Ag Alloy Wire Research Articles

Free air ball and bondability behaviours of Au-coated Ag alloy wire at various electro flame-off and bonding parameters for CMOS iBGA

Free air ball and bondability behaviours of Au-coated Ag alloy wire at various electro flame-off and bonding parameters for CMOS iBGA

Effect of large deformation on microstructure and properties of copper alloy wire

Cu–Ag alloy wire with different deformations and Ag contents of 1%, 2%, and 4% are prepared using vacuum continuous casting technology combined with cold drawing and intermediate annealing processes. The evolution of microstructure, electrical properties, and mechanical properties of the Cu–Ag alloy wire is investigated, and a relationship model between wire diameter, Ag content, and alloy wire properties is established. The results showed that the directionally solidified billets exhibited good elongation and are suitable for drawing fine wire. As the Ag content increased, the twinning content inside the Cu–Ag alloy wire also increased significantly. It is found through Strain++ characterization that there is non-uniform deformation within the wire, and the tensile and compressive strains that exist interactively tend to aggregate within the twins. The properties of the Cu–Ag alloy wire with different diameters varied significantly at different stages of deformation, showing a pronounced size effect below 1 mm. With decreasing wire diameter or increasing Ag content, the DC resistance, resistivity, and tensile strength of the Cu–Ag alloy wire gradually increased, while the conductivity decreased. The effect of Ag content on elongation is not significant. Statistical analysis revealed that the established performance model had high predictive accuracy and could provide references for the optimization of production processes for Cu–Ag alloy wire.

Study on the Effect of Cold Deformation and Heat Treatment on the Properties of Cu-Ag Alloy Wire.

The effects of various drawing parameters and annealing processes on the structure and properties of Cu-Ag wires, containing 1 wt% silver, were investigated using specialized equipment including fine wire-drawing machines, very fine wire-drawing machines, heat treatment equipment, tensile testing machines, microcomputer-controlled electronic universal testers, resistance testers, and scanning electron microscopes. The results revealed that continuous drawing of Cu-1%Ag alloy wires led to elongation of the grains, resulting in a uniform and tightly fibrous microstructure. Moreover, the tensile strength of the alloy wire increased from 670 MPa to 783.9 MPa after a single pass with a deformation of 14%. Subsequently, when the wire was drawn at a speed of 500 m/min, the tensile strength further increased to 820.1 MPa. After annealing the Փ0.08 mm Cu-1% Ag alloy wire, an increase in annealing temperature up to 500 °C resulted in the wire's tensile strength decreasing from 820.1 MPa to 377.5 MPa. Simultaneously, the elongation increased from 1.94% to 15.21%, and the resistivity decreased from 1.931 × 10-8 Ω·m to 1.723 × 10-8 Ω·m. Additionally, when annealing was conducted at a rate of 80 m/min, the wire resistivity dropped to 1.635 × 10-8 Ω·m.

Au-coated Ag alloy bonding wires with enhanced oxidation resistance for electronic packaging applications

PurposeThe purpose of this paper is to develop high-performance Au-coated Ag alloy wires (ACAA wires) and demonstrate the effect of Au coating layers on the bonding performance and oxidation resistance for stable and reliable electronic packaging applications.Design/methodology/approachACAA wire with a diameter of approximately 25 µm and Au layer thickness of approximately 100 nm were prepared by the continuous casting, plating and wire drawing method. The bonding performance of the ACAA wires were studied through bonding on 3,535 chips. The oxidation resistance of ACAA wires and Ag alloy wires (AA wires) were comparatively studied by means of chemical oxidation tests, accelerated life tests and electrochemical tests systematically.FindingsACAA wires could form axi-symmetrical spherical free air balls with controllable diameter of 1.5∼2.5 times of the wire diameter after electric flame-off process. The ball shear strength of ACAA wire was higher than that of AA wires. Most importantly, because of the surface Au coating layer, the oxidation resistance of ACAA wires was much enhanced.Research limitations/implicationsACAA wires with different lengths of heat affected zone were not developed in this study, which limited their application with different loop height requirements.Practical implicationsWith higher bonding strength and oxidation resistance, ACAA wires would be a better choice than previous reported AA wire in chip packaging which require high stability and reliability.Originality/valueThis paper provides a kind of novel ACAA wire, which possess the merits of high bonding strength and reliability, and show great potential in electronic packaging applications.

Microstructure and properties of Cu-Ag alloy prepared by continuously directional solidification

A Φ40 μm Cu-4 wt%Ag alloy wire with high strength and high electrical conductivity was prepared by continuously directional solidification and cold drawing. The evolution of microstructure and properties was investigated by means of optical microscope observation, electron microscopes observation, hardness test, tensile test, and electrical conductivity measurement. The continuous columnar-grain structure uniformly distributed along the solidification direction in the as-cast alloy. The drawing process was divided into three stages: general deformation stage, large deformation stage and extreme deformation stage. The main deformation mechanisms of the alloy in each stage are corresponding to the dislocation slip and twinning, the dynamic recovery, the evolution of layered interface, and partial dynamic recrystallization. After the deformation with the drawing strain of 11.28, the tensile strength, yield strength, and electrical conductivity of the alloys were 1048 MPa, 886 MPa, and 75.2%IACS, respectively. The main strengthening mechanism of the alloys was grain refinement strengthening according to the calculation results of the strengthening model.

Microstructural evolution and properties of Cu–20 wt% Ag alloy wire by multi-pass continuous drawing

Abstract The Cu–20 wt% Ag alloy wire rod was prepared using three-chamber vacuum cold mold vertical continuous up-casting followed by multi-pass continuous drawing. The evolution of microstructure, mechanical property, and electrical property of the Cu–20 wt% Ag alloy wire during multi-pass continuous drawing was studied. After multi-pass continuous drawing, the continuous network eutectic structure in the longitudinal section of the as-casted rod was gradually drawn into long fibers that approximately parallel to the axial direction, while the space of the continuous network eutectic structure in the transverse section is getting smaller and smaller. Both the preferred orientation of copper and silver grains are (1,1,1). With the increase of drawing strain (η), the tensile strength of Cu–20 wt% Ag alloy wire gradually increases while the elongation gradually decreases. When the diameter is drawn to 0.02 mm (η = 11.94), the tensile strength of the alloy is 1,682 MPa and elongation is 2.0%. The relationship between tensile strength, elongation, and diameter conforms to Allometric and Boltzmann functions, respectively.

Influence of Silver Addition on Structure, Martensite Transformations and Mechanical Properties of TiNi-Ag Alloy Wires for Biomedical Application.

The microstructural and functional behavior of TiNi-based wires with a silver content of 0–1.5 at.% was evaluated. The concentration range for Ag doping determined for the TiNi wires with potential for the medical industry was 0–0.2 at.%. Microstructure analysis of TiNi wires with different silver contents at room temperature indicated a multiphase structural state. Various internal structures with tangled grain boundaries were formed by intense plastic deformation. The nanocrystalline structure and phase state of wire with the minimum silver content (0.1 at.% Ag) provide full shape recovery, the greatest reversible strain, and optimal strength and ductility. TiNi ingots with a high Ag content (0.5–1.5 at.%) cracked under minimum load due to excess silver that crystallized along the grain boundaries and broke cohesion bonds between the TiNi grains.

Lumen Maintenance of Ternary Ag-Alloy Wire Bonded LED Package after Reliability Tests

In this investigation, ternary Ag-alloy wires were doped with different Pd and Au concentrations, and each wire was encapsulated in an LED package. The static and dynamic reliability were tested, and the lumen maintenance performance was examined. The static reliability tests included the sulfur test, LTSL, HTSL, and WHTSL. According to the sulfur test data, higher Pd and Au contents led to better lumen maintenance. Furthermore, the bonding wire of the LED with better lumen maintenance in the sulfur test had higher electrical resistance. The brightness decay rate of the white light LEDs was low in low-and high-temperature environments, but it was significantly higher after the WHTSL test. The dynamic reliability test after 1,000 hours of HTOL and WHTOL showed that the lumen maintenance improved with higher Pd and Au contents, indicating that doping Ag-alloy wires with sufficient amounts of Pd and Au can retard degradation due to thermal and humidity aging and oxidation reaction. Therefore, ternary Ag-Pd-Au alloy wires produced with specific drawing and annealing processes are suitable for application to mid-power white light LEDs.

Influence of Pd content on microstructure and performance of Au-coated Ag alloy wire

Au-coated Ag alloy (ACAA) wires with different Pd content were fabricated in this investigation. Microstructure, mechanical, and bonding performance of the wires were studied to understand the influence of Pd content and help optimization of the ACAA wire. Pd content ranging from 0 to 4 wt% was studied based on the usual level in commercial alloying wires. With increasing Pd content, the ingot presents more obvious dendritic crystals, and the wire shows higher recrystallization temperature with a maximum difference about 300 ℃. On the basis of a comparable elongation, higher annealing temperature is needed and higher tensile strength, Young’s modulus, hardness, electrical resistivity, and finer grains are obtained with higher Pd content. Moreover, the grains transform from 〈100〉 texture as drawn to mixed 〈100〉/〈111〉 texture partially after annealing. Less reduction in tensile performance of the heat-affected zone and higher hardness for both free air ball and wire are observed with higher Pd content. Consequently, higher wire pulling force and more pad extrusion are resulted.

Corrosion-induced degradation and its mechanism study of Cu–Al interface for Cu-wire bonding under HAST conditions

This study explores the Cu wire corrosion-related issues and provides solutions for evaluation & analysis as the Automotive Electronics Council (AEC) continually formulates industry specification and qualification procedures for evolving Cu wire-bonding technology. Evaluation of molding resin attributes concerning about the preferential Intermetallic Compounds (IMCs) corrosion and formation of oxidation layer in the Cu–Al interface during accelerated reliability stress test (bias HAST) is discussed. Failure mechanism for Cu–Al system is microgalvanic corrosion accompanied with crevice/pitting corrosion-induced deterioration in the presence of moisture and ion impurity from resin encapsulatants. Failure analysis techniques conduct on corrosion-induced broken stitch bond issue are proposed and root cause is identified. Electrochemical studies of Au, Cu, Pd-doped Cu (PCC) and Ag alloy wire presents here for further understanding the corrosion behavior in wire-bonding packages. Potentiodynamic polarization is used to investigate the electrolyte property and corrosion performance of these specimen in the corrosive medium. The results provide vital information on the Cu corrosion study not only in ball bonds but also in stitch bonds which are of practical value to the current industry under “design for reliability” (DfR) approach to product development.

Bondability of Pd and Au Containing Ag-Alloy Wires on Au Pads for LED Package

This study investigated the mechanism of the interfacial reaction of Ag-alloy wires in the gold bond pad of an LED die after a wire bonding process and their reliability. The results showed that in the as-bonded condition, no intermetallic compounds (IMCs) formed between the Ag-alloy wire and the gold bond pad and that the interfacial reaction mechanism was diffusional solid solution reaction. After 168 h of thermal aging at 200 °C, it was found that adding Pd to the Ag-alloy wire could improve the mechanical properties of the wire, and the bonding strength between the Ag-alloy wires and gold bond pad showed an increasing trend. It was speculated that increasing the thermal aging period allowed sufficient time for the diffusion reaction between the silver alloy and gold bond pad. The gold ball residual area difference indicated that adding Pd to Ag-alloy wire was more helpful than adding Au for boosting bond shear test (BST) strength in the as-bonded condition, while adding Au to a ternary Ag-alloy wire helped the bonding strength in terms of thermal stability. BST and wire pull test (WPT) specifications are suggested for Ag-alloy wire joints in LED packaging products based on tests of the critical quality parameters of Cpk.

Microstructure and bonding performance of an Au-coated Ag alloy wire

An Au-coated Ag alloy (ACAA) wire was fabricated in this work. Microstructure and bonding performance including characteristics of free air ball (FAB) and heat affected zone (HAZ) of the ACAA wire was investigated. The wire is coated by Au layer with an average thickness about 110 nm continuously via physical method. During bonding, golf-clubbed FAB and asymmetrical distribution of Au on the section is observed. Sparking and heating from the side is suggested to be the reason and the schematic is illustrated. The ACAA wire experiences certain degree of grain growth in the HAZ. Tensile strength of the HAZ and base wire is 228.86 MPa and 232.43 MPa, respectively while elongation is 15.12% and 18.84%, meaning that the HAZ present slight tensile performance reduction. The ACAA wire presents better oxidation resistance, mechanical and electrical performance than the non-coated Ag alloy (AA) wire. Despite the golf-clubbed FAB, morphology of the bonded ball and bondability testing results of the ACAA wire meet the requirement of wire bonding.

Study of wire bonding reliability of Ag-Pd-Au alloy wire with flash-gold after chlorination and sulfidation

In this study, wire is made of nanometer flash-gold (Au) film deposited onto the surface of Ag-2.5 wt% Pd-1.5 wt% Au wire (APA wire), denoted as AAPA wire. The flash-Au film increases the resistance to chlorination and sulfidation of the silver (Ag) alloy wire, which is susceptible to deterioration in a chlorine- and sulfur-rich environment. In the chlorination test, the mechanical properties of the AAPA wire were better than those of the APA wire. Chloride ions corrode the wire along grain boundaries, resulting in a change from ductile fracture to brittle fracture in the tensile test. Chloride ions preferentially corroded the aluminum (Al) pad, greatly decreasing the reliability of the ball bond. The wedge bond did not fail due to the use of Au pad. In the sulfidation test, the average diameter of the wire increased with increasing sulfidation time. After sulfidation, the electrical conductivity decreased due to the thickening of Ag2S, whose resistance is high, and a decrease in the effective channel of the electric current. In addition, the formation of Ag2S caused a consumption of Ag atoms in the Ag wire, and thus formed voids at the interface between the sulfide layer and the Ag wire. The fracture of the sulfided ball bond was located in the heat-affected zone after the tensile test. The results show that the AAPA wire has better resistance in chlorination and sulfidation environment.

Luminous Efficiency of Pd-Doped Ag-Alloy Wire Bonded LED Package after Reliability Tests

In this study, binary Ag-alloy wires were doped with different Pd concentrations, and each wire was encapsulated in an LED package. The initial optical characteristics were tested, and reliability was tested with the high temperature storage life (HTSL), high temperature operating life (HTOL) and wet high temperature operating life (WHTOL). The luminous efficiency of the Ag-alloy wire LED package was about 2% higher than that of the Au wire package, but the addition of 6% Pd to the Ag-alloy wire decreased the luminous efficiency to close to that of the Au-wire LED package. This was due to the high reflectivity of silver in the blue wavelength region, as compared to the low reflectivity of palladium. After 1,000 hours of HTOL and WHTOL, the results showed that the performance of luminous flux maintenance increased with increasing Pd content, indicating that Ag-alloy wires doped with a sufficient amount of Pd can inhibit degradation due to oxidation reaction and thermal and humidity aging. Therefore, binary Ag-Pd alloy wires produced with specific drawing and annealing processes are suitable for mid-power white light LEDs in lighting applications.

Detecting the Internal Distribution and Structure of Pd-Doped Ag Wire Bonds Using an RF Technique

A vector network analyzer was used to obtain the S parameters of Pd-doped Ag-alloy wire bonds up to 67 GHz. The alternating-current (AC) resistance was extracted at around 3 GHz, 8 GHz, 16 GHz, and 20 GHz before the resonance; it was found that Ag alloy wire bonds with 3.5 wt.% Pd had the lowest AC resistance, with consistent trends at these frequencies. Because Pd has higher resistance than silver, and at high frequencies the current tends to be distributed at the periphery (according to the skin effect), the inward movement of Pd can account for this lowest AC resistance. Analytical methods including electron probe microanalysis (EPMA), energy-dispersive x-ray spectroscopy (EDS), and focused ion beam (FIB) were employed to determine whether Pd had higher concentration in the central region for Ag wire with 3.5 wt.% Pd. EPMA and EDS showed evidence of higher Pd concentration in the central area for Ag wires with 3.5 wt.% Pd. Also, from FIB micrographs, the central area of the Ag wires with 3.5 wt.% Pd had regions of high-angle grain structure, which may explain why the Ag wires with 3.5 wt.% Pd performed better in reliability testing. It is hypothesized that regions of high-angle grain structure are associated with higher Pd concentration in the core region, but verification of this hypothesis lies beyond the scope of this work.

The Bondability and Reliability of a Ternary Ag Alloy Wire on an Al Bonding Pad under N<sub>2</sub>-Free Conditions

The Bondability and Reliability of a Ternary Ag Alloy Wire on an Al Bonding Pad under N<sub>2</sub>-Free Conditions

Complete elimination of Ag3Al in Ag wire bonds on Al by alloying In into Ag

During the electronic packaging process, when the silver (Ag) alloy wire bonds on aluminum (Al) pads, it is often desirable to avoid the formation of Ag3Al because of its low fracture toughness and low corrosion resistance. The current work reports a method to eliminate the Ag3Al through alloying indium (In) into Ag. The corresponding mechanism is studied through transmission electron microscopy (TEM) and thermodynamic modeling. The experimental results show that the formation of Ag3Al leads to the repelling of In atoms into unreacted Ag solid solution when the In concentration is low, but the Ag3Al can be fully eliminated and replaced by an Ag–In–Al ternary phase when the concentration of In is at 12%. We establish an analytical thermodynamic model for the nucleation process of Ag3Al that considers the effects of In doping, and successfully explains the underlying reason for the elimination of Ag3Al: it has to do with the necessity of repelling In into unreacted region. Reaction kinetics are further discussed and the deceleration in Ag–Al inter-diffusion is attributed to the solute–vacancy attraction. A study of defects in Ag–Al bonding wire system is conducted and the growth rate of voids is found to be low due to small volumetric change during phase transformation, which also reduces the growth rate of alumina. Compared to Ag3Al, Ag–In–Al ternary phase is experimentally proven to be softer, tougher, and more corrosion resistant, which would contribute to enhanced wire bond reliability and perhaps an increase the market share of Ag alloy bonding wires.

Evaluation of Corrosion Resistance of Ag-Alloy Bonding Wires for Electronic Packaging

The corrosion resistances of various Ag-alloy wires for the interconnection of integrated circuit and light-emitting diode packages were compared with those of traditional Au, Al, Cu, and pure Ag bonding wires through potentiodynamic polarization tests in 3.5 wt% NaCl aqueous solution. The results indicated that alloying the Ag bonding wires with Pd and Au elements decreases their thermodynamics corrosion tendency. However, the kinetics analyses of the corrosion rates from polarization curves showed that the corrosion current density of Ag-alloy wires decreases with increasing Pd and Au contents. The beneficial effects of Pd and Au alloying similarly occur in the passive region and passive current density. The results indicated that adding Pd and Au can effectively reduce the corrosion of Ag-alloy wires. Aging treatment at 600 °C for 3 h in ambient air increased the corrosion tendencies and corrosion rates of both Ag-3Pd and Ag-8Au-3Pd wires. In comparison to the corrosion of traditional Au, Al, and Cu wires, the experimental results showed that the corrosion potential of Ag-8Au-3Pd alloy wire was near that of Pd-coated Cu wire, and more noble than those of Cu wire and Al wire. The corrosion current densities increased on the order: Au wire < Ag-8Au-3Pd wire < Pd-coated Cu wire < Cu wire < Al wire. The sequence of pitting corrosion potentials was: Al wire < Cu wire < Pd-coated Cu wire < Ag-8Au-3Pd wire < Au. After corrosion tests, CuCl2 appeared on the surfaces of Cu wire and Pd-coated Cu wire, while AlCl3 was found on the corroded Al wire. The corrosion product of Ag-alloy wires was found to be AgCl, which was reported to possess high ohm resistance and could inhibit the further dissolution of Ag wire.

A study on intermetallic compound formation in Ag–Al system and evaluation of its mechanical properties by micro-indentation

In electronic packaging in recent years, silver (Ag) alloy wires have been widely adopted in wire-bond processes. The bond pads on the device chips are mostly aluminum (Al). Thus, the bonding interface is mainly Ag–Al. In recent Ag alloy wire-bond publications, it is unclear what intermetallic phases form at the interface. In this research, experiments were designed to understand the Ag–Al intermetallic compound (IMC) formation in the Ag–Al system and evaluate its mechanical properties. First, the Ag–Al alloys with compositions from 19 to 43 at.% Al were evaluated to identify the phase equilibrium and crystal structure of the Ag–Al intermetallic phases. Microstructures and phase compositions of the designed Ag–Al alloys are presented. To further study the intermetallic compound formation at the Ag/Al interface, the interfacial reaction of the Ag/Al joints at 200 °C was investigated. The µ-Ag3Al and δ-Ag2Al IMC were identified to form at the Ag/Al interface and stabilize after long-time annealing at 200 °C. At last, deformation and fracture behaviors of the bulk µ-Ag3Al and δ-Ag2Al were analyzed by the micro-indentation. The measured results reveal that µ-Ag3Al exhibits significantly higher hardness and lower fracture toughness as compared to δ-Ag2Al. Indentation crack propagation in µ-Ag3Al demonstrates the fracture characteristics of brittle materials. In the case of δ-Ag2Al, the presence of slip bands exhibits the ductility of δ-Ag2Al to endure plastic deformation prior to fracture. The effect of the mechanical properties of the µ-Ag3Al and δ-Ag2Al IMC on the Ag–Al joint reliability is discussed. New information obtained in this research is important for future study of the joint reliability and failure mechanism of Ag–Al wire bonds.

A study on electromigration-inducing intergranular fracture of fine silver alloy wires

In this study, Pd-coated Cu, Ag (purity = 4 N), and Ag alloy (Ag-8Au-3Pd) wires were employed to measure the tensile properties during current stressing using the so-called dynamic current tensile (DCT) test. Both the tensile strength and elongation of the wires decreased dramatically in the DCT test, particularly of the Ag-based wires, and the fracture morphology of the Cu-based and Ag-based wires was ductile fracture and intergranular fracture, respectively. Compared to the Cu-based wires, electromigration occurred more easily in the Ag-based wires, and it always generated voids and cracks at the grain boundaries; therefore, the fracture morphology of the Ag-based wires was intergranular fracture owing to the weakened grain boundary. Further, the results indicated that the Ag-based wires could not carry a higher current density than the Cu-based wires, primarily because their extremely low strength and elongation in current stressing might cause serious reliability problems.