Non-destructive degradation study of copper wire bond for its temperature cycling reliability evaluation

Wire bonding is essential for the electrical connection of integrated circuit (IC) devices, therefore its quality and reliability is of utmost importance. During the wire bonding process, several parameters need to be well controlled in order to achieve a well bonded wire. Furthermore, the migration to copper (Cu) wire from gold (Au) due to its high cost has resulted in an even more stringent and narrow process window. Current industrial practices to evaluate wire bond quality after the assembly and packaging process are either done destructively which may result in loss of critical information, or non-destructively which are limited by resolution, cost and time. In this work, the quality of copper wire bond is being evaluated by electrical means that is non-destructive, fast and accurate. This makes it suitable for use in the production line for wire bond quality evaluation. Experimental results showed that there is a good correlation with conventional wire assessment methods. Furthermore the electrical method is sensitive enough to pick out degraded wires that conventional methods are unable to identify.     

Ultra-fine pitch palladium-coated copper wire bonding: Effect of bonding parameters

Copper (Cu) wire bonding has become a mainstream IC assembly solution due to its significant cost savings over gold wire. However, concerns on corrosion susceptibility and package reliability have driven the industry to develop alternative materials. In recent years, palladium-coated copper (PdCu) wire has become widely used as it is believed to improve reliability. In this paper, we experimented with 0.6 ml PdCu and bare Cu wires. Palladium distribution and grain structure of the PdCu Free Air Ball (FAB) were investigated. It was observed that Electronic Flame Off (EFO) current and the cover gas type have a significant effect on palladium distribution in the FAB. The FAB hardness was measured and correlated to palladium distribution and grain structure. First bond process responses were characterized. The impact of palladium on wire bondability and wire bond intermetallic using a high temperature storage test was studied.     

Optimizing the fine-pitch copper wire bonding process with multiple quality characteristics using a grey-fuzzy Taguchi method

This paper presents an application of the grey-fuzzy Taguchi method to derive the optimum parameters for the fine-pitch copper (Cu) wire bonding process with multiple quality characteristics. Cu wire has become an important replacement material without the high manufacturing cost of gold for wire bonding processes in integrated circuit (IC) packaging. However, it sometimes fails to take advantage of the Cu material properties due to an insufficient understanding of the relationship between process inputs and outputs of the fine-pitch Cu wire bonding. To assure the accomplishment of cost savings without losing yield, the proposed grey-fuzzy Taguchi method is utilized with an L18 (2¹ × 3⁷) experimental design and grey relational analysis (GRA) to evaluate the degree of relationship between process inputs and responses, followed by transforming the multiple quality characteristics into a single performance index using a fuzzy inference system. Finally, the process parameters are optimized by the Taguchi method. The proposed optimization methodology provides superior optimization performance compared to the grey-Taguchi method and the parameter setting used in mass production in terms of the conformation experiments.     

铜线键合Cu/Al界面金属间化合物微结构研究

由于铜线具有较高的热导率、卓越的电学性能以及较低的成本,被普遍认为将逐渐代替传统的金线而在IC封装的键合工艺中得到广泛的应用。铜线键合工艺中Cu/Al界面金属间化合物(IMC)与金线键合的Au/Al IMC生长情况有很大差别,本文针对球焊键合中键合点的Cu/Al界面,将金属间化合物生长理论与分析手段相结合,研究了Cu/Al界面IMC的生长行为及其微结构。文中采用SEM测试方法,观察了IMC的形貌特点,测量并得到了IMC厚度平方正比于热处理时间的关系,计算得到了生长速率和活化能数值,并采用TEM,EDS等测试手段,进一步研究了IMC界面的微结构、成分分布及其金相结构。     

Effect of the direct current on microstructure,tensile property and bonding strength of pure silver wires

In the microelectronics assembly and packaging industry, the wire bonding has become an important process to connect lead frames and pads. In the past, gold and copper were the main materials of wire bonding. However, the cost of gold wires is getting higher nowadays and yet wire bonding cannot be wholly replaced by copper wire; thus silver wires become a novel bonding material in recent years. The reliability test of wires was a static method; this study leads electrical current into the wires to estimate the structural changing and interface properties of Al pads (positive and negative pad). After leading 90% critical fusing current density (CFCD) into a 23 μm silver wire, some grains of silver wire had grown up and formed into equal-diameter grains (EDG). After the current test, the fracture position of bonded wires moved from heat affect zone (HAZ) of electric flame-off (EFO) to the neck of HAZ. Otherwise, the current test would reduce the tensile strength of wire. The bonding strength of the positive pad was lower than that of the negative pad. The intermetallic compound (IMC) of bonding interface was AgAl₂.     

Bond reliability under humid environment for coated copper wire and bare copper wire

There is growing interest in Cu wire bonding for LSI interconnection due to cost savings and better electrical and mechanical properties. Conventional bare Cu bonding wires, in general, are severely limited in their use compared to Au wires. A coated Cu bonding wire (EX1) has been developed for LSI application. EX1 is a Pd-coated Cu wire to enhance the bondability.Bond reliability at a Cu wire bond under a humid environment is a major concern in replacing Au wires. The bond reliability of EX1 and bare Cu was compared in the reliability testing of PCT and UHAST (Unbiased HAST). The lifetimes for EX1 and the bare Cu in PCT testing were over 800 h and 250 h, respectively. Humidity reliability was significantly greater for EX1. Continuous cracking was formed at the bond interface for the bare Cu wire. Corrosion-induced deterioration would be the root cause of failure for bare Cu wires. The corrosion was a chemical reaction of Cu-Al IMC (InterMetallic Compound) and halogens (Cl, Br) from molding resins. EX1 improves the bond reliability by controlling diffusion and IMC formation at the bond interface. The excellent humidity reliability of the coated Cu wire, EX1 is suitable for LSI application.     

镀Pd Cu线键合工艺中Pd行为研究

由于Cu线热导率高、电性能好、成本低,将逐渐代替传统Au线应用于IC封装.但Cu线键合也存在Cu材料本身固有特性上的局限:易氧化、硬度高及应变强度等.表面镀Pd Cu线材料的应用则提供了一种防止Cu氧化的解决方案.然而,Cu线表面的Pd层很可能会参与到键合界面形成的行为中,带来新的问题,影响到Cu线键合的强度和可靠性.对镀Pd Cu线键合工艺中Pd的行为进行了系统的研究,使用了SEM,EDS等分析手段对cu线、烧结Cu球(FAB)、键合界面等处Pd的分布状况进行了检测,结果证明Pd的空间分布随着键合工艺的进行发生了很大的变化,同时还对产生Pd分布变化的原因进行了分析和讨论.     

Enhancing bondability with coated copper bonding wire

There is growing interest in Cu wire bonding for LSI interconnection due to the cost savings and better electrical and mechanical properties. However, the scope of use for Cu bonding wires is generally severely limited compared to Au wires; e.g. for wire oxidation, lower bondability, forming gas of N₂ + 5%H₂, and lower reliability. It is difficult for conventional bare Cu wires to achieve the target of LSI application.A coated Cu wire (EX1) has thus been developed. It is a Pd-coated Cu wire and has many advantages compared to bare Cu wires. Its stitch strength was far superior under fresh conditions and remained constant without any deterioration after being stored in air for a prolonged period. EX1 had a lifetime of over 90 days in air, as compared to just 7 days for bare Cu wire. Spherical balls were formed with pure N₂ (hydrogen-free), whereas the bare Cu produced off-center balls. Finally, the cost-effective and secure gas, pure N₂ was only available for EX1. The excellent performance of the EX1 coated Cu wire is comparable to that of Au wires, making it suitable for LSI packaging.     

基于变焦显微测量技术的键合金丝参数测量

欧美国家在半导体行业一直以来对我国实行技术封锁政策,我国许多关键技术和设备只能依靠进口,其中就包括键合金丝参数测量设备。键合金丝参数测量设备主要用于自动检测键合金丝的拱高和跨度等参数。由于键合金丝的回波损耗、驻波等微波传输特性与键合金丝的拱高、跨度等参数呈对应关系,因此可以通过测量相关参数的方法来检测键合金丝的微波传输特性是否合格。通过这一方法可以解决人工测量导致的速率低下的问题,提高键合质量检测效率,降低检测成本。本文基于变焦显微测量技术实现了键合金丝参数的测量。该方法通过自主设计的图像采集平台,获取到键合金丝的一组图像,然后进行聚焦区域提取,从而实现键合金丝的三维重建及参数测量。该方法对键合金丝拱高的测量精度<0.01 mm,相对误差<1.5%,对键合金丝跨度的测量精度<0.005 mm,相对误差<0.7%,可以满足自动检测键合金丝参数的设计需求。     

Effects of Cu and Pd addition on Au bonding wire/Al pad interfacial reactions and bond reliability

Finer pitch wire bonding technology has been needed since chips have more and finer pitch I/Os. However, finer Au wires are more prone to Au-Al bond reliability and wire sweeping problems when molded with epoxy molding compound. One of the solutions for solving these problems is to add special alloying elements to Au bonding wires. In this study, Cu and Pd were added to Au bonding wire as alloying elements. These alloyed Au bonding wires—Au-1 wt.% Cu wire and Au-1 wt.% Pd wire—were bonded on Al pads and then subsequently aged at 175°C and 200°C. Cu and Pd additions to Au bonding wire slowed down interfacial reactions and crack formation due to the formation of a Cu-rich layer and a Pd-rich layer at the interface. Wire pull testing (WPT) after thermal aging showed that Cu and Pd addition enhanced bond reliability, and Cu was more effective for improving bond reliability than Pd. In addition, comparison between the results of observation of interfacial reactions and WPT proved that crack formation was an important factor to evaluate bond reliability.     

微电子器件封装铜线键合可行性分析

虽然在集成电路封装工艺中金导线键合是主流制程,但是目前采用铜导线替代金导线键合已经在半导体封装领域形成重要研究趋势。文章对微电子封装中铜导线键合可行性进行了分析,主要包括铜导线与金导线的性能比较(包括电学性能、物理参数、机械参数等),铜导线制备和微组织结构分析,铜导线焊合中的工艺研发及铜导线焊合可靠性分析等。当今半导体生产商关注铜导线不仅是因为其价格成本优势,更由于铜导线具有良好的电学和机械特性,同时文中也介绍了铜导线键合工艺存在的诸多问题和挑战,对将来铜导线在集成电路封装中的大规模应用和发展具有一定的参考意义。     

New encapsulation development for fine pitch IC devices

The continuous reduction of chip size driven by the market demand has a significant impact on circuit design and assembly process of IC packages. Shrinking chip size and increasing I/O counts require finer bond pad pitch and bond pad size for circuitry layout. As a result, serious wire deflection during transfer molding process could make adjacent wires short, and this issue becomes more critical as a smaller wire diameter has to be applied for the finer pitch wire bonded IC devices.This paper presents a new encapsulation process development for 50 μm fine pitch plastic ball grid array package. Since reduced wire diameter decreases the bending strength of bonded wires significantly, wire deflection during molding process becomes quite serious and critical. Experiments on conventional transfer molding were conducted to evaluate wire span threshold with 23.0 μm diameter gold wire. The results show that the wire span threshold is about 4.1 mm, which is much shorter than the wire span threshold of over 5.0 mm for wire with 25.4 μm diameter. Finite element analysis shows there is a significant difference in the wire deflection between 23.0 μm gold wire and 25.4 μm gold wire diameter under the same action of mold flow. A novel encapsulation method is introduced using non-sweep solution. The wire span could be extended to over 5.0 mm with wire sweep less than 1%. Reliability tests conducted showed that all the units passed 1000 temperature cycles (−55 to 125 °C) with JEDEC moisture sensitivity level 2a (60 °C/60% relative humidity for 120 h) and 3 times reflow (peak temperature at 220–225 °C). It is believed that this solution could efficiently overcome the risk of wire short issues and improve the yield of ultra fine pitch wire bonds in high-volume production.     

Au线改Cu线的新发展及可靠性研究

Au线作为内引线一直占据着键合的主导地位,而由于Cu线具有优良的电性能和价格优势,随着键合技术的发展,以Cu丝代替Au丝作为键合用内引线已经成为现在的主要趋势。介绍了Cu线相对于Au线的优势、存在的可靠性问题,并对Cu引线键合新的发展趋势、如何进行可靠性验证等问题进行了研究。     

Role of process parameters on bondability and pad damage indicators in copper ball bonding

Cu wire bonding is one of the hottest trends in electronic packaging due to the cost and performance advantages of Cu wire over Au wire. However, there are many challenges to Cu wire bonding, one of which is the increased stress transmitted to the bond pad during bonding. This high stress is not desirable as it leads to pad damage or cratering in the Si under the pad. Another issue is pad splash in which the pad material is squeezed outside the bonded area, which in severe cases can cause Al pad thinning and depletion. To study the root cause of the increased stress, ball bonding is performed with Au and Cu wires using the same levels of ultrasound (USG), bonding force (BF), and impact force (IF). The bonding is performed on a bonding test pad with integrated piezoresistive microsensors and the in situ pad stress is measured in real time. The ultrasonic pad stress did not show any significant difference between the Au and the Cu ball bonding processes. This indicates that the cause of increased stress cannot be attributed to material properties such as hardness alone, and that the differences in bondability and bonding parameters required for the Cu process might be more influential. To achieve optimal bonding results in terms of shear force per unit area, the Cu process requires higher BF and USG settings, which are the main causes of pad damage. To understand the effect of bonding parameters IF, BF, and USG on pad stress, a detailed DOE is conducted with Cu wire. In addition to conventional bonding parameters, the effect of a non-zero USG level applied during the impact portion of the bonding (pre-bleed USG) is investigated. One of the findings is the reduction of pad damage when higher pre-bleed USG levels are used.     

Wire bond loop profile development for fine pitch-long wireassembly

Die size reductions can be achieved through “optical shrinks,” compaction of existing layouts, or redesigns to finer fab geometries. For some die the limiting factors for die size reduction are bond pad pitch and bond pad size. In these “pad limited” designs, the circuitry is concentrated in the center of the die. Precious empty space exists between the bond pads in the periphery of the die and the circuitry in the die core. The only hope for die size reductions in these designs lies in advances in assembly technology that allow for reductions in bond pad pitches and bond pad size. Fine pitch assembly poses a number of challenges for conventional wire bond technology. Reducing bond pad pitch increases the probability of ball shorting, bond wire shorting, and bond wire damage. On the other hand, decreasing the die size by reducing the bond pad pitch results in longer wire lengths thus limiting some assembly options such as moving to smaller diameter bonding wires. Wire loop profile becomes a critical factor for control in fine pitch assembly. In this paper a statistical design of experiment is used in developing a wire bond loop profile control. The effect of major bonding parameters, such as kink-height, reverse loop, loop factor, wire tension, and their impact on loop profile are analyzed. The results obtained define the bond parameter requirements that must be met in order to control the wire loop profile to optimize fine pitch wire bond assembly yields     

IC封装中键合线传输结构的仿真分析

随着高频高速集成电路制造工艺的不断进步,电子封装技术的发展也登上了一个新高度。作为微电子器件制造过程中的重要步骤之一,封装中的传输线、过孔、键合线等互连结构都可能对电路的性能产生影响,因此先进的集成电路封装设计必须要进行信号完整性分析。介绍了一种键合线互连传输结构,采用全波分析软件对模型进行仿真,着重分析与总结了键合线材料、跨距、拱高以及微带线长度、宽度五种关键设计参数对封装系统中信号完整性的影响,仿真结果对封装设计具有实际的指导作用。     

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.     

MOSFET器件引线键合技术

MOSFET器件由于高阻抗、低功耗等特点,在电脑电源、家用电器和自动控制系统等方面得到广泛应用。但由于其芯片结构的特殊性,在封装制造过程中容易受到静电、应力、环境条件等多种因素的影响。引线键合过程是影响封装成品率的关键工艺环节。引线键合是电子工程互连的重要方式,MOSFET器件通常采用超声键合的工艺进行引线互连。影响引线键合质量的因素较多,其中引线键合工艺、引线材料和设备维护是最重要的三个因素。通过实际生产过程的试验、分析和提炼,研究引线键合技术,总结了引线键合工艺、引线材料和设备维护三个方面的实践经验,为提升和稳定封装成品率提供参考。     

Iterative optimization of tail breaking force of 1 mil wire thermosonic ball bonding processes and the influence of plasma cleaning

An online tail breaking force measurement method is developed with a proximity sensor between wire clamp and horn. The wire under the tensile load measures about 1.5 cm extending from the bond location to the wire clamp. To increase the sensitivity, the bondhead speed is reduced to 2 mm/s during breaking the tail bond. It takes roughly 10 ms to break the tail bond. The force resolution of the method is estimated to be better than 5.2 mN. An automatic wire bonder used to continuously bond up to 80-wire loops while recording the on-line proximity signals. All wires are directed perpendicular to the ultrasound direction. The tail breaking force for each bond is evaluated from the signal and shown automatically on the bonder within 2 min after bonding.Results are obtained for a typical Au wire and a typical Cu wire bonding process. Both wires are 25 mm in diameter and bonded on Ag plated diepads of standard leadframes at 220 °C. An average Cu tail breaking force of higher than 50 mN is obtained if the leadframe is plasma cleaned before the bonding with 100% Ar for 5 min. This result is comparable to that obtained with Au wire. The standard deviation of the Cu tail breaking force is about twice that obtained with Au wire. The tail breaking force depends on the bonding parameters, metallization variation, and cleanliness of the bond pad. The cleanliness of the bonding pad is more important with Cu wire than with Au wire.     

基于缺陷均匀分布的集成电路制造成品率与可靠性之间的关系模型

在集成电路可制造性设计研究中,成品率与可靠性之间的关系模型备受人们关注.缺陷对成品率和可靠性的影响不仅与出现在芯片上的缺陷粒径大小有关而且与缺陷出现在芯片上的位置有关.本文主要考虑了出现在互连线上的金属丢失物缺陷对互连线的影响,分析了同一粒径的缺陷出现在互连线不同位置对互连线有效宽度的影响,给出了基于缺陷均匀分布的互连线平均有效宽度,结合已有成品率和可靠性估计模型,提出了基于缺陷位置信息的集成电路制造成品率与可靠性之间的关系模型.在工艺线稳定的情况下,利用该工艺线的制造成品率可以通过该关系式有效地估计出产品的可靠性,从而有效地缩短新产品的研发周期.