半导体封装行业中铜线键合工艺的应用

文章介绍了半导体封装行业中铜线键合工艺下,各材料及工艺参数(如框架、劈刀、设备参数、芯片铝层与铜材的匹配选择)对键合质量的影响,并总结提出如何更好地使用铜线这一新材料的规范要求。应用表明芯片铝层厚度应选择在0.025mm以上;劈刀应使用表面较粗糙的;铜线在键合工艺中使用体积比为95:5的氢、氮气混合保护气体;引线框架镀银层厚度应控制在0.03mm～0.06mm。     

DBC铜线键合工艺参数研究

铜线作为最有发展潜力的新一代键合材料,与铝线相比,具有优异的导电及导热能力。因绝缘栅双极型晶体管（IGBT）需承载大电流,采用铜线可在键合线数量不变的基础上提高电流传输能力和散热能力。采用铜线超声楔形键合对覆铜陶瓷基板（DBC）第一键合点和第二键合点的键合工艺参数进行研究,以剪切力作为衡量键合质量的标准,采用单因素分析法研究各参数对键合点强度的影响,采用正交试验确定最佳工艺参数,为铜线键合工艺的参数设定提供参考及指导方法。     

浅谈镀钯铜线的特性

随着金线价格一路上涨并创下历史新高,大型封装厂正在加大对铜线制程的投入。通过封装厂多年的摸索,发现镀钯铜线是金线很好的替代品。文章分析了镀钯铜线作为键合线材料本身的基本性质,镀钯铜线引线键合的特征和镀钯铜线PCT实验的可靠性。通过分析发现镀钯铜线材料本身有优良的导电和导热特性,同时还有很好的抗氧化性。镀钯铜线在键合过程中需要保护气体的保护,通过硬度实验发现镀钯铜线的硬度较大,因此需要在键合过程中防止弹坑的出现。通过PCT实验证实镀钯铜线具有较好的可靠性。     

键合功率对银线键合质量的影响

铜线键合技术是半导体封装的关键技术之一,影响键合质量的因素有很多。本文基于热压超声键合的方法,对影响铜线键合质量的主要工艺参数——键合功率,进行DOE研究分析。通过对键合后的产品进行焊点剪切失效模式、拉伸失效模式以及弹坑实验分析,研究键合功率对键合质量的影响,进而确定合理的工艺参数,最终应用于大批量生产。     

分立器件铜线的键合特点及其工艺研究

对铜线键合的优缺点及分立器件的结构特点进行了具体分析。根据分析结果,并结合具体的实验,给出了键合工艺条件和工艺参数对分立器件铜线键合过程的影响。此研究对提高分立器件铜线键合产品的质量及可靠性具有重要意义。     

铜线键合技术的发展与挑战

铜线键合技术近年来发展迅速,超细间距引线键合是目前铜线键合的主要发展趋势.介绍了铜线键合的防氧化措施以及键合参数的优化,并从IMC生长及焊盘铝挤出方面阐述了铜线键合的可靠性机理.针对铜线在超细间距引线键合中面临的问题,介绍了可解决这些问题的镀钯铜线的性能,并阐述了铜线的成弧能力及面临的挑战.     

铜引线键合中影响焊球硬度因素的研究

铜丝球焊由于其经济优势和优越的电气性能近来得到了普及,然而,在引线键合工艺中用铜丝取代金丝面临着一些技术上的挑战。多年来,IC芯片焊盘结构已经逐步适应了金丝球焊。铜在本质上比金硬度高,因此以铜线取代金线便引出了有关硬度的问题。研究了用25．4μm铜丝球焊中与键合机参数有关的铜焊球硬度特性。采用电子打火系统不同的电流和打火时间设置,用5％氢气和95％氮气组成的惰性保护气体形成了一个典型的25．4μm大小的铜焊球,研究了维氏硬度的焊球。用实验设计建立了第一和第二键合参数,进行了无空气焊球基本数据调整。通过改变电子打火系统参数。对硬度特性进行了进一步的测试。典型的键合球的大小和厚度的第一键合响应证实铜键合球的生产实力与电子打火系统的电流和打火时间有关．     

铜线键合塑封器件破坏性物理分析技术

铜线替代传统的金线键合已经成为半导体封装工艺发展的必然趋势,因其材料和制造工艺的特点,其破坏性物理分析方法不同于金线或铝线键合的器件。提出铜丝键合塑封器件破坏性物理分析的步骤及判据参照标准,讨论了器件激光开封技术的工艺步骤和参数值以及键合强度测试判据和典型断裂模式,以解决铜线键合塑封器件的破坏性物理分析问题。     

纳米硬度计在MEMS力学检测中的应用

纳米硬度计是一种能提供 10 3～ 10 2 μm尺度材料或结构微力学性能检测的先进仪器。采用纳米压痕技术 ,研究薄膜材料的弹性模量和硬度随压痕深度的变化规律以及薄膜厚度测量、微桥弯曲变形测量的方法。采用纳米划痕技术 ,研究薄膜的表面粗糙度、临界附着力和摩擦系数测量的方法。该仪器能广泛应用于MEMS的力学检测 ,并有望成为这一领域内的标准力学检测设备     

纳米硬度计在MEMS力学检测中的应用

纳米硬度计是一种能提供103～10-2μm尺度材料或结构微力学性能检测的先进仪器.采用纳米压痕技术,研究薄膜材料的弹性模量和硬度随压痕深度的变化规律以及薄膜厚度测量、微桥弯曲变形测量的方法.采用纳米划痕技术,研究薄膜的表面粗糙度、临界附着力和摩擦系数测量的方法.该仪器能广泛应用于MEMS的力学检测,并有望成为这一领域内的标准力学检测设备.     

Study of factors affecting the hardness of ball bonds in copper wire bonding

Copper wire bonding has gained popularity due to its economic advantage and superior electrical performance. However, copper is harder than gold, and replacing gold wire with copper wire introduces hardness related issues. This article reports investigations of the properties including microhardness of the copper balls bonded using ?25.4-μm copper wire and different combinations of electronic-flame-off (EFO) current and firing time settings with forming gas (5%H₂ and 95%N₂) as the inert cover gas. FABs with an identical diameter, obtained under different EFO firing conditions, were ball bonded with the same wire bonding parameters established using design of experiments. Microhardness tests were then performed on the cross-section of the bonded balls. The study revealed that ultrasonic generator current is the most significant factor to increase the bonded mashed ball diameter, ball shear and shear per unit area and to decrease the ball height. The microhardness of bonded copper balls is related to the EFO parameters, with FABs obtained by higher EFO current being softer. The lower hardness is attributed to the higher maximum temperature during the FAB melting state.     

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.     

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.     

Characteristic of copper wire and transient analysis on wirebonding process

In the present paper, two major analyses are achieved. In the first, experimental procedures were accomplished to measure tensile mechanical properties of copper (Cu) wire (? = 1 mil) before/after electric flame-off (EFO). Characteristics of free air ball (FAB), heat affected zone (HAZ) and thermal stable zone (TSZ) in as-drawn wire have been carefully investigated by microhardness, self-design pull test fixture, nanoindentation and atomic force microscopy (AFM). A 2nd EFO real-time technique has been conducted to reduce the strength of Cu wire and increase the bonding region. Secondary, with the obtained experimental material data, a comprehensive finite element wirebonding model based on explicit time integration software ANSYS/LS-DYNA is developed to predict the overall strain/stress distributions on the aluminum (Al) bond pad. Finite element analysis (FEA) results demonstrate that plastic deformation on Al bond pad around smashed FAB can be reduced by increasing the surface roughness on FAB. A series of comprehensive parametric studies were conducted in this research.     

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.     

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.     

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.     

A study on the tensile fracture mechanism of 15 μm copper wire after EFO process

In this study, 15 μm copper wires were bonded on substrates with thermosonic process, and the tensile fracture characteristics of FAB, as well as bonded samples, were investigated. For electronic packaging applications, all 15 μm wires were fully annealed, and the microstructures consisted of equiaxed grains. After EFO (electric flame-off) process, the microstructure of wire can be divided into three parts: (1) free air ball (FAB) with columnar grains, (2) heat-affected zone (HAZ) with equal-diameter grains, (3) annealing zone with equiaxed grains. According to tensile test results, EFO process simultaneously reduced UTS and elongation of the wire. For both FAB and bonded samples, the tensile fracture zones were either in the region of equal-diameter grains or in coarse grains located within 100 μm from the ball. And it was observed that the breakage sites appeared near the twins and the columnar grains when tensile fracture happened. Meanwhile, the relationship between hardness and microstructure of wires after EFO process were analyzed with nano-indentation. The nano-hardness value of 15 μm wire was 1.2-1.45 GPa.     

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.