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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.

Several attempts have been made to lower the price of materials in the semiconductor market. Among them, the wire used for interconnection has a disadvantage of high material price in the field of using gold. Many packages have evolved to other technologies to improve performance and are moving away from wire bonding process, but many packages still use bonding wires. In the bonding wire market, in order to reduce prices, gold is being replaced by inexpensive copper, and now the market for copper wire coated with precious metals considering workability is dominant. [1], [5] However, due to the rigid nature of copper, packages with sensitive wiring structures are difficult to apply. [2] Silver alloy wire and noble metal coated silver wire have been developed to lower the wire price of these sensitive packages. [3] In the case of the recently developed noble metal coated silver wire, unlike a silver alloy bonding wire, a spherical ball shape can be realized without using inert gas when forming a Free Air Ball. Due to these advantages, it has recently been in the spotlight as a wire for replacing Au wire. [3] In this paper, experiments were conducted on the bonding conditions of silver wire coated with noble metals. The bondability of the prepared noble metal-coated silver wire according to the Free Air Ball formation conditions was verified. Specifically, evaluation was performed according to the current change at the time of Electronic Flame Off, the type of inert gas, and the size of the Free Air Ball. The evaluation confirmed the optimum bonding conditions for the application of the noble metal coated silver wire.

Au Bonding wire, which has been widely used in the semiconductor PKG market, is being converted into Cu Bonding wire due to a sharp increase in Au price. The price of Cu is cheaper than Au and Pd coated wire products with improved reliability and workability have been introduced, bringing a large portion of Au Bonding wire usage. However, the sensitive structure of the PKG still uses Au Bonding wire, because of the problem of pad cratering due to the high workhardening of Cu, so the Au Bonding wire cannot be converted to a cheap Cu Bonding wire despite the high Au price. Recently, Ag alloy Bonding wire has been widely evaluated as a low cost Bonding wire for PKG with sensitive structure. However, like the Cu Bonding wire, a gas kit and an inert gas are required to prevent oxidation when forming a FAB (Free Air Ball, the following notation is unified FAB) for ball Bonding. In addition, the high-humidity reliability level of silver alloy Bonding wire is lower than that of Au Bonding wire. In this study, we evaluated the metal coated silver bonding wire as a next generation low cost bonding wire product to replace Au Bonding wire. In order to improve the disadvantages of the Ag alloy Bonding wire in which the elliptical FAB is formed when the inert gas is not used in the 1st ball Bonding, the noble metal coating is performed on the high purity Ag alloy Bonding wire. Gas free Bonding characteristics and high humidity reliability performance were evaluated according to the kind and thickness of noble metal. In the evaluation of high humidity reliability, metal coated silver Bonding wire showed a high humidity reliability performance more than 2 times compared to the conventional silver alloy Bonding wire. Metal coated silver Bonding wire is expected to be an alternative to the new low cost Bonding wire in the PKG market due to gas free Bonding and improved high humidity reliability performance.

Bonding wire, one of the bonding materials, has been rapidly changed from Au to Cu due to an increase in gold price. Although Pd coated Cu (Au-Pd-Cu) wires, which comprise the largest market share among Cu bonding wires, show high reliability compared to bare Cu wire in high-humidity reliability, the reliability issue still appears in certain area. In the case of Pd coated Cu wire, studies have reported that the Pd distribution on the free air ball (FAB) surface is an important factor for improving the reliability. This study confirmed that the distribution of FAB could be controlled according to the wire manufacturing conditions, and that the Pd distribution further changed depending on the change of EFO conditions. The study further confirmed that the denser Pd distribution outside bonded ball induces the higher chemical resistance at the interface between bonding pad and the bonded ball, which improves the reliability at high temperature and high humidity after PKG.

The application of an alternative method of bond monitoring during high-temperature aging is reported using a custom made test chip with piezoresistive integrated CMOS microsensors located around test bond pads. The sensor detects radial stresses originating from the bond pad and can resolve changes because of intermetallic compound (IMC) formation, voiding, or crack formation at the bond interface. Optimized Au ball bonds are aged for over 2000 h at 175 °C. It is found that stress sensors next to the bonds are capable of showing the stages of IMC growth, consumption of pad Al layers, and monitoring the formation of low-density and Al-rich IMC (AuAl2) which shows an advanced stage of aging. In particular, a first stress signal increase corresponds to the conversion of all Al above the diffusion barrier into IMCs. The second increase in stress signal after a period of stability corresponds to conversion of all Al below the barrier into IMCs. The IMC formation in these periods causes shear strength increase. After complete bond Al consumption, the bond, however, reaches maximum strength. As bond degradation starts, e.g., by lateral IMC formation, voiding, and oxide formation, as well as because of lateral pad Al transformation to IMC, the signal exhibits a strong decrease. The findings are corroborated by results obtained from classical methods such as interruptive or destructive testing including visual inspection, shear testing, cross sectioning, and by bond resistance monitoring.

We investigated the effect of Pd concentration in Pd-doped Ag wires on the humidity reliability and interfacial corrosion characteristics between Ag wire and Al metallization. Additionally, we confirmed no corrosion problem between Ag wire and noble metal (Pd, Au) metallization, even after a pressure cooker test (PCT). The chemical composition of the tested Ag wires was pure Ag, Ag-1wt% Pd and Ag-3wt% Pd. These wires were bonded to Al and noble metal (Au, Pd) metallization using a thermo-sonic bonder. The interfaces were characterized by focused ion beam (FIB), high resolution transmission electron microscope (HRTEM) and energy dispersive X-ray spectroscopy (EDS). The interface corrosion of Pd doped Ag wires was significantly reduced as the Pd concentration in the Ag wires increased. Furthermore, the Ag wires on the noble metal (Au, Pd) metallization exhibited stable reliability during the PCT.

Inhomogeneous microtexture evolution during the cold drawing process usually results in lean, sway, or sweep failure. The longitudinal fiber texture has higher stiffness than the texture and its proportion and distribution in the cross-section are critical for the bonding stability of fine gold wire. We investigated the inhomogeneous microtexture evolution of gold wire that was cold drawn through an asymmetric diamond die. In this study, the distributions of the and textures in a2 0μm diameter fine gold wire are the variables and their effects on the bonding stability of the wire were estimated by electron backscattered diffraction (EBSD) and finite element method (FEM) simulations. The use of a focused ion beam apparatus enabled a high quality of band contrast of the EBSD to be achieved in the exact half cross-sectional area of the fine gold wire. The detailed three-dimensional FEM results show that the asymmetric distribution of the textures plays a crucial role in increasing the spatial displacement of the gold bonding wire.

We report the direct observation of microstructural changes of LixSi electrode with lithium insertion. HRTEM experiments confirm that lithiated amorphous silicon forms a shell around a core made up of the unlithiated silicon and that fully lithiated silicon contains a large number of pores of which concentration increases toward the center of the particle. Chemomechanical modeling is employed in order to explain this mechanical degradation resulting from stresses in the LixSi particles with lithium insertion. Because lithiation-induced volume expansion and pulverization are the key mechanical effects that plague the performance and lifetime of high-capacity Si anodes in lithium-ion batteries, our observations and chemomechanical simulation provide important mechanistic insight for the design of advanced battery materials.

This study examined the effects of inclusions on the gold wire drawing stress using the Finite Element Method (FEM). Al2O3 and SUS304 are used as the model inclusion materials, which are frequently found on the fracture surfaces of drawn wire. The wire drawing stress showed a strong correlation with the size and yield stress of the inclusions. In the case of Al2O3, a larger diameter induced more drawing stress in the wire, whereas in case of SUS304, a larger diameter induced less drawing stress in wire. The difference was estimated quantitatively by comparing the amount of equivalent plastic strain (PEEQ) in the wire, which is the result of an interaction between the wire and inclusion. This result shows that the reason of difference in drawing stress variation is due to the yield stress of the inclusion.

Given the cost and performance advantages associated with Cu wire, it is being increasingly seen as a candidate to replace Au wire for making interconnections in first level microelectronics packaging. A Cu ball bonding process is optimized with reduced pad stress and splash, using a 25.4 μm diameter Cu wire. For ball bonds made with conventionally optimized bond force and ultrasonic settings, the shear strength is ≈140 MPa. The amount of splash extruding out of bonded ball interface is between 10 and 12 μm. It can be reduced to 3–7 μm if accepting a shear strength reduction to 50–70 MPa. For excessive ultrasonic settings, elliptical shaped Cu bonded balls are observed, with the minor axis of the ellipse in the ultrasonic direction and the major axis perpendicular to the ultrasonic direction. To quantify the direct effect of bond force and ultrasound settings on pad stress, test pads with piezoresistive microsensors integrated next to the pad and the real-time ultrasonic force signals are used. By using a lower value of bond force combined with a reduced ultrasound level, the pad stress can be reduced by 30% while achieving an average shear strength of at least 120 MPa. These process settings also aid in reducing the amount of splash by 4.3 μm.

The development of novel Cu wires for thermosonic wire bonding is time consuming and the effects of shielding gas on the electrical flame off (EFO) process is not fully understood. An online method is used in this study for characterizing Cu free air balls (FABs) formed with different shielding gas types and flow rates. The ball heights before ( H FAB ) and after deformation ( H def ) are responses of the online method and measured as functions of gas flow rate. Sudden changes in the slopes of these functions, a non-parallelity of the two functions, and a large standard deviation of the H FAB measurements all identify FAB defects. Using scanning electron microscope (SEM) images in parallel with the online measurements golf-club shaped and pointed shaped FABs are found and the conditions at which they occur are identified. In general FAB defects are thought to be caused by changes in surface tension of the molten metal during EFO due to inhomogeneous cooling or oxidation. It is found that the convective cooling effect of the shielding gas increases with flow rate up to 0.65 l/min where the bulk temperature of a thermocouple at the EFO site decreases by 19 °C. Flow rates above 0.7 l/min yield an undesirable EFO process due to an increase in oxidation which can be explained by a change in flow from laminar to turbulent. The addition of H 2 to the shielding gas reduces the oxidation of the FAB as well as providing additional thermal energy during EFO. Different Cu wire materials yield different results where some perform better than others.

A real-time signal of the stress caused by a ball bond is recorded during long-term high temperature storage (HTS) without destroying the ball bond, using a piezoresistive integrated CMOS microsensor located next to the Al bond pad (test pad) on a test chip. The sensor is sensitive to in-plane shear stress changes Δτxy that arise due to tensile or compressive stress at the test pad. While performing HTS at 200 °C during 400 h, significantly different stress signals are observed with a ball bond (test structure) compared to those observed without a ball bond (reference structure). Simultaneous to Δτxy the contact resistance of the bond was directly measured with a four-wire method in which two connection paths lead to the test pad and a second wire bond is made on top of the test ball bond. The contact resistance values measured at room temperature (25 °C) before and after HTS are 2.1 mΩ and 6.1 mΩ, respectively. Effects influencing the stress signal during HTS include volume changes by the growth of intermetallics. The stress increase initially observed during HTS shows bond shrinking corresponding to growth of Au-rich phases which was previously reported to result in volume shrinkage. A subsequent phase of signal drop is observed starting after 200 h, indicating the presence of a different mechanism partly reducing the stress built up previously, and attributed to lateral growth of Al-rich intermetallics, partially consuming the pad Al outside the bond region, and resulting in volume expansion. Finite element models are developed to support the interpretation of the stress signal features. One of the models simulates the shrinking of Au–Al material due to phase transformation. When calibrated to experimental data, the peak underpad Tresca stress level generated during such contraction is 53 MPa, located 2.4 μm inside of the 55 μm diameter bond zone.

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.

Presently, Sn-Ag-Cu (SAC) solders are most commonly used as the interconnect materials in the semiconductor package. However, their thermal fatigue and drop impact resistant properties depends on the Ag content and therefore, most semiconductor package assemblers are forced to implement multiple SAC alloys depending on intended performance. Sn-xAg-Cu solders with high Ag content (x>3 mass%) give good temperature cycling (TC) reliability but poor drop impact reliability whereas Sn-xAg-Cu solders with low Ag content (x

Solid solution-type alloying in Au wire matrix is one of the well-known methods for improving the mechanical properties of Au wire as well as the reliability of bonding interface. Palladium (Pd) uses as a typical alloying element of Au bonding wire manufacturing which makes solid solution hardening effect in Au matrix. The behavior of Pd at the Au wire-Al pad bonding interface during the thermal aging, and the effect of Pd on Au/Al interfacial reactions were investigated. Two types of Au wires, Au-0.25wt%Pd (Low Pd content) and Au-0.95wt%Pd (High Pd content), were used for the fabrication of wire-bonded test vehicles (TVs). The wirebonded TVs were thermally aged at 175°C up to 1200hours, and the formation of a Pd-accumulation layer was investigated at Au-Al bonding interface by using a cross-sectional scanning electron microscope (SEM) and an electron probe microanalysis (EPMA). The accumulation of Pd atoms was confirmed at the TVs fabricated with the Au wire with high Pd content. According the results of a transmission electron microscope (TEM), the thickness of Pd accumulation layer was about 500nm and it located between Au wire (Au-0.95wt%Pd) and Au8Al3 intermetallic compound (IMC) layer. Au4Al IMC did not detected in Au-0.95wt%Pd wire TVs. In TV with Au-0.25wt%Pd wire, the phenomenon of Pd accumulation was not observed at bonding interface after thermal aging but Au4Al formed at the interface between Au-0.25wt%Pd wire and Au8Al3 IMC. After long-term thermal aging, the bonding interface was degraded by the oxidation phenomena. According to the cross-section analysis, it was mainly due to the oxidation of Au4Al IMC and, therefore, the Au-Al bonding interface becomes vulnerable to this kind oxidation.

The rapidly solidified Si-xTiNi (x = 0.2-0.45) alloy ribbons were fabricated via melt spinning process. The thickness of the melt-spun ribbons was about 12.5 microm, and the sound section was selected for the experiment. The microstructures of the ribbons were analyzed using XRD, FE-SEM, and HR-TEM: The primary silicon particles of 30 nm-100 n min diameter were finely dispersed in the inactive buffering matrix of Si7Ni4Ti4 phase. The charge/discharge energy capacity and electrochemical properties were significantly influenced by the relative ratio of NiTi to silicon. With increasing the total amount of Ni and Ti content up to 45 at%, the amount of Si7Ni4Ti4 phase increased and the cycle performance was improved. The Si7Ni4Ti4 phase acted as a buffer for the volume expansion/contraction of Si occurring during the alloying and dealloying, and it could prevent a significant deterioration in cycle performance of the battery.

The phase change due to varying content of titanium in Si-Ni-xTi alloys and its effect on the electrochemical behavior has been investigated. Specimens were prepared by melt-spinning to reduce the microstructure scale. Results showed that silicon particles of 50-100 nm diameter and dendrites of somewhat larger scale were formed in the Si-Ni-Ti alloys ribbons. The microstructure of Si70Ni15Ti15 alloy ribbons was composed of silicon particles finely dispersed in Si7Ni4Ti4 phase. The cycle performance was improved by the formation of TiSi2 or NiSi2 phase at the presence of Si7Ni4Ti4 phase, either of which combined with Si7Ni4Ti4 phase effectively accommodated the volume change of silicon particles during cycling. The reduced scale of silicon particles contributed to the enhanced cycle efficiency as well.

Wire bond reliability testing typically consists of aging bonds in a high temperature environment for long time periods, removing samples at intervals to assess bond shear strength and characterize the bond cross sections. In this way, the degradation of the bond can be monitored at discrete time intervals, and it is determined whether the bond will be reliable under long term operation at lower temperatures. This process is labour and time consuming. An alternative process is reported using piezoresistive integrated CMOS microsensors located around test bond pads. The sensors are sensitive to radial compressive or tensile stresses occurring on the bond pad due to intermetallic formation, voiding, and crack formation at the bond interface. Two sets of identical test chips are bonded with optimized Au ball bonds and aged for 2000 h at 175 °C. One set is connected to equipment which monitors signals from the stress sensors, along with the contact resistance of the bonds. The other set of chips is destructively tested by shear tests and cross sectioning. It is found that the stress sensors are capable of indicating which stage of intermetallic growth is currently being experienced, by relating the signal to the relative density of the intermetallic compounds (IMCs) which form during aging. The sensors can detect the consumption by IMCs of each Al layer in a multilayer pad, and can monitor the formation of AuAl 2 which indicates an advanced stage of aging. Sensor signals combined with contact resistance measurements provide a valuable tool for preliminary reliability studies, and give real-time insight into microstructural changes. Drop in shear strength of a ball bond is detected by a change in the microsensor signal combined with a contact resistance increase.

Due to increasing cost of gold and technological demand for finer pitch application, Au bonding wires with finer diameter are needed. In such demands, Au bonding wire manufacturing companies produced ultra-fine Au wires with diameter of 12 μm (0.5 mil). However, interfacial reactions between ultra-fine Au wires and Al pads may cause serious reliability problems as wire diameter get smaller. Decrease in bond reliabilities may be caused by interfacial failures such as Kirkendall voids and oxidation of Au 4 Al IMCs. For the application of ultra-fine Au wires in real products, studies on interfacial reactions of ultra-fine Au wires/Al pads and effects of Pd addition on Au wire/Al pad interface which is previously proven to be enhancing bond reliabilities are necessary. In this study, interfacial reactions and failures of two types of 4N ultra-fine Au wires(12 μm)/Al pads with different bonding types were analyzed, and effects of Pd addition on interfacial reactions and failures are also investigated by using 3000, 6000, and 10000 ppm of Pd added Au wires. High temperature storage test (HTST) at 175D under air-ambient was performed to simulate an accelerated thermal exposure characteristic of device operation conditions. Intermetallic compound (IMC) phases and Pd behavior at the Au wire/Al pad interface were identified using various analytical tools. Ball pull test (BPT) was conducted to evaluate bond reliabilities depending on bonding types, wire compositions, and aging times. According to experimental results, two types of ultra-fine Au wires/Al pads showed different bonding reliabilities depending on initial inter-metallic coverage. It was found that initial intermetallic coverage determined the interface bond reliability because interfacial oxidation problem depended on initial intermetallic coverage. Addition of Pd on Au wires/Al pads significantly affected bond reliabilities depending on Pd contents. 3000 ppm of Pd added Au wires/Al pads resulted the best bond reliabilities and strongest resistance against the interfacial oxidation due to the Pd accumulation at Au 4 Al grain boundaries. However, 10000 ppm of Pd added Au wires/Al pads showed low bond reliabilities because separated Pd-rich island grains formed among Au 4 Al grains resulting in less protection of Au 4 Al from oxidation.

Ball bonding processes are optimized on Al pads with a 25.4 µm diameter Cu wire to obtain maximum average shear strengths of at least 120 MPa. To quantify the direct effect of bond force and ultrasound on the pad stress, ball bonding is performed on test pads with piezoresistive microsensors integrated next to the pad and the real-time ultrasonic signals are measured. By using a lower value of bond force combined with reduced ultrasound level, the pad stress can be reduced by 30%. An ultrasound/bond force process window for low-stress copper ball bonding is determined, which shows that a proper optimization of ultrasound and bond force leads to Cu ball bonds of high quality while transmitting lower stress to the pad during the bonding process.

There have been many studies on replacing Au bonding wire with other bonding wire materials, because the cost of Au has dramatically increased by approximately 2~3 times in recent years. When replacing part of the bond pad with a noble metal, Ag bonding wire is of particular interest due to its superior electrical properties, lower cost and similar mechanical properties as compared with Au. Ag bonding wire is thermosonically bonded to 3 kinds of bond pad (Al, Au and Pd) and aged at high temperature (175°C). Then, the bondability and interface reactions are characterized at each bond pad. In the case of Ag-Al bonding, 2 kinds of intermediate phases were observed and the composition ratios of Ag and Al in these phases were 4:1 and 2:1, respectively. After 300 hrs of aging, cracks were formed in these intermediate phases and ball-lift failure occurred. However, in the case of the noble metal bond pad, a solid solution was formed between the Ag wire and bond pad and no voids or cracks were formed. This shows the robust bonding characteristics. The diffusion layer was observed and the diffraction pattern was analyzed by TEM (Transmission Electron microscopy). The Au-Al bond reliability was also characterized by a comparative study. In this study, Ag bonding wire is proposed as an alternative to Au bonding wire for noble metal pads. Also, the thermal reliability is reviewed and the failure mechanisms are verified with various bond pads.

Various Au bonding wires which have different diameters from 25 um to 12 um, and purity of 2N and 4N are used for studying possible problems in ultra fine wire bonding. As wire diameter gets finer, grain size gets finer and portion of direction in wire texture is increased due to high working ratio. Moreover, as wire diameter gets finer, eccentricity of FAB (Free Air Ball) get worse and, for soft 4N wire, neck broken issue is occurred in 12 um bonding due to low neck strength, and for hard 2N wire, bond-off issue is occurred due to bad initial bondability. Also initial IMC (Intermetallics) coverage gets worse and HTST reliability is dramatically dropped in ultra fine wire application. Therefore, soft 2N wire which has relatively low FAB hardness and Pd-based dopant composition could be an alternative to solve these issues in ultra fine wire.

The trend of high integration and miniaturization of semiconductors has accelerated the development of gold bonding wire with smaller diameter. For the stable bonding of fine wire, it is important to characterize the wire with various diameters during the bonding process. To investigate this relationship, the experiments were done for the various sizes of wire diameter and FAB. The wire size and the FAB size were chosen for the test from 15 /spl mu/m to 25 /spl mu/m and from 1.4 WD (wire diameter) to 2.0 WD, respectively. The results showed that as the size of FAB became smaller, the size deviation of FAB increased and FAB itself was tilted to one side. When FAB was formed at the same parameter, the length of HAZ became shorter for the wire with the high temperature of recrystallization. It is also revealed that the length of HAZ decreased for the smaller size of FAB. This phenomenon is considered to be related to the beat generated during the FAB formation.