Electric Flame-off Research Articles

A Wirebonding Instrument for Insulated and Coaxial Wires

Abstract A tool has been developed that supports a novel microelectronic integration paradigm whereby interconnects between components are directly established by way of micro-coax wirebonds. A near term use-case of the tool is to facilitate rapid prototyping of high-bandwidth systems. When further matured, it will lead to the ability to rapidly integrate complex systems with hundreds or thousands of interconnects with minimal design time. Automatic stripping and bonding of coax wires having overall diameters between 50 to 150μm presents an array of process challenges that pose interesting demands on the material system of the wire and the bonding tool. This paper will review a micro-coax bonding system that is currently in development at Draper which is able to strip, feed and bond micro coax wire. The system utilizes a combination of electric-flame-off (EFO) and thermal reflow to strip outer metal shielding and polymer dielectric layers respectively. It leverages a rotary wire feed mechanism to precisely control wire position so that pre-determined wire-lengths can be established. Progress in design of the wires, tooling and software control architecture are reviewed.

The mechanism of Pd distribution in the process of FAB formation during Pd-coated Cu wire bonding

The palladium (Pd) distribution over the free air ball (FAB) and its effect on the ball’s shear strength were investigated in the Pd-coated copper (Cu) wire bonding process. It was found that for the same FAB/wire diameter ratio of 1.5, the bigger the electrical flame-off (EFO) current was, the larger would be the exposed Cu regions over FAB without Pd distribution. Combining experimental observations, a model of Pd distribution over FAB was first proposed by changing the EFO current and firing time. As the firing time increased, the coated Pd element could be dissolved into the Cu base to form a PdCu alloy at the FAB surface, to protect the bonded ball from corrosion. The Pd-coated Cu wire has higher bonding shear strength than the bare Cu wire. However, the FAB with the largest Pd coverage had the smallest ball shear strength, which revealed that the control of Pd distribution over FAB is very critical for a high-quality bonding interface.

Mechanical and Electrical Properties of Palladium-Coated Copper Wires with Flash Gold

Palladium-coated copper wire with flash gold (PCA) is a fine wire with an oxidation resistance layer. A new sulfidation test has been assessed in this work, confirming that PCA wires show better sulfidation corrosion resistance than either palladium-coated or bare copper wires. The sulfided surface of PCA was analyzed, along with its bonding strength and electrical properties. The metallurgic mechanism for formation of free air balls during the electric flame-off (EFO) process was identified. The flash gold layer of PCA wires can improve certain shortcomings, including: (1) efficiently promoting sulfidation corrosion resistance, (2) solving the problem of palladium segregation during the EFO process, (3) reducing the starting voltage, and (4) stabilizing the electrical resistivity of the bonding interface.

Characterisation of insulated Cu wire ball bonding

Insulated Cu wire technology offers potential solutions for fine and ultra-fine pitch wire bonding as the insulator coating on the bare wire prevents wires shorting problem. Most previous works focused on insulated wire stitch bonding. This paper presents a comparison study between insulated Cu and bare Cu wire ball-bonding process characterisation in the standpoints of free air ball formation, Al splash, hardness, ball bond strength and intermetallic growth at the bond interface study. Spherical and clean free air ball was formed using lower electric flame off energy compared to bare Cu wire. The study shows that insulated Cu bonding demonstrated comparable equivalent ball bond strength to bare Cu wire bonding at T0 and even after subjected to isothermal aging 175°C up to 1008 hours. Intermetallic formation was uniform at the bond interface for both wires. Insulated Cu wire demonstrates good bondability and reliability performance, suitable for fine pitch wire bonding in large-scale integration applications.

Microstructure, electric flame-off (EFO) characteristics and tensile properties of silver–lanthanum alloy wire

Silver has potential for application in the electronic packaging industry because of its great electrical and thermal properties and lower price compared to that of gold. Silver oxidizes easily, so doping lanthanum to form Ag–La alloy improves its anti-oxidation capacity. In this study, the microstructure, tensile properties, electronic flame-off (EFO) characteristics, and fusing current of Ag–La alloy wire (φ=20μm) are studied. Samples annealed at three temperatures (325°C, 375°C, and 425°C) are analyzed. According to the experimental results, after annealing at 425°C, Ag–La alloy wire recrystallized, giving it a tensile strength similar to that of pure silver wire and a uniform structure. Doping lanthanum reduced the diameter of free air balls (FABs) in the EFO process. The fusing current of Ag–La wire was about 0.45A, and the grains of Ag–La wire grew to the size of the wire diameter when a 0.4A current (90% fusing current) was applied for a long time. Ag–La alloy wire can be used in the electronic packaging industry.

Study of free air ball formation in Ag–8Au–3Pd alloy wire bonding

An innovative Ag–8Au–3Pd alloy wire has been developed as an alternative to the traditional gold wire bonding. This paper focused on the free air ball (FAB) formation of 0.7mil Ag–8Au–3Pd alloy wire, which was vital for the yield of the subsequent bonding process. During electric flame-off (EFO) process, the wire tail was melted by a high voltage spark, and then the FAB was shaped by the effects of surface tension and gravity. The EFO current was the key factor to influence the Ag–8Au–3Pd alloy FAB size and morphology due to the energy input via arc discharging. The defects including off-center and ripple appeared on the Ag–8Au–3Pd alloy FABs were discussed by cooling and solidification. It is suggested that low EFO current will effectively avoid FAB defects. The contaminants on the Ag–8Au–3Pd alloy FAB surface were analyzed by Auger electron spectroscopy (AES). Under the protection of the shielding gas, oxidation and sulfuration have been effectively prevented.

Recrystallization of Ag and Ag-La Alloy Wire in Wire Bonding Process

Sliver wire was the novel material to replaced gold wire in wire bonding process, and rare earth element was often added to improve the properties of silver wires. The annealing effect (at 225°C~275°C for 30min) on the tensile mechanical properties of silver wires with φ=20μm was investigated. In addition, the microstructural characteristics and the mechanical properties before and after an electric flame-off (EFO) process were also studied. Free-air ball (FAB) of 85μm diameter from 20μm diameter pure silver wire was too huge for bonding process, otherwise the silver wire was added 0.05 wt.% lanthanum to form Ag-La alloy wire to reduce the diameter of FAB. FAB of Ag-La alloy wire with a 55μm diameter, and can avoid short-circuited. In addition, microstructures, tensile properties and the micro-hardness of Ag-La alloy wires indicated that the best annealing temperature was 425 °C.

Effect of the direct current on microstructure, tensile property and bonding strength of pure silver wires

In the microelectronics assembly and packaging industry, the wire bonding has become an important process to connect lead frames and pads. In the past, gold and copper were the main materials of wire bonding. However, the cost of gold wires is getting higher nowadays and yet wire bonding cannot be wholly replaced by copper wire; thus silver wires become a novel bonding material in recent years. The reliability test of wires was a static method; this study leads electrical current into the wires to estimate the structural changing and interface properties of Al pads (positive and negative pad). After leading 90% critical fusing current density (CFCD) into a 23μm silver wire, some grains of silver wire had grown up and formed into equal-diameter grains (EDG). After the current test, the fracture position of bonded wires moved from heat affect zone (HAZ) of electric flame-off (EFO) to the neck of HAZ. Otherwise, the current test would reduce the tensile strength of wire. The bonding strength of the positive pad was lower than that of the negative pad. The intermetallic compound (IMC) of bonding interface was AgAl2.

Further characterization of Pd-coated copper wire on wirebonding process: from a nanoscale perspective

The objective of this research is to investigate the nanomechanical properties of ultra-thin Pd-coated copper (Cu) wire (ψ = 0.6 mil) and nanotribology along the interfacial between free air ball (FAB) and aluminum (Al) bond pad during wirebonding process. Two major analyses are conducted in the present paper. In the first, the characteristic of heat affected zone and FAB for Cu wire has been carefully experimental measured. Nanoscale interfacial tribology behavior between Cu FAB and Al pad is examined by atomic force microscopy. Secondary, the dynamic response on Al bond pad and beneath the pad during wirebonding process has been successfully predicted by finite element analysis. Micro-tensile mechanical properties of Cu wire before and after electric flame-off (EFO) process have been investigated by self-design pull test fixture. Experimental obtained hardening constant in Hall–Petch equation has significantly influence on the localize stressed area on Al pad. This would result in Al pad squeezing (large plastic deformation) around the smashed FAB during impact stage and the consequent thermosonic vibration stage. Microstructure of FAB is also carefully investigated by nanoindentation instruments. A real-time secondary EFO scheme has been conducted to reduce the strength of Cu wire and increase the bondability. All the measured data serve as material inputs for the finite element model based on explicit software ANSYS/LS-DYNA. In addition, nanoscale bondability on Cu-Al intermetallic compound is simulated by molecular dynamics. A series of comprehensive parametric studies were conducted in this research.

Effect of EFO parameters and superimposed ultrasound on work hardening behavior of palladium coated copper wire in thermosonic ball bonding

Effects of the electrical flame off (EFO) and ultrasound (US) parameters on the work hardening behavior of Pd coated Cu (PCC) free air ball (FAB) are presented and compared to those of bare Cu reported in the literature. The FABs are characterized using an online deformability method that measures in situ deformed ball height (HDEF). The levels of EFO current (IEFO) of 65, 100 and 160mA with adjusted EFO time (tEFO) are used to make 40μm diameter FABs in two different shielding gases, resulting in six experimental conditions. In a first experiment, a total of 135 samples are produced for each condition and then deformed under a 400mN deformation force (FD) without superimposing US. PCC FABs produced in nitrogen gas using IEFO and tEFO of 160mA and 0.120ms, respectively, are more deformable, having HDEF 7.1–9.2% less compared to those produced with IEFO=100mA and tEFO=0.218ms. However, the FABs produced with the higher current vary more than those with lower current, and FABs produced with forming gas vary least. HDEF of PCC FABs made in forming gas is independent of IEFO in the range from 65 to 160mA. In a second experiment, the same conditions are used except for a 20% US level (equivalent to 29mW US power) superimposed on FD during the deformation. The values of HDEF with US is 5.7–11.7% and 6.5–9.0% smaller than those of without US for the IEFO ranging from 65 to 160mA in nitrogen and forming gas, respectively.

Effect of Voltage Pattern on Intensity of Electromagnetic Emission during Electric Flame-Off Process of Ball Bonding Machine

Electromagnetic interference (EMI) can degrade device performance, particularly a sensitive device such as magnetic recording heads. A high applied voltage generates a high electric field in order to break the electrical air gap during a gold ball bonding process. This work, for the first time, investigates the influence of high voltage pattern of a gold ball bond machine on both electric field and magnetic field as an electromagnetic wave by monitoring an intensity of electromagnetic interference. The investigation was carried out by simulation of a live gold ball bond machine operation by using a finite integral technique. The simulation results reveal that the voltage pattern plays an important role in the intensity of electromagnetic radiation (EMR). In conclusion, an ideal voltage pulse of 2 kV and 4 kV generates the highest EMR detected by a monopole antenna. The induced voltage occuring at the antenna reflecting the intensity of EMI blasting was as high as 33.39 V and 71.36 V, respectively. Smoothening the voltage pattern by incrasing the rise time of voltage to 100 ns only, the induced voltage at the antenna was reduced down to 99.90% and 99.92% compairing with the case of an ideal voltage pulse for the peak EFO voltage of 2 kV and 4kV respectively. This concludes the importance of the voltage pattern during a free air ball forming process; hence lowering the risk of the highly sensitive deivce to be damaged by EMI blasting.

Microstructure, electric flame-off characteristics and tensile properties of silver bonding wires

Compared with gold wire, silver wire is cheaper and enjoys better electrical conductivity. These days copper wire is becoming more commonly used, but the reliability of its bonding still has problems in some pads. Since Ag wires have similar hardness and bonding properties to Au wires, they can be applied in some pads. In the present study, the annealing effect (at 225–275 °C for 30 min) on the tensile mechanical properties of silver wires with ϕ = 23 μm was investigated. In addition, the microstructural characteristics and the mechanical properties before and after an electric flame-off (EFO) process were also studied. Experimental results indicate that with annealing temperatures of more than 250 °C, the silver wires possessed a fully annealed structure, the tensile strength and the hardness decreased, and the elongation was raised significantly. Through recrystallization, the matrix structure transformed from long-thin grains to equiaxed grains. The microstructure of the free air ball (FAB) of the various annealed wires after an EFO process were column-like grains. The column-like grains grew from the heat-affected zone (HAZ) to the Ag ball. Under the thermal effect of EFO, the necks of the Ag balls underwent recrystallization and grain growth was induced, and the annealed Ag wires had a shorter zone of HAZ (220 μm). Additionally, the decreased hardness and the low strength of the HAZ resulted in the breakage sites of the EFO wires being in the HAZ near the Ag balls. The bonding strength and the neck-strength of the Ag wires were more than 7gf and possessed excellent bonding properties.

Characteristic of copper wire and transient analysis on wirebonding process

In the present paper, two major analyses are achieved. In the first, experimental procedures were accomplished to measure tensile mechanical properties of copper (Cu) wire ( ϕ = 1 mil) before/after electric flame-off (EFO). Characteristics of free air ball (FAB), heat affected zone (HAZ) and thermal stable zone (TSZ) in as-drawn wire have been carefully investigated by microhardness, self-design pull test fixture, nanoindentation and atomic force microscopy (AFM). A 2nd EFO real-time technique has been conducted to reduce the strength of Cu wire and increase the bonding region. Secondary, with the obtained experimental material data, a comprehensive finite element wirebonding model based on explicit time integration software ANSYS/LS-DYNA is developed to predict the overall strain/stress distributions on the aluminum (Al) bond pad. Finite element analysis (FEA) results demonstrate that plastic deformation on Al bond pad around smashed FAB can be reduced by increasing the surface roughness on FAB. A series of comprehensive parametric studies were conducted in this research.

A study on the tensile fracture mechanism of 15 μm copper wire after EFO process

In this study, 15 μm copper wires were bonded on substrates with thermosonic process, and the tensile fracture characteristics of FAB, as well as bonded samples, were investigated. For electronic packaging applications, all 15 μm wires were fully annealed, and the microstructures consisted of equiaxed grains. After EFO (electric flame-off) process, the microstructure of wire can be divided into three parts: (1) free air ball (FAB) with columnar grains, (2) heat-affected zone (HAZ) with equal-diameter grains, (3) annealing zone with equiaxed grains. According to tensile test results, EFO process simultaneously reduced UTS and elongation of the wire. For both FAB and bonded samples, the tensile fracture zones were either in the region of equal-diameter grains or in coarse grains located within 100 μm from the ball. And it was observed that the breakage sites appeared near the twins and the columnar grains when tensile fracture happened. Meanwhile, the relationship between hardness and microstructure of wires after EFO process were analyzed with nano-indentation. The nano-hardness value of 15 μm wire was 1.2–1.45 GPa.

Effect of gas type and flow rate on Cu free air ball formation in thermosonic wire bonding

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 Study on the Characteristic of the Ultra-Thin Copper Bonding Wire

In this paper, experimental procedures are developed to measure the tensile mechanical property of ultra thin Copper (Cu) wire (=1mil) before and after electric flame-off (EFO). Characteristics of free air ball (FAB), heat affected zone (HAZ) and as-drawn wire on 99.99% Cu wire have been carefully investigated by nanoindentation, microhardness and self-design pull test fixture, respectively. A secondary EFO technique has been conducted to reduce the strength of Cu wire and increase the bondability. The determined accurate material properties can then be applied to finite element analysis (FEA) for further reliability studies. A series of comprehensive parametric studies were conducted in this research.

Recrystallization, Electric Flame-Off Characteristics, and Electron Backscatter Diffraction of Copper Bonding Wires

In the present study, the neck fracture properties of annealed wire with ? = 20 ?m (0.8 mil) at 200°C ~ 300°C for 1 h and unannealed wire were compared. The microstructural characteristics, the mechanical properties and the texture transition using electron back scatter diffraction methods before and after an electric flame-off (EFO) process were also studied. Experimental results indicate that the annealing temperatures of more than 225°C, the 20 ?m copper wires possessed a fully annealed structure, the tensile strength and the hardness decreased, and the elongation was raised. Through recrystallization, the matrix structure transferred from long, thin grains to equiaxed grains and a few annealed twins. The microstructure of the free air ball (FAB) after an EFO process consisted of column-like grains, and grew from the heat-affected zone (HAZ) to the Cu ball. For the 225°C annealed and unannealed wires, their preferred orientations on the wire and the neck were ?100?//AD. Under the thermal effect of EFO, the orientation of the Cu balls were mainly ?101?//AD and ?111?//AD for annealed wires. Additionally, the hardness of the Cu balls and the strength of the neck sites of the EFO wires were able to affect the reliability of the copper wire bonding.

Free Air Ball Consistency of Palladium Coated Copper Wire

The formation of good free air ball (FAB) in palladium coated copper wire depends on many factors including the type of protection gas used, flow rate, Electric Flame Off (EFO) current, firing time and gap between the tip of wire and EFO wand. These factors can individually or, in combination of which, affect how good a ball can be formed prior to bonding onto the bond pad of a device. It is therefore important to find an optimum condition in forming good FAB so as to avoid poor bonded ball quality, or worse, non-uniform ball hardness that could lead to damage of the bond pad upon impact. The microstructure of the free air ball from palladium coated copper is unlike that of pure copper wire. It consists of palladium or palladium rich phase that can either exist partially or majority on surface of the ball making the structure non-uniform and inconsistency from one condition of EFO firing to the either. The hardness of the ball is also complicated by the non-uniform microstructure in such a way that determining the microhardness of the ball is never a straight forward characterisation. This paper covers the effect of process condition on the FAB size and quality of the wire. Microstructure and hardness of the FAB processed with different conditions are also presented and discussed.

Dynamic Analysis on Underlay Microstructure for Cu/Low-k Wafer during Wire Bonding

Two major analyses were conducted in this paper. In the first, experimental procedures are accomplished to measure the tensile mechanical properties of ultra thin gold wire (=1mil) before/after electric flame-off (EFO). Characteristics of free air ball (FAB), heat affected zone (HAZ) and as-drawn wire have been carefully investigated by nanoindentation, microhardness and self-design pull test fixture. Secondary, with the obtained experimental material data, a comprehensive finite element model using software ANSYS/LS-DYNA is successfully developed to simulate the wirebonding. Dynamic analysis is performed to evaluate the overall stress distributions on the underlay microstructure of Cu/low-k wafer. Special emphasizes are focused on the Copper via and the intermetal dielectric (IMD)/undoped silica glass (USG) dielectric microstructure.

Factors governing heat affected zone during wire bonding

In the wire bonding process of microelectronic packaging, heat affect zone (HAZ) is an important factor governing the loop profile of bonding. The height of loop is affected by the length of the HAZ. Factors governing the HAZ were studied. To investigate this relationship, experiments were done for various sizes of wire and free air ball (FAB). Electric flame-off (EFO) current, EFO time, EFO gap and recrystallization were also studied. The results show that as the size of FAB becomes larger, the length of HAZ increases. With the increase of EFO current and time, the length of HAZ becomes longer. When FAB forms at the same parameter the length of HAZ becomes shorter with the high temperature of recrystallization.