电子封装用银合金线性能的研究

研究了不同钯(Pd)含量银合金线的电阻率、铝挤出、可靠性及横截面的情况,并分析了Pd抑制银离子迁移的原理。结果表明,银合金线中加入Pd后,Pd的含量越高,线材的FAB硬度越大,铝挤出越多,Pd有助于提高合金线的可靠性,同时Pd的质量分数到3%以上时其可靠性更好。Pd能够抑制银离子迁移的原因是表面形成了一个PdO层,PdO富集在表面阻碍银离子扩散及迁移。银合金线的Ag-Al焊球界面主要形成Ag2Al及Ag3Al,Ag2Al比Ag3Al具有更高的抗腐蚀能力。     

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.     

Microstructure, electric flame-off characteristics and tensile properties of silver bonding wires

Compared with gold wire, silver wire is cheaper and enjoys better electrical conductivity. These days copper wire is becoming more commonly used, but the reliability of its bonding still has problems in some pads. Since Ag wires have similar hardness and bonding properties to Au wires, they can be applied in some pads. In the present study, the annealing effect (at 225-275 °C for 30 min) on the tensile mechanical properties of silver wires with ? = 23 μm was investigated. In addition, the microstructural characteristics and the mechanical properties before and after an electric flame-off (EFO) process were also studied. Experimental results indicate that with annealing temperatures of more than 250 °C, the silver wires possessed a fully annealed structure, the tensile strength and the hardness decreased, and the elongation was raised significantly. Through recrystallization, the matrix structure transformed from long-thin grains to equiaxed grains. The microstructure of the free air ball (FAB) of the various annealed wires after an EFO process were column-like grains. The column-like grains grew from the heat-affected zone (HAZ) to the Ag ball. Under the thermal effect of EFO, the necks of the Ag balls underwent recrystallization and grain growth was induced, and the annealed Ag wires had a shorter zone of HAZ (220 μm). Additionally, the decreased hardness and the low strength of the HAZ resulted in the breakage sites of the EFO wires being in the HAZ near the Ag balls. The bonding strength and the neck-strength of the Ag wires were more than 7gf and possessed excellent bonding properties.     

Reliability model for Al wire bonds subjected to heel crack failures

In power electronic packages wire bonding is used for the electrical contact of the chips and for interconnections on the module substrate. Limiting factors for the reliability are solder fatigue and wire bond failures. In this work we investigate the material fatigue of aluminum bonding wires stressed by cyclic lateral bonding area displacement. Bond wire heel crack failures observed by experiments are found to be strongly dependent on the loop geometry. Based on a finite element model that accounts for elastic-plastic material properties, a life-time model for the Al wire (Coffin-Manson representation) is derived from the experiments.     

Reliability of thick Al wire bonds in IGBT modules for tractionmotor drives

Reliability enhancement of thick Al wire bonds during thermal fatigue test has been investigated from a metallurgical viewpoint. Al wire bonds degrade with the increase of crack length during thermal fatigue tests with high ΔT_j due to the tensile stress generated by the thermal expansion coefficient mismatch between Al wires and Si. It is also found that cracks propagate along the small grain boundaries of Al wires at the bonding interface. It is predicted that the Al wire bonds may not degrade due to thermal fatigue if ΔT_j is controlled below 40 K, i.e., keeping it within the actual temperature fluctuation range in IGBT modules for traction motor drives. The reliability of Al wire bonds can be enhanced by increasing the grain size of the Al wire at the bonding interface. The high temperature bonding is considered to be a good candidate for enhancing the reliability of Al wire bonds     

Microstructural evolution of ultrasonic-bonded aluminum wires

The evolution of microstructural gradients, especially crystallographic texture gradients, after ultrasonic wire bonding process and after active power cycling (APC) of high purity, heavy aluminum (Al) wires is studied by electron backscatter diffraction (EBSD) and nanoindentation. The results improve the knowledge about microstructural changes and arrangements after wire bonding and during APC. After ultrasonic deformation by wire bonding, the evolution of a distinct rotated cube (RC) textured area within the wedge was proved by EBSD analysis. The RC texture is discussed as a result of shear deformation and oriented grain growth. Decreased hardness within the RC textured area provides evidence for local softening effects during wire bonding. During APC, besides crack propagation, grain coarsening as well as local low angle boundary migration occurs and the wedge texture changes to an overall random orientation. Effects of microstructure on the crack growth behavior were discussed and suggestions for the improvement of wire bond reliability were derived.     

Effects of Pd addition on Au stud bumps/Al pads interfacial reactions and bond reliability

The main purposes for developing low-alloyed Au bonding wires were to increase wire stiffness and to control the wire loop profile and heat-affected zone length. For these reasons, many alloying elements have been used for the various Au bonding wires. Although there have been many studies reported on wire strengthening mechanisms by adding alloying elements, few studies were performed on their effects on Au bonding wires and Al pad interfacial reactions. Palladium has been used as one of the important alloying elements of Au bonding wires. In this study, Au-1wt.%Pd wire was used to make Au stud bumps on Al pads, and effects of Pd on Au/Al interfacial reactions, at 150°C, 175°C, and 200°C for 0 to 1200 h thermal aging, were investigated. Cross-sectional scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and electron probe microanalysis (EMPA) were performed to identify intermetallic compound (IMC) phases and Pd behavior at the Au/Al bonding interface. According to experimental results, the dominant IMC was Au₅Al₂, and a Pd-rich layer was at the Au wire and Au-Al IMC. Moreover, Au-Al interfacial reactions were significantly affected by the Pd-rich layer. Finally, bump shear tests were performed to investigate the effects of Pd-rich layers on Au wire bond reliability, and there were three different failure modes. Cracks, accompanied with IMC growth, formed above a Pd-rich layer. Furthermore, in longer aging times, fracture occurred along the crack, which propagated from the edges of a bonding interface to the center along a Pd-rich layer.     

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.     

基于薄膜HIC金铝键合失效的工艺研究

介绍了薄膜混合集成电路(HIC)中金铝键合失效机理,提出了一种解决金铝键合失效的新工艺。分析失效机理发现,铝丝和薄膜金导带形成的金铝界面因原子扩散而形成内部空洞,出现键合根部的键合丝断裂的现象。通过改变键合区金属层结构,实现了单一金属化系统,有效避免了金属间化合物的形成。该项研究结果对陶瓷基薄膜HIC的工艺应用范围的拓宽具有参考价值。     

Wire-bond void formation during high temperature aging

Voids formed in Au-Al intermetallic phases degrade the long-term reliability of gold wire bonds to aluminum pads. In this study, a series of microstructural studies were performed to evaluate void formation in wire bonds. Voids are classified as initial, annular or minute. Probe marks and Al pad contamination are the main causes of initial voids that block alloy diffusion and slow down intermetallic growth. Annular voids are caused by the ultrasonic squeeze effect of thermosonic wire bonding. These bonding gaps may become pathways for halide species that corrode and degrade wire bonds. Minute voids are formed during the Au/sub 4/Al phase. The two Au/sub 4/Al phase textures in these voids may be due to different Au/sub 4/Al phase formation reactions or be related to grain boundary effects on the surface layer of the Au ball.     

Room temperature wedge-wedge ultrasonic bonding using aluminum coated copper wire

The purpose of this work is to evaluate the feasibility of room temperature wedge-wedge bonding using commercially available copper wires, coated with aluminum. Bonding quality, reliability and aging resistance of the wire bonds have been investigated using standard wire pull tests immediately after bonding and after accelerated life tests, including temperature storage at 125 °C, 150 °C, and 200 °C for up to 2000 h. Using focused ion beam (FIB-) preparation and high resolution electron microscopy (SEM, TEM combined with EDX X-ray analysis), results of microstructure investigations of the Al-coating/Cu wire interface as well as of the bonding interconnect formed between the coated wire and the metallization on ceramic substrate will be presented. These investigations provide background information regarding the binding mechanisms and material interactions, and contribute to assess and to avoid potential reliability risks. Due to the found advantageous bond processing behavior and increased reliability properties, our results indicate that room temperature wedge-wedge bonding of coated copper wires has a remarkable application potential, for instance in medical and other high reliability as well as high power applications. It combines all known advantages of usual copper bonding like excellent contacting behavior, high reliability and favorable material price with the possibility of processing temperature damageable components and considerable improved storage capability. Therefore, room temperature bonding using coated copper wire can also reduce cycle time, manufacturing and material costs.     

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.     

An investigation into the crystallization and electric flame-off characteristics of 20 μm copper wires

Copper wires are used in electronic packaging, however the workability and reliability still need to be improved. This work investigates the microstructural characteristics and mechanical properties of annealed wires and un-annealed wires. In addition, the interface bonding characteristics of Al pads are also studied. Experimental results indicate that at the two annealing conditions of 610 °C/0.02 s and 510 °C/0.4 s, 20 μm copper wires possessed a fully annealed structure. Compared with the un-annealed wire, the annealed tensile strength and the annealed hardness decreased, and the annealed elongation increased. Through thermal crystallization, the matrix structure transformed from long, thin grains to equiaxed grains and a few annealed twins. The microstructure of the free air ball (FAB) after an EFO process consisted of column-like grains, and grew from the heat-affected zone (HAZ) to the Cu ball. As for bonding testing, the pull strength of the bonded samples increased with increasing the Al film thickness (from 76 nm to 800 nm).     

Effects of Cu/Al intermetallic compound (IMC) on copper wire and aluminum pad bondability

Copper wire bonding is an alternative interconnection technology that serves as a viable, and cost saving alternative to gold wire bonding. Its excellent mechanical and electrical characteristics attract the high-speed, power management devices and fine-pitch applications. Copper wire bonding can be a potentially alternative interconnection technology along with flip chip interconnection. However, the growth of Cu/Al intermetallic compound (IMC) at the copper wire and aluminum interface can induce a mechanical failure and increase a potential contact resistance. In this study, the copper wire bonded chip samples were annealed at the temperature range from 150/spl deg/C to 300/spl deg/C for 2 to 250 h, respectively. The formation of Cu/Al IMC was observed and the activation energy of Cu/Al IMC growth was obtained from an Arrhenius plot (ln (growth rate) versus 1/T). The obtained activation energy was 26Kcal/mol and the behavior of IMC growth was very sensitive to the annealing temperature. To investigate the effects of IMC formation on the copper wire bondability on Al pad, ball shear tests were performed on annealed samples. For as-bonded samples, ball shear strength ranged from 240-260gf, and ball shear strength changed as a function of annealing times. For annealed samples, fracture mode changed from adhesive failure at Cu/Al interface to IMC layer or Cu wire itself. The IMC growth and the diffusion rate of aluminum and copper were closely related to failure mode changes. Micro-XRD was performed on fractured pads and balls to identify the phases of IMC and their effects on the ball bonding strength. From XRD results, it was confirmed that the major IMC was /spl gamma/-Cu/sub 9/Al/sub 4/ and it provided a strong bondability.     

银键合丝力学性能对键合质量的影响

通过调节微合金元素的含量获得3种具有不同力学性能的银键合丝.利用拉伸试验、键合试验、焊线挑断力、焊球推力测试等手段,研究了银键合丝力学性能对键合质量的影响.结果表明,在延伸率相同的条件下,随着微合金元素含量的降低,3种键合丝的断裂负荷降低,初始模量先减小后增大,键合后焊线挑断力和焊球推力均降低,电极金挤出率先减小后增大.银键合丝初始模量较低时在超声和压力的作用下易于变形,焊线内残余应力较低且第二焊点与引线框架结合较好,因此挑断测试时第二焊点与框架材料界面处不易发生脱离,有利于获得更高的键合成功率.     

Effect of annealing on the microstructure and bonding interface properties of Ag–2Pd alloy wire

This study investigated the mechanical and electrical properties of Ag–2Pd wire after thermal annealing. The thermal stability of the tested wire was examined by separately imposing static annealing at 275 °C, 325 °C and 375 °C in a vacuum environment. It was found that annealing the Ag–2Pd wire at 275 °C promoted the formation of a fully annealed structure with equiaxed grains. Annealing Ag–2Pd wire had a shorter heat affect zone (HAZ) length than those of conventional wire, and offered outstanding mechanical properties. A long-term electrical test found Ag₃(Pd)Al and Ag₂(Pd)Al compounds between the Ag–Pd ball and Al pad. These results confirmed the high-reliability properties of annealed Ag–2Pd wires for the wire bonding process.     

Development and Status of Cu Ball/Wedge Bonding in 2012

Starting in the 1980s and continuing right into the last decade, a great deal of research has been published on Cu ball/wedge (Cu B/W) wire bonding. Despite this, the technology has not been established in industrial manufacturing to any meaningful extent. Only spikes in the price of Au, improvements in equipment and techniques, and better understanding of the Cu wire-bonding process have seen Cu B/W bonding become more widespread—initially primarily for consumer goods manufacturing. Cu wire bonding is now expected to soon be used for at least 20% of all ball/wedge-bonded components, and its utilization in more sophisticated applications is around the corner. In light of this progress, the present paper comprehensively reviews the existing literature on this topic and discusses wire-bonding materials, equipment, and tools in the ongoing development of Cu B/W bonding technology. Key bonding techniques, such as flame-off, how to prevent damage to the chip (cratering), and bond formation on various common chip and substrate finishes are also described. Furthermore, apart from discussing quality assessment of Cu wire bonds in the initial state, the paper also provides an overview of Cu bonding reliability, in particular regarding Cu balls on Al metalization at high temperatures and in humidity (including under the influence of halide ions).     

铝键合丝弧高、跨度与可靠性的研究

铝丝键合作为一项半导体产品的封装工艺,被广泛用于连接半导体器件内具有铝焊盘4的芯片与其它元件。然而,如果产品内铝丝连接设计或者键合工艺参数超出铝丝材料承受能力,会降低产品的可靠性。本文以铝丝键合失效案例为起始,设计铝丝键合工艺研究试验,对铝丝所能承受的最高弧度和最大跨度进行了讨论和总结,并提出修改意见。通过对比调整前后产品内铝键合丝拉断力和拉断力标准差,证明修改后该产品可靠性明显得到了提升,也论证本文更改建议的正确性。     

Influence of surface contamination on metal/metal bond contact quality

The influence of surface cleanliness of Au/Ni coated multichip materials (MCMs), Ag plated Cu lead frames, and Al bond pads on semiconductor chips on the strength of Au wire bond contacts has been investigated. A clean surface is important for good adhesion in any kind of attachment process. Investigations by means of x-ray photoelectron spectroscopy have been performed on the bond substrates to determine the chemical composition, the nature as well as the thickness of the contamination layer. The influence of contamination on bond contact quality has been examined by pull force measurements, which is an established test method in semiconductor packaging industry for evaluating the quality of wire bonds. The results clearly show that a strong correlation between the degree of contamination of the substrate and pull strength values exists. Furthermore, a contamination thickness limiting value of 4 nm for Au and Ag substrates was determined, indicating good wire bond contact quality. The effect of plasma cleaning on wire bondability of metallic and organic (MCMs) substrates has been examined by pull force measurements. These results confirm the correlation between surface contamination and the strength of wire bond contacts for Au/Ni coated MCMs and Ag plated Cu lead frames. Atomic force microscopy measurements have been performed to determine the roughness of bond surfaces, demonstrating the importance of nanoscale characterization with regard to the bonding behavior of the substrates. Finally, bonding substrates used in integrated circuit packaging are discussed with regard to their Au wire bonding behavior. The Au wire bonding process first results in a cleaning effect of the substrate to be joined and secondly enables the change of bonding energy into frictional heat giving rise to an enhanced interdiffusion at the interface.     

封装用抗腐蚀高可靠性银合金丝

介绍一种在封装工艺中可用于替代昂贵金丝的抗腐蚀高可靠性银合金丝。利用表面钝化和固溶合金、二次中频合金熔炼和定向连续拉铸工艺,分别从成分和工艺方面提高了银合金丝的机械性能、抗腐蚀性、可靠性等一系列键合性能,解决了普通银丝在使用过程中存在的电子迁移问题,推动了键合银合金丝的广泛使用。