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.     

InP/Si键合界面热应力分析

从理论上分析了键合热应力产生的原因,在此基础上,采用双层条状金属热应力模型讨论InP/Si键合过程中应力的大小及分布情况.结果表明, 由剪切应力和晶片弯矩决定的界面正应力是晶片中心区域大面积键合失败的主要原因,同时InP/Si键合合适的退火温度应该在250～300 ℃.最后在300 ℃退火条件下很好地实现了InP/Si键合,界面几乎没有气泡,有效键合面积超过90%.     

Si/Si直接键合界面热应力模型及模拟

根据Suhir的双金属带的热应力分布理论,建立了Si/Si直接键合界面应力模型,推导出了由于高温引起的正应力、剪切应力和剥离应力的解析方程。并且应用模拟软件Matlab对热应力进行了模拟,直观地表现了键合界面应力的大小及其分布情况,对键合工艺有一定的指导意义。     

PEO-LiClO_4与铝阳极键合界面结构及力学性能

利用自制的阳极键合炉实现了PEO-LiClO4与铝的阳极键合,并且采用扫描电子显微镜(SEM)对键合界面进行了观察。结果表明,键合后铝/高分子固体电解质界面处生成了一层厚度约5μm的过渡层,分析认为该过渡层是两者得以焊合的关键。由实验得到的电流-时间的变化过程可以看出,键合区域中的导电离子是由瞬时高密度迁移转向低密度稳态迁移的。利用非线性有限元分析软件MSC.Marc研究了阳极键合件从90℃冷却到室温的残余应力和变形,并且建立了接头应力分析模型,探讨了界面应力场的分布规律,为提高键合质量提供了理论依据。对比翘曲变形前后的形状可知,试件从高温冷却后,上表面中心会向下收缩,边缘的翘曲下降,最大的翘曲发生在试件对角棱线处。模拟分析显示过渡层上等效应力是最大的,因此该层成为阳极键合接头中的薄弱部位。因此,对残余应力和变形进行分析并在此基础上提出缓解的措施是提高阳极键合性能的一个有效途径。     

Si/GaAs晶圆片键合热应力及其影响因素分析

分别利用Suhir双金属带热应力分布理论和有限元法研究了Si/GaAs晶圆片键合界面在退火过程中的热应力分布,并将热应力分布理论计算结果与有限元分析结果进行了对比验证,得到了一致的结论.根据计算分析结果,对晶圆片进行了结构热变形分析,并研究了不同因素对键合热应力的影响,提出了减小键合热应力的有效措施.     

晶片键合界面应力分布的理论分析

根据双金属带的热应力分布理论,推导了晶片键合以及薄膜键合的界面应力分布公式.对影响晶片键合的剪切应力、正应力以及剥离应力的分布特性进行了讨论.     

用有限元方法分析Si/GaAs的键合热应力

在考虑材料热膨胀系数随温度变化后,采用有限元方法结合ANSYS软件对Si/GaAs键合热应力进行了分析,研究了普通应力、轴向应力和剪切力的分布云图和沿界面的分布.同时提出了新的键合结构以减小热应力的影响,计算结果证明了该结构的有效性.     

用有限元方法分析Si/GaAs的键合热应力

在考虑材料热膨胀系数随温度变化后,采用有限元方法结合ANSYS软件对Si/GaAs键合热应力进行了分析,研究了普通应力、轴向应力和剪切力的分布云图和沿界面的分布.同时提出了新的键合结构以减小热应力的影响,计算结果证明了该结构的有效性.     

晶片键合界面应力分布的理论分析

根据双金属带的热应力分布理论,推导了晶片键合以及薄膜键合的界面应力分布公式.对影响晶片键合的剪切应力、正应力以及剥离应力的分布特性进行了讨论     

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.     

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.     

Increasing the bonding strength of chips on flex substrates using thermosonic flip-chip bonding process with nonconductive paste

Thermosonic flip-chip bonding process with a nonconductive paste (NCP) was employed to improve the processability and bonding strength of the flip-chip onto flex substrates (FCOF). A non-conductive paste was deposited on the surface of the copper electrodes over the flex substrate, and a chip with eight gold bumps bonded onto the copper electrodes by the thermosonic flip-chip bonding process.For the chips and flex substrates assembly, ultrasonic power is important in the removal of some of the non-conductive paste on the surface of copper electrodes during thermosonic bonding. Accordingly, gold stud bumps in this study were directly bonded onto copper electrodes to form successful electrical paths between chips and the flex substrate. A particular ultrasonic power resulted in some metallurgical bonding between the gold bumps and the copper electrodes, increasing the bonding strength. The ultrasonic power was not only to remove the NCP from the copper electrodes, but also formed metallurgical bonds during the thermosonic flip-chip bonding process with NCP.In this study, the parameters of the bonding of chips onto flex substrates using thermosonic flip-chip bonding process with NCP were a bonding force of 4.9 N, a curing time of 40 s, a curing temperature of 140 °C and an ultrasonic power of 14.46 W. The processability and bonding strength of flip-chips on flex substrates using thermosonic bonding process with NCP was verified in this study. This process has great potential to be applied to the packaging of consumed electronic products.     

考虑胶层影响的压电陶瓷层合结构变形模型

针对压电层合结构中胶层厚度不可忽略的情况,根据层合结构各层的正应变分布,研究了压电层合结构中压电层与基底层中的正应变与胶层内部剪应变的数学关系,再根据剪应力与剪应变的关系,建立了考虑胶层情况下压电层与基底层界面上的剪应力、剪应变分布模型,得到压电层与基底层间界面上的应力、应变分布规律。并基于该模型研究了压电层合结构的几何参数与材料物性参数对应力、应变分布的影响。最后结合压电层合结构在电载荷下的变形模型,通过与实验结果对比,验证了考虑胶层影响模型的正确性。     

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.     

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.     

Theoretical and numerical analysis of the effect of constant velocity on thermosonic bond strength

A new effective method is presented in this study to investigate the effect of constant velocity on the thermosonic bond strength. The proper relationship between the bond strength and results of FE analysis was introduced with some related theoretic equations in solid state welding. A transient non-linear dynamic finite element framework was developed, and the strain rate sensitivity of the gold ball and pad was considered. The effective normal pressure and bond surface exposure were all calculated, thus the bond strength was estimated approximately. It can be found that the bond strength increases with the increase of constant velocity, and then decreases after it exhibits the maximum. In addition, the strength distribution in bond interface was also studied. At last, the application range of the present method was discussed.     

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。     

Modeling of copper wire bonding process on high power LEDs

In this paper, a couple thermal mechanical transient dynamic finite element framework of copper wire bonding process on high power lighting emitting diodes (LEDs) is developed, which considers the thermal heating effects of friction and plastic deformation. The whole wire bonding process is simplified to consist of impact and ultrasonic vibration stages. Parametric studies are also carried out to examine the effects of ultrasonic vibration amplitude and bonding force on stress/strain distribution and friction thermal heating effect during wire bonding process. Different friction coefficients of interface between the free air ball (FAB) and the bond pad are taken in the simulation to examine the effects of friction on the stress and strain level of electrode structure. Modeling results show that the stress/strain distribution and temperature evolution of wire bonding system are significant influenced by the ultrasonic vibration amplitudes, bonding forces and friction coefficients. Discussion and comparison are conducted between the copper and the gold wire bonding processes on the high power LEDs by numerical simulation. The results have disclosed that higher stress/strain in the bond pad and the ohmic contact layer is induced during the copper wire bonding process. Therefore, the process parameters of copper wire bonding should be controlled carefully. This numerical simulation work may provide guidelines for the copper wire bonding process virtual window development of high power LEDs packaging.     

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.