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.     

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.     

Bondability window and power input for wire bonding

This paper presents a recent study by monitoring input power in wire bonding process on its performance. The instantaneous driving voltage and current to the PZT/transducer system were recorded and the input power histories for all tests were analyzed. A stable and satisfied bonding can be obtained at moderate ultrasonic power setting. A laser Doppler vibrometer was used to record the response of the structure. The initial power oscillating may represent the phase locking chaos, and the final attenuation may reflect the remains of kinetic energy in the structure. Strength of wire bonding should be attributed to the input power during the main loading segment.     

Wirebonding at higher ultrasonic frequencies: reliability and process implications

Higher-frequency ultrasonics have been utilized to improve the bondability of difficult substrates, i.e., substrates that would not bond or that bonded poorly using conventional ultrasonics (nominally at 60 kHz). A systematic study of the influence of higher-frequency ultrasonics on bond strength and the bondability of various substrates is reported. The studies were carried out using two essentially identical thermosonic ball bonding machines, one bonding at nominally 60 kHz and the other at 100 kHz. The only differences between the bonding machines were the ultrasonic generators’ operating frequency and the transducer horns. Key to the study was the ability to make the bonding experiments as controlled, repeatable, and independent of all variables (except frequency) as possible. Control techniques included setting the electronic flame-off to produce consistently sized free-air balls; monitoring the ultrasonic voltage and current waveforms; and picking force, dwell, energy, and substrate heat settings that would allow strong bonds to be formed at both frequencies. Wirebonds (ball bonds) in this study were evaluated primarily by the ball bond shear test. Statistical methods were used to determine whether the differences in the means and variances between comparable samples sets (one bonded at 60 kHz and the other bonded at 100 kHz) were significant. Results of our studies indicate that significant differences exist between bonding at nominally 60 kHz and bonding at 100 kHz. In particular, we describe effects associated with (1) the ball shear strength before and after thermal aging (temperatures up to 200 °C) for both 60- and 100-kHz bonds, (2) the influence of substrate-metallizations combinations on the geometry and strength of the bonds at the different frequencies, and (3) the sensitivity and control of the overall bonding processes.     

超声换能系统电特性实验研究

超声换能系统是引线键合设备的核心部件,对其工作特性的深入了解有助于理解引线键合过程.通过实验,观察分析了超声引线键合过程中不同劈刀安装长度对换能系统电流、电压及功率的影响,发现电流及功率在不同劈刀安装长度时有较为明显的变化.并进一步采用小波分析方法展现了电流信号在时频域内变化的细节情况,为充分了解换能系统电学特性提供了可靠依据和新的方法.     

Process windows for low-temperature Au wire bonding

The process windows are presented for low-temperature Au wire bonding on Au/Ni/Cu bond pads of varying Au-layer thicknesses metallized on an organic FR-4 printed circuit board (PCB). Three different plating techniques were used to deposit the Au layers: electrolytic plating, immersion plating, and immersion plating followed by electrolytic plating. Wide ranges of wire bond force, bond power, and bond-pad temperature were used to identify the combination of these processing parameters that can produce good wire bonds, allowing the construction of process windows. The criterion for successful bonds is no peel off for all 20 wires tested. The wire pull strengths and wire deformation ratios are measured to evaluate the bond quality after a successful wire bond. Elemental and surface characterization techniques were used to evaluate the bond-pad surfaces and are correlated to wire bondability and wire pull strength. Based on the process windows along with the pull strength data, the bond-pad metallization and bonding conditions can be further optimized for improved wire bondability and product yields. The wire bondability of the electrolytic bond pad increased with Au-layer thickness. The bond pad with an Au-layer thickness of 0.7 μm displayed the highest bondability for all bonding conditions used. The bondability of immersion bond pads was comparable to electrolytic bond pads with a similar Au thickness. Although a high temperature was beneficial to wire bondability with a wide process window, it did not improve the bond quality as measured by wire pull strength.     

芯片键合换能系统中接触界面的影响分析

接触界面对超声能量传递与振动特性的影响是各类压电换能器的共性问题。在超声芯片封装领域,各子部分之间的接触界面是影响系统超声能量传递的强非线性因素,直接影响芯片与基板的键合质量。该文通过有限元法与激光多谱勒测振仪等技术,获得系统中接触界面对超声能量传递与振动特性的影响规律,发现不合理的接触界面会引发系统多模态与频率混叠效应、超声能量输出不稳定、系统迟滞响应等,导致键合强度下降、芯片与基板倾斜、键合效率下降等封装缺陷。研究结果对理解超声键合与系统设计具有指导意义。     

Dynamics of Ultrasonic Transducer System for Thermosonic Flip Chip Bonding

Thermosonic flip chip bonding is used in certain fine pitch IC packaging for its unique features. By using this bonding process in this paper, 1 mm$, times ,$1 mm chip with 8 gold bumps has been bonded onto a silver-coated substrate. Dynamical properties of transducer system, which is the key component for providing the ultrasonic energy, have been investigated using finite element model (FEM) and measurement using impedance analyzer and laser doppler vibrometer (LDV). The simulation results show that the ultrasonic transducer vibrates by coupling with all excited modes, therefore resulting in complicated motions during bonding. The third axial mode, which includes 1.5 wavelengths and 3 nodes, is the dominant working vibration. However, this axial mode is severely disturbed by undesirable non-axial modes such as flexural modes. There are some other unwanted parasitic modes close to dominant mode. Measured velocities of the transducer horn show that the system vibrates under several modes simultaneously. The impedance measurements reveal additional frequencies overlapping the working frequency. All non-axial modes of the ultrasonic transducer disturb the bonding process and degrade the bonding quality. A subtle control is needed to obtain unique axial mode and stable vibration for high bonding quality.      

欠键合/过键合与功率设置及温度关系的研究

在热超声引线键合过程中,低功率设置和低温下容易导致欠键合,高功率设置和高温下则容易出现过键合。通过实验采集了不同超声功率设置、不同温度下大量键合过程中换能杆末端轴向的速度信号、换能杆两端驱动电压和电流信号,并测量其键合强度,计算得到不同温度下换能杆的振幅。分析了超声功率和键合温度对振幅的影响规律,解释了功率设置和键合温度导致欠键合和过键合的可能原因,建立了功率设置和温度对换能杆振幅影响的模型。这些实验数据和结果有助于进一步研究键合过程中参数间的相互影响规律。     

Development of capillaries for wire bonding of low-k ultra-fine-pitch devices

Although wire bonding has been a well-established technology for many years, the bonding tool design becomes more complex and the process is very sensitive for wire bonding of low-k ultra-fine-pitch microelectronics devices. In this study, two different types of external transition profile were considered in order to use lower ultrasonic-generator power for preventing pad damage. The ultrasonic vibration displacements of the capillaries were measured using a laser interferometer. The measurement results revealed that the amplification factor (the ratio of the vibration displacement at the capillary tip to that at the transducer point) of a capillary with a small radius transition between the bottleneck angle and the main taper angle was 37% higher than that of a capillary with a sharp transition, and this led to satisfactory results in terms of ball size, ball height, ball shear and stitch pull. To solve the ball lift problem for wire bonding of low-k ultra-fine-pitch devices, optimization of the capillary internal profile was attempted to improve bondability. Actual bonding responses were tested. Compared to a standard design, a capillary with a smaller chamfer angle, a larger inner chamfer and a larger chamfer diameter could increase the percentage of the intermetallic compound in the bond interface. Metal pad peeling and ball lift failures were not observed after an aging test.     

Two capillary solutions for ultra-fine-pitch wire bonding and insulated wire bonding

In this article, the new challenges and requirements in wire bonding are discussed, the problems in ultra-fine-pitch wire bonding and insulated wire bonding are analyzed, and then two capillary solutions to the problems are presented. Actual bonding experiments using the new capillaries were carried out and the results were satisfactory. Compared to the standard design, a new capillary design has a larger inner chamfer, a larger chamfer diameter and a smaller chamfer angle. This new capillary design has proved to improve the ball bondability and smaller ball size control for ultra-fine pitch wire bonding. A unique surface characteristic on the capillary tip surface has also been derived. The new finishing process developed creates a new surface morphology, which has relatively deep lines with no fixed directions. Compared to the standard capillary, this capillary has less slipping between the wire and the capillary tip surface in contact, and provides better coupling effect between them and better ultrasonic energy transfer. This capillary has been used to effectively improve the bondability of the stitch bonds for insulated wire bonding.     

Megasonic transducer drive utilizing MOSFET DC-to-RF inverter withoutput power of 600 W at 1 MHz

This paper describes MOSFET power inverter systems, each provided to drive a megasonic transducer with an output power of 600 W at a frequency of 1 MHz. Since the megasonic transducer is used as a resonant load with a series resistance of approximately 1 Ω, the impedance characteristic of the megasonic transducer used as an inverter load is analyzed and compared with measured data. A new method is developed to match the inverter output impedance to the load impedance of as low as 1 Ω at the resonant frequency using a high-performance output transformer which can feed RF power to the load at high efficiency. The output transformer having a primary-to-secondary winding ratio of 2 to 1 was used by the inverter to drive the megasonic transducer. Based on the analysis of the impedance characteristics of the load, two types of MOSFET dc-to-RF power inverters, a full-bridge version and a single-ended version, were designed and built. These power inverters were put into practical use in cleaners. The power conversion efficiency was greater than 80% for the full-bridge version at an output of approximately 600 W and 50% for the single-ended version at an output power of approximately 600 W. When the megasonic transducer was operated with an input power of 600 W. the operation was satisfactory     

Roughness Enhanced Au Ball Bonding of Cu Substrates

Process development studies of Au ball bumping on metallographically polished Cu substrates at ambient temperature were conducted by investigating the effect of process parameters on the ball bond shear force and the extent of bonding. These studies were performed on substrates polished with 0.06-$mu$m or 1-$mu$m abrasive solutions so as to assess the effect of surface roughness on bondability. Response surfaces were generated to illustrate the effects of ultrasonic power, bonding force, and time on bond shear force, and process windows were defined as those parametric combinations that yielded bond shear forces of 25gf or higher. After dissolving the Cu substrate away, the etched surfaces of the Au bumps were examined for bonded areas. Au–Cu ball bonds of about 65$mu$m diameter with bond shear force values higher than 25gf were obtained on 0.06-$mu$m polished substrates, but at an optimum bonding time of 1000ms. Increase in surface roughness, however, reduced the bonding time considerably, and values as low as 200ms were sufficient to yield bond shear force values higher than 25gf on 1.0-$mu$m polished substrates. Bonding on 1.0-$mu$m polished substrates not only reduced the bonding time, but also increased the maximum bond shear force and reduced the localization of bonded areas. These results suggest that a greater number of surface asperities of sufficient height on rougher substrates provide more bonding sites and hence improve the bondability.     

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.     

Numerical analysis of ultrasonic wire bonding: Part 2. Effects of bonding parameters on temperature rise

The thermo-structural analysis of ultrasonic wire bonding is performed by means of 3D finite element method. A special focus has been placed on monitoring the temperature rise during ultrasonic vibration. An equivalent method is used to simulate the wire and bond pad, where the large volumes of wire and bond pad are effectively reduced to small computational magnitudes. The history of temperature changes in the wire-bond pad-substrate interfaces influenced by varying bond forces and bond pad sizes is specifically studied. It is shown that the maximum bulk temperature obtained upon completion of ultrasonic vibration is far lower than the melting temperatures of the wire and bond pad materials, indicating that the bulk temperature rise due to ultrasonic vibration is not directly responsible for ultrasonic wire bonding. A large bond pad size usually leads to a lower temperature rise, and when the pad size reaches a certain value, the effect of bond pad size on temperature rise becomes insignificant. A higher bond force results in a marginally higher temperature rise than a lower bond force, which does not necessarily affect the wire bondability.     

Advanced process characterization for 125 kHz wire bonder ultrasonic transducers

A high frequency ultrasonic transducer for wire bonding is conceived, designed, prototyped, tested and industrially produced. The influence of each feature of the ultrasonic transducer design, such as constituent material, amplifier geometry, mounting flange, capillary fixing, on the wire bonding process results is clearly identified and carefully analyzed through thorough process tests. The assembly and aging characteristics of the transducers are measured and the scattering of the vibration properties in the mass production is statistically quantified by laser interferometer measurements and compared with that of conventional horns. The transducer is mounted on the wire bonder with a flange whose special geometry is calculated by means of FEM simulations and patented. This flange allows the mechanical stiffness of the coupling transducer-wire bonder to be dramatically increased and the parasitic mechanical dynamical vibrations at the horn tip to be significantly reduced. Process tests show that the reduction in mechanical vibrations obtained in this way allows a wire bonder scarcely capable normally to attain the 80 /spl mu/m fine pitch process, to perform easily the 60 /spl mu/m fine pitch process. A beneficial impact of the mechanical coupling horn-wire bonder on the process performance is ascertained. The use of titanium as horn material, characterized by a low thermal expansion coefficient, is proven by process tests to be effective in improving the placement accuracy of the wire bonder. The high vibration frequency of the transducer (125 kHz) is proven by process tests to be effective in improving the ball roundness and the fine pitch wire bonding capabilities of the wire bonder and in decreasing the minimum wire bonding temperature and the applied bond force. The new approach to characterize the process performance of new ultrasonic transducer designs is of general importance for wire bonding technology.     

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.     

基于NIR-AOTF的超声换能器阻抗特性分析及驱动系统设计

该文从压电超声换能器的阻抗特性出发,在对换能器的输入阻抗及匹配网络进行了深入研究的基础上设计了一种基于高速单片机和直接数字频率合成器(DDS)的NIR-AOTF驱动系统,采用软件查表法将各个频段驱动信号所对应的电压幅值控制字做成表并保存在单片机中,实现了DDS在各个频段的恒功率输出,并采用新型的宽带阻抗变换网络加载在压电换能器,最终在30~80MHz带宽范围内匹配网络 ＞-0.276dB,回波损耗 ＜-10.173dB。由于驱动电路提供功率为36dBm,实验证明换能器获得功率高于35dBm,达到超声换能器实际工作的3-4W功率要求, NIR-AOTF的0级光谱衍射效率最高达73％。     

Power and Interface Features of Thermosonic Flip-Chip Bonding

Driving voltage and current signals of piezoceramic transducer were measured directly by using digital storage oscilloscope, and interface microcharacteristics of the specimens of flip-chip bonding were inspected by using a transmission electron microscope. Results show such a trend that power curves of badly bonding were much lower than that of hard bonding, and indicated a monitoring system of ultrasonic bonding reliability. The acceleration of ultrasonic vibration was about several ten thousand times as acceleration of gravity, which activates dislocations inside the metal crystalline lattice which act as the fast diffusion channels. Dislocation diffusion is more prominent than the crystal diffusion when the temperature is low. Differing from thermal melting mechanism of the reflow bonding, the ultrasonic bonding is much faster than the reflow solder bonding.      

Bondability analysis of bond pads by thermoelectric temperature measurements

To reduce cost and enhance reliability for microelectronics applications, a complete understanding of the thermosonic bonding process is required. In particular, the question of whether melting, diffusion, or significant heating occurs along the interface during friction has often been raised. We present results obtained with a new device based on thermoelectric temperature measurements to determine the temperature at the bond interface. In addition to the temperature information, the data characterizes the bonding process in real time on a micrometer scale. The basic principle of the developed apparatus is temperature measurement by an Au-Ni thermocouple fixed within the inside chamfer of a bonding capillary. Different bond substrates with high and low bond contact quality have been investigated. The thermoelectric temperature measurements very precisely determines the bonding behavior of the bond pads. A few nanometers surface contamination on a bond pad significantly reduces the temperature rise at the bond interface and therefore impairs bondability of the substrate. These results demonstrate the sensitivity and accuracy of the measurement principle. The apparatus is a powerful tool to measure the tribology of the bond system and to characterize the bondability of different bond pads.