu-Au interface, there was no observable formation of Cu-Al IMC on as-bonded samples. Undergoing 1008 hours HTB at 175°C, only a very thin Cu-Au IMC appeared at the Cu-Au interface. After all, the Cu-Au IMC appearance hardly affects the IC's mechanical performance.

Mechanism of Electrochemical Al Deposition from Room-Temperature Chloroaluminate Ionic Liquids

and its alloy formation with other transition metals to take advant-age of the unique properties of Al. In particular, aluminum based alloy materials such as the light weight transition met-al-aluminum alloys have long been an interest because of their structural properties with high strength, environmental stabil-ity, and corrosion resistance at elevated temperature. We have studied previously the deposition of aluminum and the for-mation/growth of Au-Al intermetallic compounds directly on polycrystalline gold electrode, Au(poly),

Interfacial Degradation Mechanism of Au/Al and Alloy/Al Bonds Under High Temperature Storage Test: Contamination, Epoxy Molding Compound, Wire and Bonding Strength

In this paper, the effects of Al pad contamination, epoxy molding compound [biphenyl (BP) and ortho-cresol novolac (OCN)] and wire (Au and alloy) on the propensity for the interfacial degradation of wire bond in a quad flap package under high temperature storage (HTS) tests at 125degC, 150degC, and 170degC are meticulously investigated. The interfacial degradation intends to be explicated in regards to change in surface morphology of Au-Al and alloy-Al intermetallic compound (IMC) and bonding strength as a function of HTS test. The combination of atomic force microscope and Auger electron spectrometry reveals that initial bonding strengths from wire pull and ball shear test decrease with increasing the thickness of contamination layer on Al pad, carbon and oxygen, and subsequent surface roughness. Indeed, the plasma exposure on Al pad prior to wire bonding enhances both mechanical bonding strengths up to 10% and 15%. It is found that the failure behaviors at 125degC are dissimilar to 150degC and 170degC. We first report that Sb diffused from the OCN exists at the intermetallics of Au-Al bonds, leading to rapidly deteriorate mechanical integrity. Furthermore, inductively coupled plasma mass spectrometry affirms that the OCN is a resource of Br. Above 150degC, the interdiffusion of Br and Sb from the OCN significantly impacts the integrity of Au-Al bonds. In turn, such physical degradation mechanism governed by Sb and Br can be linearly accelerated. It is also found that in the case of Au-Al bonds, the life time with the BP is much longer than that with the OCN under the given HTS tests due to less content of halogen ions. In contrast, neither Sb nor Br was found from the intermetallic layers of alloy-Al bond encapsulated with the OCN and BP. Thus, alloy-Al bonding strengths are intact even after longer stressing. With an alloy wire having Pd as an impurity, the growth kinetics of IMC fueled by Br and Sb seems to be sluggish, providing better reliability than Au wire. Obviously, lower flame retardants and higher are critical intrinsic material properties that should be taken into account when a new epoxy molding compound is introduced to pursue cost effectiveness without loosing reliability. Finally, upon painstaking work over a wide range of analyses herein, a quality affordable guideline for the selection of wire type associated with epoxy molding compound that can ensure the long-term reliability is presented in order to secure reliable supplyline management as well as package assembler.

Research on Au-Al bonding properties based on a thermal shock environment

The thermal shock experiment of Au-Al bonding has been carried out. The mechanical characteristics, structure morphology and electrical characteristics degradation mechanism have also been researched. No cracks were observed in the bonding interface, and the bonding joints also showed good mechanical characteristics with pull stress of 3.0 to 12.0 g. Due to high temperature, the Au-Al intermetallic compound Au5Al2 with high resistibility was formed, which ultimately led to electrical failure. For the samples that were fabricated on the basis of the present technique, the bonding reliability has been evaluated. It has been found that the lifespan rule obeys the Weibull distribution, and at a high temperature of 150°C under 95% confidence level, the estimated results are η = 547 h, m = 3.83. In a room temperature environment, the Au-Al bonding samples’ lifespan has also been predicted on the basis of the rule of reliability evaluation. The result shows that the lifespan is about 20 years, while the reliability degree is 90%.

Gold–Aluminum Intermetallic Formation Kinetics

Au-Al intermetallic compounds (IMC) grow laterally (Al-rich phases) in a Fickian fashion with an activation energy of 1.0 eV, but vertical IMC thickness (Au-rich phases) grows functionally as a power law on time with a sub-Fickian exponent of 1/4, which is substantially smaller than what would be expected for bulk lattice diffusion (1/2). We conclude from the IMC thickness time exponent that an Au-rich IMC growth process is limited by grain boundary diffusion. The best bond lifetime was seen for an intermediate-thickness Al film. The activation energy and lifetime for Au-rich phase growth are each a strong function of wire impurity concentrations. We find that bond lifetime varies roughly as the square root of Pd, Cu, Pt, and As concentrations, but lifetime is not a function of Be, Ca, Fe, or Cr concentration. We find a mixture of and Au on the Al-rich side of the failing interface.