Interface microstructure effects in Au thermosonic ball bonding contacts by high reliability wire materials

Palladium-doped and (Cu, Pt)-doped high reliability gold wires were used to form wire bond interconnects on aluminum IC metallization. By isothermal annealing of wire bond samples the formation of intermetallic Au–Al phases was stimulated. SEM/EBSD investigations of the phase regions exhibited significantly slower isothermal growth rates compared to a reference gold wire. Correlated TEM, STEM–EDXS and nanobeam diffraction analyses revealed that Pd is preferentially incorporated into the Au₈Al₃ intermetallic forming a new stable phase but additionally can obviously form a new Pd-rich ternary intermetallic. In comparison, Cu dopants are also accumulated into a new Al–Au–Cu phase while Pt is rather found agglomerating within grain boundaries and interfaces. These results suggest a diffusion barrier model that allows discussing how wire doping can affect the bond contact microstructure, thus increasing the lifetime of bond contacts.     

New observations on intermetallic compound formation in gold ball bonds: general growth patterns and identification of two forms of Au4Al

Relatively little information is available on the growth patterns and metallurgy of Au–Al intermetallics in fine-pitch (FP) and ultra-fine pitch (UFP) ball bonding. This paper presents a study of the growth pattern and chemistry of intermetallic compounds formed between a 25 μm 4 N gold wire and aluminium pad metallization after isothermal ageing in air at 175 °C. The data show the intermetallics grow vertically and laterally under the ball and totally consume the Al in the bond pad at <20 h. Then, a third layer of intermetallic grows between Au₄Al and Au₅Al₂. Measurements and observations made with EDX and optical microscopy lead to the conclusion that the new compound is a different form of Au₄Al, most probably a low-temperature version of the α-Au₄Al intermetallic structure. Electrical resistance during intermetallic growth was not measured in this study but wire chemistry and bonding conditions are found to affect the thickness of the intermetallic compounds, which suggests that the resistance of ball bonds during moulding and operation can change.     

Effects of Cu and Pd addition on Au bonding wire/Al pad interfacial reactions and bond reliability

Finer pitch wire bonding technology has been needed since chips have more and finer pitch I/Os. However, finer Au wires are more prone to Au-Al bond reliability and wire sweeping problems when molded with epoxy molding compound. One of the solutions for solving these problems is to add special alloying elements to Au bonding wires. In this study, Cu and Pd were added to Au bonding wire as alloying elements. These alloyed Au bonding wires—Au-1 wt.% Cu wire and Au-1 wt.% Pd wire—were bonded on Al pads and then subsequently aged at 175°C and 200°C. Cu and Pd additions to Au bonding wire slowed down interfacial reactions and crack formation due to the formation of a Cu-rich layer and a Pd-rich layer at the interface. Wire pull testing (WPT) after thermal aging showed that Cu and Pd addition enhanced bond reliability, and Cu was more effective for improving bond reliability than Pd. In addition, comparison between the results of observation of interfacial reactions and WPT proved that crack formation was an important factor to evaluate bond reliability.     

The influence of Sn–Cu–Ni(Au) and Sn–Au intermetallic compounds on the solder joint reliability of flip chips on low temperature co-fired ceramic substrates

The formation of intermetallic compounds in the solder joint of a flip chip or chip scale package depends on the under bump metallurgy (UBM), the substrate top surface metallisation, the solder alloy and the application conditions. To evaluate the influence of intermetallic compounds on the solder joint reliability, a detailed study on the influence of the UBM, the gold finish thickness of the substrate top surface metallisation, the solder alloy and the aging conditions has been conducted. Flip chips bumped with different solder alloys were reflow-mounted on low temperature co-fired ceramic substrates. The flip chip package was then aged at high temperature and a bump shear test followed to examine the shear strength of the solder joint at certain aging intervals. It was found that the type of UBM has a great impact on the solder joint reliability. With Ni(P)/Au as the UBM, well-documented gold embrittlement was observed when the gold concentration in the eutectic SnPb solder was about 3 wt%. When Al/Ni(V)/Cu was used as the UBM, the solder joint reliability was substantially improved. Copper dissolution from the UBM into the solder gives different intermetallic formations compared to Ni(P)/Au as UBM. The addition of a small amount of copper in the solder alloy changed the mechanical property of the intermetallic compound, which is attributed to the formation of Sn–Cu–Ni(Au) intermetallic compounds. This could be used in solving the problem of the AuSn₄ embrittlement. The formation and the influence of this Sn–Cu–Ni(Au) intermetallic phase are discussed. The gold concentration in the solder joint plays a role in the formation of intermetallic compounds and consequently the solder joint reliability, especially for the Sn–Ag–Cu soldered flip chip package.     

The effect of Pd and Cu in the intermetallic growth of alloy Au wire

This is the new wire evaluation work for the reliability of the wire-bonding process. There is a trend for the plastic integrated-circuit package to function at higher junction temperature with thinner wire. New alloy Au wires have been developed to meet the reliability requirements. Two types of alloy Au wires, Au-Pd and Au-Cu, were evaluated in this study. These samples were aged between 155°C and 205°C under air from 0 h to 3,000 h. According to this study, the phase-formation sequence of Au₂Al, Au₅Al₂, and Au₄Al intermetallic is similar to the pure Au wire. There is a Pd-rich layer working as a diffusion barrier to slow down the growth rate of intermetallic phases in the Au-Pd wire. The Au-Cu wire also slowed down the growth rate with a different mechanism. Both wires have better reliability based on the microstructure examination. The reliability test results show longer working life at higher temperatures in comparison with the regular Au wire.     

Wire-bond void formation during high temperature aging

Voids formed in Au-Al intermetallic phases degrade the long-term reliability of gold wire bonds to aluminum pads. In this study, a series of microstructural studies were performed to evaluate void formation in wire bonds. Voids are classified as initial, annular or minute. Probe marks and Al pad contamination are the main causes of initial voids that block alloy diffusion and slow down intermetallic growth. Annular voids are caused by the ultrasonic squeeze effect of thermosonic wire bonding. These bonding gaps may become pathways for halide species that corrode and degrade wire bonds. Minute voids are formed during the Au/sub 4/Al phase. The two Au/sub 4/Al phase textures in these voids may be due to different Au/sub 4/Al phase formation reactions or be related to grain boundary effects on the surface layer of the Au ball.     

Effects of Pd addition on Au stud bumps/Al pads interfacial reactions and bond reliability

The main purposes for developing low-alloyed Au bonding wires were to increase wire stiffness and to control the wire loop profile and heat-affected zone length. For these reasons, many alloying elements have been used for the various Au bonding wires. Although there have been many studies reported on wire strengthening mechanisms by adding alloying elements, few studies were performed on their effects on Au bonding wires and Al pad interfacial reactions. Palladium has been used as one of the important alloying elements of Au bonding wires. In this study, Au-1wt.%Pd wire was used to make Au stud bumps on Al pads, and effects of Pd on Au/Al interfacial reactions, at 150°C, 175°C, and 200°C for 0 to 1200 h thermal aging, were investigated. Cross-sectional scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and electron probe microanalysis (EMPA) were performed to identify intermetallic compound (IMC) phases and Pd behavior at the Au/Al bonding interface. According to experimental results, the dominant IMC was Au₅Al₂, and a Pd-rich layer was at the Au wire and Au-Al IMC. Moreover, Au-Al interfacial reactions were significantly affected by the Pd-rich layer. Finally, bump shear tests were performed to investigate the effects of Pd-rich layers on Au wire bond reliability, and there were three different failure modes. Cracks, accompanied with IMC growth, formed above a Pd-rich layer. Furthermore, in longer aging times, fracture occurred along the crack, which propagated from the edges of a bonding interface to the center along a Pd-rich layer.     

Evaluation of the corrosion performance of Cu–Al intermetallic compounds and the effect of Pd addition

Copper wire has become a mainstream bonding material in fine-pitch applications due to the rising cost of gold wire. In recent years, palladium-coated copper (Pd–Cu) wire is being increasingly used to overcome some constraints posed by pure Cu wire. During wire bonding with aluminum bond pads, different intermetallic compound (IMC) phases that have been identified at the bond interface are typically CuAl₂, CuAl and Cu₉Al₄. However, the corrosion susceptibility of these IMCs has not been investigated. This paper compares the electrical impedance and corrosion performance of the three types of Cu–Al IMCs in an acidic chloride medium by employing electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization. The analysis of the potentiodynamic polarization results was performed using Tafel extrapolation. A comparison was made with pure Cu and Al. The effect of Pd alloy on the IMC corrosion performance has also been studied. Among the three Cu–Al IMCs, Cu₉Al₄ was observed to have the largest corrosion rate followed by CuAl₂ and CuAl. For the metals, Cu was observed to have the lowest corrosion rate and Al is the most easily corroded. The addition of Pd of up to 10 wt.% replacement of the Cu in the alloys slightly improves the corrosion resistance of the metals and IMCs.     

铜线键合Cu/Al界面金属间化合物微结构研究

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

Numerical and experimental analysis of the Sn3.5Ag0.75Cu solder joint reliability under thermal cycling

The Sn3.5Ag0.75Cu (SAC) solder joint reliability under thermal cycling was investigated by experiment and finite element method (FEM) analysis. SAC solder balls were reflowed on three Au metallization thicknesses, which are 0.1, 0.9, and 4.0 μm, respectively, by laser soldering. Little Cu–Ni–Au–Sn intermetallic compound (IMC) was formed at the interface of solder joints with 0.1 μm Au metallization even after 1000 thermal cycles. The morphology of AuSn₄ IMC with a small amount of Ni and Cu changed gradually from needle- to chunky-type for the solder joints with 0.9 μm Au metallization during thermal cycling. For solder joints with 4 μm Au metallization, the interfacial morphology between AuSn₄ and solder bulk became smoother, and AuSn₄ grew at the expense of AuSn and AuSn₂. The cracks mainly occurred through solder near the interface of solder/IMC on the component side for solder joints with 0.1 μm Au metallization after thermal shock, and the failure was characterized by intergranular cracking. The cracks of solder joints with 0.9 μm Au metallization were also observed at the same location, but the crack was not so significant. Only micro-cracks were found on the AuSn₄ IMC surface for solder joints with 4.0 μm Au metallization. The responses of stress and strain were investigated with nonlinear FEM, and the results correlated well with the experimental results.     

Bond reliability under humid environment for coated copper wire and bare copper wire

There is growing interest in Cu wire bonding for LSI interconnection due to cost savings and better electrical and mechanical properties. Conventional bare Cu bonding wires, in general, are severely limited in their use compared to Au wires. A coated Cu bonding wire (EX1) has been developed for LSI application. EX1 is a Pd-coated Cu wire to enhance the bondability.Bond reliability at a Cu wire bond under a humid environment is a major concern in replacing Au wires. The bond reliability of EX1 and bare Cu was compared in the reliability testing of PCT and UHAST (Unbiased HAST). The lifetimes for EX1 and the bare Cu in PCT testing were over 800 h and 250 h, respectively. Humidity reliability was significantly greater for EX1. Continuous cracking was formed at the bond interface for the bare Cu wire. Corrosion-induced deterioration would be the root cause of failure for bare Cu wires. The corrosion was a chemical reaction of Cu-Al IMC (InterMetallic Compound) and halogens (Cl, Br) from molding resins. EX1 improves the bond reliability by controlling diffusion and IMC formation at the bond interface. The excellent humidity reliability of the coated Cu wire, EX1 is suitable for LSI application.     

Nickel-palladium bond pads for copper wire bonding

The semiconductor packaging industry is undergoing a step-change transition from gold to copper wire bonding brought on by a quadrupling of gold cost over the last 8 years. The transition has been exceptionally rapid over the last 3 years and virtually all companies in the industry now have significant copper wire bonding production. Among the challenges to copper wire bonding is the damage to bond pads that had been engineered for wire bonding with the softer gold wire. This paper presents an extensive evaluation of electroless NiPd and NiPdAu bond pads that offer a much more robust alternative to the standard Al pad finish. These NiPd(Au) bond are shown to outperform Al in virtually all respects: bond strength, bond parameter window, lack of pad damage and reliability.     

金、铜丝球键合焊点的可靠性对比研究

金丝球焊是电子工业中应用最广泛的引线键合技术,但随着高密度封装的发展,铜丝球焊日益引起人们的关注。采用热压超声键合的方法,分别实现Au引线和Cu引线键合到Al-1%Si-0.5%Cu金属化焊盘。对焊点进行200℃老化实验的结果表明:铜丝球焊焊点的金属间化合物生长速率比金丝球焊焊点慢的多;铜丝球焊焊点具有比金丝球焊焊点更稳定的剪切断裂载荷,并且在一定的老化时间内铜丝球焊焊点表现出更好的力学性能;铜丝球焊焊点和金丝球焊焊点在老化后的失效模式不同。     

Wire-bond failure mechanisms in plastic encapsulated microcircuits and ceramic hybrids at high temperatures

Ceramic hybrids are the preferred solution when long-term high-temperature reliability is required, but standard plastic encapsulated microcircuits (PEMs) are an interesting alternative due to low price and high availability. Test vehicles with standard PEMs were subjected to thermal ageing at 150–175 °C. Six of eight vehicles failed after only three weeks at 175 °C, and the cause of failure was found to be microcracking at the interface between gold ball and aluminium bond pad giving rise to resistance increase. The intermetallic region was formed during high-temperature lead soldering and continued to develop during thermal ageing. The high-temperature performance of aluminium wire bonding to a selection of thick film metallizations on ceramic substrate was also investigated. Gold–palladium has previously been reported as a high-temperature solution, but we found that the mechanical strength of aluminium to gold–palladium (AuPd) degraded seriously at temperatures above 200 °C due to intermetallic formation. Aluminium to silver thick film plated with copper and nickel showed good mechanical strength and unaltered electrical resistance after four weeks thermal ageing at 250 °C.     

Observations of IMC Formation for Au Wire Bonds to Al Pads

A materials investigation of Au wire bonds to Al pads revealed the evolution of a multiphase system whose terminal phases depended on the composition of the Au wire. Scanning transmission electron microscopy/energy-dispersive spectroscopy and electron diffraction data are presented for Au/Al wire bonds using both Pd-doped, 99% pure Au wire (2N) and 99.99% pure Au wire (4N) in the as-formed state, upon completion of overmold operations, and after reflow and aging. The reacted interfaces of both the 2N and 4N bonds were found to take on a bilayer intermetallic compound (IMC) microstructure that persisted with aging and phase changes; it is the interface of this bilayer that is believed to be susceptible to mechanical degradation. Pd was found to accumulate in the IMC near the Au/IMC interface for 2N wire bonds and appears to lead to a phase evolution different from that for 4N wire that may be responsible for enhanced reliability of the 2N wire bond with high-temperature aging.     

Effect of Ag micro-particles content on the mechanical strength of the interface formed between Sn–Zn binary solder and Au/Ni/Cu bond pads

In this study, addition of Ag micro-particles with a content in the range between 0 and 4 wt.% to a Sn–Zn eutectic solder, were examined in order to understand the effect of Ag additions on the microstructural and mechanical properties as well as the thermal behavior of the composite solder formed. The shear strengths and the interfacial reactions of Sn–Zn micro-composite eutectic solders with Au/Ni/Cu ball grid array (BGA) pad metallizations were systematically investigated. Three distinct intermetallic compound (IMC) layers were formed at the solder interface of the Au/electrolytic Ni/Cu bond pads with the Sn–Zn composite alloys. The more Ag particles that were added to the Sn–Zn solder, the more Ag–Zn compound formed to thicken the uppermost IMC layer. The dissolved Ag–Zn IMCs formed in the bulk solder redeposited over the initially formed interfacial Au–Zn IMC layer, which prevented the whole IMC layer lifting-off from the pad surface. Cross-sectional studies of the interfaces were also conducted to correlate with the fracture surfaces.     

Microstructural evaluation and failure analysis of Ag wire bonded to Al pads

We found the failure mechanisms in Ag wire bonded to Al pads during the high-temperature-storage lifetime test (HTST) and the unbiased highly-accelerated temperature and humidity storage test (uHAST). The native oxide layer on the Al pads caused a ball lift. The moisture and the thermal energy during uHAST along with the Cl⁻ ion in epoxy molding compounds (EMCs) induced repetitive oxidation and reduction reactions of the Ag–Al intermetallic compounds (IMCs) with the Al pads. These repetitive reactions formed H₂ gas as a by-product causing the formation of a micro-crack. In addition, the alumina layer acted as a resistive layer in the Ag–Al IMCs. The phases of the Ag–Al IMCs were identified as Ag₂Al and Ag₃Al, and the growth rates of those IMCs were measured at 150 and 175 °C for 2000 h.     

Sequential environmental stresses tests qualification for automotive components

The purpose is to create a new qualification methodology for plastic encapsulated electronic components used in an automotive environment at high temperature. It is based on the acceleration of failure mechanisms like ball bond lift (due to intermetallic Au–Al thickness growth), by combination of environmental stresses. The delamination measurement was used as an indicator of potential assembly weaknesses. An optimized package sequential qualification test flow is proposed.     

Cu wire bond microstructure analysis and failure mechanism

In this study, copper wire bonding samples were aged at 205 °C in air from 0 h to 2000 h. It was found that the bonding of a Cu wire and an Al pad formed Cu₉Al₄, CuAl, and CuAl₂ intermetallic compounds, and an initial crack was formed by the ultrasonic squeeze effect during thermosonic wire bonding. The cracks grew towards the ball bond center with an increase in the aging time, and the Cl ions diffused through the crack into the ball center. This diffusion caused a corrosion reaction between the Cl ions and the Cu-Al intermetallic phases, which in turn caused copper wire bonding damage.     

An unusual mechanical failure mode in gold ballbonds at 50 μm pitch due to degradation at the Au–Au4Al interface during ageing in air at 175 °C

While investigating the bake test reliability of gold ball bonds in air at 175 °C an unusual failure mode was encountered that can strictly be classified as a ball lift but which has pull strengths as high as normal neck breaks. The failure mode is termed a high strength ball lift (HSBL) and is distinguished by partial de-bonding between the gold ball and intermetallic compound. The partial de-bonding occurs as a ring or annulus that starts from the outside of the ball and grows inwards toward the ball centre, occurring within the Au₄Al compound layer and between the Au₄Al and Au ball while the remaining part of the gold ball is well-bonded to the Au₄Al at the centre of the bondpad. During pull testing, the de-bonded outer ring acts as a notch that propagates a blunt crack into the gold ball resulting in high pull strengths. As ageing time increases the de-bonded region grows larger and advances inwards towards the ball centre. EDX of both types of ball-lifts indicates higher levels of oxygen on the Au₄Al surface that suggests annulus growth may be partially due to oxidation of the intermetallic. The number of HSBL failures and the point in time at which they occur is observed to depend on the bonded ball squash height, a relatively easily controlled parameter in gold ballbonding. When devices with targeted squash heights (TSH) of 6 μm, 7 μm and 8 μm are isothermally aged, zero ball lifts are only obtained at 7 μm squash height. HSBL failures are encountered earlier at the 6 μm TSH than at 8 μm TSH and as the de-bonded region progresses further inwards during baking, the HSBL’s are observed to transition to low strength ball lifts (LSBL) as the load-bearing area of the ball is reduced. The dependence of failure mode on squash height is believed to be related to interactions between bonded ball stress, intrinsic intermetallic growth stresses and thermo-chemical degradation of the Au₄Al intermetallic compound.