Reliability of assembly of chips and flex substrates using thermosonic flip-chip bonding process with a non-conductive paste

This study investigates the reliability of the assembly of chips and flex substrates using the thermosonic flip-chip bonding process with non-conductive paste (NCP). The high-temperature storage (HTS) test, the temperature cycling test (TCT), the pressure cooker test (PCT) and the high-temperature/high-humidity (HT/HH) test were conducted to examine the reliability of chips that are bonded on flex substrates. The environmental parameters used in the various reliability tests were consistent with the JEDEC standards. After the reliability tests, a peeling test was performed and the microstructure of the tested specimen observed to evaluate further the reliability.The bonding strength increased with the storage period in the HTS test. After the peeling test, a layer of copper electrodes was observed to be stuck on gold bumps over the fractured morphology of the chips when the chips and flex substrates were assembled using an ultrasonic power of 14.46 W, indicating that the bonding strength between the gold bumps and the copper electrodes was even higher than the adhesive strength of the layers that were deposited on the flex substrates. The HTS test yielded sufficient thermal energy to promote atomic interdiffusion between gold bumps and copper electrodes. Metallurgical bonding between the gold bump and the copper electrode occurred, improving the bonding strength. In the assembly of chips and flex substrates without the application of ultrasonic power in bonding process, the adhesive strength of NCP was highly reliable after HTS test, because the bonding strength was maintained after HTS test for various storage periods. The typical failure mode of PCT was interfacial delamination between NCP and flex substrates. Approximately 80% of the specimens exhibited full separation after PCT at 336 h when chips and flex substrates were assembled without applied ultrasonic power to the bonding process, revealing that the NCP cannot withstand the PCT and lost its adhesive strength. Applying an adequate ultrasonic power of 14.46 W in the bonding process not only improved the bonding strength, but also enabled the bonding strength to be maintained at high level after PCT. The high bonding strength was attributable to the strong bonding of the gold bumps on the copper electrodes after PCT for various storage periods. This experimental result demonstrates that ultrasonic power can increase the reliability of PCT on chips and flex substrates that were assembled with the NCP. The bonding strength of the gold bumps on the flex substrates did not change significantly after the TCT, revealing the great reliability of TCT on chips and flex substrates that were assembled using the thermosonic flip-chip bonding process with the NCP. The bonding strength of chips bonded to flex substrates increased with the storage periods of the HT/HH test if ultrasonic power was applied to bonding process. Neither delamination nor any defect at the bonding interface was observed. The reliability of the HT/HH test for chips bonded on flex substrates using the thermosonic flip-chip process with the NCP fulfills the requirements stated in the JEDEC standards.According to the experimental findings of various reliability tests, the chips that were bonded to flex substrates using the thermosonic bonding process with NCP met the JEDEC specifications; with the exception of the adhesive strength of NCP under PCT which must be improved.     

Gold Stud Bumps Flip-Chip Bonded onto Copper Electrodes with Titanium and Silver Layers

The purpose of this study was to develop the thermosonic flip-chip bonding process for gold stud bumps bonded onto copper electrodes on an alumina substrate. Copper electrodes were deposited with silver as the bonding layer and with titanium as the diffusion barrier layer. Deposition of these layers on copper electrodes improves the bonding quality between the gold stud bumps and copper electrodes. With appropriate bonding parameters, 100% bondability was achieved. Bonding strength between the gold stud bumps and copper electrodes was much higher than the value converted from the standards of the Joint Electron Device Engineering Council (JEDEC). The effects of process parameters, including bonding force, ultrasonic power, and bonding time, on bonding strength were also investigated. Experimental results indicate that bonding strength increased as bonding force and ultrasonic power increased and did not deteriorate after prolonged storage at elevated temperatures. Thus, the reliability of the high-temperature storage (HTS) test for gold stud bumps flip-chip bonded onto a silver bonding layer and titanium diffusion barrier layer is not a concern. Deposition of these two layers on copper electrodes is an effective and direct method for thermosonic flip-chip bonding of gold stud bumps to a substrate, and ensures excellent bond quality. Applications such as flip-chip bonding of chips with low pin counts or light-emitting diode (LED) packaging are appropriate.     

Enhancement of the Bondability and Die-Shear Force of Chip–Flex Substrate Assemblies by Depositing a Nickel Layer on the Flex Substrate

A nickel layer and a silver bonding layer have been deposited on copper electrodes over flex substrates to improve the bondability and die-shear force performance of chip?Cflex substrate assemblies when using the thermosonic flip-chip bonding process. For bonding temperature of 200°C, the maximum die-shear force was achieved by combining parameter values of 20.66?W ultrasonic power, 625?gf bonding force, and 0.5?s bonding time. The improved bondability and die-shear force could be attributed to better transfer of ultrasonic power across the bonding interface during thermosonic flip-chip bonding, owing to the high rigidity of the copper electrodes provided by the nickel layer. Experimental results also indicated that high bonding load is necessary at elevated ultrasonic power range to provide firm contact between the bumps and electrodes to enable smooth ultrasonic power transfer across the bonding interface. Moreover, prolonged bonding time caused cracks between the bumps and flex substrate. Close examination of the fracture morphologies after die-shear testing and after ultrasonic separation provided insight into the die-shear force performance as influenced by the process parameters and by the deposition of the nickel layer on the copper electrodes over the flex substrate.     

Increasing bondability and ball-shear force of gold balls thermosonic bonding to flex substrates by depositing a nickel layer

To improve the bondability and ball-shear force of gold balls that are thermosonically bonded to copper electrodes over flex substrates, a nickel layer was deposited on the surface of the copper electrodes to increase their rigidity. A silver layer was then deposited on the nickel layer to prevent oxidation of the copper electrodes during the thermosonic bonding process. This nickel layer was expected to enhance the rigidity of copper electrodes over the flex substrates, increasing the thermosonic bonding efficiency of gold balls to copper electrodes over the flex substrates.Deposition the nickel layer on the copper electrodes improved the elastic modulus of the flex substrates, indicating that the nickel layer is effective in enhancing the rigidity of copper electrodes over the flex substrates. The bondability and ball-shear force of gold balls that are thermosonically bonded to copper electrodes increases with the thickness of the nickel layer given fixed bonding parameters. One hundred percent bondability and high ball-shear force can be achieved when gold balls are thermosonically bonded to copper electrodes with the deposition of a 0.5 μm-thick nickel layer. Herein, the ball-shear force was higher than that specified in JEDEC standards. Furthermore, gold balls that were thermosonically bonded to copper electrodes with a nickel layer had a large bonded area with an extensive scrape, while gold balls that were thermosonically bonded to copper electrodes without a nickel layer had a blank surface morphology. This experimental result was similar to that of tests of the elastic modulus of flex substrates, similarity can be used to explain that the effectiveness of the nickel layer in increasing the rigidity of copper electrodes, increasing the bonding efficiency at the bonding interface between gold balls and copper electrodes during thermosonic bonding process. After ball-shear test, a layer that was stuck on the ball bond was observed at the location of fracture of the ball bonds for gold balls they were thermosonically boned on copper electrodes with 0.5 μm-thick nickel layer. This observation implies that the ball-shear force of the gold balls that were bonded on the copper electrodes exceeded even the adhesive force of the layers that were deposited on the copper electrodes.The deposition of a 0.5 μm-thick nickel layer on copper electrodes over flex substrates improved the rigidity of the copper electrodes; the ultrasonic power could be propagated to the bonding interface between the gold balls and the copper electrodes, increasing the bondability and ball-shear force.     

HTS and PCT Reliability of Chips and Flex Substrates Assembled Using a Thermosonic Flip-Chip Bonding Process

This study assesses the high-temperature storage (HTS) test and the pressure-cooker test (PCT) reliability of an assembly of chips and flexible substrates. After the chips were bonded onto the flexible substrates, specimens were utilized to assess the HTS test and PCT reliability. After the PCT and HTS tests, the die-shear test was applied to examine changes in die-shear forces. The microstructure of the test specimens was analyzed to evaluate reliability and to identify possible failure mechanisms. When the duration of the HTS test was increased, the percentage of gold bumps that peeled off from the surface of the copper pads on the chip side increased, and a crack was present at the bonding interface between the gold bumps and chip bond pads. This crack was due to thermal stress generated during the HTS test, and degraded the die-shear force of the assembly of chips and flexible substrates. After the PCT, the crack was present at the interface between deposited layers of copper electrodes after the specimens were subjected to the PCT for various durations. Moisture penetrated into the deposited layers of the copper electrodes, deposited layers lost their adhesion, and the crack progressed from the corner into the central bond area as the test duration increased. To improve the PCT reliability of assemblies of chips and flexible substrates using the thermosonic flip-chip bonding process, one must prevent moisture from penetrating into deposited layers of copper electrodes and prevent crack formation at the interface between nickel and copper layers. Underfill would be an effective approach to prevent moisture from penetrating into deposited layers during the PCT, thereby improving the reliability of the samples during the PCT.     

Increasing bondability and bonding strength of gold stud bumps onto copper pads with a deposited titanium barrier layer

A flip-chip assembly is an attractive scheme for use in high performance and miniaturized microelectronics packaging. Wafer bumping is essential before chips can be flip-bonded to a substrate. Wafer bumping can be used for mechanical-single point stud bump bonding (SBB), and is based on conventional thermosonic wire bonding. This work proposes depositing a titanium barrier layer between the copper film and the silver bonding layer to achieve perfect bondability and sufficiently strong thermosonic bonding between a stud bump and the copper pad.A titanium layer was deposited on the copper pads to prevent copper atoms from out-diffusing during thermosonic stud bump bonding. A silver film was then deposited on the surface of the titanium film as a bonding layer to increase the bondability and bonding strength for stud bumps onto copper pads. The integration of the silver bonding layer with a diffusion barrier layer of titanium on the copper pads yielded 100% bondability between the stud bump and pads. The strength of bonding between the gold bumps on the copper pads significantly exceeds the minimum average values in JEDEC specifications. The diffusion barrier layer of titanium effectively prevents copper atoms from out-diffusing to the silver bonding layer surface during thermosonic bonding, which fact can be interpreted with reference to the experimental results of energy dispersive spectrometry (EDS) and analyses of Auger depth profiles. This diffusion barrier layer of titanium efficiently provides perfect bondability and sufficiently strong bonding between a stud bump and copper pads with a silver bonding layer.     

Thermosonic bonding of gold wire onto a copper pad with titanium thin-film deposition

A novel thermosonic (TS) bonding process for gold wire bonded onto chips with copper interconnects was successfully developed by depositing a thin, titanium passivation layer on a copper pad. The copper pad oxidizes easily at elevated temperature during TS wire bonding. The bondability and bonding strength of the Au ball onto copper pads are significantly deteriorated if a copper-oxide film exists. To overcome this intrinsic drawback of the copper pad, a titanium thin film was deposited onto the copper pad to improve the bondability and bonding strength. The thickness of the titanium passivation layer is crucial to bondability and bonding strength. An appropriate, titanium film thickness of 3.7 nm is proposed in this work. One hundred percent bondability and high bonding strength was achieved. A thicker titanium film results in poor bond-ability and lower bonding strength, because the thicker titanium film cannot be removed by an appropriate range of ultrasonic power during TS bonding. The protective mechanism of the titanium passivation layer was interpreted by the results of field-emission Auger electron spectroscopy (FEAES) and electron spectroscopy for chemical analysis (ESCA). Titanium dioxide (TiO₂), formed during the die-saw and die-mount processes, plays an important role in preventing the copper pad from oxidizing. Reliability of the high-temperature storage (HTS) test for a gold ball bonded on the copper pad with a 3.7-nm titanium passivation layer was verified. The bonding strength did not degrade after prolonged storage at elevated temperature. This novel process could be applied to chips with copper interconnect packaging in the TS wire-bonding process.     

RFID芯片超声倒装封装技术研究

超声倒装是近年来芯片封装领域中快速发展的一种倒装技术,具有连接强度高、接触电阻低、可靠性高、低温下短时完成和成本低的优势,特别适合较少凸点的RFID芯片封装。在镀Ni/Au铜基板上进行了RFID芯片超声倒装焊接实验,金凸点与镀Ni/Au铜基板之间实现了冶金结合,获得了良好的力学与电气性能,满足射频要求。     

Thermosonic flip-chip bonding for SAW filter

A flip-chip bonding (FCB) method suitable for the surface acoustic wave (SAW) filter was developed. In this method, the gold-ball bumps formed on the chip are directly bonded onto the ceramic substrate by thermosonic bonding. After FCB, they are sealed with a cap without using underfill resin. To obtain high bond strength, characteristic properties of the substrate electrode and the ball bump, were optimized. Furthermore, bondability has been improved by adopting a ramp-up loading profile. The reliability test was carried out with 6-pin SAW chips, and we confirmed the sufficient reliability of bonds.     

Thermosonic bonding of gold wire onto silver bonding layer on the bond pads of chips with copper interconnects

A copper pad oxidizes easily at elevated temperatures during thermosonic wire bonding for chips with copper interconnects. The bondability and bonding strength of a gold wire onto a bare copper pad are seriously degraded by the formation of a copper oxide film. A new bonding approach is proposed to overcome this intrinsic drawback of the copper pad. A silver layer is deposited as a bonding layer on the surface of copper pads. Both the ball-shear force and the wire-pull force of a gold wire bonded onto copper pads with silver bonding layers far exceed the minimum values stated in the JEDEC standard and MIL specifications. The silver bonding layer improves bonding between the gold ball and copper pads. The reliability of gold ball bonds on a bond pad is verified in a high-temperature storage (HTS) test. The bonding strength increases with the storage time and far exceeds that required by the relevant industrial codes. The superior bondability and high strength after the HTS test were interpreted with reference to the results of electron probe x-ray microanalyzer (EPMA) analysis. This use of a silver bonding layer may make the fabrication of copper chips simpler than by other protective schemes.     

Oxidation of copper pads and its influence on the quality of Au/Cu bonds during thermosonic wire bonding process

To understand the copper oxide effect on the bondability of gold wire onto a copper pad, thermosonic gold wire bonding to a copper pad was conducted at 90–200 °C under an air atmosphere. The bondability and bonding strength of the Au/Cu bonds were investigated. The bondability and bonding strength were far below the minimum requirements stated in industrial codes. At elevated bonding temperature of 200 °C, the bondability and bonding strength deteriorated mainly due to hydroxide and copper oxide formation on the copper pad. Oxide formation occurred if no appropriate oxide preventive schemes were applied. At lower bonding temperature, 90 °C, poor bondability and low bonding strength were mainly attributed to insufficient thermal energy for atomic inter-diffusion between the gold ball and copper pad.Copper pad oxidation was investigated using an electron spectroscopy for chemical analysis (ESCA) and thermogravimetric analysis (TGA). An activation energy of 35 kJ/mol for copper pad oxidation was obtained from TGA. This implies that different mechanisms govern the oxidation of copper pad and bulk copper. Hydroxide and copper oxide were identified based on the shifted binding energy. Cu(OH)₂ forms mainly on the top surface of copper pads and the underlying layer consists mainly of CuO. The hydroxide concentration increased with increasing the heating temperatures. After heating at 200 °C, the hydroxide concentration on the copper pad surface was approximately six times that at 90 °C. Protective measures such as passivation layer deposition or using shielding gas are critical for thermosonic wire bonding on chips with copper interconnects.     

功率型LED芯片的热超声倒装技术

结合功率型GaN基蓝光LED芯片的电极分布,在硅载体上电镀制作了金凸点,然后通过热超声倒装焊接技术将LED芯片焊接到载体硅片上.结果表明,在合适的热超声参数范围内,焊接后的功率型LED光电特性和出光一致性较好,证明了热超声倒装焊接技术是一种可靠有效的功率型光电子器件互连技术.     

Development of a thermosonic wire-bonding process for gold wire bonding to copper pads using argon shielding

To improve the bondability and ensure the reliability of Au/Cu ball bonds of the thermosonic (TS) wire-bonding process, an argon-shielding atmosphere was applied to prevent the copper pad from oxidizing. With argon shielding in the TS wire-bonding process, 100% gold wire attached on a copper pad can be achieved at the bonding temperature of 180°C and above. The ball-shear and wire-pull forces far exceed the minimum requirements specified in the related industrial codes. In a suitable range of bonding parameters, increasing bonding parameters resulted in greater bonding strength. However, if bonding parameters exceed the suitable range, the bonding strength is deteriorated. The reliability of the high-temperature storage (HTS) test for Au/Cu ball bonds was verified in this study. The bonding strength of Au/Cu ball bonds increases slightly with prolonged storage duration because of diffusion between the gold ball and copper pad during the HTS test. As a whole, argon shielding is a successful way to ensure the Au/Cu ball bond in the TS wire-bonding process applied for packaging of chips with copper interconnects.     

Fabrication and thermal analysis of flip-chip light-emitting diodes with different numbers of Au stub bumps

The thermal performance of flip-chip (FC) light-emitting diodes (LEDs) with different numbers of Au stub bumps has been investigated by using thermosonic bonder. The LEDs were mounted on the aluminium nitride (AlN) sub-mounts which have superior thermal conductivity (230 W/mK), and the high power Chip-on-Plate (COP) package was proposed to be used for our measurement. In order to understand the thermal performance of the high power FC-LEDs, the experimental measurement and finite-element model (FRM) numerical simulation have been used. It is found that the thermal performance of our 1 × 1 mm² FC-LEDs can only be improved when using at least 6 Au stub bumps as interconnected metals. Moreover, the surface temperature of FC-LEDs is significantly reduced while using 20 Au stub bumps.     

Scanning white light interferometry in quality control of single-point tape automated bonding

We report on applying a scanning white light interferometry (SWLI) for quality control of aluminum lead single-point tape automated bonding (spTAB). A spTAB process was used to connect Al leads on a thin polyimide flex to Al bond pads on a flexible Al-polyimide cable. In the experiment three different bonding process parameters, i.e. bond force, ultrasonic power, and ultrasonic treatment time were varied in order to maximize the pull force. A custom built scanning white-light interferometer was used to measure the bond height in order to correlate this parameter with the tensile bond force. This force was obtained in a destructive way by a consecutive pull test. All bonds with a height within (7.22 ± 1.80 μm), possessed a tensile strength exceeding 85 mN. This was verified by a separate validation measurement where the pull force of bonds complying with the height requirement was recorded. Based on the 3D observations the conditions for an acceptable bond quality were revisited and refined.     

Microstructural study of copper free air balls in thermosonic wire bonding

Copper wires are increasingly used in place of gold wires for making bonded interconnections in microelectronics. In this paper, a microstructural study is reported of cross-sectioned free air balls (FABs) made with 23 μm diameter copper bonding wire. It was found that the FAB is comprised of a few columnar grains and a large number of fine subgrains formed within the columnar grains around the periphery of the FAB. It was determined that conduction through the wire was the dominant heat loss mechanism during cooling, and the solidification process started from the wire-ball interface and proceeded across the diameter then outward towards the ball periphery.The microstructure of the Cu ball bond after thermosonic bonding was investigated. The result showed that the subgrain orientations were changed in the bonding process. It is evident that metal flow along the bonding interface was from the central area to the bond periphery during thermosonic bonding.     

Laser-assisted bumping for flip chip assembly

A novel laser-assisted chip bumping technique is presented in which bumps are fabricated on a carrier and subsequently transferred onto silicon chips by a laser-driven release process. Copper bumps with gold bonding layers and intermediate nickel barriers are fabricated on quartz wafers with pre-deposited polyimide layers, using UV lithography and electroplating. The bumps are thermosonically bonded to their respective chips and then released from the carrier by laser machining of the polyimide layer, using light incident through the carrier. Bumps of 60 to 85 μm diameter and 50 μm height at a pitch of 127 μm have been fabricated in peripheral arrays. Parallel bonding and subsequent transfer of arrays of 28 bumps onto test chips have been successfully demonstrated. Individual bump shear tests have been performed on a sample of 13 test chips, showing an average bond strength of 26 gf per bump     

In situ ultrasonic force signals during low-temperature thermosonic copper wire bonding

Ultrasonic in situ force signals from integrated piezo-resistive microsensors were used previously to describe the interfacial stick-slip motion as the most important mechanism in thermosonic Au wire ball bonding to Al pads. The same experimental method is applied here with a hard and a soft Cu wire type. The signals are compared with those obtained from ball bonds with standard Au wire. Prior to carrying out the microsensor measurements, the bonding processes are optimized to obtain consistent bonded ball diameters of 60 μm yielding average shear strengths of at least 110 MPa at a process temperature of 110 °C. The results of the process optimization show that the shear strength c_pk values of Cu ball bonds are almost twice as large as that of the Au ball bonds. The in situ ultrasonic force during Cu ball bonding process is found to be about 30% higher than that measured during the Au ball bonding process. The analysis of the microsensor signal harmonics leads to the conclusion that the stick-slip frictional behavior is significantly less pronounced in the Cu ball bonding process. The bond growth with Cu is approximately 2.5 times faster than with Au. Ball bonds made with the softer Cu wire show higher shear strengths while experiencing about 5% lower ultrasonic force than those made with the harder Cu wire.     

Effect of Gold Stud Bump Topology on Reliability of Flip Chip on Flex Interconnects

The flip chip technique using conductive adhesives have emerged as a good alternative to solder flip chip methods. Different approaches of the interconnection mechanism using conductive adhesives have been developed. In this paper, test chips with gold stud bumps are flip-chipped with conductive adhesives onto a flexible substrate. An experimental study to characterize the bonding process parameters is reported. Initial results from the environmental studies show that thermal shock test causes negligible failure. On the other hand, high humidity test causes considerable failure in flip chip on flex assemblies. Improvements in the reliability of the assembly are achieved by modifying the shape of the gold stud bumps.     

Interconnect resistance characteristics of several flip-chip bumping and assembly techniques

Several flip-chip interconnection methods were compared by measuring interconnect resistance before and after exposure to environments including pre-conditioning, 85°C/85% RH exposure, 150°C storage, and 0–100°C temperature cycling. The goal was to determine an acceptable low-cost, reliable method for bumping and assembling chips to flexible or rigid substrates using flip-chip assembly techniques. Alternative flip-chip bumping methods are compared to a traditional wafer solder bumping method. Flip-chip interconnection methods evaluated included high lead content solder, silver filled conductive adhesive, and gold stud bumps. Under bump metallurgies evaluated included bare aluminum, evaporated Cr/Cr–Cu/Cu, and electroless nickel plating.