Free Air Ball Formation Research Articles

High Robustness of Coated-Ag Wire Bonding

Formation of free air ball (FAB) within the range of certain processing parameters leads to excellent bondability (distributed intermetallic coverage (IMC) at the center and periphery of the bonded ball interface, bonded ball concentricity, etc.) on bonding to aluminum (Al) bond pad and Au/Ag plated surfaces. The wire revealed zero stoppage and 0.8M bond touchdowns when tested under laboratory conditions. Thermal aging of bonded interface showed absence of Kirkendall voids until 192h at 250°C where Al diffuses into Ag bonded ball up to a depth of 5.3µm, consuming the complete Al bond pad and thus transforming into silver-aluminide containing 5 to 12wt% of Al and rest silver (Ag). Traces of coated gold (Au) and alloyed palladium (Pd) are noticed at the bond interface.

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.

Application of laser heating and its effects on bonding strengths for varied copper wire purity levels

Gold as the medium of interconnection is rapidly being replaced by copper as a means of creating connectivity in integrated circuit packaging. However, the use of copper wire is known to pose many threats in integrated circuit packaging, especially for devices with very thin die structures. The process of wire bonding which involves the formation of free air ball can be risky to the quality of integrated circuit packaging. The impact of hard copper free air ball onto the bonding surfaces may damage the bond pad and the silicon die. These challenges require innovative solutions that are able to be applied to integrated circuit packaging with minimal thermal and mechanical effect on the device. Through recent research, it is apparent that the laser processing technology can provide viable innovative solutions to electronic packaging. This research paper investigates the application of laser heating on copper wire bonding technology and its effects on bonding strength for three copper wire purity types, 99.9% purity (3 N), 99.99% purity (4 N), and 99.999% purity (5 N) copper wires. Wire bonding was performed on bond pads consisting of NiPdAu metallization under 16 different temperature conditions. Results of this study showed that different copper purity levels exhibit different bonding strengths with the application of laser heating. It is shown in this research that significantly higher as-bonded ball shear strengths were obtained when laser heating was applied at all three copper wire purity types. The results of this study can be helpful for integrated circuit packaging with copper wire bonding on thin die structures. Copper wires with lower purity levels are shown as a viable option by further improving the wire bonding quality with the application of laser heating without the need to use excessive bonding temperature, higher bonding force, and excessive ultrasonic energy.Gold as the medium of interconnection is rapidly being replaced by copper as a means of creating connectivity in integrated circuit packaging. However, the use of copper wire is known to pose many threats in integrated circuit packaging, especially for devices with very thin die structures. The process of wire bonding which involves the formation of free air ball can be risky to the quality of integrated circuit packaging. The impact of hard copper free air ball onto the bonding surfaces may damage the bond pad and the silicon die. These challenges require innovative solutions that are able to be applied to integrated circuit packaging with minimal thermal and mechanical effect on the device. Through recent research, it is apparent that the laser processing technology can provide viable innovative solutions to electronic packaging. This research paper investigates the application of laser heating on copper wire bonding technology and its effects on bonding strength for three copper wire purity types, 99.9...

Effect of Process Parameters on Free Air Ball Integrity in Copper and Palladium-Coated Copper Bonding Wires

The free air ball (FAB) formation processes of fine diameter Cu wires were observed, and the effects of process parameters were studied on the basis of high-speed camera observation. It was revealed that the FAB formation process is characterized by preheating before melting, melting that accompanies ball rising, and solidification periods. The relationships between the electronic flame-off (EFO) condition and the FAB formation were summarized for both bare Cu and palladium-coated Cu (PCC) wires. Further study was performed on the changes in FAB shape with different EFO conditions, wire types, and FAB sizes. Off-centered FABs were observed in the bare Cu wire whereas they were rarely observed in the PCC wire. The off-center tended to be affected by shielding gas condition, EFO torch gap, and FAB size. The mechanisms of the off-centered FAB formation were proposed through the high-speed camera observations. The optimization of process parameters to achieve defect-free, consistent FAB is discussed based on these findings, which is useful for the next generation high-density packaging.

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.

Effect of Process Parameters on Free Air Ball Integrity in Copper and Palladium-coated Copper Bonding Wires

The Free Air Ball (FAB) formation processes of fine diameter Cu wires were observed, and the effects of process parameters were studied on the basis of high-speed camera observation. It was revealed that the FAB formation process is characterized by preheating before melting, melting that accompanies ball rising, and solidification periods. The relationships between the electronic flame-off (EFO) condition and the FAB formation were summarized for both bare Cu and palladium-coated Cu wires. Further study was performed on the changes in FAB shape with different EFO conditions, wire types, and FAB sizes. Off-centered FABs were observed in the bare Cu wire whereas they were rarely observed in the Pd-coated Cu wire. The off-center tended to be affected by shielding gas condition, EFO torch gap, and FAB size. The mechanisms of the off-centered FAB formation were proposed through the high speed camera observations. The optimization of process parameters to achieve defect-free, consistent FAB is discussed based on these findings, which is useful for the next generation high density packaging.

Advancement in Thermosonic Bonding Wire

Coated copper (Cu) and alloyed silver (Ag) wires are the new developments in bonding wire sector and this paper discusses on its bonding performances. Palladium (Pd) coated Cu wire replaces rapidly with Au wire. A thin gold (Au) coating over Pd coated Cu wire revealed wider 2nd bond process window and superior 2nd bond bondability, termed as Au Flash Pd Coated Cu Wire (AFPC). The free air ball (FAB) formation, looping, 1st bond and reliability performance of the AFPC wire satisfies the basic requirements. The benefits of doped/alloyed Cu wire are also presented. Good reflectance properties of Ag wire replaces with Au wire in LED sector. Moreover, soft nature of Ag wire and its FAB near to Au, turn the focus to Ag as a potential bonding wire. Detailed examination on alloying additions, Ag electro migration, reliability, bondability, FAB formation, etc., are currently examined and discussed. During bonding, purging with forming gas is a common practice irrespective of the wires used (bare/alloyed/coated Cu or Ag wires).

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.

Insulated Cu wire free air ball characterization

Insulated Cu wire technology has immense potential for fine pitch wire bonding interconnection. Understanding the behavior of the insulated Cu free air ball (FAB) formation is crucial for wire bonding process. The FAB formation, size, shape and cleanliness under different conditions for 20μm insulated Cu wire were investigated using SEM, FESEM and FTIR surface analysis. The results were compared with that of bare Cu wire. Consistently spherical residue free FAB of insulated Cu wire were formed using forming gas. The samples with insulated Cu wire consistently produced larger FAB than that of bare Cu wire, indicating that the energy required for free air ball formation is lower. Basic bonding performances in terms of ball bond strength, intermetallic (IMC) coverage growth and stitch bond strength of insulated Cu wire at time zero are also discussed in the paper.

Dynamics of Free Air Ball Formation in Thermosonic Wire Bonding

In thermosonic wire bonding, a free air ball (FAB) created by electronic flame-off (EFO) is crucial to a reliable first bond. To reveal the mechanisms of FAB and ripple formation, the dynamic FAB formation process was monitored and analyzed with a high-speed camera and digital data acquisition system. FAB formation was found to occur in four stages: initiation, preheating, melting, and solidification. The FAB formation speed and model depend only on the EFO current. The FAB formation speed can be as high as 90.4 μm/ms at an EFO current of 60 mA. A minimum EFO time is required for FAB formation, which depends on the EFO current. A semiempirical model FAB=ηln[ <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> - 0.5 <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">min</sub> )]- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> , was developed to describe the relationship between the FAB size and the EFO time at different EFO currents. The ripple formation mechanism was investigated by analyzing the solidification stage images. Transient droplet vibration caused by high-speed rolling-up and skin-forming solidification in the molten gold drop caused by a large temperature gradient are two major factors in ripple formation. This analysis is expected to be useful for understanding the FAB formation mechanism and optimizing the EFO parameters.

Silver pick up during tail formation in thermosonic wire bonding process

Cu ball bonding processes are significantly less robust than Au ball bonding processes. One reason for this are higher variations observed, e.g. in the free-air ball (FAB) formation process. There is a strong influence the tail bond process has on subsequent FAB formation. Tail tips and bond fractographs made with Cu and Au wires are investigated using scanning electron microscopy. Au and Cu wires pick up Ag material (Ag pick up) from the Ag metallization of Cu leadframe diepads during wire tail breaking. Ag pick up by Cu wire is more dominant than that by Au wire. Ultrasonic friction energy is necessary in order for Ag material pick up to occur. The impact force plays an important role for Ag pick up; less pick up is observed with a higher impact force. The hardness of the free-air ball (FAB) with Ag pick up is reduced by up to 4% compared to that of a FAB made from a tail broken at the neck of a ball bond and therefore having no Ag pick up. No significant change of FAB diameter is observed in these two cases.

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.