热超声倒装键合过程中的非线性动力学行为

在作为微器件电气互连主要手段的热超声键合工艺中,微细键合区域内金属变形/变性/互连所需的能量,来自于超声波功率源通过换能系统所施加的微幅压剪动载.对热超声倒装键合过程的研究说明,PZT换能系统在热超声倒装键合工艺过程中的非线性动力学行为,如换能系统启动后的初值敏感性和不确定性,键合工具与换能杆之间的不稳定动力耦合,倒装芯片运动的奇异相轨线等,是深入研究键合机理以及提高工艺可靠性的重要关键.     

热超声倒装键合过程中的非线性动力学行为

在作为微器件电气互连主要手段的热超声键合工艺中,微细键合区域内金属变形/变性/互连所需的能量,来自于超声波功率源通过换能系统所施加的微幅压剪动载.对热超声倒装键合过程的研究说明,PZT换能系统在热超声倒装键合工艺过程中的非线性动力学行为,如换能系统启动后的初值敏感性和不确定性,键合工具与换能杆之间的不稳定动力耦合,倒装芯片运动的奇异相轨线等,是深入研究键合机理以及提高工艺可靠性的重要关键.     

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.     

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。     

热超声键合换能系统接触界面非线性振动研究

为了减小热超声键合换能系统的振动稳定性、提高键合强度,从超声波在热超声键合换能系统中的传播出发,建立了超声波在接触界面处传播的微观模型.研究表明,当静应力较小时,输出的超声波不完整,材料内部质点的有效振动较小;当静应力逐渐增大时,材料进入弹性变形阶段,输出的超声波波形与输入的超声波的波形一致,材料内部质点的有效振动最大;当静应力太大时,材料进入塑性变形区域,材料内部质点的有效振动减小.通过在芯片倒装键合实验台上实验,测量在不同压电陶瓷片预紧力下,变幅杆的振动速度和芯片的键合强度,证明了所提出模型的正确性.     

Low-Stress Thermosonic Copper Ball Bonding

Thermosonic ball bonding processes on test chips with Al metallized bonding pads are optimized with one Au and two Cu wire types, all 25 $mu{hbox {m}}$ diameter, obtaining average shear strengths of more than 120 MPa. The process temperature is $sim {hbox {110}}~^{circ}{hbox {C}}$. Ball bonds made with Cu wire show at least 15% higher shear strength than those made with Au wire. The estimated maximum shear strength $c_{pk}$ value determined for Cu ball bonding $(c_{pk}=3.7pm 1.2)$ is almost 1.5 times as large as that of the Au ball bonding process $(c_{pk}=2.3pm 0.9)$, where $LSL$ is 65.2 MPa. However, the ultrasound level required for Cu is approximately 1.3 times than that required for Au. Consequently, about 30% higher ultrasonic forces induced to the bonding pad are measured using integrated real-time microsensors. The accompanying higher stresses increase the risk of underpad damage. One way to reduce ultrasonic bonding stresses is by choosing the softer of the two Cu wire types, resulting in a measured ultrasonic force reduction of about 5%. A second way is to reduce the ultrasound level. While this causes the average shear strength to fall by 15%, the ultrasonic force falls by 9%. The $c_{pk}$ value does not change significantly, suggesting that a successful Cu ball bonding operation can be run with about 0.9 times the conventionally optimized ultrasound level. The process adjusted in this way reduces the extra stress observed with Cu wire compared to that observed with Au wire by 42%.      

混合集成电路金丝热超声球焊工艺及应用发展

混合集成电路工艺已成为今后电源和器件等微型化模块快速发展的关键瓶颈,在简要介绍内部引线键合技术的基础上,重点讨论HIC和MCM中广泛应用的直径25.4μm金丝热超声球焊工艺的技术特点、材料特性、工艺过程、工艺控制、工艺设计、目检要求、强度测试和应力试验,介绍了该工艺在近期的应用与发展.     

焊料凸点倒装焊技术

介绍了倒芯片面阵式凸点制作、多层陶瓷基板焊盘制作及倒装焊各关键技术 ,并成功地获得了芯片与基板的互连。     

热超声倒装过程中的建模和多参量仿真

热超声键合是一个极其复杂的瞬态过程,利用常规手段不易了解此局部区域内的瞬态特性.针对这个问题,基于MSC.Marc大型非线性有限元分析软件建立了热超声倒装的几何模型,利用其强大的非线性分析能力对热声倒装进行了热力耦合有限元分析,得出了不同摩擦状况对正应力和切应力的分布及大小的影响,金凸点的塑性应变在键合界面上的分布及演变规律,切向位移加载前后键合界面所受正应力和切应力的大小及分布的变化情况,仿真结果对热超声倒装芯片连接工艺的理论研究有着重要的参考价值.     

FPC产品倒装片工艺的改善

本文主要分析了智能卡产品倒装片工艺固有的位置精度问题。通过数据分析,找出了设备长期工作后,精度下降引发产品倒装问题的主要原因。同时,采用AutoCAD软件对倒装片智能卡(Chip Card)产品的布局分析,并结合设备精度、FPC(挠性线路板)制造精度等因素,进行倒装片工艺的仿真模拟,进而可以确定需要改进的设计布局图中的关键尺寸,从而彻底地解决了引发倒装片工艺精度差的问题。     

热超声键合换能系统设计及性能分析

在热超声键合系统中,超声换能系统是键合装备的核心部分。基于等效电路法和解析法设计换能系统,获得换能系统的基本结构尺寸;基于有限元方法建立系统的仿真模型,获得系统的振动模态特性;采用阻抗分析仪与Doppler测振仪对系统进行性能分析。实验表明,实测频率与设计频率吻合,换能系统阻抗为12.7Ω,振动位移达到3.725μm,满足超声芯片封装的工艺要求。     

芯片封装互连新工艺热超声倒装焊的发展现状

介绍了一种芯片封装互连新工艺热超声倒装芯片连接工艺.在比较当前多种芯片封装方式的基础上,总结了这一工艺的特点及优越性,并详细论述了当前这一工艺的技术进展与理论研究状况,指出该工艺是芯片封装领域中具有发展潜力的新工艺.     

一种用于MEMS器件的Au-In倒装焊技术

介绍了Au-In键合在MEMS芯片封装中的应用.根据现有的工艺设备和实验条件对制备铟凸点阵列进行工艺设计,对铟凸点制备技术进行了研究,最终在硅圆片上制备了6 μm高的铟凸点阵列.在150～300℃下成功的进行了Au-In倒装键合实验.在300℃,0.3 MPa压力下键合的剪切强度达到了5 MPa.     

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.      

倒装键合中助焊剂涂敷的工艺优化

助焊剂涂敷是C4凸点焊料的倒装键合中的关键工艺步骤之一,涂敷均匀和稳定性决定了回流焊后整体成品的质量和可靠性,同时影响倒装键合设备的生产效率。在实际生产中,现有的助焊剂涂敷系统影响设备提升生产效率,并且暴露出生产过程中助焊剂泄漏量过大的问题。通过分析现有涂敷系统的问题和助焊剂泄漏的成因,优化设计了一种更高效的助焊剂涂敷系统,有效提升了设备生产率,使泄漏量对生产的影响降低到最小。     

温度因素对热超声键合强度的影响实验研究

主要研究了热超声键合过程中环境温度对键合强度的影响规律.分析了键合强度与键合温度之间的关系.实验研究发现,最佳键合"窗口"出现在200～240℃,此时键合强度可达20g左右.对推荐使用的大于5.4g的键合强度标准,本实验条件下可键合"窗口"为120～360℃.这些实验现象和分析结果为后期的键合参数匹配规律和系统动态特性研究打下基础.     

热超声倒装焊在制作大功率GaN基LED中的应用

设计了适合于倒装的大功率GaN基LED芯片结构,在倒装基板硅片上制作了金凸点,采用热超声倒装焊接(FCB)技术将芯片倒装在基板上.测试结果表明获得的大面积金凸点连接的剪切力最高达53.93 g/bump,焊接后的GaN基绿光LED在350 mA工作电流下正向电压为3.0 V.将热超声倒装焊接技术用于制作大功率GaN基LED器件,能起到良好的机械互连和电气互连.     

Thermosonic Wire Bonding Process Simulation and Bond Pad Over Active Stress Analysis

In this paper, a transient nonlinear dynamic finite element framework is developed, which integrates the wire bonding process and the silicon devices under bond pad. Two major areas are addressed: one is the impact of assembly 1st wire bonding process and another one is the impact of device layout below the bond pad. Simulation includes the ultrasonic transient dynamic bonding process and the stress wave transferred to bond pad device and silicon in the 1st bond. The Pierce strain rate dependent model is introduced to model the impact stain hardening effect. Ultrasonic amplitude and frequency are studied and discussed for the bonding process. In addition, different layouts of device metallization under bond pad are analyzed and discussed for the efforts to reduce the dynamic impact response of the bond pad over active design. Modeling discloses the stress and deformation impacts to both wire bonding and pad below device with strain rate, different ultrasonic amplitudes and frequencies, different friction coefficients, as well as different bond pad thickness and device layout under pad. The residual stress, after cooling down to a lower temperature, is discussed for the impact of substrate temperature.     

Investigation of Thermosonic Wire Bonding Resistance of Gold Wire Onto Copper Pad

This paper discusses the electric performance for thermosonic wire bonding of gold wire onto copper pads. Various methods normally used to improve bondability were investigated including the bare copper pads with argon shielding gas and the copper pads with cupric oxide film, cuprous oxide film, and silver film. The micro-contact theory was used to determine the effective contact area. The circuit contact resistance was measured for each sample and was presented in terms of ultrasound power and effective contact area. The results show that the increase in the effective contact area leads to a lower circuit contact resistance before reaching a minimum value, and further increase in the effective contact area would not have noticeable effect on the resistance.      

Growth of Intermetallic Compounds in Thermosonic Copper Wire Bonding on Aluminum Metallization

Interface evolution caused by thermal aging under different temperatures and durations was investigated by means of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that approximately 30-nm-thick and discontinuous Cu-Al intermetallic compounds (IMCs) were present in the initial bonds before aging. Cu-Al IMCs grew under thermal aging with increasing aging time. The growth kinetics of the Cu-Al IMCs was correlated to the diffusion process during aging; their combined activation energy was estimated to be 1.01 eV. Initially, Al-rich Cu-Al IMCs formed in the as-bonded state and early stage of aging treatment. Cu₉Al₄ was identified by selected-area electron diffraction (SAD) as the only type of Cu-Al IMC present after thermal aging at 250°C for 100 h; this is attributed to the relatively short supply of aluminum to the interfacial reaction.